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 SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder);
71 bool SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder);
72 bool SimplifyUnreachable(UnreachableInst *UI);
73 bool SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder);
74 bool SimplifyIndirectBr(IndirectBrInst *IBI);
75 bool SimplifyUncondBranch(BranchInst *BI, IRBuilder <> &Builder);
76 bool SimplifyCondBranch(BranchInst *BI, IRBuilder <>&Builder);
79 explicit SimplifyCFGOpt(const TargetData *td) : TD(td) {}
80 bool run(BasicBlock *BB);
84 /// SafeToMergeTerminators - Return true if it is safe to merge these two
85 /// terminator instructions together.
87 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
88 if (SI1 == SI2) return false; // Can't merge with self!
90 // It is not safe to merge these two switch instructions if they have a common
91 // successor, and if that successor has a PHI node, and if *that* PHI node has
92 // conflicting incoming values from the two switch blocks.
93 BasicBlock *SI1BB = SI1->getParent();
94 BasicBlock *SI2BB = SI2->getParent();
95 SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
97 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
98 if (SI1Succs.count(*I))
99 for (BasicBlock::iterator BBI = (*I)->begin();
100 isa<PHINode>(BBI); ++BBI) {
101 PHINode *PN = cast<PHINode>(BBI);
102 if (PN->getIncomingValueForBlock(SI1BB) !=
103 PN->getIncomingValueForBlock(SI2BB))
110 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
111 /// now be entries in it from the 'NewPred' block. The values that will be
112 /// flowing into the PHI nodes will be the same as those coming in from
113 /// ExistPred, an existing predecessor of Succ.
114 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
115 BasicBlock *ExistPred) {
116 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
119 for (BasicBlock::iterator I = Succ->begin();
120 (PN = dyn_cast<PHINode>(I)); ++I)
121 PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred);
125 /// GetIfCondition - Given a basic block (BB) with two predecessors (and at
126 /// least one PHI node in it), check to see if the merge at this block is due
127 /// to an "if condition". If so, return the boolean condition that determines
128 /// which entry into BB will be taken. Also, return by references the block
129 /// that will be entered from if the condition is true, and the block that will
130 /// be entered if the condition is false.
132 /// This does no checking to see if the true/false blocks have large or unsavory
133 /// instructions in them.
134 static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
135 BasicBlock *&IfFalse) {
136 PHINode *SomePHI = cast<PHINode>(BB->begin());
137 assert(SomePHI->getNumIncomingValues() == 2 &&
138 "Function can only handle blocks with 2 predecessors!");
139 BasicBlock *Pred1 = SomePHI->getIncomingBlock(0);
140 BasicBlock *Pred2 = SomePHI->getIncomingBlock(1);
142 // We can only handle branches. Other control flow will be lowered to
143 // branches if possible anyway.
144 BranchInst *Pred1Br = dyn_cast<BranchInst>(Pred1->getTerminator());
145 BranchInst *Pred2Br = dyn_cast<BranchInst>(Pred2->getTerminator());
146 if (Pred1Br == 0 || Pred2Br == 0)
149 // Eliminate code duplication by ensuring that Pred1Br is conditional if
151 if (Pred2Br->isConditional()) {
152 // If both branches are conditional, we don't have an "if statement". In
153 // reality, we could transform this case, but since the condition will be
154 // required anyway, we stand no chance of eliminating it, so the xform is
155 // probably not profitable.
156 if (Pred1Br->isConditional())
159 std::swap(Pred1, Pred2);
160 std::swap(Pred1Br, Pred2Br);
163 if (Pred1Br->isConditional()) {
164 // The only thing we have to watch out for here is to make sure that Pred2
165 // doesn't have incoming edges from other blocks. If it does, the condition
166 // doesn't dominate BB.
167 if (Pred2->getSinglePredecessor() == 0)
170 // If we found a conditional branch predecessor, make sure that it branches
171 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
172 if (Pred1Br->getSuccessor(0) == BB &&
173 Pred1Br->getSuccessor(1) == Pred2) {
176 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
177 Pred1Br->getSuccessor(1) == BB) {
181 // We know that one arm of the conditional goes to BB, so the other must
182 // go somewhere unrelated, and this must not be an "if statement".
186 return Pred1Br->getCondition();
189 // Ok, if we got here, both predecessors end with an unconditional branch to
190 // BB. Don't panic! If both blocks only have a single (identical)
191 // predecessor, and THAT is a conditional branch, then we're all ok!
192 BasicBlock *CommonPred = Pred1->getSinglePredecessor();
193 if (CommonPred == 0 || CommonPred != Pred2->getSinglePredecessor())
196 // Otherwise, if this is a conditional branch, then we can use it!
197 BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator());
198 if (BI == 0) return 0;
200 assert(BI->isConditional() && "Two successors but not conditional?");
201 if (BI->getSuccessor(0) == Pred1) {
208 return BI->getCondition();
211 /// ComputeSpeculuationCost - Compute an abstract "cost" of speculating the
212 /// given instruction, which is assumed to be safe to speculate. 1 means
213 /// cheap, 2 means less cheap, and UINT_MAX means prohibitively expensive.
214 static unsigned ComputeSpeculationCost(const User *I) {
215 assert(isSafeToSpeculativelyExecute(I) &&
216 "Instruction is not safe to speculatively execute!");
217 switch (Operator::getOpcode(I)) {
219 // In doubt, be conservative.
221 case Instruction::GetElementPtr:
222 // GEPs are cheap if all indices are constant.
223 if (!cast<GEPOperator>(I)->hasAllConstantIndices())
226 case Instruction::Load:
227 case Instruction::Add:
228 case Instruction::Sub:
229 case Instruction::And:
230 case Instruction::Or:
231 case Instruction::Xor:
232 case Instruction::Shl:
233 case Instruction::LShr:
234 case Instruction::AShr:
235 case Instruction::ICmp:
236 case Instruction::Trunc:
237 case Instruction::ZExt:
238 case Instruction::SExt:
239 return 1; // These are all cheap.
241 case Instruction::Call:
242 case Instruction::Select:
247 /// DominatesMergePoint - If we have a merge point of an "if condition" as
248 /// accepted above, return true if the specified value dominates the block. We
249 /// don't handle the true generality of domination here, just a special case
250 /// which works well enough for us.
252 /// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
253 /// see if V (which must be an instruction) and its recursive operands
254 /// that do not dominate BB have a combined cost lower than CostRemaining and
255 /// are non-trapping. If both are true, the instruction is inserted into the
256 /// set and true is returned.
258 /// The cost for most non-trapping instructions is defined as 1 except for
259 /// Select whose cost is 2.
261 /// After this function returns, CostRemaining is decreased by the cost of
262 /// V plus its non-dominating operands. If that cost is greater than
263 /// CostRemaining, false is returned and CostRemaining is undefined.
264 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
265 SmallPtrSet<Instruction*, 4> *AggressiveInsts,
266 unsigned &CostRemaining) {
267 Instruction *I = dyn_cast<Instruction>(V);
269 // Non-instructions all dominate instructions, but not all constantexprs
270 // can be executed unconditionally.
271 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
276 BasicBlock *PBB = I->getParent();
278 // We don't want to allow weird loops that might have the "if condition" in
279 // the bottom of this block.
280 if (PBB == BB) return false;
282 // If this instruction is defined in a block that contains an unconditional
283 // branch to BB, then it must be in the 'conditional' part of the "if
284 // statement". If not, it definitely dominates the region.
285 BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator());
286 if (BI == 0 || BI->isConditional() || BI->getSuccessor(0) != BB)
289 // If we aren't allowing aggressive promotion anymore, then don't consider
290 // instructions in the 'if region'.
291 if (AggressiveInsts == 0) return false;
293 // If we have seen this instruction before, don't count it again.
294 if (AggressiveInsts->count(I)) return true;
296 // Okay, it looks like the instruction IS in the "condition". Check to
297 // see if it's a cheap instruction to unconditionally compute, and if it
298 // only uses stuff defined outside of the condition. If so, hoist it out.
299 if (!isSafeToSpeculativelyExecute(I))
302 unsigned Cost = ComputeSpeculationCost(I);
304 if (Cost > CostRemaining)
307 CostRemaining -= Cost;
309 // Okay, we can only really hoist these out if their operands do
310 // not take us over the cost threshold.
311 for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
312 if (!DominatesMergePoint(*i, BB, AggressiveInsts, CostRemaining))
314 // Okay, it's safe to do this! Remember this instruction.
315 AggressiveInsts->insert(I);
319 /// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr
320 /// and PointerNullValue. Return NULL if value is not a constant int.
321 static ConstantInt *GetConstantInt(Value *V, const TargetData *TD) {
322 // Normal constant int.
323 ConstantInt *CI = dyn_cast<ConstantInt>(V);
324 if (CI || !TD || !isa<Constant>(V) || !V->getType()->isPointerTy())
327 // This is some kind of pointer constant. Turn it into a pointer-sized
328 // ConstantInt if possible.
329 IntegerType *PtrTy = TD->getIntPtrType(V->getContext());
331 // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
332 if (isa<ConstantPointerNull>(V))
333 return ConstantInt::get(PtrTy, 0);
335 // IntToPtr const int.
336 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
337 if (CE->getOpcode() == Instruction::IntToPtr)
338 if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
339 // The constant is very likely to have the right type already.
340 if (CI->getType() == PtrTy)
343 return cast<ConstantInt>
344 (ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
349 /// GatherConstantCompares - Given a potentially 'or'd or 'and'd together
350 /// collection of icmp eq/ne instructions that compare a value against a
351 /// constant, return the value being compared, and stick the constant into the
354 GatherConstantCompares(Value *V, std::vector<ConstantInt*> &Vals, Value *&Extra,
355 const TargetData *TD, bool isEQ, unsigned &UsedICmps) {
356 Instruction *I = dyn_cast<Instruction>(V);
357 if (I == 0) return 0;
359 // If this is an icmp against a constant, handle this as one of the cases.
360 if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
361 if (ConstantInt *C = GetConstantInt(I->getOperand(1), TD)) {
362 if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ:ICmpInst::ICMP_NE)) {
365 return I->getOperand(0);
368 // If we have "x ult 3" comparison, for example, then we can add 0,1,2 to
371 ConstantRange::makeICmpRegion(ICI->getPredicate(), C->getValue());
373 // If this is an and/!= check then we want to optimize "x ugt 2" into
376 Span = Span.inverse();
378 // If there are a ton of values, we don't want to make a ginormous switch.
379 if (Span.getSetSize().ugt(8) || Span.isEmptySet())
382 for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
383 Vals.push_back(ConstantInt::get(V->getContext(), Tmp));
385 return I->getOperand(0);
390 // Otherwise, we can only handle an | or &, depending on isEQ.
391 if (I->getOpcode() != (isEQ ? Instruction::Or : Instruction::And))
394 unsigned NumValsBeforeLHS = Vals.size();
395 unsigned UsedICmpsBeforeLHS = UsedICmps;
396 if (Value *LHS = GatherConstantCompares(I->getOperand(0), Vals, Extra, TD,
398 unsigned NumVals = Vals.size();
399 unsigned UsedICmpsBeforeRHS = UsedICmps;
400 if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, TD,
404 Vals.resize(NumVals);
405 UsedICmps = UsedICmpsBeforeRHS;
408 // The RHS of the or/and can't be folded in and we haven't used "Extra" yet,
409 // set it and return success.
410 if (Extra == 0 || Extra == I->getOperand(1)) {
411 Extra = I->getOperand(1);
415 Vals.resize(NumValsBeforeLHS);
416 UsedICmps = UsedICmpsBeforeLHS;
420 // If the LHS can't be folded in, but Extra is available and RHS can, try to
422 if (Extra == 0 || Extra == I->getOperand(0)) {
423 Value *OldExtra = Extra;
424 Extra = I->getOperand(0);
425 if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, TD,
428 assert(Vals.size() == NumValsBeforeLHS);
435 static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) {
436 Instruction *Cond = 0;
437 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
438 Cond = dyn_cast<Instruction>(SI->getCondition());
439 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
440 if (BI->isConditional())
441 Cond = dyn_cast<Instruction>(BI->getCondition());
442 } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
443 Cond = dyn_cast<Instruction>(IBI->getAddress());
446 TI->eraseFromParent();
447 if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond);
450 /// isValueEqualityComparison - Return true if the specified terminator checks
451 /// to see if a value is equal to constant integer value.
452 Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) {
454 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
455 // Do not permit merging of large switch instructions into their
456 // predecessors unless there is only one predecessor.
457 if (SI->getNumSuccessors()*std::distance(pred_begin(SI->getParent()),
458 pred_end(SI->getParent())) <= 128)
459 CV = SI->getCondition();
460 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
461 if (BI->isConditional() && BI->getCondition()->hasOneUse())
462 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition()))
463 if ((ICI->getPredicate() == ICmpInst::ICMP_EQ ||
464 ICI->getPredicate() == ICmpInst::ICMP_NE) &&
465 GetConstantInt(ICI->getOperand(1), TD))
466 CV = ICI->getOperand(0);
468 // Unwrap any lossless ptrtoint cast.
469 if (TD && CV && CV->getType() == TD->getIntPtrType(CV->getContext()))
470 if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV))
471 CV = PTII->getOperand(0);
475 /// GetValueEqualityComparisonCases - Given a value comparison instruction,
476 /// decode all of the 'cases' that it represents and return the 'default' block.
477 BasicBlock *SimplifyCFGOpt::
478 GetValueEqualityComparisonCases(TerminatorInst *TI,
479 std::vector<std::pair<ConstantInt*,
480 BasicBlock*> > &Cases) {
481 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
482 Cases.reserve(SI->getNumCases());
483 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i)
484 Cases.push_back(std::make_pair(i.getCaseValue(),
485 i.getCaseSuccessor()));
486 return SI->getDefaultDest();
489 BranchInst *BI = cast<BranchInst>(TI);
490 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
491 Cases.push_back(std::make_pair(GetConstantInt(ICI->getOperand(1), TD),
492 BI->getSuccessor(ICI->getPredicate() ==
493 ICmpInst::ICMP_NE)));
494 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
498 /// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries
499 /// in the list that match the specified block.
500 static void EliminateBlockCases(BasicBlock *BB,
501 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
502 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
503 if (Cases[i].second == BB) {
504 Cases.erase(Cases.begin()+i);
509 /// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
512 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
513 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
514 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
516 // Make V1 be smaller than V2.
517 if (V1->size() > V2->size())
520 if (V1->size() == 0) return false;
521 if (V1->size() == 1) {
523 ConstantInt *TheVal = (*V1)[0].first;
524 for (unsigned i = 0, e = V2->size(); i != e; ++i)
525 if (TheVal == (*V2)[i].first)
529 // Otherwise, just sort both lists and compare element by element.
530 array_pod_sort(V1->begin(), V1->end());
531 array_pod_sort(V2->begin(), V2->end());
532 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
533 while (i1 != e1 && i2 != e2) {
534 if ((*V1)[i1].first == (*V2)[i2].first)
536 if ((*V1)[i1].first < (*V2)[i2].first)
544 /// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
545 /// terminator instruction and its block is known to only have a single
546 /// predecessor block, check to see if that predecessor is also a value
547 /// comparison with the same value, and if that comparison determines the
548 /// outcome of this comparison. If so, simplify TI. This does a very limited
549 /// form of jump threading.
550 bool SimplifyCFGOpt::
551 SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
553 IRBuilder<> &Builder) {
554 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
555 if (!PredVal) return false; // Not a value comparison in predecessor.
557 Value *ThisVal = isValueEqualityComparison(TI);
558 assert(ThisVal && "This isn't a value comparison!!");
559 if (ThisVal != PredVal) return false; // Different predicates.
561 // Find out information about when control will move from Pred to TI's block.
562 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
563 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
565 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
567 // Find information about how control leaves this block.
568 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
569 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
570 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
572 // If TI's block is the default block from Pred's comparison, potentially
573 // simplify TI based on this knowledge.
574 if (PredDef == TI->getParent()) {
575 // If we are here, we know that the value is none of those cases listed in
576 // PredCases. If there are any cases in ThisCases that are in PredCases, we
578 if (!ValuesOverlap(PredCases, ThisCases))
581 if (isa<BranchInst>(TI)) {
582 // Okay, one of the successors of this condbr is dead. Convert it to a
584 assert(ThisCases.size() == 1 && "Branch can only have one case!");
585 // Insert the new branch.
586 Instruction *NI = Builder.CreateBr(ThisDef);
589 // Remove PHI node entries for the dead edge.
590 ThisCases[0].second->removePredecessor(TI->getParent());
592 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
593 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
595 EraseTerminatorInstAndDCECond(TI);
599 SwitchInst *SI = cast<SwitchInst>(TI);
600 // Okay, TI has cases that are statically dead, prune them away.
601 SmallPtrSet<Constant*, 16> DeadCases;
602 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
603 DeadCases.insert(PredCases[i].first);
605 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
606 << "Through successor TI: " << *TI);
608 for (SwitchInst::CaseIt i = SI->case_end(), e = SI->case_begin(); i != e;) {
610 if (DeadCases.count(i.getCaseValue())) {
611 i.getCaseSuccessor()->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 /// MultiplyAndLosePrecision - Multiplies A and B, then returns the result. In
1472 /// the event of overflow, logically-shifts all four inputs right until the
1474 static APInt MultiplyAndLosePrecision(APInt &A, APInt &B, APInt &C, APInt &D,
1475 unsigned &BitsLost) {
1477 bool Overflow = false;
1478 APInt Result = A.umul_ov(B, Overflow);
1480 APInt MaxB = APInt::getMaxValue(A.getBitWidth()).udiv(A);
1484 } while (B.ugt(MaxB));
1485 A = A.lshr(BitsLost);
1486 C = C.lshr(BitsLost);
1487 D = D.lshr(BitsLost);
1494 /// FoldBranchToCommonDest - If this basic block is simple enough, and if a
1495 /// predecessor branches to us and one of our successors, fold the block into
1496 /// the predecessor and use logical operations to pick the right destination.
1497 bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
1498 BasicBlock *BB = BI->getParent();
1500 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
1501 if (Cond == 0 || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
1502 Cond->getParent() != BB || !Cond->hasOneUse())
1505 // Only allow this if the condition is a simple instruction that can be
1506 // executed unconditionally. It must be in the same block as the branch, and
1507 // must be at the front of the block.
1508 BasicBlock::iterator FrontIt = BB->front();
1510 // Ignore dbg intrinsics.
1511 while (isa<DbgInfoIntrinsic>(FrontIt)) ++FrontIt;
1513 // Allow a single instruction to be hoisted in addition to the compare
1514 // that feeds the branch. We later ensure that any values that _it_ uses
1515 // were also live in the predecessor, so that we don't unnecessarily create
1516 // register pressure or inhibit out-of-order execution.
1517 Instruction *BonusInst = 0;
1518 if (&*FrontIt != Cond &&
1519 FrontIt->hasOneUse() && *FrontIt->use_begin() == Cond &&
1520 isSafeToSpeculativelyExecute(FrontIt)) {
1521 BonusInst = &*FrontIt;
1524 // Ignore dbg intrinsics.
1525 while (isa<DbgInfoIntrinsic>(FrontIt)) ++FrontIt;
1528 // Only a single bonus inst is allowed.
1529 if (&*FrontIt != Cond)
1532 // Make sure the instruction after the condition is the cond branch.
1533 BasicBlock::iterator CondIt = Cond; ++CondIt;
1535 // Ingore dbg intrinsics.
1536 while (isa<DbgInfoIntrinsic>(CondIt)) ++CondIt;
1541 // Cond is known to be a compare or binary operator. Check to make sure that
1542 // neither operand is a potentially-trapping constant expression.
1543 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
1546 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
1550 // Finally, don't infinitely unroll conditional loops.
1551 BasicBlock *TrueDest = BI->getSuccessor(0);
1552 BasicBlock *FalseDest = BI->getSuccessor(1);
1553 if (TrueDest == BB || FalseDest == BB)
1556 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1557 BasicBlock *PredBlock = *PI;
1558 BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
1560 // Check that we have two conditional branches. If there is a PHI node in
1561 // the common successor, verify that the same value flows in from both
1563 if (PBI == 0 || PBI->isUnconditional() || !SafeToMergeTerminators(BI, PBI))
1566 // Determine if the two branches share a common destination.
1567 Instruction::BinaryOps Opc;
1568 bool InvertPredCond = false;
1570 if (PBI->getSuccessor(0) == TrueDest)
1571 Opc = Instruction::Or;
1572 else if (PBI->getSuccessor(1) == FalseDest)
1573 Opc = Instruction::And;
1574 else if (PBI->getSuccessor(0) == FalseDest)
1575 Opc = Instruction::And, InvertPredCond = true;
1576 else if (PBI->getSuccessor(1) == TrueDest)
1577 Opc = Instruction::Or, InvertPredCond = true;
1581 // Ensure that any values used in the bonus instruction are also used
1582 // by the terminator of the predecessor. This means that those values
1583 // must already have been resolved, so we won't be inhibiting the
1584 // out-of-order core by speculating them earlier.
1586 // Collect the values used by the bonus inst
1587 SmallPtrSet<Value*, 4> UsedValues;
1588 for (Instruction::op_iterator OI = BonusInst->op_begin(),
1589 OE = BonusInst->op_end(); OI != OE; ++OI) {
1591 if (!isa<Constant>(V))
1592 UsedValues.insert(V);
1595 SmallVector<std::pair<Value*, unsigned>, 4> Worklist;
1596 Worklist.push_back(std::make_pair(PBI->getOperand(0), 0));
1598 // Walk up to four levels back up the use-def chain of the predecessor's
1599 // terminator to see if all those values were used. The choice of four
1600 // levels is arbitrary, to provide a compile-time-cost bound.
1601 while (!Worklist.empty()) {
1602 std::pair<Value*, unsigned> Pair = Worklist.back();
1603 Worklist.pop_back();
1605 if (Pair.second >= 4) continue;
1606 UsedValues.erase(Pair.first);
1607 if (UsedValues.empty()) break;
1609 if (Instruction *I = dyn_cast<Instruction>(Pair.first)) {
1610 for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
1612 Worklist.push_back(std::make_pair(OI->get(), Pair.second+1));
1616 if (!UsedValues.empty()) return false;
1619 DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
1620 IRBuilder<> Builder(PBI);
1622 // If we need to invert the condition in the pred block to match, do so now.
1623 if (InvertPredCond) {
1624 Value *NewCond = PBI->getCondition();
1626 if (NewCond->hasOneUse() && isa<CmpInst>(NewCond)) {
1627 CmpInst *CI = cast<CmpInst>(NewCond);
1628 CI->setPredicate(CI->getInversePredicate());
1630 NewCond = Builder.CreateNot(NewCond,
1631 PBI->getCondition()->getName()+".not");
1634 PBI->setCondition(NewCond);
1635 PBI->swapSuccessors();
1638 // If we have a bonus inst, clone it into the predecessor block.
1639 Instruction *NewBonus = 0;
1641 NewBonus = BonusInst->clone();
1642 PredBlock->getInstList().insert(PBI, NewBonus);
1643 NewBonus->takeName(BonusInst);
1644 BonusInst->setName(BonusInst->getName()+".old");
1647 // Clone Cond into the predecessor basic block, and or/and the
1648 // two conditions together.
1649 Instruction *New = Cond->clone();
1650 if (BonusInst) New->replaceUsesOfWith(BonusInst, NewBonus);
1651 PredBlock->getInstList().insert(PBI, New);
1652 New->takeName(Cond);
1653 Cond->setName(New->getName()+".old");
1655 Instruction *NewCond =
1656 cast<Instruction>(Builder.CreateBinOp(Opc, PBI->getCondition(),
1658 PBI->setCondition(NewCond);
1659 if (PBI->getSuccessor(0) == BB) {
1660 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1661 PBI->setSuccessor(0, TrueDest);
1663 if (PBI->getSuccessor(1) == BB) {
1664 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1665 PBI->setSuccessor(1, FalseDest);
1668 // TODO: If BB is reachable from all paths through PredBlock, then we
1669 // could replace PBI's branch probabilities with BI's.
1671 // Merge probability data into PredBlock's branch.
1673 if (ExtractBranchMetadata(PBI, C, D) && ExtractBranchMetadata(BI, A, B)) {
1674 // Given IR which does:
1676 // br i1 %x, label %bbB, label %bbC
1678 // br i1 %y, label %bbD, label %bbC
1679 // Let's call the probability that we take the edge from %bbA to %bbB
1680 // 'a', from %bbA to %bbC, 'b', from %bbB to %bbD 'c' and from %bbB to
1681 // %bbC probability 'd'.
1683 // We transform the IR into:
1685 // br i1 %z, label %bbD, label %bbC
1686 // where the probability of going to %bbD is (a*c) and going to bbC is
1689 // Probabilities aren't stored as ratios directly. Using branch weights,
1691 // (a*c)% = A*C, (b+(a*d))% = A*D+B*C+B*D.
1693 // In the event of overflow, we want to drop the LSB of the input
1697 // Ignore overflow result on ProbTrue.
1698 APInt ProbTrue = MultiplyAndLosePrecision(A, C, B, D, BitsLost);
1700 APInt Tmp1 = MultiplyAndLosePrecision(B, D, A, C, BitsLost);
1702 ProbTrue = ProbTrue.lshr(BitsLost*2);
1705 APInt Tmp2 = MultiplyAndLosePrecision(A, D, C, B, BitsLost);
1707 ProbTrue = ProbTrue.lshr(BitsLost*2);
1708 Tmp1 = Tmp1.lshr(BitsLost*2);
1711 APInt Tmp3 = MultiplyAndLosePrecision(B, C, A, D, BitsLost);
1713 ProbTrue = ProbTrue.lshr(BitsLost*2);
1714 Tmp1 = Tmp1.lshr(BitsLost*2);
1715 Tmp2 = Tmp2.lshr(BitsLost*2);
1718 bool Overflow1 = false, Overflow2 = false;
1719 APInt Tmp4 = Tmp2.uadd_ov(Tmp3, Overflow1);
1720 APInt ProbFalse = Tmp4.uadd_ov(Tmp1, Overflow2);
1722 if (Overflow1 || Overflow2) {
1723 ProbTrue = ProbTrue.lshr(1);
1724 Tmp1 = Tmp1.lshr(1);
1725 Tmp2 = Tmp2.lshr(1);
1726 Tmp3 = Tmp3.lshr(1);
1728 ProbFalse = Tmp4 + Tmp1;
1731 // The sum of branch weights must fit in 32-bits.
1732 if (ProbTrue.isNegative() && ProbFalse.isNegative()) {
1733 ProbTrue = ProbTrue.lshr(1);
1734 ProbFalse = ProbFalse.lshr(1);
1737 if (ProbTrue != ProbFalse) {
1738 // Normalize the result.
1739 APInt GCD = APIntOps::GreatestCommonDivisor(ProbTrue, ProbFalse);
1740 ProbTrue = ProbTrue.udiv(GCD);
1741 ProbFalse = ProbFalse.udiv(GCD);
1743 LLVMContext &Context = BI->getContext();
1745 Ops[0] = BI->getMetadata(LLVMContext::MD_prof)->getOperand(0);
1746 Ops[1] = ConstantInt::get(Context, ProbTrue);
1747 Ops[2] = ConstantInt::get(Context, ProbFalse);
1748 PBI->setMetadata(LLVMContext::MD_prof, MDNode::get(Context, Ops));
1750 PBI->setMetadata(LLVMContext::MD_prof, NULL);
1753 PBI->setMetadata(LLVMContext::MD_prof, NULL);
1756 // Copy any debug value intrinsics into the end of PredBlock.
1757 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
1758 if (isa<DbgInfoIntrinsic>(*I))
1759 I->clone()->insertBefore(PBI);
1766 /// SimplifyCondBranchToCondBranch - If we have a conditional branch as a
1767 /// predecessor of another block, this function tries to simplify it. We know
1768 /// that PBI and BI are both conditional branches, and BI is in one of the
1769 /// successor blocks of PBI - PBI branches to BI.
1770 static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
1771 assert(PBI->isConditional() && BI->isConditional());
1772 BasicBlock *BB = BI->getParent();
1774 // If this block ends with a branch instruction, and if there is a
1775 // predecessor that ends on a branch of the same condition, make
1776 // this conditional branch redundant.
1777 if (PBI->getCondition() == BI->getCondition() &&
1778 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1779 // Okay, the outcome of this conditional branch is statically
1780 // knowable. If this block had a single pred, handle specially.
1781 if (BB->getSinglePredecessor()) {
1782 // Turn this into a branch on constant.
1783 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1784 BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
1786 return true; // Nuke the branch on constant.
1789 // Otherwise, if there are multiple predecessors, insert a PHI that merges
1790 // in the constant and simplify the block result. Subsequent passes of
1791 // simplifycfg will thread the block.
1792 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1793 pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
1794 PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()),
1795 std::distance(PB, PE),
1796 BI->getCondition()->getName() + ".pr",
1798 // Okay, we're going to insert the PHI node. Since PBI is not the only
1799 // predecessor, compute the PHI'd conditional value for all of the preds.
1800 // Any predecessor where the condition is not computable we keep symbolic.
1801 for (pred_iterator PI = PB; PI != PE; ++PI) {
1802 BasicBlock *P = *PI;
1803 if ((PBI = dyn_cast<BranchInst>(P->getTerminator())) &&
1804 PBI != BI && PBI->isConditional() &&
1805 PBI->getCondition() == BI->getCondition() &&
1806 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1807 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1808 NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
1811 NewPN->addIncoming(BI->getCondition(), P);
1815 BI->setCondition(NewPN);
1820 // If this is a conditional branch in an empty block, and if any
1821 // predecessors is a conditional branch to one of our destinations,
1822 // fold the conditions into logical ops and one cond br.
1823 BasicBlock::iterator BBI = BB->begin();
1824 // Ignore dbg intrinsics.
1825 while (isa<DbgInfoIntrinsic>(BBI))
1831 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
1836 if (PBI->getSuccessor(0) == BI->getSuccessor(0))
1838 else if (PBI->getSuccessor(0) == BI->getSuccessor(1))
1839 PBIOp = 0, BIOp = 1;
1840 else if (PBI->getSuccessor(1) == BI->getSuccessor(0))
1841 PBIOp = 1, BIOp = 0;
1842 else if (PBI->getSuccessor(1) == BI->getSuccessor(1))
1847 // Check to make sure that the other destination of this branch
1848 // isn't BB itself. If so, this is an infinite loop that will
1849 // keep getting unwound.
1850 if (PBI->getSuccessor(PBIOp) == BB)
1853 // Do not perform this transformation if it would require
1854 // insertion of a large number of select instructions. For targets
1855 // without predication/cmovs, this is a big pessimization.
1856 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1858 unsigned NumPhis = 0;
1859 for (BasicBlock::iterator II = CommonDest->begin();
1860 isa<PHINode>(II); ++II, ++NumPhis)
1861 if (NumPhis > 2) // Disable this xform.
1864 // Finally, if everything is ok, fold the branches to logical ops.
1865 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
1867 DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
1868 << "AND: " << *BI->getParent());
1871 // If OtherDest *is* BB, then BB is a basic block with a single conditional
1872 // branch in it, where one edge (OtherDest) goes back to itself but the other
1873 // exits. We don't *know* that the program avoids the infinite loop
1874 // (even though that seems likely). If we do this xform naively, we'll end up
1875 // recursively unpeeling the loop. Since we know that (after the xform is
1876 // done) that the block *is* infinite if reached, we just make it an obviously
1877 // infinite loop with no cond branch.
1878 if (OtherDest == BB) {
1879 // Insert it at the end of the function, because it's either code,
1880 // or it won't matter if it's hot. :)
1881 BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(),
1882 "infloop", BB->getParent());
1883 BranchInst::Create(InfLoopBlock, InfLoopBlock);
1884 OtherDest = InfLoopBlock;
1887 DEBUG(dbgs() << *PBI->getParent()->getParent());
1889 // BI may have other predecessors. Because of this, we leave
1890 // it alone, but modify PBI.
1892 // Make sure we get to CommonDest on True&True directions.
1893 Value *PBICond = PBI->getCondition();
1894 IRBuilder<true, NoFolder> Builder(PBI);
1896 PBICond = Builder.CreateNot(PBICond, PBICond->getName()+".not");
1898 Value *BICond = BI->getCondition();
1900 BICond = Builder.CreateNot(BICond, BICond->getName()+".not");
1902 // Merge the conditions.
1903 Value *Cond = Builder.CreateOr(PBICond, BICond, "brmerge");
1905 // Modify PBI to branch on the new condition to the new dests.
1906 PBI->setCondition(Cond);
1907 PBI->setSuccessor(0, CommonDest);
1908 PBI->setSuccessor(1, OtherDest);
1910 // OtherDest may have phi nodes. If so, add an entry from PBI's
1911 // block that are identical to the entries for BI's block.
1912 AddPredecessorToBlock(OtherDest, PBI->getParent(), BB);
1914 // We know that the CommonDest already had an edge from PBI to
1915 // it. If it has PHIs though, the PHIs may have different
1916 // entries for BB and PBI's BB. If so, insert a select to make
1919 for (BasicBlock::iterator II = CommonDest->begin();
1920 (PN = dyn_cast<PHINode>(II)); ++II) {
1921 Value *BIV = PN->getIncomingValueForBlock(BB);
1922 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
1923 Value *PBIV = PN->getIncomingValue(PBBIdx);
1925 // Insert a select in PBI to pick the right value.
1926 Value *NV = cast<SelectInst>
1927 (Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName()+".mux"));
1928 PN->setIncomingValue(PBBIdx, NV);
1932 DEBUG(dbgs() << "INTO: " << *PBI->getParent());
1933 DEBUG(dbgs() << *PBI->getParent()->getParent());
1935 // This basic block is probably dead. We know it has at least
1936 // one fewer predecessor.
1940 // SimplifyTerminatorOnSelect - Simplifies a terminator by replacing it with a
1941 // branch to TrueBB if Cond is true or to FalseBB if Cond is false.
1942 // Takes care of updating the successors and removing the old terminator.
1943 // Also makes sure not to introduce new successors by assuming that edges to
1944 // non-successor TrueBBs and FalseBBs aren't reachable.
1945 static bool SimplifyTerminatorOnSelect(TerminatorInst *OldTerm, Value *Cond,
1946 BasicBlock *TrueBB, BasicBlock *FalseBB){
1947 // Remove any superfluous successor edges from the CFG.
1948 // First, figure out which successors to preserve.
1949 // If TrueBB and FalseBB are equal, only try to preserve one copy of that
1951 BasicBlock *KeepEdge1 = TrueBB;
1952 BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : 0;
1954 // Then remove the rest.
1955 for (unsigned I = 0, E = OldTerm->getNumSuccessors(); I != E; ++I) {
1956 BasicBlock *Succ = OldTerm->getSuccessor(I);
1957 // Make sure only to keep exactly one copy of each edge.
1958 if (Succ == KeepEdge1)
1960 else if (Succ == KeepEdge2)
1963 Succ->removePredecessor(OldTerm->getParent());
1966 IRBuilder<> Builder(OldTerm);
1967 Builder.SetCurrentDebugLocation(OldTerm->getDebugLoc());
1969 // Insert an appropriate new terminator.
1970 if ((KeepEdge1 == 0) && (KeepEdge2 == 0)) {
1971 if (TrueBB == FalseBB)
1972 // We were only looking for one successor, and it was present.
1973 // Create an unconditional branch to it.
1974 Builder.CreateBr(TrueBB);
1976 // We found both of the successors we were looking for.
1977 // Create a conditional branch sharing the condition of the select.
1978 Builder.CreateCondBr(Cond, TrueBB, FalseBB);
1979 } else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
1980 // Neither of the selected blocks were successors, so this
1981 // terminator must be unreachable.
1982 new UnreachableInst(OldTerm->getContext(), OldTerm);
1984 // One of the selected values was a successor, but the other wasn't.
1985 // Insert an unconditional branch to the one that was found;
1986 // the edge to the one that wasn't must be unreachable.
1988 // Only TrueBB was found.
1989 Builder.CreateBr(TrueBB);
1991 // Only FalseBB was found.
1992 Builder.CreateBr(FalseBB);
1995 EraseTerminatorInstAndDCECond(OldTerm);
1999 // SimplifySwitchOnSelect - Replaces
2000 // (switch (select cond, X, Y)) on constant X, Y
2001 // with a branch - conditional if X and Y lead to distinct BBs,
2002 // unconditional otherwise.
2003 static bool SimplifySwitchOnSelect(SwitchInst *SI, SelectInst *Select) {
2004 // Check for constant integer values in the select.
2005 ConstantInt *TrueVal = dyn_cast<ConstantInt>(Select->getTrueValue());
2006 ConstantInt *FalseVal = dyn_cast<ConstantInt>(Select->getFalseValue());
2007 if (!TrueVal || !FalseVal)
2010 // Find the relevant condition and destinations.
2011 Value *Condition = Select->getCondition();
2012 BasicBlock *TrueBB = SI->findCaseValue(TrueVal).getCaseSuccessor();
2013 BasicBlock *FalseBB = SI->findCaseValue(FalseVal).getCaseSuccessor();
2015 // Perform the actual simplification.
2016 return SimplifyTerminatorOnSelect(SI, Condition, TrueBB, FalseBB);
2019 // SimplifyIndirectBrOnSelect - Replaces
2020 // (indirectbr (select cond, blockaddress(@fn, BlockA),
2021 // blockaddress(@fn, BlockB)))
2023 // (br cond, BlockA, BlockB).
2024 static bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI) {
2025 // Check that both operands of the select are block addresses.
2026 BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
2027 BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
2031 // Extract the actual blocks.
2032 BasicBlock *TrueBB = TBA->getBasicBlock();
2033 BasicBlock *FalseBB = FBA->getBasicBlock();
2035 // Perform the actual simplification.
2036 return SimplifyTerminatorOnSelect(IBI, SI->getCondition(), TrueBB, FalseBB);
2039 /// TryToSimplifyUncondBranchWithICmpInIt - This is called when we find an icmp
2040 /// instruction (a seteq/setne with a constant) as the only instruction in a
2041 /// block that ends with an uncond branch. We are looking for a very specific
2042 /// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
2043 /// this case, we merge the first two "or's of icmp" into a switch, but then the
2044 /// default value goes to an uncond block with a seteq in it, we get something
2047 /// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
2049 /// %tmp = icmp eq i8 %A, 92
2052 /// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
2054 /// We prefer to split the edge to 'end' so that there is a true/false entry to
2055 /// the PHI, merging the third icmp into the switch.
2056 static bool TryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
2057 const TargetData *TD,
2058 IRBuilder<> &Builder) {
2059 BasicBlock *BB = ICI->getParent();
2061 // If the block has any PHIs in it or the icmp has multiple uses, it is too
2063 if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse()) return false;
2065 Value *V = ICI->getOperand(0);
2066 ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
2068 // The pattern we're looking for is where our only predecessor is a switch on
2069 // 'V' and this block is the default case for the switch. In this case we can
2070 // fold the compared value into the switch to simplify things.
2071 BasicBlock *Pred = BB->getSinglePredecessor();
2072 if (Pred == 0 || !isa<SwitchInst>(Pred->getTerminator())) return false;
2074 SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
2075 if (SI->getCondition() != V)
2078 // If BB is reachable on a non-default case, then we simply know the value of
2079 // V in this block. Substitute it and constant fold the icmp instruction
2081 if (SI->getDefaultDest() != BB) {
2082 ConstantInt *VVal = SI->findCaseDest(BB);
2083 assert(VVal && "Should have a unique destination value");
2084 ICI->setOperand(0, VVal);
2086 if (Value *V = SimplifyInstruction(ICI, TD)) {
2087 ICI->replaceAllUsesWith(V);
2088 ICI->eraseFromParent();
2090 // BB is now empty, so it is likely to simplify away.
2091 return SimplifyCFG(BB) | true;
2094 // Ok, the block is reachable from the default dest. If the constant we're
2095 // comparing exists in one of the other edges, then we can constant fold ICI
2097 if (SI->findCaseValue(Cst) != SI->case_default()) {
2099 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
2100 V = ConstantInt::getFalse(BB->getContext());
2102 V = ConstantInt::getTrue(BB->getContext());
2104 ICI->replaceAllUsesWith(V);
2105 ICI->eraseFromParent();
2106 // BB is now empty, so it is likely to simplify away.
2107 return SimplifyCFG(BB) | true;
2110 // The use of the icmp has to be in the 'end' block, by the only PHI node in
2112 BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
2113 PHINode *PHIUse = dyn_cast<PHINode>(ICI->use_back());
2114 if (PHIUse == 0 || PHIUse != &SuccBlock->front() ||
2115 isa<PHINode>(++BasicBlock::iterator(PHIUse)))
2118 // If the icmp is a SETEQ, then the default dest gets false, the new edge gets
2120 Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
2121 Constant *NewCst = ConstantInt::getFalse(BB->getContext());
2123 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
2124 std::swap(DefaultCst, NewCst);
2126 // Replace ICI (which is used by the PHI for the default value) with true or
2127 // false depending on if it is EQ or NE.
2128 ICI->replaceAllUsesWith(DefaultCst);
2129 ICI->eraseFromParent();
2131 // Okay, the switch goes to this block on a default value. Add an edge from
2132 // the switch to the merge point on the compared value.
2133 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "switch.edge",
2134 BB->getParent(), BB);
2135 SI->addCase(Cst, NewBB);
2137 // NewBB branches to the phi block, add the uncond branch and the phi entry.
2138 Builder.SetInsertPoint(NewBB);
2139 Builder.SetCurrentDebugLocation(SI->getDebugLoc());
2140 Builder.CreateBr(SuccBlock);
2141 PHIUse->addIncoming(NewCst, NewBB);
2145 /// SimplifyBranchOnICmpChain - The specified branch is a conditional branch.
2146 /// Check to see if it is branching on an or/and chain of icmp instructions, and
2147 /// fold it into a switch instruction if so.
2148 static bool SimplifyBranchOnICmpChain(BranchInst *BI, const TargetData *TD,
2149 IRBuilder<> &Builder) {
2150 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
2151 if (Cond == 0) return false;
2154 // Change br (X == 0 | X == 1), T, F into a switch instruction.
2155 // If this is a bunch of seteq's or'd together, or if it's a bunch of
2156 // 'setne's and'ed together, collect them.
2158 std::vector<ConstantInt*> Values;
2159 bool TrueWhenEqual = true;
2160 Value *ExtraCase = 0;
2161 unsigned UsedICmps = 0;
2163 if (Cond->getOpcode() == Instruction::Or) {
2164 CompVal = GatherConstantCompares(Cond, Values, ExtraCase, TD, true,
2166 } else if (Cond->getOpcode() == Instruction::And) {
2167 CompVal = GatherConstantCompares(Cond, Values, ExtraCase, TD, false,
2169 TrueWhenEqual = false;
2172 // If we didn't have a multiply compared value, fail.
2173 if (CompVal == 0) return false;
2175 // Avoid turning single icmps into a switch.
2179 // There might be duplicate constants in the list, which the switch
2180 // instruction can't handle, remove them now.
2181 array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate);
2182 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
2184 // If Extra was used, we require at least two switch values to do the
2185 // transformation. A switch with one value is just an cond branch.
2186 if (ExtraCase && Values.size() < 2) return false;
2188 // Figure out which block is which destination.
2189 BasicBlock *DefaultBB = BI->getSuccessor(1);
2190 BasicBlock *EdgeBB = BI->getSuccessor(0);
2191 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
2193 BasicBlock *BB = BI->getParent();
2195 DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size()
2196 << " cases into SWITCH. BB is:\n" << *BB);
2198 // If there are any extra values that couldn't be folded into the switch
2199 // then we evaluate them with an explicit branch first. Split the block
2200 // right before the condbr to handle it.
2202 BasicBlock *NewBB = BB->splitBasicBlock(BI, "switch.early.test");
2203 // Remove the uncond branch added to the old block.
2204 TerminatorInst *OldTI = BB->getTerminator();
2205 Builder.SetInsertPoint(OldTI);
2208 Builder.CreateCondBr(ExtraCase, EdgeBB, NewBB);
2210 Builder.CreateCondBr(ExtraCase, NewBB, EdgeBB);
2212 OldTI->eraseFromParent();
2214 // If there are PHI nodes in EdgeBB, then we need to add a new entry to them
2215 // for the edge we just added.
2216 AddPredecessorToBlock(EdgeBB, BB, NewBB);
2218 DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase
2219 << "\nEXTRABB = " << *BB);
2223 Builder.SetInsertPoint(BI);
2224 // Convert pointer to int before we switch.
2225 if (CompVal->getType()->isPointerTy()) {
2226 assert(TD && "Cannot switch on pointer without TargetData");
2227 CompVal = Builder.CreatePtrToInt(CompVal,
2228 TD->getIntPtrType(CompVal->getContext()),
2232 // Create the new switch instruction now.
2233 SwitchInst *New = Builder.CreateSwitch(CompVal, DefaultBB, Values.size());
2235 // Add all of the 'cases' to the switch instruction.
2236 for (unsigned i = 0, e = Values.size(); i != e; ++i)
2237 New->addCase(Values[i], EdgeBB);
2239 // We added edges from PI to the EdgeBB. As such, if there were any
2240 // PHI nodes in EdgeBB, they need entries to be added corresponding to
2241 // the number of edges added.
2242 for (BasicBlock::iterator BBI = EdgeBB->begin();
2243 isa<PHINode>(BBI); ++BBI) {
2244 PHINode *PN = cast<PHINode>(BBI);
2245 Value *InVal = PN->getIncomingValueForBlock(BB);
2246 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
2247 PN->addIncoming(InVal, BB);
2250 // Erase the old branch instruction.
2251 EraseTerminatorInstAndDCECond(BI);
2253 DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n');
2257 bool SimplifyCFGOpt::SimplifyResume(ResumeInst *RI, IRBuilder<> &Builder) {
2258 // If this is a trivial landing pad that just continues unwinding the caught
2259 // exception then zap the landing pad, turning its invokes into calls.
2260 BasicBlock *BB = RI->getParent();
2261 LandingPadInst *LPInst = dyn_cast<LandingPadInst>(BB->getFirstNonPHI());
2262 if (RI->getValue() != LPInst)
2263 // Not a landing pad, or the resume is not unwinding the exception that
2264 // caused control to branch here.
2267 // Check that there are no other instructions except for debug intrinsics.
2268 BasicBlock::iterator I = LPInst, E = RI;
2270 if (!isa<DbgInfoIntrinsic>(I))
2273 // Turn all invokes that unwind here into calls and delete the basic block.
2274 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE;) {
2275 InvokeInst *II = cast<InvokeInst>((*PI++)->getTerminator());
2276 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
2277 // Insert a call instruction before the invoke.
2278 CallInst *Call = CallInst::Create(II->getCalledValue(), Args, "", II);
2280 Call->setCallingConv(II->getCallingConv());
2281 Call->setAttributes(II->getAttributes());
2282 Call->setDebugLoc(II->getDebugLoc());
2284 // Anything that used the value produced by the invoke instruction now uses
2285 // the value produced by the call instruction. Note that we do this even
2286 // for void functions and calls with no uses so that the callgraph edge is
2288 II->replaceAllUsesWith(Call);
2289 BB->removePredecessor(II->getParent());
2291 // Insert a branch to the normal destination right before the invoke.
2292 BranchInst::Create(II->getNormalDest(), II);
2294 // Finally, delete the invoke instruction!
2295 II->eraseFromParent();
2298 // The landingpad is now unreachable. Zap it.
2299 BB->eraseFromParent();
2303 bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI, IRBuilder<> &Builder) {
2304 BasicBlock *BB = RI->getParent();
2305 if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false;
2307 // Find predecessors that end with branches.
2308 SmallVector<BasicBlock*, 8> UncondBranchPreds;
2309 SmallVector<BranchInst*, 8> CondBranchPreds;
2310 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
2311 BasicBlock *P = *PI;
2312 TerminatorInst *PTI = P->getTerminator();
2313 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
2314 if (BI->isUnconditional())
2315 UncondBranchPreds.push_back(P);
2317 CondBranchPreds.push_back(BI);
2321 // If we found some, do the transformation!
2322 if (!UncondBranchPreds.empty() && DupRet) {
2323 while (!UncondBranchPreds.empty()) {
2324 BasicBlock *Pred = UncondBranchPreds.pop_back_val();
2325 DEBUG(dbgs() << "FOLDING: " << *BB
2326 << "INTO UNCOND BRANCH PRED: " << *Pred);
2327 (void)FoldReturnIntoUncondBranch(RI, BB, Pred);
2330 // If we eliminated all predecessors of the block, delete the block now.
2331 if (pred_begin(BB) == pred_end(BB))
2332 // We know there are no successors, so just nuke the block.
2333 BB->eraseFromParent();
2338 // Check out all of the conditional branches going to this return
2339 // instruction. If any of them just select between returns, change the
2340 // branch itself into a select/return pair.
2341 while (!CondBranchPreds.empty()) {
2342 BranchInst *BI = CondBranchPreds.pop_back_val();
2344 // Check to see if the non-BB successor is also a return block.
2345 if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
2346 isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
2347 SimplifyCondBranchToTwoReturns(BI, Builder))
2353 bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
2354 BasicBlock *BB = UI->getParent();
2356 bool Changed = false;
2358 // If there are any instructions immediately before the unreachable that can
2359 // be removed, do so.
2360 while (UI != BB->begin()) {
2361 BasicBlock::iterator BBI = UI;
2363 // Do not delete instructions that can have side effects which might cause
2364 // the unreachable to not be reachable; specifically, calls and volatile
2365 // operations may have this effect.
2366 if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI)) break;
2368 if (BBI->mayHaveSideEffects()) {
2369 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
2370 if (SI->isVolatile())
2372 } else if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
2373 if (LI->isVolatile())
2375 } else if (AtomicRMWInst *RMWI = dyn_cast<AtomicRMWInst>(BBI)) {
2376 if (RMWI->isVolatile())
2378 } else if (AtomicCmpXchgInst *CXI = dyn_cast<AtomicCmpXchgInst>(BBI)) {
2379 if (CXI->isVolatile())
2381 } else if (!isa<FenceInst>(BBI) && !isa<VAArgInst>(BBI) &&
2382 !isa<LandingPadInst>(BBI)) {
2385 // Note that deleting LandingPad's here is in fact okay, although it
2386 // involves a bit of subtle reasoning. If this inst is a LandingPad,
2387 // all the predecessors of this block will be the unwind edges of Invokes,
2388 // and we can therefore guarantee this block will be erased.
2391 // Delete this instruction (any uses are guaranteed to be dead)
2392 if (!BBI->use_empty())
2393 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
2394 BBI->eraseFromParent();
2398 // If the unreachable instruction is the first in the block, take a gander
2399 // at all of the predecessors of this instruction, and simplify them.
2400 if (&BB->front() != UI) return Changed;
2402 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
2403 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
2404 TerminatorInst *TI = Preds[i]->getTerminator();
2405 IRBuilder<> Builder(TI);
2406 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
2407 if (BI->isUnconditional()) {
2408 if (BI->getSuccessor(0) == BB) {
2409 new UnreachableInst(TI->getContext(), TI);
2410 TI->eraseFromParent();
2414 if (BI->getSuccessor(0) == BB) {
2415 Builder.CreateBr(BI->getSuccessor(1));
2416 EraseTerminatorInstAndDCECond(BI);
2417 } else if (BI->getSuccessor(1) == BB) {
2418 Builder.CreateBr(BI->getSuccessor(0));
2419 EraseTerminatorInstAndDCECond(BI);
2423 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
2424 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
2426 if (i.getCaseSuccessor() == BB) {
2427 BB->removePredecessor(SI->getParent());
2432 // If the default value is unreachable, figure out the most popular
2433 // destination and make it the default.
2434 if (SI->getDefaultDest() == BB) {
2435 std::map<BasicBlock*, std::pair<unsigned, unsigned> > Popularity;
2436 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
2438 std::pair<unsigned, unsigned> &entry =
2439 Popularity[i.getCaseSuccessor()];
2440 if (entry.first == 0) {
2442 entry.second = i.getCaseIndex();
2448 // Find the most popular block.
2449 unsigned MaxPop = 0;
2450 unsigned MaxIndex = 0;
2451 BasicBlock *MaxBlock = 0;
2452 for (std::map<BasicBlock*, std::pair<unsigned, unsigned> >::iterator
2453 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
2454 if (I->second.first > MaxPop ||
2455 (I->second.first == MaxPop && MaxIndex > I->second.second)) {
2456 MaxPop = I->second.first;
2457 MaxIndex = I->second.second;
2458 MaxBlock = I->first;
2462 // Make this the new default, allowing us to delete any explicit
2464 SI->setDefaultDest(MaxBlock);
2467 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
2469 if (isa<PHINode>(MaxBlock->begin()))
2470 for (unsigned i = 0; i != MaxPop-1; ++i)
2471 MaxBlock->removePredecessor(SI->getParent());
2473 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
2475 if (i.getCaseSuccessor() == MaxBlock) {
2481 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
2482 if (II->getUnwindDest() == BB) {
2483 // Convert the invoke to a call instruction. This would be a good
2484 // place to note that the call does not throw though.
2485 BranchInst *BI = Builder.CreateBr(II->getNormalDest());
2486 II->removeFromParent(); // Take out of symbol table
2488 // Insert the call now...
2489 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end()-3);
2490 Builder.SetInsertPoint(BI);
2491 CallInst *CI = Builder.CreateCall(II->getCalledValue(),
2492 Args, II->getName());
2493 CI->setCallingConv(II->getCallingConv());
2494 CI->setAttributes(II->getAttributes());
2495 // If the invoke produced a value, the call does now instead.
2496 II->replaceAllUsesWith(CI);
2503 // If this block is now dead, remove it.
2504 if (pred_begin(BB) == pred_end(BB) &&
2505 BB != &BB->getParent()->getEntryBlock()) {
2506 // We know there are no successors, so just nuke the block.
2507 BB->eraseFromParent();
2514 /// TurnSwitchRangeIntoICmp - Turns a switch with that contains only a
2515 /// integer range comparison into a sub, an icmp and a branch.
2516 static bool TurnSwitchRangeIntoICmp(SwitchInst *SI, IRBuilder<> &Builder) {
2517 assert(SI->getNumCases() > 1 && "Degenerate switch?");
2519 // Make sure all cases point to the same destination and gather the values.
2520 SmallVector<ConstantInt *, 16> Cases;
2521 SwitchInst::CaseIt I = SI->case_begin();
2522 Cases.push_back(I.getCaseValue());
2523 SwitchInst::CaseIt PrevI = I++;
2524 for (SwitchInst::CaseIt E = SI->case_end(); I != E; PrevI = I++) {
2525 if (PrevI.getCaseSuccessor() != I.getCaseSuccessor())
2527 Cases.push_back(I.getCaseValue());
2529 assert(Cases.size() == SI->getNumCases() && "Not all cases gathered");
2531 // Sort the case values, then check if they form a range we can transform.
2532 array_pod_sort(Cases.begin(), Cases.end(), ConstantIntSortPredicate);
2533 for (unsigned I = 1, E = Cases.size(); I != E; ++I) {
2534 if (Cases[I-1]->getValue() != Cases[I]->getValue()+1)
2538 Constant *Offset = ConstantExpr::getNeg(Cases.back());
2539 Constant *NumCases = ConstantInt::get(Offset->getType(), SI->getNumCases());
2541 Value *Sub = SI->getCondition();
2542 if (!Offset->isNullValue())
2543 Sub = Builder.CreateAdd(Sub, Offset, Sub->getName()+".off");
2544 Value *Cmp = Builder.CreateICmpULT(Sub, NumCases, "switch");
2545 Builder.CreateCondBr(
2546 Cmp, SI->case_begin().getCaseSuccessor(), SI->getDefaultDest());
2548 // Prune obsolete incoming values off the successor's PHI nodes.
2549 for (BasicBlock::iterator BBI = SI->case_begin().getCaseSuccessor()->begin();
2550 isa<PHINode>(BBI); ++BBI) {
2551 for (unsigned I = 0, E = SI->getNumCases()-1; I != E; ++I)
2552 cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
2554 SI->eraseFromParent();
2559 /// EliminateDeadSwitchCases - Compute masked bits for the condition of a switch
2560 /// and use it to remove dead cases.
2561 static bool EliminateDeadSwitchCases(SwitchInst *SI) {
2562 Value *Cond = SI->getCondition();
2563 unsigned Bits = cast<IntegerType>(Cond->getType())->getBitWidth();
2564 APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
2565 ComputeMaskedBits(Cond, APInt::getAllOnesValue(Bits), KnownZero, KnownOne);
2567 // Gather dead cases.
2568 SmallVector<ConstantInt*, 8> DeadCases;
2569 for (SwitchInst::CaseIt I = SI->case_begin(), E = SI->case_end(); I != E; ++I) {
2570 if ((I.getCaseValue()->getValue() & KnownZero) != 0 ||
2571 (I.getCaseValue()->getValue() & KnownOne) != KnownOne) {
2572 DeadCases.push_back(I.getCaseValue());
2573 DEBUG(dbgs() << "SimplifyCFG: switch case '"
2574 << I.getCaseValue() << "' is dead.\n");
2578 // Remove dead cases from the switch.
2579 for (unsigned I = 0, E = DeadCases.size(); I != E; ++I) {
2580 SwitchInst::CaseIt Case = SI->findCaseValue(DeadCases[I]);
2581 assert(Case != SI->case_default() &&
2582 "Case was not found. Probably mistake in DeadCases forming.");
2583 // Prune unused values from PHI nodes.
2584 Case.getCaseSuccessor()->removePredecessor(SI->getParent());
2585 SI->removeCase(Case);
2588 return !DeadCases.empty();
2591 /// FindPHIForConditionForwarding - If BB would be eligible for simplification
2592 /// by TryToSimplifyUncondBranchFromEmptyBlock (i.e. it is empty and terminated
2593 /// by an unconditional branch), look at the phi node for BB in the successor
2594 /// block and see if the incoming value is equal to CaseValue. If so, return
2595 /// the phi node, and set PhiIndex to BB's index in the phi node.
2596 static PHINode *FindPHIForConditionForwarding(ConstantInt *CaseValue,
2599 if (BB->getFirstNonPHIOrDbg() != BB->getTerminator())
2600 return NULL; // BB must be empty to be a candidate for simplification.
2601 if (!BB->getSinglePredecessor())
2602 return NULL; // BB must be dominated by the switch.
2604 BranchInst *Branch = dyn_cast<BranchInst>(BB->getTerminator());
2605 if (!Branch || !Branch->isUnconditional())
2606 return NULL; // Terminator must be unconditional branch.
2608 BasicBlock *Succ = Branch->getSuccessor(0);
2610 BasicBlock::iterator I = Succ->begin();
2611 while (PHINode *PHI = dyn_cast<PHINode>(I++)) {
2612 int Idx = PHI->getBasicBlockIndex(BB);
2613 assert(Idx >= 0 && "PHI has no entry for predecessor?");
2615 Value *InValue = PHI->getIncomingValue(Idx);
2616 if (InValue != CaseValue) continue;
2625 /// ForwardSwitchConditionToPHI - Try to forward the condition of a switch
2626 /// instruction to a phi node dominated by the switch, if that would mean that
2627 /// some of the destination blocks of the switch can be folded away.
2628 /// Returns true if a change is made.
2629 static bool ForwardSwitchConditionToPHI(SwitchInst *SI) {
2630 typedef DenseMap<PHINode*, SmallVector<int,4> > ForwardingNodesMap;
2631 ForwardingNodesMap ForwardingNodes;
2633 for (SwitchInst::CaseIt I = SI->case_begin(), E = SI->case_end(); I != E; ++I) {
2634 ConstantInt *CaseValue = I.getCaseValue();
2635 BasicBlock *CaseDest = I.getCaseSuccessor();
2638 PHINode *PHI = FindPHIForConditionForwarding(CaseValue, CaseDest,
2642 ForwardingNodes[PHI].push_back(PhiIndex);
2645 bool Changed = false;
2647 for (ForwardingNodesMap::iterator I = ForwardingNodes.begin(),
2648 E = ForwardingNodes.end(); I != E; ++I) {
2649 PHINode *Phi = I->first;
2650 SmallVector<int,4> &Indexes = I->second;
2652 if (Indexes.size() < 2) continue;
2654 for (size_t I = 0, E = Indexes.size(); I != E; ++I)
2655 Phi->setIncomingValue(Indexes[I], SI->getCondition());
2662 bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
2663 // If this switch is too complex to want to look at, ignore it.
2664 if (!isValueEqualityComparison(SI))
2667 BasicBlock *BB = SI->getParent();
2669 // If we only have one predecessor, and if it is a branch on this value,
2670 // see if that predecessor totally determines the outcome of this switch.
2671 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
2672 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred, Builder))
2673 return SimplifyCFG(BB) | true;
2675 Value *Cond = SI->getCondition();
2676 if (SelectInst *Select = dyn_cast<SelectInst>(Cond))
2677 if (SimplifySwitchOnSelect(SI, Select))
2678 return SimplifyCFG(BB) | true;
2680 // If the block only contains the switch, see if we can fold the block
2681 // away into any preds.
2682 BasicBlock::iterator BBI = BB->begin();
2683 // Ignore dbg intrinsics.
2684 while (isa<DbgInfoIntrinsic>(BBI))
2687 if (FoldValueComparisonIntoPredecessors(SI, Builder))
2688 return SimplifyCFG(BB) | true;
2690 // Try to transform the switch into an icmp and a branch.
2691 if (TurnSwitchRangeIntoICmp(SI, Builder))
2692 return SimplifyCFG(BB) | true;
2694 // Remove unreachable cases.
2695 if (EliminateDeadSwitchCases(SI))
2696 return SimplifyCFG(BB) | true;
2698 if (ForwardSwitchConditionToPHI(SI))
2699 return SimplifyCFG(BB) | true;
2704 bool SimplifyCFGOpt::SimplifyIndirectBr(IndirectBrInst *IBI) {
2705 BasicBlock *BB = IBI->getParent();
2706 bool Changed = false;
2708 // Eliminate redundant destinations.
2709 SmallPtrSet<Value *, 8> Succs;
2710 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
2711 BasicBlock *Dest = IBI->getDestination(i);
2712 if (!Dest->hasAddressTaken() || !Succs.insert(Dest)) {
2713 Dest->removePredecessor(BB);
2714 IBI->removeDestination(i);
2720 if (IBI->getNumDestinations() == 0) {
2721 // If the indirectbr has no successors, change it to unreachable.
2722 new UnreachableInst(IBI->getContext(), IBI);
2723 EraseTerminatorInstAndDCECond(IBI);
2727 if (IBI->getNumDestinations() == 1) {
2728 // If the indirectbr has one successor, change it to a direct branch.
2729 BranchInst::Create(IBI->getDestination(0), IBI);
2730 EraseTerminatorInstAndDCECond(IBI);
2734 if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
2735 if (SimplifyIndirectBrOnSelect(IBI, SI))
2736 return SimplifyCFG(BB) | true;
2741 bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI, IRBuilder<> &Builder){
2742 BasicBlock *BB = BI->getParent();
2744 // If the Terminator is the only non-phi instruction, simplify the block.
2745 BasicBlock::iterator I = BB->getFirstNonPHIOrDbgOrLifetime();
2746 if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
2747 TryToSimplifyUncondBranchFromEmptyBlock(BB))
2750 // If the only instruction in the block is a seteq/setne comparison
2751 // against a constant, try to simplify the block.
2752 if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
2753 if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
2754 for (++I; isa<DbgInfoIntrinsic>(I); ++I)
2756 if (I->isTerminator() &&
2757 TryToSimplifyUncondBranchWithICmpInIt(ICI, TD, Builder))
2765 bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
2766 BasicBlock *BB = BI->getParent();
2768 // Conditional branch
2769 if (isValueEqualityComparison(BI)) {
2770 // If we only have one predecessor, and if it is a branch on this value,
2771 // see if that predecessor totally determines the outcome of this
2773 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
2774 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred, Builder))
2775 return SimplifyCFG(BB) | true;
2777 // This block must be empty, except for the setcond inst, if it exists.
2778 // Ignore dbg intrinsics.
2779 BasicBlock::iterator I = BB->begin();
2780 // Ignore dbg intrinsics.
2781 while (isa<DbgInfoIntrinsic>(I))
2784 if (FoldValueComparisonIntoPredecessors(BI, Builder))
2785 return SimplifyCFG(BB) | true;
2786 } else if (&*I == cast<Instruction>(BI->getCondition())){
2788 // Ignore dbg intrinsics.
2789 while (isa<DbgInfoIntrinsic>(I))
2791 if (&*I == BI && FoldValueComparisonIntoPredecessors(BI, Builder))
2792 return SimplifyCFG(BB) | true;
2796 // Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction.
2797 if (SimplifyBranchOnICmpChain(BI, TD, Builder))
2800 // If this basic block is ONLY a compare and a branch, and if a predecessor
2801 // branches to us and one of our successors, fold the comparison into the
2802 // predecessor and use logical operations to pick the right destination.
2803 if (FoldBranchToCommonDest(BI))
2804 return SimplifyCFG(BB) | true;
2806 // We have a conditional branch to two blocks that are only reachable
2807 // from BI. We know that the condbr dominates the two blocks, so see if
2808 // there is any identical code in the "then" and "else" blocks. If so, we
2809 // can hoist it up to the branching block.
2810 if (BI->getSuccessor(0)->getSinglePredecessor() != 0) {
2811 if (BI->getSuccessor(1)->getSinglePredecessor() != 0) {
2812 if (HoistThenElseCodeToIf(BI))
2813 return SimplifyCFG(BB) | true;
2815 // If Successor #1 has multiple preds, we may be able to conditionally
2816 // execute Successor #0 if it branches to successor #1.
2817 TerminatorInst *Succ0TI = BI->getSuccessor(0)->getTerminator();
2818 if (Succ0TI->getNumSuccessors() == 1 &&
2819 Succ0TI->getSuccessor(0) == BI->getSuccessor(1))
2820 if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0)))
2821 return SimplifyCFG(BB) | true;
2823 } else if (BI->getSuccessor(1)->getSinglePredecessor() != 0) {
2824 // If Successor #0 has multiple preds, we may be able to conditionally
2825 // execute Successor #1 if it branches to successor #0.
2826 TerminatorInst *Succ1TI = BI->getSuccessor(1)->getTerminator();
2827 if (Succ1TI->getNumSuccessors() == 1 &&
2828 Succ1TI->getSuccessor(0) == BI->getSuccessor(0))
2829 if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1)))
2830 return SimplifyCFG(BB) | true;
2833 // If this is a branch on a phi node in the current block, thread control
2834 // through this block if any PHI node entries are constants.
2835 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
2836 if (PN->getParent() == BI->getParent())
2837 if (FoldCondBranchOnPHI(BI, TD))
2838 return SimplifyCFG(BB) | true;
2840 // Scan predecessor blocks for conditional branches.
2841 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
2842 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
2843 if (PBI != BI && PBI->isConditional())
2844 if (SimplifyCondBranchToCondBranch(PBI, BI))
2845 return SimplifyCFG(BB) | true;
2850 /// Check if passing a value to an instruction will cause undefined behavior.
2851 static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
2852 Constant *C = dyn_cast<Constant>(V);
2856 if (!I->hasOneUse()) // Only look at single-use instructions, for compile time
2859 if (C->isNullValue()) {
2860 Instruction *Use = I->use_back();
2862 // Now make sure that there are no instructions in between that can alter
2863 // control flow (eg. calls)
2864 for (BasicBlock::iterator i = ++BasicBlock::iterator(I); &*i != Use; ++i)
2865 if (i == I->getParent()->end() || i->mayHaveSideEffects())
2868 // Look through GEPs. A load from a GEP derived from NULL is still undefined
2869 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Use))
2870 if (GEP->getPointerOperand() == I)
2871 return passingValueIsAlwaysUndefined(V, GEP);
2873 // Look through bitcasts.
2874 if (BitCastInst *BC = dyn_cast<BitCastInst>(Use))
2875 return passingValueIsAlwaysUndefined(V, BC);
2877 // Load from null is undefined.
2878 if (LoadInst *LI = dyn_cast<LoadInst>(Use))
2879 return LI->getPointerAddressSpace() == 0;
2881 // Store to null is undefined.
2882 if (StoreInst *SI = dyn_cast<StoreInst>(Use))
2883 return SI->getPointerAddressSpace() == 0 && SI->getPointerOperand() == I;
2888 /// If BB has an incoming value that will always trigger undefined behavior
2889 /// (eg. null pointer dereference), remove the branch leading here.
2890 static bool removeUndefIntroducingPredecessor(BasicBlock *BB) {
2891 for (BasicBlock::iterator i = BB->begin();
2892 PHINode *PHI = dyn_cast<PHINode>(i); ++i)
2893 for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i)
2894 if (passingValueIsAlwaysUndefined(PHI->getIncomingValue(i), PHI)) {
2895 TerminatorInst *T = PHI->getIncomingBlock(i)->getTerminator();
2896 IRBuilder<> Builder(T);
2897 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
2898 BB->removePredecessor(PHI->getIncomingBlock(i));
2899 // Turn uncoditional branches into unreachables and remove the dead
2900 // destination from conditional branches.
2901 if (BI->isUnconditional())
2902 Builder.CreateUnreachable();
2904 Builder.CreateBr(BI->getSuccessor(0) == BB ? BI->getSuccessor(1) :
2905 BI->getSuccessor(0));
2906 BI->eraseFromParent();
2909 // TODO: SwitchInst.
2915 bool SimplifyCFGOpt::run(BasicBlock *BB) {
2916 bool Changed = false;
2918 assert(BB && BB->getParent() && "Block not embedded in function!");
2919 assert(BB->getTerminator() && "Degenerate basic block encountered!");
2921 // Remove basic blocks that have no predecessors (except the entry block)...
2922 // or that just have themself as a predecessor. These are unreachable.
2923 if ((pred_begin(BB) == pred_end(BB) &&
2924 BB != &BB->getParent()->getEntryBlock()) ||
2925 BB->getSinglePredecessor() == BB) {
2926 DEBUG(dbgs() << "Removing BB: \n" << *BB);
2927 DeleteDeadBlock(BB);
2931 // Check to see if we can constant propagate this terminator instruction
2933 Changed |= ConstantFoldTerminator(BB, true);
2935 // Check for and eliminate duplicate PHI nodes in this block.
2936 Changed |= EliminateDuplicatePHINodes(BB);
2938 // Check for and remove branches that will always cause undefined behavior.
2939 Changed |= removeUndefIntroducingPredecessor(BB);
2941 // Merge basic blocks into their predecessor if there is only one distinct
2942 // pred, and if there is only one distinct successor of the predecessor, and
2943 // if there are no PHI nodes.
2945 if (MergeBlockIntoPredecessor(BB))
2948 IRBuilder<> Builder(BB);
2950 // If there is a trivial two-entry PHI node in this basic block, and we can
2951 // eliminate it, do so now.
2952 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
2953 if (PN->getNumIncomingValues() == 2)
2954 Changed |= FoldTwoEntryPHINode(PN, TD);
2956 Builder.SetInsertPoint(BB->getTerminator());
2957 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
2958 if (BI->isUnconditional()) {
2959 if (SimplifyUncondBranch(BI, Builder)) return true;
2961 if (SimplifyCondBranch(BI, Builder)) return true;
2963 } else if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
2964 if (SimplifyReturn(RI, Builder)) return true;
2965 } else if (ResumeInst *RI = dyn_cast<ResumeInst>(BB->getTerminator())) {
2966 if (SimplifyResume(RI, Builder)) return true;
2967 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
2968 if (SimplifySwitch(SI, Builder)) return true;
2969 } else if (UnreachableInst *UI =
2970 dyn_cast<UnreachableInst>(BB->getTerminator())) {
2971 if (SimplifyUnreachable(UI)) return true;
2972 } else if (IndirectBrInst *IBI =
2973 dyn_cast<IndirectBrInst>(BB->getTerminator())) {
2974 if (SimplifyIndirectBr(IBI)) return true;
2980 /// SimplifyCFG - This function is used to do simplification of a CFG. For
2981 /// example, it adjusts branches to branches to eliminate the extra hop, it
2982 /// eliminates unreachable basic blocks, and does other "peephole" optimization
2983 /// of the CFG. It returns true if a modification was made.
2985 bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) {
2986 return SimplifyCFGOpt(TD).run(BB);