1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // Peephole optimize the CFG.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "simplifycfg"
15 #include "llvm/Transforms/Utils/Local.h"
16 #include "llvm/Constants.h"
17 #include "llvm/DerivedTypes.h"
18 #include "llvm/GlobalVariable.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/IntrinsicInst.h"
21 #include "llvm/LLVMContext.h"
22 #include "llvm/Metadata.h"
23 #include "llvm/Operator.h"
24 #include "llvm/Type.h"
25 #include "llvm/Analysis/InstructionSimplify.h"
26 #include "llvm/Analysis/ValueTracking.h"
27 #include "llvm/Target/TargetData.h"
28 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
29 #include "llvm/ADT/DenseMap.h"
30 #include "llvm/ADT/SetVector.h"
31 #include "llvm/ADT/SmallVector.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/ADT/Statistic.h"
34 #include "llvm/ADT/STLExtras.h"
35 #include "llvm/Support/CFG.h"
36 #include "llvm/Support/CommandLine.h"
37 #include "llvm/Support/ConstantRange.h"
38 #include "llvm/Support/Debug.h"
39 #include "llvm/Support/IRBuilder.h"
40 #include "llvm/Support/NoFolder.h"
41 #include "llvm/Support/raw_ostream.h"
47 static cl::opt<unsigned>
48 PHINodeFoldingThreshold("phi-node-folding-threshold", cl::Hidden, cl::init(1),
49 cl::desc("Control the amount of phi node folding to perform (default = 1)"));
52 DupRet("simplifycfg-dup-ret", cl::Hidden, cl::init(false),
53 cl::desc("Duplicate return instructions into unconditional branches"));
55 STATISTIC(NumSpeculations, "Number of speculative executed instructions");
58 class SimplifyCFGOpt {
59 const TargetData *const TD;
61 Value *isValueEqualityComparison(TerminatorInst *TI);
62 BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI,
63 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases);
64 bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
66 IRBuilder<> &Builder);
67 bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI,
68 IRBuilder<> &Builder);
70 bool SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder);
71 bool SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder);
72 bool SimplifyUnwind(UnwindInst *UI, IRBuilder<> &Builder);
73 bool SimplifyUnreachable(UnreachableInst *UI);
74 bool SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder);
75 bool SimplifyIndirectBr(IndirectBrInst *IBI);
76 bool SimplifyUncondBranch(BranchInst *BI, IRBuilder <> &Builder);
77 bool SimplifyCondBranch(BranchInst *BI, IRBuilder <>&Builder);
80 explicit SimplifyCFGOpt(const TargetData *td) : TD(td) {}
81 bool run(BasicBlock *BB);
85 /// SafeToMergeTerminators - Return true if it is safe to merge these two
86 /// terminator instructions together.
88 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
89 if (SI1 == SI2) return false; // Can't merge with self!
91 // It is not safe to merge these two switch instructions if they have a common
92 // successor, and if that successor has a PHI node, and if *that* PHI node has
93 // conflicting incoming values from the two switch blocks.
94 BasicBlock *SI1BB = SI1->getParent();
95 BasicBlock *SI2BB = SI2->getParent();
96 SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
98 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
99 if (SI1Succs.count(*I))
100 for (BasicBlock::iterator BBI = (*I)->begin();
101 isa<PHINode>(BBI); ++BBI) {
102 PHINode *PN = cast<PHINode>(BBI);
103 if (PN->getIncomingValueForBlock(SI1BB) !=
104 PN->getIncomingValueForBlock(SI2BB))
111 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
112 /// now be entries in it from the 'NewPred' block. The values that will be
113 /// flowing into the PHI nodes will be the same as those coming in from
114 /// ExistPred, an existing predecessor of Succ.
115 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
116 BasicBlock *ExistPred) {
117 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
120 for (BasicBlock::iterator I = Succ->begin();
121 (PN = dyn_cast<PHINode>(I)); ++I)
122 PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred);
126 /// GetIfCondition - Given a basic block (BB) with two predecessors (and at
127 /// least one PHI node in it), check to see if the merge at this block is due
128 /// to an "if condition". If so, return the boolean condition that determines
129 /// which entry into BB will be taken. Also, return by references the block
130 /// that will be entered from if the condition is true, and the block that will
131 /// be entered if the condition is false.
133 /// This does no checking to see if the true/false blocks have large or unsavory
134 /// instructions in them.
135 static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
136 BasicBlock *&IfFalse) {
137 PHINode *SomePHI = cast<PHINode>(BB->begin());
138 assert(SomePHI->getNumIncomingValues() == 2 &&
139 "Function can only handle blocks with 2 predecessors!");
140 BasicBlock *Pred1 = SomePHI->getIncomingBlock(0);
141 BasicBlock *Pred2 = SomePHI->getIncomingBlock(1);
143 // We can only handle branches. Other control flow will be lowered to
144 // branches if possible anyway.
145 BranchInst *Pred1Br = dyn_cast<BranchInst>(Pred1->getTerminator());
146 BranchInst *Pred2Br = dyn_cast<BranchInst>(Pred2->getTerminator());
147 if (Pred1Br == 0 || Pred2Br == 0)
150 // Eliminate code duplication by ensuring that Pred1Br is conditional if
152 if (Pred2Br->isConditional()) {
153 // If both branches are conditional, we don't have an "if statement". In
154 // reality, we could transform this case, but since the condition will be
155 // required anyway, we stand no chance of eliminating it, so the xform is
156 // probably not profitable.
157 if (Pred1Br->isConditional())
160 std::swap(Pred1, Pred2);
161 std::swap(Pred1Br, Pred2Br);
164 if (Pred1Br->isConditional()) {
165 // The only thing we have to watch out for here is to make sure that Pred2
166 // doesn't have incoming edges from other blocks. If it does, the condition
167 // doesn't dominate BB.
168 if (Pred2->getSinglePredecessor() == 0)
171 // If we found a conditional branch predecessor, make sure that it branches
172 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
173 if (Pred1Br->getSuccessor(0) == BB &&
174 Pred1Br->getSuccessor(1) == Pred2) {
177 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
178 Pred1Br->getSuccessor(1) == BB) {
182 // We know that one arm of the conditional goes to BB, so the other must
183 // go somewhere unrelated, and this must not be an "if statement".
187 return Pred1Br->getCondition();
190 // Ok, if we got here, both predecessors end with an unconditional branch to
191 // BB. Don't panic! If both blocks only have a single (identical)
192 // predecessor, and THAT is a conditional branch, then we're all ok!
193 BasicBlock *CommonPred = Pred1->getSinglePredecessor();
194 if (CommonPred == 0 || CommonPred != Pred2->getSinglePredecessor())
197 // Otherwise, if this is a conditional branch, then we can use it!
198 BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator());
199 if (BI == 0) return 0;
201 assert(BI->isConditional() && "Two successors but not conditional?");
202 if (BI->getSuccessor(0) == Pred1) {
209 return BI->getCondition();
212 /// ComputeSpeculuationCost - Compute an abstract "cost" of speculating the
213 /// given instruction, which is assumed to be safe to speculate. 1 means
214 /// cheap, 2 means less cheap, and UINT_MAX means prohibitively expensive.
215 static unsigned ComputeSpeculationCost(const User *I) {
216 assert(isSafeToSpeculativelyExecute(I) &&
217 "Instruction is not safe to speculatively execute!");
218 switch (Operator::getOpcode(I)) {
220 // In doubt, be conservative.
222 case Instruction::GetElementPtr:
223 // GEPs are cheap if all indices are constant.
224 if (!cast<GEPOperator>(I)->hasAllConstantIndices())
227 case Instruction::Load:
228 case Instruction::Add:
229 case Instruction::Sub:
230 case Instruction::And:
231 case Instruction::Or:
232 case Instruction::Xor:
233 case Instruction::Shl:
234 case Instruction::LShr:
235 case Instruction::AShr:
236 case Instruction::ICmp:
237 case Instruction::Trunc:
238 case Instruction::ZExt:
239 case Instruction::SExt:
240 return 1; // These are all cheap.
242 case Instruction::Call:
243 case Instruction::Select:
248 /// DominatesMergePoint - If we have a merge point of an "if condition" as
249 /// accepted above, return true if the specified value dominates the block. We
250 /// don't handle the true generality of domination here, just a special case
251 /// which works well enough for us.
253 /// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
254 /// see if V (which must be an instruction) and its recursive operands
255 /// that do not dominate BB have a combined cost lower than CostRemaining and
256 /// are non-trapping. If both are true, the instruction is inserted into the
257 /// set and true is returned.
259 /// The cost for most non-trapping instructions is defined as 1 except for
260 /// Select whose cost is 2.
262 /// After this function returns, CostRemaining is decreased by the cost of
263 /// V plus its non-dominating operands. If that cost is greater than
264 /// CostRemaining, false is returned and CostRemaining is undefined.
265 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
266 SmallPtrSet<Instruction*, 4> *AggressiveInsts,
267 unsigned &CostRemaining) {
268 Instruction *I = dyn_cast<Instruction>(V);
270 // Non-instructions all dominate instructions, but not all constantexprs
271 // can be executed unconditionally.
272 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
277 BasicBlock *PBB = I->getParent();
279 // We don't want to allow weird loops that might have the "if condition" in
280 // the bottom of this block.
281 if (PBB == BB) return false;
283 // If this instruction is defined in a block that contains an unconditional
284 // branch to BB, then it must be in the 'conditional' part of the "if
285 // statement". If not, it definitely dominates the region.
286 BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator());
287 if (BI == 0 || BI->isConditional() || BI->getSuccessor(0) != BB)
290 // If we aren't allowing aggressive promotion anymore, then don't consider
291 // instructions in the 'if region'.
292 if (AggressiveInsts == 0) return false;
294 // If we have seen this instruction before, don't count it again.
295 if (AggressiveInsts->count(I)) return true;
297 // Okay, it looks like the instruction IS in the "condition". Check to
298 // see if it's a cheap instruction to unconditionally compute, and if it
299 // only uses stuff defined outside of the condition. If so, hoist it out.
300 if (!isSafeToSpeculativelyExecute(I))
303 unsigned Cost = ComputeSpeculationCost(I);
305 if (Cost > CostRemaining)
308 CostRemaining -= Cost;
310 // Okay, we can only really hoist these out if their operands do
311 // not take us over the cost threshold.
312 for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
313 if (!DominatesMergePoint(*i, BB, AggressiveInsts, CostRemaining))
315 // Okay, it's safe to do this! Remember this instruction.
316 AggressiveInsts->insert(I);
320 /// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr
321 /// and PointerNullValue. Return NULL if value is not a constant int.
322 static ConstantInt *GetConstantInt(Value *V, const TargetData *TD) {
323 // Normal constant int.
324 ConstantInt *CI = dyn_cast<ConstantInt>(V);
325 if (CI || !TD || !isa<Constant>(V) || !V->getType()->isPointerTy())
328 // This is some kind of pointer constant. Turn it into a pointer-sized
329 // ConstantInt if possible.
330 IntegerType *PtrTy = TD->getIntPtrType(V->getContext());
332 // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
333 if (isa<ConstantPointerNull>(V))
334 return ConstantInt::get(PtrTy, 0);
336 // IntToPtr const int.
337 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
338 if (CE->getOpcode() == Instruction::IntToPtr)
339 if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
340 // The constant is very likely to have the right type already.
341 if (CI->getType() == PtrTy)
344 return cast<ConstantInt>
345 (ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
350 /// GatherConstantCompares - Given a potentially 'or'd or 'and'd together
351 /// collection of icmp eq/ne instructions that compare a value against a
352 /// constant, return the value being compared, and stick the constant into the
355 GatherConstantCompares(Value *V, std::vector<ConstantInt*> &Vals, Value *&Extra,
356 const TargetData *TD, bool isEQ, unsigned &UsedICmps) {
357 Instruction *I = dyn_cast<Instruction>(V);
358 if (I == 0) return 0;
360 // If this is an icmp against a constant, handle this as one of the cases.
361 if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
362 if (ConstantInt *C = GetConstantInt(I->getOperand(1), TD)) {
363 if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ:ICmpInst::ICMP_NE)) {
366 return I->getOperand(0);
369 // If we have "x ult 3" comparison, for example, then we can add 0,1,2 to
372 ConstantRange::makeICmpRegion(ICI->getPredicate(), C->getValue());
374 // If this is an and/!= check then we want to optimize "x ugt 2" into
377 Span = Span.inverse();
379 // If there are a ton of values, we don't want to make a ginormous switch.
380 if (Span.getSetSize().ugt(8) || Span.isEmptySet() ||
381 // We don't handle wrapped sets yet.
385 for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
386 Vals.push_back(ConstantInt::get(V->getContext(), Tmp));
388 return I->getOperand(0);
393 // Otherwise, we can only handle an | or &, depending on isEQ.
394 if (I->getOpcode() != (isEQ ? Instruction::Or : Instruction::And))
397 unsigned NumValsBeforeLHS = Vals.size();
398 unsigned UsedICmpsBeforeLHS = UsedICmps;
399 if (Value *LHS = GatherConstantCompares(I->getOperand(0), Vals, Extra, TD,
401 unsigned NumVals = Vals.size();
402 unsigned UsedICmpsBeforeRHS = UsedICmps;
403 if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, TD,
407 Vals.resize(NumVals);
408 UsedICmps = UsedICmpsBeforeRHS;
411 // The RHS of the or/and can't be folded in and we haven't used "Extra" yet,
412 // set it and return success.
413 if (Extra == 0 || Extra == I->getOperand(1)) {
414 Extra = I->getOperand(1);
418 Vals.resize(NumValsBeforeLHS);
419 UsedICmps = UsedICmpsBeforeLHS;
423 // If the LHS can't be folded in, but Extra is available and RHS can, try to
425 if (Extra == 0 || Extra == I->getOperand(0)) {
426 Value *OldExtra = Extra;
427 Extra = I->getOperand(0);
428 if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, TD,
431 assert(Vals.size() == NumValsBeforeLHS);
438 static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) {
439 Instruction *Cond = 0;
440 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
441 Cond = dyn_cast<Instruction>(SI->getCondition());
442 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
443 if (BI->isConditional())
444 Cond = dyn_cast<Instruction>(BI->getCondition());
445 } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
446 Cond = dyn_cast<Instruction>(IBI->getAddress());
449 TI->eraseFromParent();
450 if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond);
453 /// isValueEqualityComparison - Return true if the specified terminator checks
454 /// to see if a value is equal to constant integer value.
455 Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) {
457 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
458 // Do not permit merging of large switch instructions into their
459 // predecessors unless there is only one predecessor.
460 if (SI->getNumSuccessors()*std::distance(pred_begin(SI->getParent()),
461 pred_end(SI->getParent())) <= 128)
462 CV = SI->getCondition();
463 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
464 if (BI->isConditional() && BI->getCondition()->hasOneUse())
465 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition()))
466 if ((ICI->getPredicate() == ICmpInst::ICMP_EQ ||
467 ICI->getPredicate() == ICmpInst::ICMP_NE) &&
468 GetConstantInt(ICI->getOperand(1), TD))
469 CV = ICI->getOperand(0);
471 // Unwrap any lossless ptrtoint cast.
472 if (TD && CV && CV->getType() == TD->getIntPtrType(CV->getContext()))
473 if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV))
474 CV = PTII->getOperand(0);
478 /// GetValueEqualityComparisonCases - Given a value comparison instruction,
479 /// decode all of the 'cases' that it represents and return the 'default' block.
480 BasicBlock *SimplifyCFGOpt::
481 GetValueEqualityComparisonCases(TerminatorInst *TI,
482 std::vector<std::pair<ConstantInt*,
483 BasicBlock*> > &Cases) {
484 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
485 Cases.reserve(SI->getNumCases());
486 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
487 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
488 return SI->getDefaultDest();
491 BranchInst *BI = cast<BranchInst>(TI);
492 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
493 Cases.push_back(std::make_pair(GetConstantInt(ICI->getOperand(1), TD),
494 BI->getSuccessor(ICI->getPredicate() ==
495 ICmpInst::ICMP_NE)));
496 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
500 /// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries
501 /// in the list that match the specified block.
502 static void EliminateBlockCases(BasicBlock *BB,
503 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
504 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
505 if (Cases[i].second == BB) {
506 Cases.erase(Cases.begin()+i);
511 /// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
514 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
515 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
516 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
518 // Make V1 be smaller than V2.
519 if (V1->size() > V2->size())
522 if (V1->size() == 0) return false;
523 if (V1->size() == 1) {
525 ConstantInt *TheVal = (*V1)[0].first;
526 for (unsigned i = 0, e = V2->size(); i != e; ++i)
527 if (TheVal == (*V2)[i].first)
531 // Otherwise, just sort both lists and compare element by element.
532 array_pod_sort(V1->begin(), V1->end());
533 array_pod_sort(V2->begin(), V2->end());
534 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
535 while (i1 != e1 && i2 != e2) {
536 if ((*V1)[i1].first == (*V2)[i2].first)
538 if ((*V1)[i1].first < (*V2)[i2].first)
546 /// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
547 /// terminator instruction and its block is known to only have a single
548 /// predecessor block, check to see if that predecessor is also a value
549 /// comparison with the same value, and if that comparison determines the
550 /// outcome of this comparison. If so, simplify TI. This does a very limited
551 /// form of jump threading.
552 bool SimplifyCFGOpt::
553 SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
555 IRBuilder<> &Builder) {
556 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
557 if (!PredVal) return false; // Not a value comparison in predecessor.
559 Value *ThisVal = isValueEqualityComparison(TI);
560 assert(ThisVal && "This isn't a value comparison!!");
561 if (ThisVal != PredVal) return false; // Different predicates.
563 // Find out information about when control will move from Pred to TI's block.
564 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
565 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
567 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
569 // Find information about how control leaves this block.
570 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
571 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
572 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
574 // If TI's block is the default block from Pred's comparison, potentially
575 // simplify TI based on this knowledge.
576 if (PredDef == TI->getParent()) {
577 // If we are here, we know that the value is none of those cases listed in
578 // PredCases. If there are any cases in ThisCases that are in PredCases, we
580 if (!ValuesOverlap(PredCases, ThisCases))
583 if (isa<BranchInst>(TI)) {
584 // Okay, one of the successors of this condbr is dead. Convert it to a
586 assert(ThisCases.size() == 1 && "Branch can only have one case!");
587 // Insert the new branch.
588 Instruction *NI = Builder.CreateBr(ThisDef);
591 // Remove PHI node entries for the dead edge.
592 ThisCases[0].second->removePredecessor(TI->getParent());
594 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
595 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
597 EraseTerminatorInstAndDCECond(TI);
601 SwitchInst *SI = cast<SwitchInst>(TI);
602 // Okay, TI has cases that are statically dead, prune them away.
603 SmallPtrSet<Constant*, 16> DeadCases;
604 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
605 DeadCases.insert(PredCases[i].first);
607 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
608 << "Through successor TI: " << *TI);
610 for (unsigned i = SI->getNumCases()-1; i != 0; --i)
611 if (DeadCases.count(SI->getCaseValue(i))) {
612 SI->getSuccessor(i)->removePredecessor(TI->getParent());
616 DEBUG(dbgs() << "Leaving: " << *TI << "\n");
620 // Otherwise, TI's block must correspond to some matched value. Find out
621 // which value (or set of values) this is.
622 ConstantInt *TIV = 0;
623 BasicBlock *TIBB = TI->getParent();
624 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
625 if (PredCases[i].second == TIBB) {
627 return false; // Cannot handle multiple values coming to this block.
628 TIV = PredCases[i].first;
630 assert(TIV && "No edge from pred to succ?");
632 // Okay, we found the one constant that our value can be if we get into TI's
633 // BB. Find out which successor will unconditionally be branched to.
634 BasicBlock *TheRealDest = 0;
635 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
636 if (ThisCases[i].first == TIV) {
637 TheRealDest = ThisCases[i].second;
641 // If not handled by any explicit cases, it is handled by the default case.
642 if (TheRealDest == 0) TheRealDest = ThisDef;
644 // Remove PHI node entries for dead edges.
645 BasicBlock *CheckEdge = TheRealDest;
646 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
647 if (*SI != CheckEdge)
648 (*SI)->removePredecessor(TIBB);
652 // Insert the new branch.
653 Instruction *NI = Builder.CreateBr(TheRealDest);
656 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
657 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
659 EraseTerminatorInstAndDCECond(TI);
664 /// ConstantIntOrdering - This class implements a stable ordering of constant
665 /// integers that does not depend on their address. This is important for
666 /// applications that sort ConstantInt's to ensure uniqueness.
667 struct ConstantIntOrdering {
668 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
669 return LHS->getValue().ult(RHS->getValue());
674 static int ConstantIntSortPredicate(const void *P1, const void *P2) {
675 const ConstantInt *LHS = *(const ConstantInt**)P1;
676 const ConstantInt *RHS = *(const ConstantInt**)P2;
677 if (LHS->getValue().ult(RHS->getValue()))
679 if (LHS->getValue() == RHS->getValue())
684 /// FoldValueComparisonIntoPredecessors - The specified terminator is a value
685 /// equality comparison instruction (either a switch or a branch on "X == c").
686 /// See if any of the predecessors of the terminator block are value comparisons
687 /// on the same value. If so, and if safe to do so, fold them together.
688 bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI,
689 IRBuilder<> &Builder) {
690 BasicBlock *BB = TI->getParent();
691 Value *CV = isValueEqualityComparison(TI); // CondVal
692 assert(CV && "Not a comparison?");
693 bool Changed = false;
695 SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
696 while (!Preds.empty()) {
697 BasicBlock *Pred = Preds.pop_back_val();
699 // See if the predecessor is a comparison with the same value.
700 TerminatorInst *PTI = Pred->getTerminator();
701 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
703 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
704 // Figure out which 'cases' to copy from SI to PSI.
705 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
706 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
708 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
709 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
711 // Based on whether the default edge from PTI goes to BB or not, fill in
712 // PredCases and PredDefault with the new switch cases we would like to
714 SmallVector<BasicBlock*, 8> NewSuccessors;
716 if (PredDefault == BB) {
717 // If this is the default destination from PTI, only the edges in TI
718 // that don't occur in PTI, or that branch to BB will be activated.
719 std::set<ConstantInt*, ConstantIntOrdering> PTIHandled;
720 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
721 if (PredCases[i].second != BB)
722 PTIHandled.insert(PredCases[i].first);
724 // The default destination is BB, we don't need explicit targets.
725 std::swap(PredCases[i], PredCases.back());
726 PredCases.pop_back();
730 // Reconstruct the new switch statement we will be building.
731 if (PredDefault != BBDefault) {
732 PredDefault->removePredecessor(Pred);
733 PredDefault = BBDefault;
734 NewSuccessors.push_back(BBDefault);
736 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
737 if (!PTIHandled.count(BBCases[i].first) &&
738 BBCases[i].second != BBDefault) {
739 PredCases.push_back(BBCases[i]);
740 NewSuccessors.push_back(BBCases[i].second);
744 // If this is not the default destination from PSI, only the edges
745 // in SI that occur in PSI with a destination of BB will be
747 std::set<ConstantInt*, ConstantIntOrdering> PTIHandled;
748 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
749 if (PredCases[i].second == BB) {
750 PTIHandled.insert(PredCases[i].first);
751 std::swap(PredCases[i], PredCases.back());
752 PredCases.pop_back();
756 // Okay, now we know which constants were sent to BB from the
757 // predecessor. Figure out where they will all go now.
758 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
759 if (PTIHandled.count(BBCases[i].first)) {
760 // If this is one we are capable of getting...
761 PredCases.push_back(BBCases[i]);
762 NewSuccessors.push_back(BBCases[i].second);
763 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
766 // If there are any constants vectored to BB that TI doesn't handle,
767 // they must go to the default destination of TI.
768 for (std::set<ConstantInt*, ConstantIntOrdering>::iterator I =
770 E = PTIHandled.end(); I != E; ++I) {
771 PredCases.push_back(std::make_pair(*I, BBDefault));
772 NewSuccessors.push_back(BBDefault);
776 // Okay, at this point, we know which new successor Pred will get. Make
777 // sure we update the number of entries in the PHI nodes for these
779 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
780 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
782 Builder.SetInsertPoint(PTI);
783 // Convert pointer to int before we switch.
784 if (CV->getType()->isPointerTy()) {
785 assert(TD && "Cannot switch on pointer without TargetData");
786 CV = Builder.CreatePtrToInt(CV, TD->getIntPtrType(CV->getContext()),
790 // Now that the successors are updated, create the new Switch instruction.
791 SwitchInst *NewSI = Builder.CreateSwitch(CV, PredDefault,
793 NewSI->setDebugLoc(PTI->getDebugLoc());
794 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
795 NewSI->addCase(PredCases[i].first, PredCases[i].second);
797 EraseTerminatorInstAndDCECond(PTI);
799 // Okay, last check. If BB is still a successor of PSI, then we must
800 // have an infinite loop case. If so, add an infinitely looping block
801 // to handle the case to preserve the behavior of the code.
802 BasicBlock *InfLoopBlock = 0;
803 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
804 if (NewSI->getSuccessor(i) == BB) {
805 if (InfLoopBlock == 0) {
806 // Insert it at the end of the function, because it's either code,
807 // or it won't matter if it's hot. :)
808 InfLoopBlock = BasicBlock::Create(BB->getContext(),
809 "infloop", BB->getParent());
810 BranchInst::Create(InfLoopBlock, InfLoopBlock);
812 NewSI->setSuccessor(i, InfLoopBlock);
821 // isSafeToHoistInvoke - If we would need to insert a select that uses the
822 // value of this invoke (comments in HoistThenElseCodeToIf explain why we
823 // would need to do this), we can't hoist the invoke, as there is nowhere
824 // to put the select in this case.
825 static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
826 Instruction *I1, Instruction *I2) {
827 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
829 for (BasicBlock::iterator BBI = SI->begin();
830 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
831 Value *BB1V = PN->getIncomingValueForBlock(BB1);
832 Value *BB2V = PN->getIncomingValueForBlock(BB2);
833 if (BB1V != BB2V && (BB1V==I1 || BB2V==I2)) {
841 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
842 /// BB2, hoist any common code in the two blocks up into the branch block. The
843 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
844 static bool HoistThenElseCodeToIf(BranchInst *BI) {
845 // This does very trivial matching, with limited scanning, to find identical
846 // instructions in the two blocks. In particular, we don't want to get into
847 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
848 // such, we currently just scan for obviously identical instructions in an
850 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
851 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
853 BasicBlock::iterator BB1_Itr = BB1->begin();
854 BasicBlock::iterator BB2_Itr = BB2->begin();
856 Instruction *I1 = BB1_Itr++, *I2 = BB2_Itr++;
857 // Skip debug info if it is not identical.
858 DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
859 DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
860 if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
861 while (isa<DbgInfoIntrinsic>(I1))
863 while (isa<DbgInfoIntrinsic>(I2))
866 if (isa<PHINode>(I1) || !I1->isIdenticalToWhenDefined(I2) ||
867 (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)))
870 // If we get here, we can hoist at least one instruction.
871 BasicBlock *BIParent = BI->getParent();
874 // If we are hoisting the terminator instruction, don't move one (making a
875 // broken BB), instead clone it, and remove BI.
876 if (isa<TerminatorInst>(I1))
877 goto HoistTerminator;
879 // For a normal instruction, we just move one to right before the branch,
880 // then replace all uses of the other with the first. Finally, we remove
881 // the now redundant second instruction.
882 BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
883 if (!I2->use_empty())
884 I2->replaceAllUsesWith(I1);
885 I1->intersectOptionalDataWith(I2);
886 I2->eraseFromParent();
890 // Skip debug info if it is not identical.
891 DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
892 DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
893 if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
894 while (isa<DbgInfoIntrinsic>(I1))
896 while (isa<DbgInfoIntrinsic>(I2))
899 } while (I1->isIdenticalToWhenDefined(I2));
904 // It may not be possible to hoist an invoke.
905 if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))
908 // Okay, it is safe to hoist the terminator.
909 Instruction *NT = I1->clone();
910 BIParent->getInstList().insert(BI, NT);
911 if (!NT->getType()->isVoidTy()) {
912 I1->replaceAllUsesWith(NT);
913 I2->replaceAllUsesWith(NT);
917 IRBuilder<true, NoFolder> Builder(NT);
918 // Hoisting one of the terminators from our successor is a great thing.
919 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
920 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
921 // nodes, so we insert select instruction to compute the final result.
922 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
923 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
925 for (BasicBlock::iterator BBI = SI->begin();
926 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
927 Value *BB1V = PN->getIncomingValueForBlock(BB1);
928 Value *BB2V = PN->getIncomingValueForBlock(BB2);
929 if (BB1V == BB2V) continue;
931 // These values do not agree. Insert a select instruction before NT
932 // that determines the right value.
933 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
935 SI = cast<SelectInst>
936 (Builder.CreateSelect(BI->getCondition(), BB1V, BB2V,
937 BB1V->getName()+"."+BB2V->getName()));
939 // Make the PHI node use the select for all incoming values for BB1/BB2
940 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
941 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
942 PN->setIncomingValue(i, SI);
946 // Update any PHI nodes in our new successors.
947 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
948 AddPredecessorToBlock(*SI, BIParent, BB1);
950 EraseTerminatorInstAndDCECond(BI);
954 /// SpeculativelyExecuteBB - Given a conditional branch that goes to BB1
955 /// and an BB2 and the only successor of BB1 is BB2, hoist simple code
956 /// (for now, restricted to a single instruction that's side effect free) from
957 /// the BB1 into the branch block to speculatively execute it.
962 /// br i1 %t1, label %BB1, label %BB2
971 /// %t3 = select i1 %t1, %t2, %t3
972 static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
973 // Only speculatively execution a single instruction (not counting the
974 // terminator) for now.
975 Instruction *HInst = NULL;
976 Instruction *Term = BB1->getTerminator();
977 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end();
979 Instruction *I = BBI;
981 if (isa<DbgInfoIntrinsic>(I)) continue;
982 if (I == Term) break;
989 BasicBlock *BIParent = BI->getParent();
991 // Check the instruction to be hoisted, if there is one.
993 // Don't hoist the instruction if it's unsafe or expensive.
994 if (!isSafeToSpeculativelyExecute(HInst))
996 if (ComputeSpeculationCost(HInst) > PHINodeFoldingThreshold)
999 // Do not hoist the instruction if any of its operands are defined but not
1000 // used in this BB. The transformation will prevent the operand from
1001 // being sunk into the use block.
1002 for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
1004 Instruction *OpI = dyn_cast<Instruction>(*i);
1005 if (OpI && OpI->getParent() == BIParent &&
1006 !OpI->mayHaveSideEffects() &&
1007 !OpI->isUsedInBasicBlock(BIParent))
1012 // Be conservative for now. FP select instruction can often be expensive.
1013 Value *BrCond = BI->getCondition();
1014 if (isa<FCmpInst>(BrCond))
1017 // If BB1 is actually on the false edge of the conditional branch, remember
1018 // to swap the select operands later.
1019 bool Invert = false;
1020 if (BB1 != BI->getSuccessor(0)) {
1021 assert(BB1 == BI->getSuccessor(1) && "No edge from 'if' block?");
1025 // Collect interesting PHIs, and scan for hazards.
1026 SmallSetVector<std::pair<Value *, Value *>, 4> PHIs;
1027 BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0);
1028 for (BasicBlock::iterator I = BB2->begin();
1029 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
1030 Value *BB1V = PN->getIncomingValueForBlock(BB1);
1031 Value *BIParentV = PN->getIncomingValueForBlock(BIParent);
1033 // Skip PHIs which are trivial.
1034 if (BB1V == BIParentV)
1037 // Check for saftey.
1038 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BB1V)) {
1039 // An unfolded ConstantExpr could end up getting expanded into
1040 // Instructions. Don't speculate this and another instruction at
1044 if (!isSafeToSpeculativelyExecute(CE))
1046 if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold)
1050 // Ok, we may insert a select for this PHI.
1051 PHIs.insert(std::make_pair(BB1V, BIParentV));
1054 // If there are no PHIs to process, bail early. This helps ensure idempotence
1059 // If we get here, we can hoist the instruction and if-convert.
1060 DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *BB1 << "\n";);
1062 // Hoist the instruction.
1064 BIParent->getInstList().splice(BI, BB1->getInstList(), HInst);
1066 // Insert selects and rewrite the PHI operands.
1067 IRBuilder<true, NoFolder> Builder(BI);
1068 for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
1069 Value *TrueV = PHIs[i].first;
1070 Value *FalseV = PHIs[i].second;
1072 // Create a select whose true value is the speculatively executed value and
1073 // false value is the previously determined FalseV.
1076 SI = cast<SelectInst>
1077 (Builder.CreateSelect(BrCond, FalseV, TrueV,
1078 FalseV->getName() + "." + TrueV->getName()));
1080 SI = cast<SelectInst>
1081 (Builder.CreateSelect(BrCond, TrueV, FalseV,
1082 TrueV->getName() + "." + FalseV->getName()));
1084 // Make the PHI node use the select for all incoming values for "then" and
1086 for (BasicBlock::iterator I = BB2->begin();
1087 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
1088 unsigned BB1I = PN->getBasicBlockIndex(BB1);
1089 unsigned BIParentI = PN->getBasicBlockIndex(BIParent);
1090 Value *BB1V = PN->getIncomingValue(BB1I);
1091 Value *BIParentV = PN->getIncomingValue(BIParentI);
1092 if (TrueV == BB1V && FalseV == BIParentV) {
1093 PN->setIncomingValue(BB1I, SI);
1094 PN->setIncomingValue(BIParentI, SI);
1103 /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
1104 /// across this block.
1105 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
1106 BranchInst *BI = cast<BranchInst>(BB->getTerminator());
1109 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
1110 if (isa<DbgInfoIntrinsic>(BBI))
1112 if (Size > 10) return false; // Don't clone large BB's.
1115 // We can only support instructions that do not define values that are
1116 // live outside of the current basic block.
1117 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
1119 Instruction *U = cast<Instruction>(*UI);
1120 if (U->getParent() != BB || isa<PHINode>(U)) return false;
1123 // Looks ok, continue checking.
1129 /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
1130 /// that is defined in the same block as the branch and if any PHI entries are
1131 /// constants, thread edges corresponding to that entry to be branches to their
1132 /// ultimate destination.
1133 static bool FoldCondBranchOnPHI(BranchInst *BI, const TargetData *TD) {
1134 BasicBlock *BB = BI->getParent();
1135 PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
1136 // NOTE: we currently cannot transform this case if the PHI node is used
1137 // outside of the block.
1138 if (!PN || PN->getParent() != BB || !PN->hasOneUse())
1141 // Degenerate case of a single entry PHI.
1142 if (PN->getNumIncomingValues() == 1) {
1143 FoldSingleEntryPHINodes(PN->getParent());
1147 // Now we know that this block has multiple preds and two succs.
1148 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
1150 // Okay, this is a simple enough basic block. See if any phi values are
1152 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1153 ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i));
1154 if (CB == 0 || !CB->getType()->isIntegerTy(1)) continue;
1156 // Okay, we now know that all edges from PredBB should be revectored to
1157 // branch to RealDest.
1158 BasicBlock *PredBB = PN->getIncomingBlock(i);
1159 BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
1161 if (RealDest == BB) continue; // Skip self loops.
1162 // Skip if the predecessor's terminator is an indirect branch.
1163 if (isa<IndirectBrInst>(PredBB->getTerminator())) continue;
1165 // The dest block might have PHI nodes, other predecessors and other
1166 // difficult cases. Instead of being smart about this, just insert a new
1167 // block that jumps to the destination block, effectively splitting
1168 // the edge we are about to create.
1169 BasicBlock *EdgeBB = BasicBlock::Create(BB->getContext(),
1170 RealDest->getName()+".critedge",
1171 RealDest->getParent(), RealDest);
1172 BranchInst::Create(RealDest, EdgeBB);
1174 // Update PHI nodes.
1175 AddPredecessorToBlock(RealDest, EdgeBB, BB);
1177 // BB may have instructions that are being threaded over. Clone these
1178 // instructions into EdgeBB. We know that there will be no uses of the
1179 // cloned instructions outside of EdgeBB.
1180 BasicBlock::iterator InsertPt = EdgeBB->begin();
1181 DenseMap<Value*, Value*> TranslateMap; // Track translated values.
1182 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
1183 if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
1184 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
1187 // Clone the instruction.
1188 Instruction *N = BBI->clone();
1189 if (BBI->hasName()) N->setName(BBI->getName()+".c");
1191 // Update operands due to translation.
1192 for (User::op_iterator i = N->op_begin(), e = N->op_end();
1194 DenseMap<Value*, Value*>::iterator PI = TranslateMap.find(*i);
1195 if (PI != TranslateMap.end())
1199 // Check for trivial simplification.
1200 if (Value *V = SimplifyInstruction(N, TD)) {
1201 TranslateMap[BBI] = V;
1202 delete N; // Instruction folded away, don't need actual inst
1204 // Insert the new instruction into its new home.
1205 EdgeBB->getInstList().insert(InsertPt, N);
1206 if (!BBI->use_empty())
1207 TranslateMap[BBI] = N;
1211 // Loop over all of the edges from PredBB to BB, changing them to branch
1212 // to EdgeBB instead.
1213 TerminatorInst *PredBBTI = PredBB->getTerminator();
1214 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
1215 if (PredBBTI->getSuccessor(i) == BB) {
1216 BB->removePredecessor(PredBB);
1217 PredBBTI->setSuccessor(i, EdgeBB);
1220 // Recurse, simplifying any other constants.
1221 return FoldCondBranchOnPHI(BI, TD) | true;
1227 /// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry
1228 /// PHI node, see if we can eliminate it.
1229 static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) {
1230 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1231 // statement", which has a very simple dominance structure. Basically, we
1232 // are trying to find the condition that is being branched on, which
1233 // subsequently causes this merge to happen. We really want control
1234 // dependence information for this check, but simplifycfg can't keep it up
1235 // to date, and this catches most of the cases we care about anyway.
1236 BasicBlock *BB = PN->getParent();
1237 BasicBlock *IfTrue, *IfFalse;
1238 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
1240 // Don't bother if the branch will be constant folded trivially.
1241 isa<ConstantInt>(IfCond))
1244 // Okay, we found that we can merge this two-entry phi node into a select.
1245 // Doing so would require us to fold *all* two entry phi nodes in this block.
1246 // At some point this becomes non-profitable (particularly if the target
1247 // doesn't support cmov's). Only do this transformation if there are two or
1248 // fewer PHI nodes in this block.
1249 unsigned NumPhis = 0;
1250 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
1254 // Loop over the PHI's seeing if we can promote them all to select
1255 // instructions. While we are at it, keep track of the instructions
1256 // that need to be moved to the dominating block.
1257 SmallPtrSet<Instruction*, 4> AggressiveInsts;
1258 unsigned MaxCostVal0 = PHINodeFoldingThreshold,
1259 MaxCostVal1 = PHINodeFoldingThreshold;
1261 for (BasicBlock::iterator II = BB->begin(); isa<PHINode>(II);) {
1262 PHINode *PN = cast<PHINode>(II++);
1263 if (Value *V = SimplifyInstruction(PN, TD)) {
1264 PN->replaceAllUsesWith(V);
1265 PN->eraseFromParent();
1269 if (!DominatesMergePoint(PN->getIncomingValue(0), BB, &AggressiveInsts,
1271 !DominatesMergePoint(PN->getIncomingValue(1), BB, &AggressiveInsts,
1276 // If we folded the the first phi, PN dangles at this point. Refresh it. If
1277 // we ran out of PHIs then we simplified them all.
1278 PN = dyn_cast<PHINode>(BB->begin());
1279 if (PN == 0) return true;
1281 // Don't fold i1 branches on PHIs which contain binary operators. These can
1282 // often be turned into switches and other things.
1283 if (PN->getType()->isIntegerTy(1) &&
1284 (isa<BinaryOperator>(PN->getIncomingValue(0)) ||
1285 isa<BinaryOperator>(PN->getIncomingValue(1)) ||
1286 isa<BinaryOperator>(IfCond)))
1289 // If we all PHI nodes are promotable, check to make sure that all
1290 // instructions in the predecessor blocks can be promoted as well. If
1291 // not, we won't be able to get rid of the control flow, so it's not
1292 // worth promoting to select instructions.
1293 BasicBlock *DomBlock = 0;
1294 BasicBlock *IfBlock1 = PN->getIncomingBlock(0);
1295 BasicBlock *IfBlock2 = PN->getIncomingBlock(1);
1296 if (cast<BranchInst>(IfBlock1->getTerminator())->isConditional()) {
1299 DomBlock = *pred_begin(IfBlock1);
1300 for (BasicBlock::iterator I = IfBlock1->begin();!isa<TerminatorInst>(I);++I)
1301 if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) {
1302 // This is not an aggressive instruction that we can promote.
1303 // Because of this, we won't be able to get rid of the control
1304 // flow, so the xform is not worth it.
1309 if (cast<BranchInst>(IfBlock2->getTerminator())->isConditional()) {
1312 DomBlock = *pred_begin(IfBlock2);
1313 for (BasicBlock::iterator I = IfBlock2->begin();!isa<TerminatorInst>(I);++I)
1314 if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) {
1315 // This is not an aggressive instruction that we can promote.
1316 // Because of this, we won't be able to get rid of the control
1317 // flow, so the xform is not worth it.
1322 DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond << " T: "
1323 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1325 // If we can still promote the PHI nodes after this gauntlet of tests,
1326 // do all of the PHI's now.
1327 Instruction *InsertPt = DomBlock->getTerminator();
1328 IRBuilder<true, NoFolder> Builder(InsertPt);
1330 // Move all 'aggressive' instructions, which are defined in the
1331 // conditional parts of the if's up to the dominating block.
1333 DomBlock->getInstList().splice(InsertPt,
1334 IfBlock1->getInstList(), IfBlock1->begin(),
1335 IfBlock1->getTerminator());
1337 DomBlock->getInstList().splice(InsertPt,
1338 IfBlock2->getInstList(), IfBlock2->begin(),
1339 IfBlock2->getTerminator());
1341 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
1342 // Change the PHI node into a select instruction.
1343 Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1344 Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1347 cast<SelectInst>(Builder.CreateSelect(IfCond, TrueVal, FalseVal, ""));
1348 PN->replaceAllUsesWith(NV);
1350 PN->eraseFromParent();
1353 // At this point, IfBlock1 and IfBlock2 are both empty, so our if statement
1354 // has been flattened. Change DomBlock to jump directly to our new block to
1355 // avoid other simplifycfg's kicking in on the diamond.
1356 TerminatorInst *OldTI = DomBlock->getTerminator();
1357 Builder.SetInsertPoint(OldTI);
1358 Builder.CreateBr(BB);
1359 OldTI->eraseFromParent();
1363 /// SimplifyCondBranchToTwoReturns - If we found a conditional branch that goes
1364 /// to two returning blocks, try to merge them together into one return,
1365 /// introducing a select if the return values disagree.
1366 static bool SimplifyCondBranchToTwoReturns(BranchInst *BI,
1367 IRBuilder<> &Builder) {
1368 assert(BI->isConditional() && "Must be a conditional branch");
1369 BasicBlock *TrueSucc = BI->getSuccessor(0);
1370 BasicBlock *FalseSucc = BI->getSuccessor(1);
1371 ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator());
1372 ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator());
1374 // Check to ensure both blocks are empty (just a return) or optionally empty
1375 // with PHI nodes. If there are other instructions, merging would cause extra
1376 // computation on one path or the other.
1377 if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator())
1379 if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator())
1382 Builder.SetInsertPoint(BI);
1383 // Okay, we found a branch that is going to two return nodes. If
1384 // there is no return value for this function, just change the
1385 // branch into a return.
1386 if (FalseRet->getNumOperands() == 0) {
1387 TrueSucc->removePredecessor(BI->getParent());
1388 FalseSucc->removePredecessor(BI->getParent());
1389 Builder.CreateRetVoid();
1390 EraseTerminatorInstAndDCECond(BI);
1394 // Otherwise, figure out what the true and false return values are
1395 // so we can insert a new select instruction.
1396 Value *TrueValue = TrueRet->getReturnValue();
1397 Value *FalseValue = FalseRet->getReturnValue();
1399 // Unwrap any PHI nodes in the return blocks.
1400 if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue))
1401 if (TVPN->getParent() == TrueSucc)
1402 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1403 if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue))
1404 if (FVPN->getParent() == FalseSucc)
1405 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1407 // In order for this transformation to be safe, we must be able to
1408 // unconditionally execute both operands to the return. This is
1409 // normally the case, but we could have a potentially-trapping
1410 // constant expression that prevents this transformation from being
1412 if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue))
1415 if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue))
1419 // Okay, we collected all the mapped values and checked them for sanity, and
1420 // defined to really do this transformation. First, update the CFG.
1421 TrueSucc->removePredecessor(BI->getParent());
1422 FalseSucc->removePredecessor(BI->getParent());
1424 // Insert select instructions where needed.
1425 Value *BrCond = BI->getCondition();
1427 // Insert a select if the results differ.
1428 if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) {
1429 } else if (isa<UndefValue>(TrueValue)) {
1430 TrueValue = FalseValue;
1432 TrueValue = Builder.CreateSelect(BrCond, TrueValue,
1433 FalseValue, "retval");
1437 Value *RI = !TrueValue ?
1438 Builder.CreateRetVoid() : Builder.CreateRet(TrueValue);
1442 DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
1443 << "\n " << *BI << "NewRet = " << *RI
1444 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc);
1446 EraseTerminatorInstAndDCECond(BI);
1451 /// ExtractBranchMetadata - Given a conditional BranchInstruction, retrieve the
1452 /// probabilities of the branch taking each edge. Fills in the two APInt
1453 /// parameters and return true, or returns false if no or invalid metadata was
1455 static bool ExtractBranchMetadata(BranchInst *BI,
1456 APInt &ProbTrue, APInt &ProbFalse) {
1457 assert(BI->isConditional() &&
1458 "Looking for probabilities on unconditional branch?");
1459 MDNode *ProfileData = BI->getMetadata(LLVMContext::MD_prof);
1460 if (!ProfileData || ProfileData->getNumOperands() != 3) return false;
1461 ConstantInt *CITrue = dyn_cast<ConstantInt>(ProfileData->getOperand(1));
1462 ConstantInt *CIFalse = dyn_cast<ConstantInt>(ProfileData->getOperand(2));
1463 if (!CITrue || !CIFalse) return false;
1464 ProbTrue = CITrue->getValue();
1465 ProbFalse = CIFalse->getValue();
1466 assert(ProbTrue.getBitWidth() == 32 && ProbFalse.getBitWidth() == 32 &&
1467 "Branch probability metadata must be 32-bit integers");
1471 /// FoldBranchToCommonDest - If this basic block is simple enough, and if a
1472 /// predecessor branches to us and one of our successors, fold the block into
1473 /// the predecessor and use logical operations to pick the right destination.
1474 bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
1475 BasicBlock *BB = BI->getParent();
1477 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
1478 if (Cond == 0 || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
1479 Cond->getParent() != BB || !Cond->hasOneUse())
1482 // Only allow this if the condition is a simple instruction that can be
1483 // executed unconditionally. It must be in the same block as the branch, and
1484 // must be at the front of the block.
1485 BasicBlock::iterator FrontIt = BB->front();
1487 // Ignore dbg intrinsics.
1488 while (isa<DbgInfoIntrinsic>(FrontIt)) ++FrontIt;
1490 // Allow a single instruction to be hoisted in addition to the compare
1491 // that feeds the branch. We later ensure that any values that _it_ uses
1492 // were also live in the predecessor, so that we don't unnecessarily create
1493 // register pressure or inhibit out-of-order execution.
1494 Instruction *BonusInst = 0;
1495 if (&*FrontIt != Cond &&
1496 FrontIt->hasOneUse() && *FrontIt->use_begin() == Cond &&
1497 isSafeToSpeculativelyExecute(FrontIt)) {
1498 BonusInst = &*FrontIt;
1501 // Ignore dbg intrinsics.
1502 while (isa<DbgInfoIntrinsic>(FrontIt)) ++FrontIt;
1505 // Only a single bonus inst is allowed.
1506 if (&*FrontIt != Cond)
1509 // Make sure the instruction after the condition is the cond branch.
1510 BasicBlock::iterator CondIt = Cond; ++CondIt;
1512 // Ingore dbg intrinsics.
1513 while (isa<DbgInfoIntrinsic>(CondIt)) ++CondIt;
1518 // Cond is known to be a compare or binary operator. Check to make sure that
1519 // neither operand is a potentially-trapping constant expression.
1520 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
1523 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
1527 // Finally, don't infinitely unroll conditional loops.
1528 BasicBlock *TrueDest = BI->getSuccessor(0);
1529 BasicBlock *FalseDest = BI->getSuccessor(1);
1530 if (TrueDest == BB || FalseDest == BB)
1533 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1534 BasicBlock *PredBlock = *PI;
1535 BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
1537 // Check that we have two conditional branches. If there is a PHI node in
1538 // the common successor, verify that the same value flows in from both
1540 if (PBI == 0 || PBI->isUnconditional() || !SafeToMergeTerminators(BI, PBI))
1543 // Determine if the two branches share a common destination.
1544 Instruction::BinaryOps Opc;
1545 bool InvertPredCond = false;
1547 if (PBI->getSuccessor(0) == TrueDest)
1548 Opc = Instruction::Or;
1549 else if (PBI->getSuccessor(1) == FalseDest)
1550 Opc = Instruction::And;
1551 else if (PBI->getSuccessor(0) == FalseDest)
1552 Opc = Instruction::And, InvertPredCond = true;
1553 else if (PBI->getSuccessor(1) == TrueDest)
1554 Opc = Instruction::Or, InvertPredCond = true;
1558 // Ensure that any values used in the bonus instruction are also used
1559 // by the terminator of the predecessor. This means that those values
1560 // must already have been resolved, so we won't be inhibiting the
1561 // out-of-order core by speculating them earlier.
1563 // Collect the values used by the bonus inst
1564 SmallPtrSet<Value*, 4> UsedValues;
1565 for (Instruction::op_iterator OI = BonusInst->op_begin(),
1566 OE = BonusInst->op_end(); OI != OE; ++OI) {
1568 if (!isa<Constant>(V))
1569 UsedValues.insert(V);
1572 SmallVector<std::pair<Value*, unsigned>, 4> Worklist;
1573 Worklist.push_back(std::make_pair(PBI->getOperand(0), 0));
1575 // Walk up to four levels back up the use-def chain of the predecessor's
1576 // terminator to see if all those values were used. The choice of four
1577 // levels is arbitrary, to provide a compile-time-cost bound.
1578 while (!Worklist.empty()) {
1579 std::pair<Value*, unsigned> Pair = Worklist.back();
1580 Worklist.pop_back();
1582 if (Pair.second >= 4) continue;
1583 UsedValues.erase(Pair.first);
1584 if (UsedValues.empty()) break;
1586 if (Instruction *I = dyn_cast<Instruction>(Pair.first)) {
1587 for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
1589 Worklist.push_back(std::make_pair(OI->get(), Pair.second+1));
1593 if (!UsedValues.empty()) return false;
1596 DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
1597 IRBuilder<> Builder(PBI);
1599 // If we need to invert the condition in the pred block to match, do so now.
1600 if (InvertPredCond) {
1601 Value *NewCond = PBI->getCondition();
1603 if (NewCond->hasOneUse() && isa<CmpInst>(NewCond)) {
1604 CmpInst *CI = cast<CmpInst>(NewCond);
1605 CI->setPredicate(CI->getInversePredicate());
1607 NewCond = Builder.CreateNot(NewCond,
1608 PBI->getCondition()->getName()+".not");
1611 PBI->setCondition(NewCond);
1612 PBI->swapSuccessors();
1615 // If we have a bonus inst, clone it into the predecessor block.
1616 Instruction *NewBonus = 0;
1618 NewBonus = BonusInst->clone();
1619 PredBlock->getInstList().insert(PBI, NewBonus);
1620 NewBonus->takeName(BonusInst);
1621 BonusInst->setName(BonusInst->getName()+".old");
1624 // Clone Cond into the predecessor basic block, and or/and the
1625 // two conditions together.
1626 Instruction *New = Cond->clone();
1627 if (BonusInst) New->replaceUsesOfWith(BonusInst, NewBonus);
1628 PredBlock->getInstList().insert(PBI, New);
1629 New->takeName(Cond);
1630 Cond->setName(New->getName()+".old");
1632 Instruction *NewCond =
1633 cast<Instruction>(Builder.CreateBinOp(Opc, PBI->getCondition(),
1635 PBI->setCondition(NewCond);
1636 if (PBI->getSuccessor(0) == BB) {
1637 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1638 PBI->setSuccessor(0, TrueDest);
1640 if (PBI->getSuccessor(1) == BB) {
1641 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1642 PBI->setSuccessor(1, FalseDest);
1645 // TODO: If BB is reachable from all paths through PredBlock, then we
1646 // could replace PBI's branch probabilities with BI's.
1648 // Merge probability data into PredBlock's branch.
1650 if (ExtractBranchMetadata(PBI, C, D) && ExtractBranchMetadata(BI, A, B)) {
1651 // Given IR which does:
1653 // br i1 %x, label %bbB, label %bbC
1655 // br i1 %y, label %bbD, label %bbC
1656 // Let's call the probability that we take the edge from %bbA to %bbB
1657 // 'a', from %bbA to %bbC, 'b', from %bbB to %bbD 'c' and from %bbB to
1658 // %bbC probability 'd'.
1660 // We transform the IR into:
1662 // br i1 %z, label %bbD, label %bbC
1663 // where the probability of going to %bbD is (a*c) and going to bbC is
1666 // Probabilities aren't stored as ratios directly. Using branch weights,
1668 // (a*c)% = A*C, (b+(a*d))% = A*D+B*C+B*D.
1670 bool Overflow1 = false, Overflow2 = false, Overflow3 = false;
1671 bool Overflow4 = false, Overflow5 = false, Overflow6 = false;
1672 APInt ProbTrue = A.umul_ov(C, Overflow1);
1674 APInt Tmp1 = A.umul_ov(D, Overflow2);
1675 APInt Tmp2 = B.umul_ov(C, Overflow3);
1676 APInt Tmp3 = B.umul_ov(D, Overflow4);
1677 APInt Tmp4 = Tmp1.uadd_ov(Tmp2, Overflow5);
1678 APInt ProbFalse = Tmp4.uadd_ov(Tmp3, Overflow6);
1680 APInt GCD = APIntOps::GreatestCommonDivisor(ProbTrue, ProbFalse);
1681 ProbTrue = ProbTrue.udiv(GCD);
1682 ProbFalse = ProbFalse.udiv(GCD);
1684 if (Overflow1 || Overflow2 || Overflow3 || Overflow4 || Overflow5 ||
1686 DEBUG(dbgs() << "Overflow recomputing branch weight on: " << *PBI
1687 << "when merging with: " << *BI);
1688 PBI->setMetadata(LLVMContext::MD_prof, NULL);
1690 LLVMContext &Context = BI->getContext();
1692 Ops[0] = BI->getMetadata(LLVMContext::MD_prof)->getOperand(0);
1693 Ops[1] = ConstantInt::get(Context, ProbTrue);
1694 Ops[2] = ConstantInt::get(Context, ProbFalse);
1695 PBI->setMetadata(LLVMContext::MD_prof, MDNode::get(Context, Ops));
1698 PBI->setMetadata(LLVMContext::MD_prof, NULL);
1701 // Copy any debug value intrinsics into the end of PredBlock.
1702 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
1703 if (isa<DbgInfoIntrinsic>(*I))
1704 I->clone()->insertBefore(PBI);
1711 /// SimplifyCondBranchToCondBranch - If we have a conditional branch as a
1712 /// predecessor of another block, this function tries to simplify it. We know
1713 /// that PBI and BI are both conditional branches, and BI is in one of the
1714 /// successor blocks of PBI - PBI branches to BI.
1715 static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
1716 assert(PBI->isConditional() && BI->isConditional());
1717 BasicBlock *BB = BI->getParent();
1719 // If this block ends with a branch instruction, and if there is a
1720 // predecessor that ends on a branch of the same condition, make
1721 // this conditional branch redundant.
1722 if (PBI->getCondition() == BI->getCondition() &&
1723 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1724 // Okay, the outcome of this conditional branch is statically
1725 // knowable. If this block had a single pred, handle specially.
1726 if (BB->getSinglePredecessor()) {
1727 // Turn this into a branch on constant.
1728 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1729 BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
1731 return true; // Nuke the branch on constant.
1734 // Otherwise, if there are multiple predecessors, insert a PHI that merges
1735 // in the constant and simplify the block result. Subsequent passes of
1736 // simplifycfg will thread the block.
1737 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1738 pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
1739 PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()),
1740 std::distance(PB, PE),
1741 BI->getCondition()->getName() + ".pr",
1743 // Okay, we're going to insert the PHI node. Since PBI is not the only
1744 // predecessor, compute the PHI'd conditional value for all of the preds.
1745 // Any predecessor where the condition is not computable we keep symbolic.
1746 for (pred_iterator PI = PB; PI != PE; ++PI) {
1747 BasicBlock *P = *PI;
1748 if ((PBI = dyn_cast<BranchInst>(P->getTerminator())) &&
1749 PBI != BI && PBI->isConditional() &&
1750 PBI->getCondition() == BI->getCondition() &&
1751 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1752 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1753 NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
1756 NewPN->addIncoming(BI->getCondition(), P);
1760 BI->setCondition(NewPN);
1765 // If this is a conditional branch in an empty block, and if any
1766 // predecessors is a conditional branch to one of our destinations,
1767 // fold the conditions into logical ops and one cond br.
1768 BasicBlock::iterator BBI = BB->begin();
1769 // Ignore dbg intrinsics.
1770 while (isa<DbgInfoIntrinsic>(BBI))
1776 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
1781 if (PBI->getSuccessor(0) == BI->getSuccessor(0))
1783 else if (PBI->getSuccessor(0) == BI->getSuccessor(1))
1784 PBIOp = 0, BIOp = 1;
1785 else if (PBI->getSuccessor(1) == BI->getSuccessor(0))
1786 PBIOp = 1, BIOp = 0;
1787 else if (PBI->getSuccessor(1) == BI->getSuccessor(1))
1792 // Check to make sure that the other destination of this branch
1793 // isn't BB itself. If so, this is an infinite loop that will
1794 // keep getting unwound.
1795 if (PBI->getSuccessor(PBIOp) == BB)
1798 // Do not perform this transformation if it would require
1799 // insertion of a large number of select instructions. For targets
1800 // without predication/cmovs, this is a big pessimization.
1801 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1803 unsigned NumPhis = 0;
1804 for (BasicBlock::iterator II = CommonDest->begin();
1805 isa<PHINode>(II); ++II, ++NumPhis)
1806 if (NumPhis > 2) // Disable this xform.
1809 // Finally, if everything is ok, fold the branches to logical ops.
1810 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
1812 DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
1813 << "AND: " << *BI->getParent());
1816 // If OtherDest *is* BB, then BB is a basic block with a single conditional
1817 // branch in it, where one edge (OtherDest) goes back to itself but the other
1818 // exits. We don't *know* that the program avoids the infinite loop
1819 // (even though that seems likely). If we do this xform naively, we'll end up
1820 // recursively unpeeling the loop. Since we know that (after the xform is
1821 // done) that the block *is* infinite if reached, we just make it an obviously
1822 // infinite loop with no cond branch.
1823 if (OtherDest == BB) {
1824 // Insert it at the end of the function, because it's either code,
1825 // or it won't matter if it's hot. :)
1826 BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(),
1827 "infloop", BB->getParent());
1828 BranchInst::Create(InfLoopBlock, InfLoopBlock);
1829 OtherDest = InfLoopBlock;
1832 DEBUG(dbgs() << *PBI->getParent()->getParent());
1834 // BI may have other predecessors. Because of this, we leave
1835 // it alone, but modify PBI.
1837 // Make sure we get to CommonDest on True&True directions.
1838 Value *PBICond = PBI->getCondition();
1839 IRBuilder<true, NoFolder> Builder(PBI);
1841 PBICond = Builder.CreateNot(PBICond, PBICond->getName()+".not");
1843 Value *BICond = BI->getCondition();
1845 BICond = Builder.CreateNot(BICond, BICond->getName()+".not");
1847 // Merge the conditions.
1848 Value *Cond = Builder.CreateOr(PBICond, BICond, "brmerge");
1850 // Modify PBI to branch on the new condition to the new dests.
1851 PBI->setCondition(Cond);
1852 PBI->setSuccessor(0, CommonDest);
1853 PBI->setSuccessor(1, OtherDest);
1855 // OtherDest may have phi nodes. If so, add an entry from PBI's
1856 // block that are identical to the entries for BI's block.
1857 AddPredecessorToBlock(OtherDest, PBI->getParent(), BB);
1859 // We know that the CommonDest already had an edge from PBI to
1860 // it. If it has PHIs though, the PHIs may have different
1861 // entries for BB and PBI's BB. If so, insert a select to make
1864 for (BasicBlock::iterator II = CommonDest->begin();
1865 (PN = dyn_cast<PHINode>(II)); ++II) {
1866 Value *BIV = PN->getIncomingValueForBlock(BB);
1867 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
1868 Value *PBIV = PN->getIncomingValue(PBBIdx);
1870 // Insert a select in PBI to pick the right value.
1871 Value *NV = cast<SelectInst>
1872 (Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName()+".mux"));
1873 PN->setIncomingValue(PBBIdx, NV);
1877 DEBUG(dbgs() << "INTO: " << *PBI->getParent());
1878 DEBUG(dbgs() << *PBI->getParent()->getParent());
1880 // This basic block is probably dead. We know it has at least
1881 // one fewer predecessor.
1885 // SimplifyTerminatorOnSelect - Simplifies a terminator by replacing it with a
1886 // branch to TrueBB if Cond is true or to FalseBB if Cond is false.
1887 // Takes care of updating the successors and removing the old terminator.
1888 // Also makes sure not to introduce new successors by assuming that edges to
1889 // non-successor TrueBBs and FalseBBs aren't reachable.
1890 static bool SimplifyTerminatorOnSelect(TerminatorInst *OldTerm, Value *Cond,
1891 BasicBlock *TrueBB, BasicBlock *FalseBB){
1892 // Remove any superfluous successor edges from the CFG.
1893 // First, figure out which successors to preserve.
1894 // If TrueBB and FalseBB are equal, only try to preserve one copy of that
1896 BasicBlock *KeepEdge1 = TrueBB;
1897 BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : 0;
1899 // Then remove the rest.
1900 for (unsigned I = 0, E = OldTerm->getNumSuccessors(); I != E; ++I) {
1901 BasicBlock *Succ = OldTerm->getSuccessor(I);
1902 // Make sure only to keep exactly one copy of each edge.
1903 if (Succ == KeepEdge1)
1905 else if (Succ == KeepEdge2)
1908 Succ->removePredecessor(OldTerm->getParent());
1911 IRBuilder<> Builder(OldTerm);
1912 Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc());
1914 // Insert an appropriate new terminator.
1915 if ((KeepEdge1 == 0) && (KeepEdge2 == 0)) {
1916 if (TrueBB == FalseBB)
1917 // We were only looking for one successor, and it was present.
1918 // Create an unconditional branch to it.
1919 Builder.CreateBr(TrueBB);
1921 // We found both of the successors we were looking for.
1922 // Create a conditional branch sharing the condition of the select.
1923 Builder.CreateCondBr(Cond, TrueBB, FalseBB);
1924 } else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
1925 // Neither of the selected blocks were successors, so this
1926 // terminator must be unreachable.
1927 new UnreachableInst(OldTerm->getContext(), OldTerm);
1929 // One of the selected values was a successor, but the other wasn't.
1930 // Insert an unconditional branch to the one that was found;
1931 // the edge to the one that wasn't must be unreachable.
1933 // Only TrueBB was found.
1934 Builder.CreateBr(TrueBB);
1936 // Only FalseBB was found.
1937 Builder.CreateBr(FalseBB);
1940 EraseTerminatorInstAndDCECond(OldTerm);
1944 // SimplifySwitchOnSelect - Replaces
1945 // (switch (select cond, X, Y)) on constant X, Y
1946 // with a branch - conditional if X and Y lead to distinct BBs,
1947 // unconditional otherwise.
1948 static bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select) {
1949 // Check for constant integer values in the select.
1950 ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
1951 ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
1952 if (!TrueVal || !FalseVal)
1955 // Find the relevant condition and destinations.
1956 Value *Condition = Select->getCondition();
1957 BasicBlock *TrueBB = SI->getSuccessor(SI->findCaseValue(TrueVal));
1958 BasicBlock *FalseBB = SI->getSuccessor(SI->findCaseValue(FalseVal));
1960 // Perform the actual simplification.
1961 return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB);
1964 // SimplifyIndirectBrOnSelect - Replaces
1965 // (indirectbr (select cond, blockaddress(@fn, BlockA),
1966 // blockaddress(@fn, BlockB)))
1968 // (br cond, BlockA, BlockB).
1969 static bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI) {
1970 // Check that both operands of the select are block addresses.
1971 BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
1972 BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
1976 // Extract the actual blocks.
1977 BasicBlock *TrueBB = TBA->getBasicBlock();
1978 BasicBlock *FalseBB = FBA->getBasicBlock();
1980 // Perform the actual simplification.
1981 return SimplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB);
1984 /// TryToSimplifyUncondBranchWithICmpInIt - This is called when we find an icmp
1985 /// instruction (a seteq/setne with a constant) as the only instruction in a
1986 /// block that ends with an uncond branch. We are looking for a very specific
1987 /// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
1988 /// this case, we merge the first two "or's of icmp" into a switch, but then the
1989 /// default value goes to an uncond block with a seteq in it, we get something
1992 /// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
1994 /// %tmp = icmp eq i8 %A, 92
1997 /// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
1999 /// We prefer to split the edge to 'end' so that there is a true/false entry to
2000 /// the PHI, merging the third icmp into the switch.
2001 static bool TryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
2002 const TargetData *TD,
2003 IRBuilder<> &Builder) {
2004 BasicBlock *BB = ICI->getParent();
2006 // If the block has any PHIs in it or the icmp has multiple uses, it is too
2008 if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse()) return false;
2010 Value *V = ICI->getOperand(0);
2011 ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
2013 // The pattern we're looking for is where our only predecessor is a switch on
2014 // 'V' and this block is the default case for the switch. In this case we can
2015 // fold the compared value into the switch to simplify things.
2016 BasicBlock *Pred = BB->getSinglePredecessor();
2017 if (Pred == 0 || !isa<SwitchInst>(Pred->getTerminator())) return false;
2019 SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
2020 if (SI->getCondition() != V)
2023 // If BB is reachable on a non-default case, then we simply know the value of
2024 // V in this block. Substitute it and constant fold the icmp instruction
2026 if (SI->getDefaultDest() != BB) {
2027 ConstantInt *VVal = SI->findCaseDest(BB);
2028 assert(VVal && "Should have a unique destination value");
2029 ICI->setOperand(0, VVal);
2031 if (Value *V = SimplifyInstruction(ICI, TD)) {
2032 ICI->replaceAllUsesWith(V);
2033 ICI->eraseFromParent();
2035 // BB is now empty, so it is likely to simplify away.
2036 return SimplifyCFG(BB) | true;
2039 // Ok, the block is reachable from the default dest. If the constant we're
2040 // comparing exists in one of the other edges, then we can constant fold ICI
2042 if (SI->findCaseValue(Cst) != 0) {
2044 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
2045 V = ConstantInt::getFalse(BB->getContext());
2047 V = ConstantInt::getTrue(BB->getContext());
2049 ICI->replaceAllUsesWith(V);
2050 ICI->eraseFromParent();
2051 // BB is now empty, so it is likely to simplify away.
2052 return SimplifyCFG(BB) | true;
2055 // The use of the icmp has to be in the 'end' block, by the only PHI node in
2057 BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
2058 PHINode *PHIUse = dyn_cast<PHINode>(ICI->use_back());
2059 if (PHIUse == 0 || PHIUse != &SuccBlock->front() ||
2060 isa<PHINode>(++BasicBlock::iterator(PHIUse)))
2063 // If the icmp is a SETEQ, then the default dest gets false, the new edge gets
2065 Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
2066 Constant *NewCst = ConstantInt::getFalse(BB->getContext());
2068 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
2069 std::swap(DefaultCst, NewCst);
2071 // Replace ICI (which is used by the PHI for the default value) with true or
2072 // false depending on if it is EQ or NE.
2073 ICI->replaceAllUsesWith(DefaultCst);
2074 ICI->eraseFromParent();
2076 // Okay, the switch goes to this block on a default value. Add an edge from
2077 // the switch to the merge point on the compared value.
2078 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "switch.edge",
2079 BB->getParent(), BB);
2080 SI->addCase(Cst, NewBB);
2082 // NewBB branches to the phi block, add the uncond branch and the phi entry.
2083 Builder.SetInsertPoint(NewBB);
2084 Builder.SetCurrentDebugLocation(SI->getDebugLoc());
2085 Builder.CreateBr(SuccBlock);
2086 PHIUse->addIncoming(NewCst, NewBB);
2090 /// SimplifyBranchOnICmpChain - The specified branch is a conditional branch.
2091 /// Check to see if it is branching on an or/and chain of icmp instructions, and
2092 /// fold it into a switch instruction if so.
2093 static bool SimplifyBranchOnICmpChain(BranchInst *BI, const TargetData *TD,
2094 IRBuilder<> &Builder) {
2095 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
2096 if (Cond == 0) return false;
2099 // Change br (X == 0 | X == 1), T, F into a switch instruction.
2100 // If this is a bunch of seteq's or'd together, or if it's a bunch of
2101 // 'setne's and'ed together, collect them.
2103 std::vector<ConstantInt*> Values;
2104 bool TrueWhenEqual = true;
2105 Value *ExtraCase = 0;
2106 unsigned UsedICmps = 0;
2108 if (Cond->getOpcode() == Instruction::Or) {
2109 CompVal = GatherConstantCompares(Cond, Values, ExtraCase, TD, true,
2111 } else if (Cond->getOpcode() == Instruction::And) {
2112 CompVal = GatherConstantCompares(Cond, Values, ExtraCase, TD, false,
2114 TrueWhenEqual = false;
2117 // If we didn't have a multiply compared value, fail.
2118 if (CompVal == 0) return false;
2120 // Avoid turning single icmps into a switch.
2124 // There might be duplicate constants in the list, which the switch
2125 // instruction can't handle, remove them now.
2126 array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate);
2127 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
2129 // If Extra was used, we require at least two switch values to do the
2130 // transformation. A switch with one value is just an cond branch.
2131 if (ExtraCase && Values.size() < 2) return false;
2133 // Figure out which block is which destination.
2134 BasicBlock *DefaultBB = BI->getSuccessor(1);
2135 BasicBlock *EdgeBB = BI->getSuccessor(0);
2136 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
2138 BasicBlock *BB = BI->getParent();
2140 DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
2141 << " cases into SWITCH. BB is:\n" << *BB);
2143 // If there are any extra values that couldn't be folded into the switch
2144 // then we evaluate them with an explicit branch first. Split the block
2145 // right before the condbr to handle it.
2147 BasicBlock *NewBB = BB->splitBasicBlock(BI, "switch.early.test");
2148 // Remove the uncond branch added to the old block.
2149 TerminatorInst *OldTI = BB->getTerminator();
2150 Builder.SetInsertPoint(OldTI);
2153 Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB);
2155 Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
2157 OldTI->eraseFromParent();
2159 // If there are PHI nodes in EdgeBB, then we need to add a new entry to them
2160 // for the edge we just added.
2161 AddPredecessorToBlock(EdgeBB, BB, NewBB);
2163 DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase
2164 << "\nEXTRABB = " << *BB);
2168 Builder.SetInsertPoint(BI);
2169 // Convert pointer to int before we switch.
2170 if (CompVal->getType()->isPointerTy()) {
2171 assert(TD && "Cannot switch on pointer without TargetData");
2172 CompVal = Builder.CreatePtrToInt(CompVal,
2173 TD->getIntPtrType(CompVal->getContext()),
2177 // Create the new switch instruction now.
2178 SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
2180 // Add all of the 'cases' to the switch instruction.
2181 for (unsigned i = 0, e = Values.size(); i != e; ++i)
2182 New->addCase(Values[i], EdgeBB);
2184 // We added edges from PI to the EdgeBB. As such, if there were any
2185 // PHI nodes in EdgeBB, they need entries to be added corresponding to
2186 // the number of edges added.
2187 for (BasicBlock::iterator BBI = EdgeBB->begin();
2188 isa<PHINode>(BBI); ++BBI) {
2189 PHINode *PN = cast<PHINode>(BBI);
2190 Value *InVal = PN->getIncomingValueForBlock(BB);
2191 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
2192 PN->addIncoming(InVal, BB);
2195 // Erase the old branch instruction.
2196 EraseTerminatorInstAndDCECond(BI);
2198 DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n');
2202 bool SimplifyCFGOpt::SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder) {
2203 // If this is a trivial landing pad that just continues unwinding the caught
2204 // exception then zap the landing pad, turning its invokes into calls.
2205 BasicBlock *BB = RI->getParent();
2206 LandingPadInst *LPInst = dyn_cast<LandingPadInst>(BB->getFirstNonPHI());
2207 if (RI->getValue() != LPInst)
2208 // Not a landing pad, or the resume is not unwinding the exception that
2209 // caused control to branch here.
2212 // Check that there are no other instructions except for debug intrinsics.
2213 BasicBlock::iterator I = LPInst, E = RI;
2215 if (!isa<DbgInfoIntrinsic>(I))
2218 // Turn all invokes that unwind here into calls and delete the basic block.
2219 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) {
2220 InvokeInst *II = cast<InvokeInst>((*PI++)->getTerminator());
2221 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
2222 // Insert a call instruction before the invoke.
2223 CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
2225 Call->setCallingConv(II->getCallingConv());
2226 Call->setAttributes(II->getAttributes());
2227 Call->setDebugLoc(II->getDebugLoc());
2229 // Anything that used the value produced by the invoke instruction now uses
2230 // the value produced by the call instruction. Note that we do this even
2231 // for void functions and calls with no uses so that the callgraph edge is
2233 II->replaceAllUsesWith(Call);
2234 BB->removePredecessor(II->getParent());
2236 // Insert a branch to the normal destination right before the invoke.
2237 BranchInst::Create(II->getNormalDest(), II);
2239 // Finally, delete the invoke instruction!
2240 II->eraseFromParent();
2243 // The landingpad is now unreachable. Zap it.
2244 BB->eraseFromParent();
2248 bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
2249 BasicBlock *BB = RI->getParent();
2250 if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false;
2252 // Find predecessors that end with branches.
2253 SmallVector<BasicBlock*, 8> UncondBranchPreds;
2254 SmallVector<BranchInst*, 8> CondBranchPreds;
2255 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
2256 BasicBlock *P = *PI;
2257 TerminatorInst *PTI = P->getTerminator();
2258 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
2259 if (BI->isUnconditional())
2260 UncondBranchPreds.push_back(P);
2262 CondBranchPreds.push_back(BI);
2266 // If we found some, do the transformation!
2267 if (!UncondBranchPreds.empty() && DupRet) {
2268 while (!UncondBranchPreds.empty()) {
2269 BasicBlock *Pred = UncondBranchPreds.pop_back_val();
2270 DEBUG(dbgs() << "FOLDING: " << *BB
2271 << "INTO UNCOND BRANCH PRED: " << *Pred);
2272 (void)FoldReturnIntoUncondBranch(RI, BB, Pred);
2275 // If we eliminated all predecessors of the block, delete the block now.
2276 if (pred_begin(BB) == pred_end(BB))
2277 // We know there are no successors, so just nuke the block.
2278 BB->eraseFromParent();
2283 // Check out all of the conditional branches going to this return
2284 // instruction. If any of them just select between returns, change the
2285 // branch itself into a select/return pair.
2286 while (!CondBranchPreds.empty()) {
2287 BranchInst *BI = CondBranchPreds.pop_back_val();
2289 // Check to see if the non-BB successor is also a return block.
2290 if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
2291 isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
2292 SimplifyCondBranchToTwoReturns(BI, Builder))
2298 bool SimplifyCFGOpt::SimplifyUnwind(UnwindInst *UI, IRBuilder<> &Builder) {
2299 // Check to see if the first instruction in this block is just an unwind.
2300 // If so, replace any invoke instructions which use this as an exception
2301 // destination with call instructions.
2302 BasicBlock *BB = UI->getParent();
2303 if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false;
2305 bool Changed = false;
2306 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
2307 while (!Preds.empty()) {
2308 BasicBlock *Pred = Preds.back();
2309 InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator());
2310 if (II && II->getUnwindDest() == BB) {
2311 // Insert a new branch instruction before the invoke, because this
2312 // is now a fall through.
2313 Builder.SetInsertPoint(II);
2314 BranchInst *BI = Builder.CreateBr(II->getNormalDest());
2315 Pred->getInstList().remove(II); // Take out of symbol table
2317 // Insert the call now.
2318 SmallVector<Value*,8> Args(II->op_begin(), II->op_end()-3);
2319 Builder.SetInsertPoint(BI);
2320 CallInst *CI = Builder.CreateCall(II->getCalledValue(),
2321 Args, II->getName());
2322 CI->setCallingConv(II->getCallingConv());
2323 CI->setAttributes(II->getAttributes());
2324 // If the invoke produced a value, the Call now does instead.
2325 II->replaceAllUsesWith(CI);
2333 // If this block is now dead (and isn't the entry block), remove it.
2334 if (pred_begin(BB) == pred_end(BB) &&
2335 BB != &BB->getParent()->getEntryBlock()) {
2336 // We know there are no successors, so just nuke the block.
2337 BB->eraseFromParent();
2344 bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
2345 BasicBlock *BB = UI->getParent();
2347 bool Changed = false;
2349 // If there are any instructions immediately before the unreachable that can
2350 // be removed, do so.
2351 while (UI != BB->begin()) {
2352 BasicBlock::iterator BBI = UI;
2354 // Do not delete instructions that can have side effects which might cause
2355 // the unreachable to not be reachable; specifically, calls and volatile
2356 // operations may have this effect.
2357 if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI)) break;
2359 if (BBI->mayHaveSideEffects()) {
2360 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
2361 if (SI->isVolatile())
2363 } else if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
2364 if (LI->isVolatile())
2366 } else if (AtomicRMWInst *RMWI = dyn_cast<AtomicRMWInst>(BBI)) {
2367 if (RMWI->isVolatile())
2369 } else if (AtomicCmpXchgInst *CXI = dyn_cast<AtomicCmpXchgInst>(BBI)) {
2370 if (CXI->isVolatile())
2372 } else if (!isa<FenceInst>(BBI) && !isa<VAArgInst>(BBI) &&
2373 !isa<LandingPadInst>(BBI)) {
2376 // Note that deleting LandingPad's here is in fact okay, although it
2377 // involves a bit of subtle reasoning. If this inst is a LandingPad,
2378 // all the predecessors of this block will be the unwind edges of Invokes,
2379 // and we can therefore guarantee this block will be erased.
2382 // Delete this instruction (any uses are guaranteed to be dead)
2383 if (!BBI->use_empty())
2384 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
2385 BBI->eraseFromParent();
2389 // If the unreachable instruction is the first in the block, take a gander
2390 // at all of the predecessors of this instruction, and simplify them.
2391 if (&BB->front() != UI) return Changed;
2393 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
2394 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
2395 TerminatorInst *TI = Preds[i]->getTerminator();
2396 IRBuilder<> Builder(TI);
2397 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
2398 if (BI->isUnconditional()) {
2399 if (BI->getSuccessor(0) == BB) {
2400 new UnreachableInst(TI->getContext(), TI);
2401 TI->eraseFromParent();
2405 if (BI->getSuccessor(0) == BB) {
2406 Builder.CreateBr(BI->getSuccessor(1));
2407 EraseTerminatorInstAndDCECond(BI);
2408 } else if (BI->getSuccessor(1) == BB) {
2409 Builder.CreateBr(BI->getSuccessor(0));
2410 EraseTerminatorInstAndDCECond(BI);
2414 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
2415 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
2416 if (SI->getSuccessor(i) == BB) {
2417 BB->removePredecessor(SI->getParent());
2422 // If the default value is unreachable, figure out the most popular
2423 // destination and make it the default.
2424 if (SI->getSuccessor(0) == BB) {
2425 std::map<BasicBlock*, std::pair<unsigned, unsigned> > Popularity;
2426 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
2427 std::pair<unsigned, unsigned> &entry =
2428 Popularity[SI->getSuccessor(i)];
2429 if (entry.first == 0) {
2437 // Find the most popular block.
2438 unsigned MaxPop = 0;
2439 unsigned MaxIndex = 0;
2440 BasicBlock *MaxBlock = 0;
2441 for (std::map<BasicBlock*, std::pair<unsigned, unsigned> >::iterator
2442 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
2443 if (I->second.first > MaxPop ||
2444 (I->second.first == MaxPop && MaxIndex > I->second.second)) {
2445 MaxPop = I->second.first;
2446 MaxIndex = I->second.second;
2447 MaxBlock = I->first;
2451 // Make this the new default, allowing us to delete any explicit
2453 SI->setSuccessor(0, MaxBlock);
2456 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
2458 if (isa<PHINode>(MaxBlock->begin()))
2459 for (unsigned i = 0; i != MaxPop-1; ++i)
2460 MaxBlock->removePredecessor(SI->getParent());
2462 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
2463 if (SI->getSuccessor(i) == MaxBlock) {
2469 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
2470 if (II->getUnwindDest() == BB) {
2471 // Convert the invoke to a call instruction. This would be a good
2472 // place to note that the call does not throw though.
2473 BranchInst *BI = Builder.CreateBr(II->getNormalDest());
2474 II->removeFromParent(); // Take out of symbol table
2476 // Insert the call now...
2477 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end()-3);
2478 Builder.SetInsertPoint(BI);
2479 CallInst *CI = Builder.CreateCall(II->getCalledValue(),
2480 Args, II->getName());
2481 CI->setCallingConv(II->getCallingConv());
2482 CI->setAttributes(II->getAttributes());
2483 // If the invoke produced a value, the call does now instead.
2484 II->replaceAllUsesWith(CI);
2491 // If this block is now dead, remove it.
2492 if (pred_begin(BB) == pred_end(BB) &&
2493 BB != &BB->getParent()->getEntryBlock()) {
2494 // We know there are no successors, so just nuke the block.
2495 BB->eraseFromParent();
2502 /// TurnSwitchRangeIntoICmp - Turns a switch with that contains only a
2503 /// integer range comparison into a sub, an icmp and a branch.
2504 static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) {
2505 assert(SI->getNumCases() > 2 && "Degenerate switch?");
2507 // Make sure all cases point to the same destination and gather the values.
2508 SmallVector<ConstantInt *, 16> Cases;
2509 Cases.push_back(SI->getCaseValue(1));
2510 for (unsigned I = 2, E = SI->getNumCases(); I != E; ++I) {
2511 if (SI->getSuccessor(I-1) != SI->getSuccessor(I))
2513 Cases.push_back(SI->getCaseValue(I));
2515 assert(Cases.size() == SI->getNumCases()-1 && "Not all cases gathered");
2517 // Sort the case values, then check if they form a range we can transform.
2518 array_pod_sort(Cases.begin(), Cases.end(), ConstantIntSortPredicate);
2519 for (unsigned I = 1, E = Cases.size(); I != E; ++I) {
2520 if (Cases[I-1]->getValue() != Cases[I]->getValue()+1)
2524 Constant *Offset = ConstantExpr::getNeg(Cases.back());
2525 Constant *NumCases = ConstantInt::get(Offset->getType(), SI->getNumCases()-1);
2527 Value *Sub = SI->getCondition();
2528 if (!Offset->isNullValue())
2529 Sub = Builder.CreateAdd(Sub, Offset, Sub->getName()+".off");
2530 Value *Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
2531 Builder.CreateCondBr(Cmp, SI->getSuccessor(1), SI->getDefaultDest());
2533 // Prune obsolete incoming values off the successor's PHI nodes.
2534 for (BasicBlock::iterator BBI = SI->getSuccessor(1)->begin();
2535 isa<PHINode>(BBI); ++BBI) {
2536 for (unsigned I = 0, E = SI->getNumCases()-2; I != E; ++I)
2537 cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
2539 SI->eraseFromParent();
2544 /// EliminateDeadSwitchCases - Compute masked bits for the condition of a switch
2545 /// and use it to remove dead cases.
2546 static bool EliminateDeadSwitchCases(SwitchInst *SI) {
2547 Value *Cond = SI->getCondition();
2548 unsigned Bits = cast<IntegerType>(Cond->getType())->getBitWidth();
2549 APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
2550 ComputeMaskedBits(Cond, APInt::getAllOnesValue(Bits), KnownZero, KnownOne);
2552 // Gather dead cases.
2553 SmallVector<ConstantInt*, 8> DeadCases;
2554 for (unsigned I = 1, E = SI->getNumCases(); I != E; ++I) {
2555 if ((SI->getCaseValue(I)->getValue() & KnownZero) != 0 ||
2556 (SI->getCaseValue(I)->getValue() & KnownOne) != KnownOne) {
2557 DeadCases.push_back(SI->getCaseValue(I));
2558 DEBUG(dbgs() << "SimplifyCFG: switch case '"
2559 << SI->getCaseValue(I)->getValue() << "' is dead.\n");
2563 // Remove dead cases from the switch.
2564 for (unsigned I = 0, E = DeadCases.size(); I != E; ++I) {
2565 unsigned Case = SI->findCaseValue(DeadCases[I]);
2566 // Prune unused values from PHI nodes.
2567 SI->getSuccessor(Case)->removePredecessor(SI->getParent());
2568 SI->removeCase(Case);
2571 return !DeadCases.empty();
2574 /// FindPHIForConditionForwarding - If BB would be eligible for simplification
2575 /// by TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
2576 /// by an unconditional branch), look at the phi node for BB in the successor
2577 /// block and see if the incoming value is equal to CaseValue. If so, return
2578 /// the phi node, and set PhiIndex to BB's index in the phi node.
2579 static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue,
2582 if (BB->getFirstNonPHIOrDbg() != BB->getTerminator())
2583 return NULL; // BB must be empty to be a candidate for simplification.
2584 if (!BB->getSinglePredecessor())
2585 return NULL; // BB must be dominated by the switch.
2587 BranchInst *Branch = dyn_cast<BranchInst>(BB->getTerminator());
2588 if (!Branch || !Branch->isUnconditional())
2589 return NULL; // Terminator must be unconditional branch.
2591 BasicBlock *Succ = Branch->getSuccessor(0);
2593 BasicBlock::iterator I = Succ->begin();
2594 while (PHINode *PHI = dyn_cast<PHINode>(I++)) {
2595 int Idx = PHI->getBasicBlockIndex(BB);
2596 assert(Idx >= 0 && "PHI has no entry for predecessor?");
2598 Value *InValue = PHI->getIncomingValue(Idx);
2599 if (InValue != CaseValue) continue;
2608 /// ForwardSwitchConditionToPHI - Try to forward the condition of a switch
2609 /// instruction to a phi node dominated by the switch, if that would mean that
2610 /// some of the destination blocks of the switch can be folded away.
2611 /// Returns true if a change is made.
2612 static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
2613 typedef DenseMap<PHINode*, SmallVector<int,4> > ForwardingNodesMap;
2614 ForwardingNodesMap ForwardingNodes;
2616 for (unsigned I = 1; I < SI->getNumCases(); ++I) { // 0 is the default case.
2617 ConstantInt *CaseValue = SI->getCaseValue(I);
2618 BasicBlock *CaseDest = SI->getSuccessor(I);
2621 PHINode *PHI = FindPHIForConditionForwarding(CaseValue, CaseDest,
2625 ForwardingNodes[PHI].push_back(PhiIndex);
2628 bool Changed = false;
2630 for (ForwardingNodesMap::iterator I = ForwardingNodes.begin(),
2631 E = ForwardingNodes.end(); I != E; ++I) {
2632 PHINode *Phi = I->first;
2633 SmallVector<int,4> &Indexes = I->second;
2635 if (Indexes.size() < 2) continue;
2637 for (size_t I = 0, E = Indexes.size(); I != E; ++I)
2638 Phi->setIncomingValue(Indexes[I], SI->getCondition());
2645 bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
2646 // If this switch is too complex to want to look at, ignore it.
2647 if (!isValueEqualityComparison(SI))
2650 BasicBlock *BB = SI->getParent();
2652 // If we only have one predecessor, and if it is a branch on this value,
2653 // see if that predecessor totally determines the outcome of this switch.
2654 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
2655 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
2656 return SimplifyCFG(BB) | true;
2658 Value *Cond = SI->getCondition();
2659 if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
2660 if (SimplifySwitchOnSelect(SI, Select))
2661 return SimplifyCFG(BB) | true;
2663 // If the block only contains the switch, see if we can fold the block
2664 // away into any preds.
2665 BasicBlock::iterator BBI = BB->begin();
2666 // Ignore dbg intrinsics.
2667 while (isa<DbgInfoIntrinsic>(BBI))
2670 if (FoldValueComparisonIntoPredecessors(SI, Builder))
2671 return SimplifyCFG(BB) | true;
2673 // Try to transform the switch into an icmp and a branch.
2674 if (TurnSwitchRangeIntoICmp(SI, Builder))
2675 return SimplifyCFG(BB) | true;
2677 // Remove unreachable cases.
2678 if (EliminateDeadSwitchCases(SI))
2679 return SimplifyCFG(BB) | true;
2681 if (ForwardSwitchConditionToPHI(SI))
2682 return SimplifyCFG(BB) | true;
2687 bool SimplifyCFGOpt::SimplifyIndirectBr(IndirectBrInst *IBI) {
2688 BasicBlock *BB = IBI->getParent();
2689 bool Changed = false;
2691 // Eliminate redundant destinations.
2692 SmallPtrSet<Value *, 8> Succs;
2693 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
2694 BasicBlock *Dest = IBI->getDestination(i);
2695 if (!Dest->hasAddressTaken() || !Succs.insert(Dest)) {
2696 Dest->removePredecessor(BB);
2697 IBI->removeDestination(i);
2703 if (IBI->getNumDestinations() == 0) {
2704 // If the indirectbr has no successors, change it to unreachable.
2705 new UnreachableInst(IBI->getContext(), IBI);
2706 EraseTerminatorInstAndDCECond(IBI);
2710 if (IBI->getNumDestinations() == 1) {
2711 // If the indirectbr has one successor, change it to a direct branch.
2712 BranchInst::Create(IBI->getDestination(0), IBI);
2713 EraseTerminatorInstAndDCECond(IBI);
2717 if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
2718 if (SimplifyIndirectBrOnSelect(IBI, SI))
2719 return SimplifyCFG(BB) | true;
2724 bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder){
2725 BasicBlock *BB = BI->getParent();
2727 // If the Terminator is the only non-phi instruction, simplify the block.
2728 BasicBlock::iterator I = BB->getFirstNonPHIOrDbgOrLifetime();
2729 if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
2730 TryToSimplifyUncondBranchFromEmptyBlock(BB))
2733 // If the only instruction in the block is a seteq/setne comparison
2734 // against a constant, try to simplify the block.
2735 if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
2736 if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
2737 for (++I; isa<DbgInfoIntrinsic>(I); ++I)
2739 if (I->isTerminator() &&
2740 TryToSimplifyUncondBranchWithICmpInIt(ICI, TD, Builder))
2748 bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
2749 BasicBlock *BB = BI->getParent();
2751 // Conditional branch
2752 if (isValueEqualityComparison(BI)) {
2753 // If we only have one predecessor, and if it is a branch on this value,
2754 // see if that predecessor totally determines the outcome of this
2756 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
2757 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
2758 return SimplifyCFG(BB) | true;
2760 // This block must be empty, except for the setcond inst, if it exists.
2761 // Ignore dbg intrinsics.
2762 BasicBlock::iterator I = BB->begin();
2763 // Ignore dbg intrinsics.
2764 while (isa<DbgInfoIntrinsic>(I))
2767 if (FoldValueComparisonIntoPredecessors(BI, Builder))
2768 return SimplifyCFG(BB) | true;
2769 } else if (&*I == cast<Instruction>(BI->getCondition())){
2771 // Ignore dbg intrinsics.
2772 while (isa<DbgInfoIntrinsic>(I))
2774 if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder))
2775 return SimplifyCFG(BB) | true;
2779 // Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction.
2780 if (SimplifyBranchOnICmpChain(BI, TD, Builder))
2783 // If this basic block is ONLY a compare and a branch, and if a predecessor
2784 // branches to us and one of our successors, fold the comparison into the
2785 // predecessor and use logical operations to pick the right destination.
2786 if (FoldBranchToCommonDest(BI))
2787 return SimplifyCFG(BB) | true;
2789 // We have a conditional branch to two blocks that are only reachable
2790 // from BI. We know that the condbr dominates the two blocks, so see if
2791 // there is any identical code in the "then" and "else" blocks. If so, we
2792 // can hoist it up to the branching block.
2793 if (BI->getSuccessor(0)->getSinglePredecessor() != 0) {
2794 if (BI->getSuccessor(1)->getSinglePredecessor() != 0) {
2795 if (HoistThenElseCodeToIf(BI))
2796 return SimplifyCFG(BB) | true;
2798 // If Successor #1 has multiple preds, we may be able to conditionally
2799 // execute Successor #0 if it branches to successor #1.
2800 TerminatorInst *Succ0TI = BI->getSuccessor(0)->getTerminator();
2801 if (Succ0TI->getNumSuccessors() == 1 &&
2802 Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
2803 if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0)))
2804 return SimplifyCFG(BB) | true;
2806 } else if (BI->getSuccessor(1)->getSinglePredecessor() != 0) {
2807 // If Successor #0 has multiple preds, we may be able to conditionally
2808 // execute Successor #1 if it branches to successor #0.
2809 TerminatorInst *Succ1TI = BI->getSuccessor(1)->getTerminator();
2810 if (Succ1TI->getNumSuccessors() == 1 &&
2811 Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
2812 if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1)))
2813 return SimplifyCFG(BB) | true;
2816 // If this is a branch on a phi node in the current block, thread control
2817 // through this block if any PHI node entries are constants.
2818 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
2819 if (PN->getParent() == BI->getParent())
2820 if (FoldCondBranchOnPHI(BI, TD))
2821 return SimplifyCFG(BB) | true;
2823 // Scan predecessor blocks for conditional branches.
2824 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
2825 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
2826 if (PBI != BI && PBI->isConditional())
2827 if (SimplifyCondBranchToCondBranch(PBI, BI))
2828 return SimplifyCFG(BB) | true;
2833 /// Check if passing a value to an instruction will cause undefined behavior.
2834 static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
2835 Constant *C = dyn_cast<Constant>(V);
2839 if (!I->hasOneUse()) // Only look at single-use instructions, for compile time
2842 if (C->isNullValue()) {
2843 Instruction *Use = I->use_back();
2845 // Now make sure that there are no instructions in between that can alter
2846 // control flow (eg. calls)
2847 for (BasicBlock::iterator i = ++BasicBlock::iterator(I); &*i != Use; ++i)
2848 if (i == I->getParent()->end() || i->mayHaveSideEffects())
2851 // Look through GEPs. A load from a GEP derived from NULL is still undefined
2852 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
2853 if (GEP->getPointerOperand() == I)
2854 return passingValueIsAlwaysUndefined(V, GEP);
2856 // Look through bitcasts.
2857 if (BitCastInst *BC = dyn_cast<BitCastInst>(Use))
2858 return passingValueIsAlwaysUndefined(V, BC);
2860 // Load from null is undefined.
2861 if (LoadInst *LI = dyn_cast<LoadInst>(Use))
2862 return LI->getPointerAddressSpace() == 0;
2864 // Store to null is undefined.
2865 if (StoreInst *SI = dyn_cast<StoreInst>(Use))
2866 return SI->getPointerAddressSpace() == 0 && SI->getPointerOperand() == I;
2871 /// If BB has an incoming value that will always trigger undefined behavior
2872 /// (eg. null pointer dereference), remove the branch leading here.
2873 static bool removeUndefIntroducingPredecessor(BasicBlock *BB) {
2874 for (BasicBlock::iterator i = BB->begin();
2875 PHINode *PHI = dyn_cast<PHINode>(i); ++i)
2876 for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i)
2877 if (passingValueIsAlwaysUndefined(PHI->getIncomingValue(i), PHI)) {
2878 TerminatorInst *T = PHI->getIncomingBlock(i)->getTerminator();
2879 IRBuilder<> Builder(T);
2880 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
2881 BB->removePredecessor(PHI->getIncomingBlock(i));
2882 // Turn uncoditional branches into unreachables and remove the dead
2883 // destination from conditional branches.
2884 if (BI->isUnconditional())
2885 Builder.CreateUnreachable();
2887 Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1) :
2888 BI->getSuccessor(0));
2889 BI->eraseFromParent();
2892 // TODO: SwitchInst.
2898 bool SimplifyCFGOpt::run(BasicBlock *BB) {
2899 bool Changed = false;
2901 assert(BB && BB->getParent() && "Block not embedded in function!");
2902 assert(BB->getTerminator() && "Degenerate basic block encountered!");
2904 // Remove basic blocks that have no predecessors (except the entry block)...
2905 // or that just have themself as a predecessor. These are unreachable.
2906 if ((pred_begin(BB) == pred_end(BB) &&
2907 BB != &BB->getParent()->getEntryBlock()) ||
2908 BB->getSinglePredecessor() == BB) {
2909 DEBUG(dbgs() << "Removing BB: \n" << *BB);
2910 DeleteDeadBlock(BB);
2914 // Check to see if we can constant propagate this terminator instruction
2916 Changed |= ConstantFoldTerminator(BB, true);
2918 // Check for and eliminate duplicate PHI nodes in this block.
2919 Changed |= EliminateDuplicatePHINodes(BB);
2921 // Check for and remove branches that will always cause undefined behavior.
2922 Changed |= removeUndefIntroducingPredecessor(BB);
2924 // Merge basic blocks into their predecessor if there is only one distinct
2925 // pred, and if there is only one distinct successor of the predecessor, and
2926 // if there are no PHI nodes.
2928 if (MergeBlockIntoPredecessor(BB))
2931 IRBuilder<> Builder(BB);
2933 // If there is a trivial two-entry PHI node in this basic block, and we can
2934 // eliminate it, do so now.
2935 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
2936 if (PN->getNumIncomingValues() == 2)
2937 Changed |= FoldTwoEntryPHINode(PN, TD);
2939 Builder.SetInsertPoint(BB->getTerminator());
2940 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
2941 if (BI->isUnconditional()) {
2942 if (SimplifyUncondBranch(BI, Builder)) return true;
2944 if (SimplifyCondBranch(BI, Builder)) return true;
2946 } else if (ResumeInst *RI = dyn_cast<ResumeInst>(BB->getTerminator())) {
2947 if (SimplifyResume(RI, Builder)) return true;
2948 } else if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
2949 if (SimplifyReturn(RI, Builder)) return true;
2950 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
2951 if (SimplifySwitch(SI, Builder)) return true;
2952 } else if (UnreachableInst *UI =
2953 dyn_cast<UnreachableInst>(BB->getTerminator())) {
2954 if (SimplifyUnreachable(UI)) return true;
2955 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->getTerminator())) {
2956 if (SimplifyUnwind(UI, Builder)) return true;
2957 } else if (IndirectBrInst *IBI =
2958 dyn_cast<IndirectBrInst>(BB->getTerminator())) {
2959 if (SimplifyIndirectBr(IBI)) return true;
2965 /// SimplifyCFG - This function is used to do simplification of a CFG. For
2966 /// example, it adjusts branches to branches to eliminate the extra hop, it
2967 /// eliminates unreachable basic blocks, and does other "peephole" optimization
2968 /// of the CFG. It returns true if a modification was made.
2970 bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) {
2971 return SimplifyCFGOpt(TD).run(BB);