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/Instructions.h"
18 #include "llvm/IntrinsicInst.h"
19 #include "llvm/Type.h"
20 #include "llvm/DerivedTypes.h"
21 #include "llvm/GlobalVariable.h"
22 #include "llvm/Support/CFG.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 #include "llvm/Analysis/ConstantFolding.h"
26 #include "llvm/Target/TargetData.h"
27 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
28 #include "llvm/ADT/DenseMap.h"
29 #include "llvm/ADT/SmallVector.h"
30 #include "llvm/ADT/SmallPtrSet.h"
31 #include "llvm/ADT/Statistic.h"
32 #include "llvm/ADT/STLExtras.h"
38 STATISTIC(NumSpeculations, "Number of speculative executed instructions");
41 class SimplifyCFGOpt {
42 const TargetData *const TD;
44 Value *isValueEqualityComparison(TerminatorInst *TI);
45 BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI,
46 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases);
47 bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
49 bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI);
52 explicit SimplifyCFGOpt(const TargetData *td) : TD(td) {}
53 bool run(BasicBlock *BB);
57 /// SafeToMergeTerminators - Return true if it is safe to merge these two
58 /// terminator instructions together.
60 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
61 if (SI1 == SI2) return false; // Can't merge with self!
63 // It is not safe to merge these two switch instructions if they have a common
64 // successor, and if that successor has a PHI node, and if *that* PHI node has
65 // conflicting incoming values from the two switch blocks.
66 BasicBlock *SI1BB = SI1->getParent();
67 BasicBlock *SI2BB = SI2->getParent();
68 SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
70 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
71 if (SI1Succs.count(*I))
72 for (BasicBlock::iterator BBI = (*I)->begin();
73 isa<PHINode>(BBI); ++BBI) {
74 PHINode *PN = cast<PHINode>(BBI);
75 if (PN->getIncomingValueForBlock(SI1BB) !=
76 PN->getIncomingValueForBlock(SI2BB))
83 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
84 /// now be entries in it from the 'NewPred' block. The values that will be
85 /// flowing into the PHI nodes will be the same as those coming in from
86 /// ExistPred, an existing predecessor of Succ.
87 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
88 BasicBlock *ExistPred) {
89 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
90 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
91 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
94 for (BasicBlock::iterator I = Succ->begin();
95 (PN = dyn_cast<PHINode>(I)); ++I)
96 PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred);
100 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
101 /// presumably PHI nodes in it), check to see if the merge at this block is due
102 /// to an "if condition". If so, return the boolean condition that determines
103 /// which entry into BB will be taken. Also, return by references the block
104 /// that will be entered from if the condition is true, and the block that will
105 /// be entered if the condition is false.
108 static Value *GetIfCondition(BasicBlock *BB,
109 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
110 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
111 "Function can only handle blocks with 2 predecessors!");
112 BasicBlock *Pred1 = *pred_begin(BB);
113 BasicBlock *Pred2 = *++pred_begin(BB);
115 // We can only handle branches. Other control flow will be lowered to
116 // branches if possible anyway.
117 if (!isa<BranchInst>(Pred1->getTerminator()) ||
118 !isa<BranchInst>(Pred2->getTerminator()))
120 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
121 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
123 // Eliminate code duplication by ensuring that Pred1Br is conditional if
125 if (Pred2Br->isConditional()) {
126 // If both branches are conditional, we don't have an "if statement". In
127 // reality, we could transform this case, but since the condition will be
128 // required anyway, we stand no chance of eliminating it, so the xform is
129 // probably not profitable.
130 if (Pred1Br->isConditional())
133 std::swap(Pred1, Pred2);
134 std::swap(Pred1Br, Pred2Br);
137 if (Pred1Br->isConditional()) {
138 // If we found a conditional branch predecessor, make sure that it branches
139 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
140 if (Pred1Br->getSuccessor(0) == BB &&
141 Pred1Br->getSuccessor(1) == Pred2) {
144 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
145 Pred1Br->getSuccessor(1) == BB) {
149 // We know that one arm of the conditional goes to BB, so the other must
150 // go somewhere unrelated, and this must not be an "if statement".
154 // The only thing we have to watch out for here is to make sure that Pred2
155 // doesn't have incoming edges from other blocks. If it does, the condition
156 // doesn't dominate BB.
157 if (++pred_begin(Pred2) != pred_end(Pred2))
160 return Pred1Br->getCondition();
163 // Ok, if we got here, both predecessors end with an unconditional branch to
164 // BB. Don't panic! If both blocks only have a single (identical)
165 // predecessor, and THAT is a conditional branch, then we're all ok!
166 if (pred_begin(Pred1) == pred_end(Pred1) ||
167 ++pred_begin(Pred1) != pred_end(Pred1) ||
168 pred_begin(Pred2) == pred_end(Pred2) ||
169 ++pred_begin(Pred2) != pred_end(Pred2) ||
170 *pred_begin(Pred1) != *pred_begin(Pred2))
173 // Otherwise, if this is a conditional branch, then we can use it!
174 BasicBlock *CommonPred = *pred_begin(Pred1);
175 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
176 assert(BI->isConditional() && "Two successors but not conditional?");
177 if (BI->getSuccessor(0) == Pred1) {
184 return BI->getCondition();
189 /// DominatesMergePoint - If we have a merge point of an "if condition" as
190 /// accepted above, return true if the specified value dominates the block. We
191 /// don't handle the true generality of domination here, just a special case
192 /// which works well enough for us.
194 /// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
195 /// see if V (which must be an instruction) is cheap to compute and is
196 /// non-trapping. If both are true, the instruction is inserted into the set
197 /// and true is returned.
198 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
199 std::set<Instruction*> *AggressiveInsts) {
200 Instruction *I = dyn_cast<Instruction>(V);
202 // Non-instructions all dominate instructions, but not all constantexprs
203 // can be executed unconditionally.
204 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
209 BasicBlock *PBB = I->getParent();
211 // We don't want to allow weird loops that might have the "if condition" in
212 // the bottom of this block.
213 if (PBB == BB) return false;
215 // If this instruction is defined in a block that contains an unconditional
216 // branch to BB, then it must be in the 'conditional' part of the "if
218 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
219 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
220 if (!AggressiveInsts) return false;
221 // Okay, it looks like the instruction IS in the "condition". Check to
222 // see if it's a cheap instruction to unconditionally compute, and if it
223 // only uses stuff defined outside of the condition. If so, hoist it out.
224 if (!I->isSafeToSpeculativelyExecute())
227 switch (I->getOpcode()) {
228 default: return false; // Cannot hoist this out safely.
229 case Instruction::Load: {
230 // We have to check to make sure there are no instructions before the
231 // load in its basic block, as we are going to hoist the loop out to
233 BasicBlock::iterator IP = PBB->begin();
234 while (isa<DbgInfoIntrinsic>(IP))
236 if (IP != BasicBlock::iterator(I))
240 case Instruction::Add:
241 case Instruction::Sub:
242 case Instruction::And:
243 case Instruction::Or:
244 case Instruction::Xor:
245 case Instruction::Shl:
246 case Instruction::LShr:
247 case Instruction::AShr:
248 case Instruction::ICmp:
249 break; // These are all cheap and non-trapping instructions.
252 // Okay, we can only really hoist these out if their operands are not
253 // defined in the conditional region.
254 for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i)
255 if (!DominatesMergePoint(*i, BB, 0))
257 // Okay, it's safe to do this! Remember this instruction.
258 AggressiveInsts->insert(I);
264 /// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr
265 /// and PointerNullValue. Return NULL if value is not a constant int.
266 static ConstantInt *GetConstantInt(Value *V, const TargetData *TD) {
267 // Normal constant int.
268 ConstantInt *CI = dyn_cast<ConstantInt>(V);
269 if (CI || !TD || !isa<Constant>(V) || !V->getType()->isPointerTy())
272 // This is some kind of pointer constant. Turn it into a pointer-sized
273 // ConstantInt if possible.
274 const IntegerType *PtrTy = TD->getIntPtrType(V->getContext());
276 // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*).
277 if (isa<ConstantPointerNull>(V))
278 return ConstantInt::get(PtrTy, 0);
280 // IntToPtr const int.
281 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
282 if (CE->getOpcode() == Instruction::IntToPtr)
283 if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) {
284 // The constant is very likely to have the right type already.
285 if (CI->getType() == PtrTy)
288 return cast<ConstantInt>
289 (ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false));
294 /// GatherConstantSetEQs - Given a potentially 'or'd together collection of
295 /// icmp_eq instructions that compare a value against a constant, return the
296 /// value being compared, and stick the constant into the Values vector.
298 GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values,
299 const TargetData *TD) {
300 Instruction *Inst = dyn_cast<Instruction>(V);
301 if (Inst == 0) return 0;
303 if (Inst->getOpcode() == Instruction::ICmp &&
304 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_EQ) {
305 if (ConstantInt *C = GetConstantInt(Inst->getOperand(1), TD)) {
307 return Inst->getOperand(0);
309 if (ConstantInt *C = GetConstantInt(Inst->getOperand(0), TD)) {
311 return Inst->getOperand(1);
313 } else if (Inst->getOpcode() == Instruction::Or) {
314 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values, TD))
315 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values, TD))
322 /// GatherConstantSetNEs - Given a potentially 'and'd together collection of
323 /// setne instructions that compare a value against a constant, return the value
324 /// being compared, and stick the constant into the Values vector.
326 GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values,
327 const TargetData *TD) {
328 Instruction *Inst = dyn_cast<Instruction>(V);
329 if (Inst == 0) return 0;
331 if (Inst->getOpcode() == Instruction::ICmp &&
332 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_NE) {
333 if (ConstantInt *C = GetConstantInt(Inst->getOperand(1), TD)) {
335 return Inst->getOperand(0);
337 if (ConstantInt *C = GetConstantInt(Inst->getOperand(0), TD)) {
339 return Inst->getOperand(1);
341 } else if (Inst->getOpcode() == Instruction::And) {
342 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values, TD))
343 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values, TD))
350 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
351 /// bunch of comparisons of one value against constants, return the value and
352 /// the constants being compared.
353 static bool GatherValueComparisons(Value *CondV, Value *&CompVal,
354 std::vector<ConstantInt*> &Values,
355 const TargetData *TD) {
356 Instruction *Cond = dyn_cast<Instruction>(CondV);
357 if (Cond == 0) return false;
359 if (Cond->getOpcode() == Instruction::Or) {
360 CompVal = GatherConstantSetEQs(Cond, Values, TD);
362 // Return true to indicate that the condition is true if the CompVal is
363 // equal to one of the constants.
366 if (Cond->getOpcode() == Instruction::And) {
367 CompVal = GatherConstantSetNEs(Cond, Values, TD);
369 // Return false to indicate that the condition is false if the CompVal is
370 // equal to one of the constants.
376 static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) {
377 Instruction* Cond = 0;
378 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
379 Cond = dyn_cast<Instruction>(SI->getCondition());
380 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
381 if (BI->isConditional())
382 Cond = dyn_cast<Instruction>(BI->getCondition());
383 } else if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(TI)) {
384 Cond = dyn_cast<Instruction>(IBI->getAddress());
387 TI->eraseFromParent();
388 if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond);
391 /// isValueEqualityComparison - Return true if the specified terminator checks
392 /// to see if a value is equal to constant integer value.
393 Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) {
395 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
396 // Do not permit merging of large switch instructions into their
397 // predecessors unless there is only one predecessor.
398 if (SI->getNumSuccessors()*std::distance(pred_begin(SI->getParent()),
399 pred_end(SI->getParent())) <= 128)
400 CV = SI->getCondition();
401 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
402 if (BI->isConditional() && BI->getCondition()->hasOneUse())
403 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition()))
404 if ((ICI->getPredicate() == ICmpInst::ICMP_EQ ||
405 ICI->getPredicate() == ICmpInst::ICMP_NE) &&
406 GetConstantInt(ICI->getOperand(1), TD))
407 CV = ICI->getOperand(0);
409 // Unwrap any lossless ptrtoint cast.
410 if (TD && CV && CV->getType() == TD->getIntPtrType(CV->getContext()))
411 if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV))
412 CV = PTII->getOperand(0);
416 /// GetValueEqualityComparisonCases - Given a value comparison instruction,
417 /// decode all of the 'cases' that it represents and return the 'default' block.
418 BasicBlock *SimplifyCFGOpt::
419 GetValueEqualityComparisonCases(TerminatorInst *TI,
420 std::vector<std::pair<ConstantInt*,
421 BasicBlock*> > &Cases) {
422 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
423 Cases.reserve(SI->getNumCases());
424 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
425 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
426 return SI->getDefaultDest();
429 BranchInst *BI = cast<BranchInst>(TI);
430 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
431 Cases.push_back(std::make_pair(GetConstantInt(ICI->getOperand(1), TD),
432 BI->getSuccessor(ICI->getPredicate() ==
433 ICmpInst::ICMP_NE)));
434 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
438 /// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries
439 /// in the list that match the specified block.
440 static void EliminateBlockCases(BasicBlock *BB,
441 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
442 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
443 if (Cases[i].second == BB) {
444 Cases.erase(Cases.begin()+i);
449 /// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
452 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
453 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
454 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
456 // Make V1 be smaller than V2.
457 if (V1->size() > V2->size())
460 if (V1->size() == 0) return false;
461 if (V1->size() == 1) {
463 ConstantInt *TheVal = (*V1)[0].first;
464 for (unsigned i = 0, e = V2->size(); i != e; ++i)
465 if (TheVal == (*V2)[i].first)
469 // Otherwise, just sort both lists and compare element by element.
470 array_pod_sort(V1->begin(), V1->end());
471 array_pod_sort(V2->begin(), V2->end());
472 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
473 while (i1 != e1 && i2 != e2) {
474 if ((*V1)[i1].first == (*V2)[i2].first)
476 if ((*V1)[i1].first < (*V2)[i2].first)
484 /// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
485 /// terminator instruction and its block is known to only have a single
486 /// predecessor block, check to see if that predecessor is also a value
487 /// comparison with the same value, and if that comparison determines the
488 /// outcome of this comparison. If so, simplify TI. This does a very limited
489 /// form of jump threading.
490 bool SimplifyCFGOpt::
491 SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
493 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
494 if (!PredVal) return false; // Not a value comparison in predecessor.
496 Value *ThisVal = isValueEqualityComparison(TI);
497 assert(ThisVal && "This isn't a value comparison!!");
498 if (ThisVal != PredVal) return false; // Different predicates.
500 // Find out information about when control will move from Pred to TI's block.
501 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
502 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
504 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
506 // Find information about how control leaves this block.
507 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
508 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
509 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
511 // If TI's block is the default block from Pred's comparison, potentially
512 // simplify TI based on this knowledge.
513 if (PredDef == TI->getParent()) {
514 // If we are here, we know that the value is none of those cases listed in
515 // PredCases. If there are any cases in ThisCases that are in PredCases, we
517 if (!ValuesOverlap(PredCases, ThisCases))
520 if (isa<BranchInst>(TI)) {
521 // Okay, one of the successors of this condbr is dead. Convert it to a
523 assert(ThisCases.size() == 1 && "Branch can only have one case!");
524 // Insert the new branch.
525 Instruction *NI = BranchInst::Create(ThisDef, TI);
528 // Remove PHI node entries for the dead edge.
529 ThisCases[0].second->removePredecessor(TI->getParent());
531 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
532 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
534 EraseTerminatorInstAndDCECond(TI);
538 SwitchInst *SI = cast<SwitchInst>(TI);
539 // Okay, TI has cases that are statically dead, prune them away.
540 SmallPtrSet<Constant*, 16> DeadCases;
541 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
542 DeadCases.insert(PredCases[i].first);
544 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
545 << "Through successor TI: " << *TI);
547 for (unsigned i = SI->getNumCases()-1; i != 0; --i)
548 if (DeadCases.count(SI->getCaseValue(i))) {
549 SI->getSuccessor(i)->removePredecessor(TI->getParent());
553 DEBUG(dbgs() << "Leaving: " << *TI << "\n");
557 // Otherwise, TI's block must correspond to some matched value. Find out
558 // which value (or set of values) this is.
559 ConstantInt *TIV = 0;
560 BasicBlock *TIBB = TI->getParent();
561 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
562 if (PredCases[i].second == TIBB) {
564 return false; // Cannot handle multiple values coming to this block.
565 TIV = PredCases[i].first;
567 assert(TIV && "No edge from pred to succ?");
569 // Okay, we found the one constant that our value can be if we get into TI's
570 // BB. Find out which successor will unconditionally be branched to.
571 BasicBlock *TheRealDest = 0;
572 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
573 if (ThisCases[i].first == TIV) {
574 TheRealDest = ThisCases[i].second;
578 // If not handled by any explicit cases, it is handled by the default case.
579 if (TheRealDest == 0) TheRealDest = ThisDef;
581 // Remove PHI node entries for dead edges.
582 BasicBlock *CheckEdge = TheRealDest;
583 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
584 if (*SI != CheckEdge)
585 (*SI)->removePredecessor(TIBB);
589 // Insert the new branch.
590 Instruction *NI = BranchInst::Create(TheRealDest, TI);
593 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator()
594 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
596 EraseTerminatorInstAndDCECond(TI);
601 /// ConstantIntOrdering - This class implements a stable ordering of constant
602 /// integers that does not depend on their address. This is important for
603 /// applications that sort ConstantInt's to ensure uniqueness.
604 struct ConstantIntOrdering {
605 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
606 return LHS->getValue().ult(RHS->getValue());
611 static int ConstantIntSortPredicate(const void *P1, const void *P2) {
612 const ConstantInt *LHS = *(const ConstantInt**)P1;
613 const ConstantInt *RHS = *(const ConstantInt**)P2;
614 return LHS->getValue().ult(RHS->getValue());
617 /// FoldValueComparisonIntoPredecessors - The specified terminator is a value
618 /// equality comparison instruction (either a switch or a branch on "X == c").
619 /// See if any of the predecessors of the terminator block are value comparisons
620 /// on the same value. If so, and if safe to do so, fold them together.
621 bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
622 BasicBlock *BB = TI->getParent();
623 Value *CV = isValueEqualityComparison(TI); // CondVal
624 assert(CV && "Not a comparison?");
625 bool Changed = false;
627 SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
628 while (!Preds.empty()) {
629 BasicBlock *Pred = Preds.pop_back_val();
631 // See if the predecessor is a comparison with the same value.
632 TerminatorInst *PTI = Pred->getTerminator();
633 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
635 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
636 // Figure out which 'cases' to copy from SI to PSI.
637 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
638 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
640 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
641 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
643 // Based on whether the default edge from PTI goes to BB or not, fill in
644 // PredCases and PredDefault with the new switch cases we would like to
646 SmallVector<BasicBlock*, 8> NewSuccessors;
648 if (PredDefault == BB) {
649 // If this is the default destination from PTI, only the edges in TI
650 // that don't occur in PTI, or that branch to BB will be activated.
651 std::set<ConstantInt*, ConstantIntOrdering> PTIHandled;
652 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
653 if (PredCases[i].second != BB)
654 PTIHandled.insert(PredCases[i].first);
656 // The default destination is BB, we don't need explicit targets.
657 std::swap(PredCases[i], PredCases.back());
658 PredCases.pop_back();
662 // Reconstruct the new switch statement we will be building.
663 if (PredDefault != BBDefault) {
664 PredDefault->removePredecessor(Pred);
665 PredDefault = BBDefault;
666 NewSuccessors.push_back(BBDefault);
668 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
669 if (!PTIHandled.count(BBCases[i].first) &&
670 BBCases[i].second != BBDefault) {
671 PredCases.push_back(BBCases[i]);
672 NewSuccessors.push_back(BBCases[i].second);
676 // If this is not the default destination from PSI, only the edges
677 // in SI that occur in PSI with a destination of BB will be
679 std::set<ConstantInt*, ConstantIntOrdering> PTIHandled;
680 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
681 if (PredCases[i].second == BB) {
682 PTIHandled.insert(PredCases[i].first);
683 std::swap(PredCases[i], PredCases.back());
684 PredCases.pop_back();
688 // Okay, now we know which constants were sent to BB from the
689 // predecessor. Figure out where they will all go now.
690 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
691 if (PTIHandled.count(BBCases[i].first)) {
692 // If this is one we are capable of getting...
693 PredCases.push_back(BBCases[i]);
694 NewSuccessors.push_back(BBCases[i].second);
695 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
698 // If there are any constants vectored to BB that TI doesn't handle,
699 // they must go to the default destination of TI.
700 for (std::set<ConstantInt*, ConstantIntOrdering>::iterator I =
702 E = PTIHandled.end(); I != E; ++I) {
703 PredCases.push_back(std::make_pair(*I, BBDefault));
704 NewSuccessors.push_back(BBDefault);
708 // Okay, at this point, we know which new successor Pred will get. Make
709 // sure we update the number of entries in the PHI nodes for these
711 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
712 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
714 // Convert pointer to int before we switch.
715 if (CV->getType()->isPointerTy()) {
716 assert(TD && "Cannot switch on pointer without TargetData");
717 CV = new PtrToIntInst(CV, TD->getIntPtrType(CV->getContext()),
721 // Now that the successors are updated, create the new Switch instruction.
722 SwitchInst *NewSI = SwitchInst::Create(CV, PredDefault,
723 PredCases.size(), PTI);
724 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
725 NewSI->addCase(PredCases[i].first, PredCases[i].second);
727 EraseTerminatorInstAndDCECond(PTI);
729 // Okay, last check. If BB is still a successor of PSI, then we must
730 // have an infinite loop case. If so, add an infinitely looping block
731 // to handle the case to preserve the behavior of the code.
732 BasicBlock *InfLoopBlock = 0;
733 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
734 if (NewSI->getSuccessor(i) == BB) {
735 if (InfLoopBlock == 0) {
736 // Insert it at the end of the function, because it's either code,
737 // or it won't matter if it's hot. :)
738 InfLoopBlock = BasicBlock::Create(BB->getContext(),
739 "infloop", BB->getParent());
740 BranchInst::Create(InfLoopBlock, InfLoopBlock);
742 NewSI->setSuccessor(i, InfLoopBlock);
751 // isSafeToHoistInvoke - If we would need to insert a select that uses the
752 // value of this invoke (comments in HoistThenElseCodeToIf explain why we
753 // would need to do this), we can't hoist the invoke, as there is nowhere
754 // to put the select in this case.
755 static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
756 Instruction *I1, Instruction *I2) {
757 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
759 for (BasicBlock::iterator BBI = SI->begin();
760 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
761 Value *BB1V = PN->getIncomingValueForBlock(BB1);
762 Value *BB2V = PN->getIncomingValueForBlock(BB2);
763 if (BB1V != BB2V && (BB1V==I1 || BB2V==I2)) {
771 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
772 /// BB2, hoist any common code in the two blocks up into the branch block. The
773 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
774 static bool HoistThenElseCodeToIf(BranchInst *BI) {
775 // This does very trivial matching, with limited scanning, to find identical
776 // instructions in the two blocks. In particular, we don't want to get into
777 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
778 // such, we currently just scan for obviously identical instructions in an
780 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
781 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
783 BasicBlock::iterator BB1_Itr = BB1->begin();
784 BasicBlock::iterator BB2_Itr = BB2->begin();
786 Instruction *I1 = BB1_Itr++, *I2 = BB2_Itr++;
787 while (isa<DbgInfoIntrinsic>(I1))
789 while (isa<DbgInfoIntrinsic>(I2))
791 if (I1->getOpcode() != I2->getOpcode() || isa<PHINode>(I1) ||
792 !I1->isIdenticalToWhenDefined(I2) ||
793 (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)))
796 // If we get here, we can hoist at least one instruction.
797 BasicBlock *BIParent = BI->getParent();
800 // If we are hoisting the terminator instruction, don't move one (making a
801 // broken BB), instead clone it, and remove BI.
802 if (isa<TerminatorInst>(I1))
803 goto HoistTerminator;
805 // For a normal instruction, we just move one to right before the branch,
806 // then replace all uses of the other with the first. Finally, we remove
807 // the now redundant second instruction.
808 BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
809 if (!I2->use_empty())
810 I2->replaceAllUsesWith(I1);
811 I1->intersectOptionalDataWith(I2);
812 BB2->getInstList().erase(I2);
815 while (isa<DbgInfoIntrinsic>(I1))
818 while (isa<DbgInfoIntrinsic>(I2))
820 } while (I1->getOpcode() == I2->getOpcode() &&
821 I1->isIdenticalToWhenDefined(I2));
826 // It may not be possible to hoist an invoke.
827 if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))
830 // Okay, it is safe to hoist the terminator.
831 Instruction *NT = I1->clone();
832 BIParent->getInstList().insert(BI, NT);
833 if (!NT->getType()->isVoidTy()) {
834 I1->replaceAllUsesWith(NT);
835 I2->replaceAllUsesWith(NT);
839 // Hoisting one of the terminators from our successor is a great thing.
840 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
841 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
842 // nodes, so we insert select instruction to compute the final result.
843 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
844 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
846 for (BasicBlock::iterator BBI = SI->begin();
847 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
848 Value *BB1V = PN->getIncomingValueForBlock(BB1);
849 Value *BB2V = PN->getIncomingValueForBlock(BB2);
850 if (BB1V == BB2V) continue;
852 // These values do not agree. Insert a select instruction before NT
853 // that determines the right value.
854 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
856 SI = SelectInst::Create(BI->getCondition(), BB1V, BB2V,
857 BB1V->getName()+"."+BB2V->getName(), NT);
858 // Make the PHI node use the select for all incoming values for BB1/BB2
859 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
860 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
861 PN->setIncomingValue(i, SI);
865 // Update any PHI nodes in our new successors.
866 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
867 AddPredecessorToBlock(*SI, BIParent, BB1);
869 EraseTerminatorInstAndDCECond(BI);
873 /// SpeculativelyExecuteBB - Given a conditional branch that goes to BB1
874 /// and an BB2 and the only successor of BB1 is BB2, hoist simple code
875 /// (for now, restricted to a single instruction that's side effect free) from
876 /// the BB1 into the branch block to speculatively execute it.
877 static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
878 // Only speculatively execution a single instruction (not counting the
879 // terminator) for now.
880 Instruction *HInst = NULL;
881 Instruction *Term = BB1->getTerminator();
882 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end();
884 Instruction *I = BBI;
886 if (isa<DbgInfoIntrinsic>(I)) continue;
887 if (I == Term) break;
896 // Be conservative for now. FP select instruction can often be expensive.
897 Value *BrCond = BI->getCondition();
898 if (isa<FCmpInst>(BrCond))
901 // If BB1 is actually on the false edge of the conditional branch, remember
902 // to swap the select operands later.
904 if (BB1 != BI->getSuccessor(0)) {
905 assert(BB1 == BI->getSuccessor(1) && "No edge from 'if' block?");
912 // br i1 %t1, label %BB1, label %BB2
921 // %t3 = select i1 %t1, %t2, %t3
922 switch (HInst->getOpcode()) {
923 default: return false; // Not safe / profitable to hoist.
924 case Instruction::Add:
925 case Instruction::Sub:
926 // Not worth doing for vector ops.
927 if (HInst->getType()->isVectorTy())
930 case Instruction::And:
931 case Instruction::Or:
932 case Instruction::Xor:
933 case Instruction::Shl:
934 case Instruction::LShr:
935 case Instruction::AShr:
936 // Don't mess with vector operations.
937 if (HInst->getType()->isVectorTy())
939 break; // These are all cheap and non-trapping instructions.
942 // If the instruction is obviously dead, don't try to predicate it.
943 if (HInst->use_empty()) {
944 HInst->eraseFromParent();
948 // Can we speculatively execute the instruction? And what is the value
949 // if the condition is false? Consider the phi uses, if the incoming value
950 // from the "if" block are all the same V, then V is the value of the
951 // select if the condition is false.
952 BasicBlock *BIParent = BI->getParent();
953 SmallVector<PHINode*, 4> PHIUses;
954 Value *FalseV = NULL;
956 BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0);
957 for (Value::use_iterator UI = HInst->use_begin(), E = HInst->use_end();
959 // Ignore any user that is not a PHI node in BB2. These can only occur in
960 // unreachable blocks, because they would not be dominated by the instr.
961 PHINode *PN = dyn_cast<PHINode>(*UI);
962 if (!PN || PN->getParent() != BB2)
964 PHIUses.push_back(PN);
966 Value *PHIV = PN->getIncomingValueForBlock(BIParent);
969 else if (FalseV != PHIV)
970 return false; // Inconsistent value when condition is false.
973 assert(FalseV && "Must have at least one user, and it must be a PHI");
975 // Do not hoist the instruction if any of its operands are defined but not
976 // used in this BB. The transformation will prevent the operand from
977 // being sunk into the use block.
978 for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
980 Instruction *OpI = dyn_cast<Instruction>(*i);
981 if (OpI && OpI->getParent() == BIParent &&
982 !OpI->isUsedInBasicBlock(BIParent))
986 // If we get here, we can hoist the instruction. Try to place it
987 // before the icmp instruction preceding the conditional branch.
988 BasicBlock::iterator InsertPos = BI;
989 if (InsertPos != BIParent->begin())
991 // Skip debug info between condition and branch.
992 while (InsertPos != BIParent->begin() && isa<DbgInfoIntrinsic>(InsertPos))
994 if (InsertPos == BrCond && !isa<PHINode>(BrCond)) {
995 SmallPtrSet<Instruction *, 4> BB1Insns;
996 for(BasicBlock::iterator BB1I = BB1->begin(), BB1E = BB1->end();
997 BB1I != BB1E; ++BB1I)
998 BB1Insns.insert(BB1I);
999 for(Value::use_iterator UI = BrCond->use_begin(), UE = BrCond->use_end();
1001 Instruction *Use = cast<Instruction>(*UI);
1002 if (!BB1Insns.count(Use)) continue;
1004 // If BrCond uses the instruction that place it just before
1005 // branch instruction.
1011 BIParent->getInstList().splice(InsertPos, BB1->getInstList(), HInst);
1013 // Create a select whose true value is the speculatively executed value and
1014 // false value is the previously determined FalseV.
1017 SI = SelectInst::Create(BrCond, FalseV, HInst,
1018 FalseV->getName() + "." + HInst->getName(), BI);
1020 SI = SelectInst::Create(BrCond, HInst, FalseV,
1021 HInst->getName() + "." + FalseV->getName(), BI);
1023 // Make the PHI node use the select for all incoming values for "then" and
1025 for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) {
1026 PHINode *PN = PHIUses[i];
1027 for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j)
1028 if (PN->getIncomingBlock(j) == BB1 || PN->getIncomingBlock(j) == BIParent)
1029 PN->setIncomingValue(j, SI);
1036 /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
1037 /// across this block.
1038 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
1039 BranchInst *BI = cast<BranchInst>(BB->getTerminator());
1042 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
1043 if (isa<DbgInfoIntrinsic>(BBI))
1045 if (Size > 10) return false; // Don't clone large BB's.
1048 // We can only support instructions that do not define values that are
1049 // live outside of the current basic block.
1050 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
1052 Instruction *U = cast<Instruction>(*UI);
1053 if (U->getParent() != BB || isa<PHINode>(U)) return false;
1056 // Looks ok, continue checking.
1062 /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
1063 /// that is defined in the same block as the branch and if any PHI entries are
1064 /// constants, thread edges corresponding to that entry to be branches to their
1065 /// ultimate destination.
1066 static bool FoldCondBranchOnPHI(BranchInst *BI) {
1067 BasicBlock *BB = BI->getParent();
1068 PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
1069 // NOTE: we currently cannot transform this case if the PHI node is used
1070 // outside of the block.
1071 if (!PN || PN->getParent() != BB || !PN->hasOneUse())
1074 // Degenerate case of a single entry PHI.
1075 if (PN->getNumIncomingValues() == 1) {
1076 FoldSingleEntryPHINodes(PN->getParent());
1080 // Now we know that this block has multiple preds and two succs.
1081 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
1083 // Okay, this is a simple enough basic block. See if any phi values are
1085 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1086 ConstantInt *CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i));
1087 if (CB == 0 || !CB->getType()->isIntegerTy(1)) continue;
1089 // Okay, we now know that all edges from PredBB should be revectored to
1090 // branch to RealDest.
1091 BasicBlock *PredBB = PN->getIncomingBlock(i);
1092 BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
1094 if (RealDest == BB) continue; // Skip self loops.
1096 // The dest block might have PHI nodes, other predecessors and other
1097 // difficult cases. Instead of being smart about this, just insert a new
1098 // block that jumps to the destination block, effectively splitting
1099 // the edge we are about to create.
1100 BasicBlock *EdgeBB = BasicBlock::Create(BB->getContext(),
1101 RealDest->getName()+".critedge",
1102 RealDest->getParent(), RealDest);
1103 BranchInst::Create(RealDest, EdgeBB);
1105 for (BasicBlock::iterator BBI = RealDest->begin();
1106 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
1107 Value *V = PN->getIncomingValueForBlock(BB);
1108 PN->addIncoming(V, EdgeBB);
1111 // BB may have instructions that are being threaded over. Clone these
1112 // instructions into EdgeBB. We know that there will be no uses of the
1113 // cloned instructions outside of EdgeBB.
1114 BasicBlock::iterator InsertPt = EdgeBB->begin();
1115 DenseMap<Value*, Value*> TranslateMap; // Track translated values.
1116 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
1117 if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
1118 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
1121 // Clone the instruction.
1122 Instruction *N = BBI->clone();
1123 if (BBI->hasName()) N->setName(BBI->getName()+".c");
1125 // Update operands due to translation.
1126 for (User::op_iterator i = N->op_begin(), e = N->op_end();
1128 DenseMap<Value*, Value*>::iterator PI = TranslateMap.find(*i);
1129 if (PI != TranslateMap.end())
1133 // Check for trivial simplification.
1134 if (Constant *C = ConstantFoldInstruction(N)) {
1135 TranslateMap[BBI] = C;
1136 delete N; // Constant folded away, don't need actual inst
1138 // Insert the new instruction into its new home.
1139 EdgeBB->getInstList().insert(InsertPt, N);
1140 if (!BBI->use_empty())
1141 TranslateMap[BBI] = N;
1145 // Loop over all of the edges from PredBB to BB, changing them to branch
1146 // to EdgeBB instead.
1147 TerminatorInst *PredBBTI = PredBB->getTerminator();
1148 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
1149 if (PredBBTI->getSuccessor(i) == BB) {
1150 BB->removePredecessor(PredBB);
1151 PredBBTI->setSuccessor(i, EdgeBB);
1154 // Recurse, simplifying any other constants.
1155 return FoldCondBranchOnPHI(BI) | true;
1161 /// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry
1162 /// PHI node, see if we can eliminate it.
1163 static bool FoldTwoEntryPHINode(PHINode *PN) {
1164 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1165 // statement", which has a very simple dominance structure. Basically, we
1166 // are trying to find the condition that is being branched on, which
1167 // subsequently causes this merge to happen. We really want control
1168 // dependence information for this check, but simplifycfg can't keep it up
1169 // to date, and this catches most of the cases we care about anyway.
1171 BasicBlock *BB = PN->getParent();
1172 BasicBlock *IfTrue, *IfFalse;
1173 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
1174 if (!IfCond) return false;
1176 // Okay, we found that we can merge this two-entry phi node into a select.
1177 // Doing so would require us to fold *all* two entry phi nodes in this block.
1178 // At some point this becomes non-profitable (particularly if the target
1179 // doesn't support cmov's). Only do this transformation if there are two or
1180 // fewer PHI nodes in this block.
1181 unsigned NumPhis = 0;
1182 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
1186 DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond << " T: "
1187 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1189 // Loop over the PHI's seeing if we can promote them all to select
1190 // instructions. While we are at it, keep track of the instructions
1191 // that need to be moved to the dominating block.
1192 std::set<Instruction*> AggressiveInsts;
1194 BasicBlock::iterator AfterPHIIt = BB->begin();
1195 while (isa<PHINode>(AfterPHIIt)) {
1196 PHINode *PN = cast<PHINode>(AfterPHIIt++);
1197 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) {
1198 if (PN->getIncomingValue(0) != PN)
1199 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1201 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
1202 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
1203 &AggressiveInsts) ||
1204 !DominatesMergePoint(PN->getIncomingValue(1), BB,
1205 &AggressiveInsts)) {
1210 // If we all PHI nodes are promotable, check to make sure that all
1211 // instructions in the predecessor blocks can be promoted as well. If
1212 // not, we won't be able to get rid of the control flow, so it's not
1213 // worth promoting to select instructions.
1214 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0;
1215 PN = cast<PHINode>(BB->begin());
1216 BasicBlock *Pred = PN->getIncomingBlock(0);
1217 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1219 DomBlock = *pred_begin(Pred);
1220 for (BasicBlock::iterator I = Pred->begin();
1221 !isa<TerminatorInst>(I); ++I)
1222 if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) {
1223 // This is not an aggressive instruction that we can promote.
1224 // Because of this, we won't be able to get rid of the control
1225 // flow, so the xform is not worth it.
1230 Pred = PN->getIncomingBlock(1);
1231 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1233 DomBlock = *pred_begin(Pred);
1234 for (BasicBlock::iterator I = Pred->begin();
1235 !isa<TerminatorInst>(I); ++I)
1236 if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) {
1237 // This is not an aggressive instruction that we can promote.
1238 // Because of this, we won't be able to get rid of the control
1239 // flow, so the xform is not worth it.
1244 // If we can still promote the PHI nodes after this gauntlet of tests,
1245 // do all of the PHI's now.
1247 // Move all 'aggressive' instructions, which are defined in the
1248 // conditional parts of the if's up to the dominating block.
1250 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1251 IfBlock1->getInstList(), IfBlock1->begin(),
1252 IfBlock1->getTerminator());
1254 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1255 IfBlock2->getInstList(), IfBlock2->begin(),
1256 IfBlock2->getTerminator());
1258 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
1259 // Change the PHI node into a select instruction.
1260 Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1261 Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1263 Value *NV = SelectInst::Create(IfCond, TrueVal, FalseVal, "", AfterPHIIt);
1264 PN->replaceAllUsesWith(NV);
1267 BB->getInstList().erase(PN);
1272 /// SimplifyCondBranchToTwoReturns - If we found a conditional branch that goes
1273 /// to two returning blocks, try to merge them together into one return,
1274 /// introducing a select if the return values disagree.
1275 static bool SimplifyCondBranchToTwoReturns(BranchInst *BI) {
1276 assert(BI->isConditional() && "Must be a conditional branch");
1277 BasicBlock *TrueSucc = BI->getSuccessor(0);
1278 BasicBlock *FalseSucc = BI->getSuccessor(1);
1279 ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator());
1280 ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator());
1282 // Check to ensure both blocks are empty (just a return) or optionally empty
1283 // with PHI nodes. If there are other instructions, merging would cause extra
1284 // computation on one path or the other.
1285 if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator())
1287 if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator())
1290 // Okay, we found a branch that is going to two return nodes. If
1291 // there is no return value for this function, just change the
1292 // branch into a return.
1293 if (FalseRet->getNumOperands() == 0) {
1294 TrueSucc->removePredecessor(BI->getParent());
1295 FalseSucc->removePredecessor(BI->getParent());
1296 ReturnInst::Create(BI->getContext(), 0, BI);
1297 EraseTerminatorInstAndDCECond(BI);
1301 // Otherwise, figure out what the true and false return values are
1302 // so we can insert a new select instruction.
1303 Value *TrueValue = TrueRet->getReturnValue();
1304 Value *FalseValue = FalseRet->getReturnValue();
1306 // Unwrap any PHI nodes in the return blocks.
1307 if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue))
1308 if (TVPN->getParent() == TrueSucc)
1309 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1310 if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue))
1311 if (FVPN->getParent() == FalseSucc)
1312 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1314 // In order for this transformation to be safe, we must be able to
1315 // unconditionally execute both operands to the return. This is
1316 // normally the case, but we could have a potentially-trapping
1317 // constant expression that prevents this transformation from being
1319 if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue))
1322 if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue))
1326 // Okay, we collected all the mapped values and checked them for sanity, and
1327 // defined to really do this transformation. First, update the CFG.
1328 TrueSucc->removePredecessor(BI->getParent());
1329 FalseSucc->removePredecessor(BI->getParent());
1331 // Insert select instructions where needed.
1332 Value *BrCond = BI->getCondition();
1334 // Insert a select if the results differ.
1335 if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) {
1336 } else if (isa<UndefValue>(TrueValue)) {
1337 TrueValue = FalseValue;
1339 TrueValue = SelectInst::Create(BrCond, TrueValue,
1340 FalseValue, "retval", BI);
1344 Value *RI = !TrueValue ?
1345 ReturnInst::Create(BI->getContext(), BI) :
1346 ReturnInst::Create(BI->getContext(), TrueValue, BI);
1349 DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
1350 << "\n " << *BI << "NewRet = " << *RI
1351 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc);
1353 EraseTerminatorInstAndDCECond(BI);
1358 /// FoldBranchToCommonDest - If this basic block is ONLY a setcc and a branch,
1359 /// and if a predecessor branches to us and one of our successors, fold the
1360 /// setcc into the predecessor and use logical operations to pick the right
1362 bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
1363 BasicBlock *BB = BI->getParent();
1364 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
1365 if (Cond == 0 || (!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) ||
1366 Cond->getParent() != BB || !Cond->hasOneUse())
1369 // Only allow this if the condition is a simple instruction that can be
1370 // executed unconditionally. It must be in the same block as the branch, and
1371 // must be at the front of the block.
1372 BasicBlock::iterator FrontIt = BB->front();
1373 // Ignore dbg intrinsics.
1374 while(isa<DbgInfoIntrinsic>(FrontIt))
1377 // Allow a single instruction to be hoisted in addition to the compare
1378 // that feeds the branch. We later ensure that any values that _it_ uses
1379 // were also live in the predecessor, so that we don't unnecessarily create
1380 // register pressure or inhibit out-of-order execution.
1381 Instruction *BonusInst = 0;
1382 if (&*FrontIt != Cond &&
1383 FrontIt->hasOneUse() && *FrontIt->use_begin() == Cond &&
1384 FrontIt->isSafeToSpeculativelyExecute()) {
1385 BonusInst = &*FrontIt;
1389 // Only a single bonus inst is allowed.
1390 if (&*FrontIt != Cond)
1393 // Make sure the instruction after the condition is the cond branch.
1394 BasicBlock::iterator CondIt = Cond; ++CondIt;
1395 // Ingore dbg intrinsics.
1396 while(isa<DbgInfoIntrinsic>(CondIt))
1398 if (&*CondIt != BI) {
1399 assert (!isa<DbgInfoIntrinsic>(CondIt) && "Hey do not forget debug info!");
1403 // Cond is known to be a compare or binary operator. Check to make sure that
1404 // neither operand is a potentially-trapping constant expression.
1405 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0)))
1408 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1)))
1413 // Finally, don't infinitely unroll conditional loops.
1414 BasicBlock *TrueDest = BI->getSuccessor(0);
1415 BasicBlock *FalseDest = BI->getSuccessor(1);
1416 if (TrueDest == BB || FalseDest == BB)
1419 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1420 BasicBlock *PredBlock = *PI;
1421 BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator());
1423 // Check that we have two conditional branches. If there is a PHI node in
1424 // the common successor, verify that the same value flows in from both
1426 if (PBI == 0 || PBI->isUnconditional() ||
1427 !SafeToMergeTerminators(BI, PBI))
1430 // Ensure that any values used in the bonus instruction are also used
1431 // by the terminator of the predecessor. This means that those values
1432 // must already have been resolved, so we won't be inhibiting the
1433 // out-of-order core by speculating them earlier.
1435 // Collect the values used by the bonus inst
1436 SmallPtrSet<Value*, 4> UsedValues;
1437 for (Instruction::op_iterator OI = BonusInst->op_begin(),
1438 OE = BonusInst->op_end(); OI != OE; ++OI) {
1440 if (!isa<Constant>(V))
1441 UsedValues.insert(V);
1444 SmallVector<std::pair<Value*, unsigned>, 4> Worklist;
1445 Worklist.push_back(std::make_pair(PBI->getOperand(0), 0));
1447 // Walk up to four levels back up the use-def chain of the predecessor's
1448 // terminator to see if all those values were used. The choice of four
1449 // levels is arbitrary, to provide a compile-time-cost bound.
1450 while (!Worklist.empty()) {
1451 std::pair<Value*, unsigned> Pair = Worklist.back();
1452 Worklist.pop_back();
1454 if (Pair.second >= 4) continue;
1455 UsedValues.erase(Pair.first);
1456 if (UsedValues.empty()) break;
1458 if (Instruction* I = dyn_cast<Instruction>(Pair.first)) {
1459 for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
1461 Worklist.push_back(std::make_pair(OI->get(), Pair.second+1));
1465 if (!UsedValues.empty()) return false;
1468 Instruction::BinaryOps Opc;
1469 bool InvertPredCond = false;
1471 if (PBI->getSuccessor(0) == TrueDest)
1472 Opc = Instruction::Or;
1473 else if (PBI->getSuccessor(1) == FalseDest)
1474 Opc = Instruction::And;
1475 else if (PBI->getSuccessor(0) == FalseDest)
1476 Opc = Instruction::And, InvertPredCond = true;
1477 else if (PBI->getSuccessor(1) == TrueDest)
1478 Opc = Instruction::Or, InvertPredCond = true;
1482 DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB);
1484 // If we need to invert the condition in the pred block to match, do so now.
1485 if (InvertPredCond) {
1487 BinaryOperator::CreateNot(PBI->getCondition(),
1488 PBI->getCondition()->getName()+".not", PBI);
1489 PBI->setCondition(NewCond);
1490 BasicBlock *OldTrue = PBI->getSuccessor(0);
1491 BasicBlock *OldFalse = PBI->getSuccessor(1);
1492 PBI->setSuccessor(0, OldFalse);
1493 PBI->setSuccessor(1, OldTrue);
1496 // If we have a bonus inst, clone it into the predecessor block.
1497 Instruction *NewBonus = 0;
1499 NewBonus = BonusInst->clone();
1500 PredBlock->getInstList().insert(PBI, NewBonus);
1501 NewBonus->takeName(BonusInst);
1502 BonusInst->setName(BonusInst->getName()+".old");
1505 // Clone Cond into the predecessor basic block, and or/and the
1506 // two conditions together.
1507 Instruction *New = Cond->clone();
1508 if (BonusInst) New->replaceUsesOfWith(BonusInst, NewBonus);
1509 PredBlock->getInstList().insert(PBI, New);
1510 New->takeName(Cond);
1511 Cond->setName(New->getName()+".old");
1513 Value *NewCond = BinaryOperator::Create(Opc, PBI->getCondition(),
1514 New, "or.cond", PBI);
1515 PBI->setCondition(NewCond);
1516 if (PBI->getSuccessor(0) == BB) {
1517 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1518 PBI->setSuccessor(0, TrueDest);
1520 if (PBI->getSuccessor(1) == BB) {
1521 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1522 PBI->setSuccessor(1, FalseDest);
1529 /// SimplifyCondBranchToCondBranch - If we have a conditional branch as a
1530 /// predecessor of another block, this function tries to simplify it. We know
1531 /// that PBI and BI are both conditional branches, and BI is in one of the
1532 /// successor blocks of PBI - PBI branches to BI.
1533 static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
1534 assert(PBI->isConditional() && BI->isConditional());
1535 BasicBlock *BB = BI->getParent();
1537 // If this block ends with a branch instruction, and if there is a
1538 // predecessor that ends on a branch of the same condition, make
1539 // this conditional branch redundant.
1540 if (PBI->getCondition() == BI->getCondition() &&
1541 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1542 // Okay, the outcome of this conditional branch is statically
1543 // knowable. If this block had a single pred, handle specially.
1544 if (BB->getSinglePredecessor()) {
1545 // Turn this into a branch on constant.
1546 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1547 BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
1549 return true; // Nuke the branch on constant.
1552 // Otherwise, if there are multiple predecessors, insert a PHI that merges
1553 // in the constant and simplify the block result. Subsequent passes of
1554 // simplifycfg will thread the block.
1555 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1556 PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()),
1557 BI->getCondition()->getName() + ".pr",
1559 // Okay, we're going to insert the PHI node. Since PBI is not the only
1560 // predecessor, compute the PHI'd conditional value for all of the preds.
1561 // Any predecessor where the condition is not computable we keep symbolic.
1562 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1563 BasicBlock *P = *PI;
1564 if ((PBI = dyn_cast<BranchInst>(P->getTerminator())) &&
1565 PBI != BI && PBI->isConditional() &&
1566 PBI->getCondition() == BI->getCondition() &&
1567 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1568 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1569 NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
1572 NewPN->addIncoming(BI->getCondition(), P);
1576 BI->setCondition(NewPN);
1581 // If this is a conditional branch in an empty block, and if any
1582 // predecessors is a conditional branch to one of our destinations,
1583 // fold the conditions into logical ops and one cond br.
1584 BasicBlock::iterator BBI = BB->begin();
1585 // Ignore dbg intrinsics.
1586 while (isa<DbgInfoIntrinsic>(BBI))
1592 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BI->getCondition()))
1597 if (PBI->getSuccessor(0) == BI->getSuccessor(0))
1599 else if (PBI->getSuccessor(0) == BI->getSuccessor(1))
1600 PBIOp = 0, BIOp = 1;
1601 else if (PBI->getSuccessor(1) == BI->getSuccessor(0))
1602 PBIOp = 1, BIOp = 0;
1603 else if (PBI->getSuccessor(1) == BI->getSuccessor(1))
1608 // Check to make sure that the other destination of this branch
1609 // isn't BB itself. If so, this is an infinite loop that will
1610 // keep getting unwound.
1611 if (PBI->getSuccessor(PBIOp) == BB)
1614 // Do not perform this transformation if it would require
1615 // insertion of a large number of select instructions. For targets
1616 // without predication/cmovs, this is a big pessimization.
1617 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1619 unsigned NumPhis = 0;
1620 for (BasicBlock::iterator II = CommonDest->begin();
1621 isa<PHINode>(II); ++II, ++NumPhis)
1622 if (NumPhis > 2) // Disable this xform.
1625 // Finally, if everything is ok, fold the branches to logical ops.
1626 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
1628 DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent()
1629 << "AND: " << *BI->getParent());
1632 // If OtherDest *is* BB, then BB is a basic block with a single conditional
1633 // branch in it, where one edge (OtherDest) goes back to itself but the other
1634 // exits. We don't *know* that the program avoids the infinite loop
1635 // (even though that seems likely). If we do this xform naively, we'll end up
1636 // recursively unpeeling the loop. Since we know that (after the xform is
1637 // done) that the block *is* infinite if reached, we just make it an obviously
1638 // infinite loop with no cond branch.
1639 if (OtherDest == BB) {
1640 // Insert it at the end of the function, because it's either code,
1641 // or it won't matter if it's hot. :)
1642 BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(),
1643 "infloop", BB->getParent());
1644 BranchInst::Create(InfLoopBlock, InfLoopBlock);
1645 OtherDest = InfLoopBlock;
1648 DEBUG(dbgs() << *PBI->getParent()->getParent());
1650 // BI may have other predecessors. Because of this, we leave
1651 // it alone, but modify PBI.
1653 // Make sure we get to CommonDest on True&True directions.
1654 Value *PBICond = PBI->getCondition();
1656 PBICond = BinaryOperator::CreateNot(PBICond,
1657 PBICond->getName()+".not",
1659 Value *BICond = BI->getCondition();
1661 BICond = BinaryOperator::CreateNot(BICond,
1662 BICond->getName()+".not",
1664 // Merge the conditions.
1665 Value *Cond = BinaryOperator::CreateOr(PBICond, BICond, "brmerge", PBI);
1667 // Modify PBI to branch on the new condition to the new dests.
1668 PBI->setCondition(Cond);
1669 PBI->setSuccessor(0, CommonDest);
1670 PBI->setSuccessor(1, OtherDest);
1672 // OtherDest may have phi nodes. If so, add an entry from PBI's
1673 // block that are identical to the entries for BI's block.
1675 for (BasicBlock::iterator II = OtherDest->begin();
1676 (PN = dyn_cast<PHINode>(II)); ++II) {
1677 Value *V = PN->getIncomingValueForBlock(BB);
1678 PN->addIncoming(V, PBI->getParent());
1681 // We know that the CommonDest already had an edge from PBI to
1682 // it. If it has PHIs though, the PHIs may have different
1683 // entries for BB and PBI's BB. If so, insert a select to make
1685 for (BasicBlock::iterator II = CommonDest->begin();
1686 (PN = dyn_cast<PHINode>(II)); ++II) {
1687 Value *BIV = PN->getIncomingValueForBlock(BB);
1688 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
1689 Value *PBIV = PN->getIncomingValue(PBBIdx);
1691 // Insert a select in PBI to pick the right value.
1692 Value *NV = SelectInst::Create(PBICond, PBIV, BIV,
1693 PBIV->getName()+".mux", PBI);
1694 PN->setIncomingValue(PBBIdx, NV);
1698 DEBUG(dbgs() << "INTO: " << *PBI->getParent());
1699 DEBUG(dbgs() << *PBI->getParent()->getParent());
1701 // This basic block is probably dead. We know it has at least
1702 // one fewer predecessor.
1706 // SimplifyIndirectBrOnSelect - Replaces
1707 // (indirectbr (select cond, blockaddress(@fn, BlockA),
1708 // blockaddress(@fn, BlockB)))
1710 // (br cond, BlockA, BlockB).
1711 static bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI) {
1712 // Check that both operands of the select are block addresses.
1713 BlockAddress *TBA = dyn_cast<BlockAddress>(SI->getTrueValue());
1714 BlockAddress *FBA = dyn_cast<BlockAddress>(SI->getFalseValue());
1718 // Extract the actual blocks.
1719 BasicBlock *TrueBB = TBA->getBasicBlock();
1720 BasicBlock *FalseBB = FBA->getBasicBlock();
1722 // Remove any superfluous successor edges from the CFG.
1723 // First, figure out which successors to preserve.
1724 // If TrueBB and FalseBB are equal, only try to preserve one copy of that
1726 BasicBlock *KeepEdge1 = TrueBB;
1727 BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : 0;
1729 // Then remove the rest.
1730 for (unsigned I = 0, E = IBI->getNumSuccessors(); I != E; ++I) {
1731 BasicBlock *Succ = IBI->getSuccessor(I);
1732 // Make sure only to keep exactly one copy of each edge.
1733 if (Succ == KeepEdge1)
1735 else if (Succ == KeepEdge2)
1738 Succ->removePredecessor(IBI->getParent());
1741 // Insert an appropriate new terminator.
1742 if ((KeepEdge1 == 0) && (KeepEdge2 == 0)) {
1743 if (TrueBB == FalseBB)
1744 // We were only looking for one successor, and it was present.
1745 // Create an unconditional branch to it.
1746 BranchInst::Create(TrueBB, IBI);
1748 // We found both of the successors we were looking for.
1749 // Create a conditional branch sharing the condition of the select.
1750 BranchInst::Create(TrueBB, FalseBB, SI->getCondition(), IBI);
1751 } else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) {
1752 // Neither of the selected blocks were successors, so this
1753 // indirectbr must be unreachable.
1754 new UnreachableInst(IBI->getContext(), IBI);
1756 // One of the selected values was a successor, but the other wasn't.
1757 // Insert an unconditional branch to the one that was found;
1758 // the edge to the one that wasn't must be unreachable.
1760 // Only TrueBB was found.
1761 BranchInst::Create(TrueBB, IBI);
1763 // Only FalseBB was found.
1764 BranchInst::Create(FalseBB, IBI);
1767 EraseTerminatorInstAndDCECond(IBI);
1771 /// TryToSimplifyUncondBranchWithICmpInIt - This is called when we find an icmp
1772 /// instruction (a seteq/setne with a constant) as the only instruction in a
1773 /// block that ends with an uncond branch. We are looking for a very specific
1774 /// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In
1775 /// this case, we merge the first two "or's of icmp" into a switch, but then the
1776 /// default value goes to an uncond block with a seteq in it, we get something
1779 /// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
1781 /// %tmp = icmp eq i8 %A, 92
1784 /// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
1786 /// We prefer to split the edge to 'end' so that there is a true/false entry to
1787 /// the PHI, merging the third icmp into the switch.
1788 static bool TryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI) {
1789 BasicBlock *BB = ICI->getParent();
1790 // If the block has any PHIs in it or the icmp has multiple uses, it is too
1792 if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse()) return false;
1794 Value *V = ICI->getOperand(0);
1795 ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
1797 // The pattern we're looking for is where our only predecessor is a switch on
1798 // 'V' and this block is the default case for the switch. In this case we can
1799 // fold the compared value into the switch to simplify things.
1800 BasicBlock *Pred = BB->getSinglePredecessor();
1801 if (Pred == 0 || !isa<SwitchInst>(Pred->getTerminator())) return false;
1803 SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
1804 if (SI->getCondition() != V)
1807 // If BB is reachable on a non-default case, then we simply know the value of
1808 // V in this block. Substitute it and constant fold the icmp instruction
1810 if (SI->getDefaultDest() != BB) {
1811 ConstantInt *VVal = SI->findCaseDest(BB);
1812 assert(VVal && "Should have a unique destination value");
1813 ICI->setOperand(0, VVal);
1815 if (Constant *C = ConstantFoldInstruction(ICI)) {
1816 ICI->replaceAllUsesWith(C);
1817 ICI->eraseFromParent();
1819 // BB is now empty, so it is likely to simplify away.
1820 return SimplifyCFG(BB) | true;
1823 // The use of the icmp has to be in the 'end' block, by the only PHI node in
1825 BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
1826 PHINode *PHIUse = dyn_cast<PHINode>(ICI->use_back());
1827 if (PHIUse == 0 || PHIUse != &SuccBlock->front() ||
1828 isa<PHINode>(++BasicBlock::iterator(PHIUse)))
1831 // If the icmp is a SETEQ, then the default dest gets false, the new edge gets
1833 Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
1834 Constant *NewCst = ConstantInt::getFalse(BB->getContext());
1836 if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
1837 std::swap(DefaultCst, NewCst);
1839 // Replace ICI (which is used by the PHI for the default value) with true or
1840 // false depending on if it is EQ or NE.
1841 ICI->replaceAllUsesWith(DefaultCst);
1842 ICI->eraseFromParent();
1844 // Okay, the switch goes to this block on a default value. Add an edge from
1845 // the switch to the merge point on the compared value.
1846 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "switch.edge",
1847 BB->getParent(), BB);
1848 SI->addCase(Cst, NewBB);
1850 // NewBB branches to the phi block, add the uncond branch and the phi entry.
1851 BranchInst::Create(SuccBlock, NewBB);
1852 PHIUse->addIncoming(NewCst, NewBB);
1856 bool SimplifyCFGOpt::run(BasicBlock *BB) {
1857 bool Changed = false;
1858 Function *Fn = BB->getParent();
1860 assert(BB && Fn && "Block not embedded in function!");
1861 assert(BB->getTerminator() && "Degenerate basic block encountered!");
1863 // Remove basic blocks that have no predecessors (except the entry block)...
1864 // or that just have themself as a predecessor. These are unreachable.
1865 if ((pred_begin(BB) == pred_end(BB) && BB != &Fn->getEntryBlock()) ||
1866 BB->getSinglePredecessor() == BB) {
1867 DEBUG(dbgs() << "Removing BB: \n" << *BB);
1868 DeleteDeadBlock(BB);
1872 // Check to see if we can constant propagate this terminator instruction
1874 Changed |= ConstantFoldTerminator(BB);
1876 // Check for and eliminate duplicate PHI nodes in this block.
1877 Changed |= EliminateDuplicatePHINodes(BB);
1879 // If there is a trivial two-entry PHI node in this basic block, and we can
1880 // eliminate it, do so now.
1881 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1882 if (PN->getNumIncomingValues() == 2)
1883 Changed |= FoldTwoEntryPHINode(PN);
1885 // If this is a returning block with only PHI nodes in it, fold the return
1886 // instruction into any unconditional branch predecessors.
1888 // If any predecessor is a conditional branch that just selects among
1889 // different return values, fold the replace the branch/return with a select
1891 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1892 if (BB->getFirstNonPHIOrDbg()->isTerminator()) {
1893 // Find predecessors that end with branches.
1894 SmallVector<BasicBlock*, 8> UncondBranchPreds;
1895 SmallVector<BranchInst*, 8> CondBranchPreds;
1896 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1897 BasicBlock *P = *PI;
1898 TerminatorInst *PTI = P->getTerminator();
1899 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) {
1900 if (BI->isUnconditional())
1901 UncondBranchPreds.push_back(P);
1903 CondBranchPreds.push_back(BI);
1907 // If we found some, do the transformation!
1908 if (!UncondBranchPreds.empty()) {
1909 while (!UncondBranchPreds.empty()) {
1910 BasicBlock *Pred = UncondBranchPreds.pop_back_val();
1911 DEBUG(dbgs() << "FOLDING: " << *BB
1912 << "INTO UNCOND BRANCH PRED: " << *Pred);
1913 Instruction *UncondBranch = Pred->getTerminator();
1914 // Clone the return and add it to the end of the predecessor.
1915 Instruction *NewRet = RI->clone();
1916 Pred->getInstList().push_back(NewRet);
1918 // If the return instruction returns a value, and if the value was a
1919 // PHI node in "BB", propagate the right value into the return.
1920 for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
1922 if (PHINode *PN = dyn_cast<PHINode>(*i))
1923 if (PN->getParent() == BB)
1924 *i = PN->getIncomingValueForBlock(Pred);
1926 // Update any PHI nodes in the returning block to realize that we no
1927 // longer branch to them.
1928 BB->removePredecessor(Pred);
1929 Pred->getInstList().erase(UncondBranch);
1932 // If we eliminated all predecessors of the block, delete the block now.
1933 if (pred_begin(BB) == pred_end(BB))
1934 // We know there are no successors, so just nuke the block.
1935 Fn->getBasicBlockList().erase(BB);
1940 // Check out all of the conditional branches going to this return
1941 // instruction. If any of them just select between returns, change the
1942 // branch itself into a select/return pair.
1943 while (!CondBranchPreds.empty()) {
1944 BranchInst *BI = CondBranchPreds.pop_back_val();
1946 // Check to see if the non-BB successor is also a return block.
1947 if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) &&
1948 isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) &&
1949 SimplifyCondBranchToTwoReturns(BI))
1953 } else if (isa<UnwindInst>(BB->begin())) {
1954 // Check to see if the first instruction in this block is just an unwind.
1955 // If so, replace any invoke instructions which use this as an exception
1956 // destination with call instructions.
1958 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
1959 while (!Preds.empty()) {
1960 BasicBlock *Pred = Preds.back();
1961 InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator());
1962 if (II && II->getUnwindDest() == BB) {
1963 // Insert a new branch instruction before the invoke, because this
1964 // is now a fall through.
1965 BranchInst *BI = BranchInst::Create(II->getNormalDest(), II);
1966 Pred->getInstList().remove(II); // Take out of symbol table
1968 // Insert the call now.
1969 SmallVector<Value*,8> Args(II->op_begin(), II->op_end()-3);
1970 CallInst *CI = CallInst::Create(II->getCalledValue(),
1971 Args.begin(), Args.end(),
1973 CI->setCallingConv(II->getCallingConv());
1974 CI->setAttributes(II->getAttributes());
1975 // If the invoke produced a value, the Call now does instead.
1976 II->replaceAllUsesWith(CI);
1984 // If this block is now dead (and isn't the entry block), remove it.
1985 if (pred_begin(BB) == pred_end(BB) && BB != &Fn->getEntryBlock()) {
1986 // We know there are no successors, so just nuke the block.
1987 Fn->getBasicBlockList().erase(BB);
1991 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1992 if (isValueEqualityComparison(SI)) {
1993 // If we only have one predecessor, and if it is a branch on this value,
1994 // see if that predecessor totally determines the outcome of this switch.
1995 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1996 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
1997 return SimplifyCFG(BB) || 1;
1999 // If the block only contains the switch, see if we can fold the block
2000 // away into any preds.
2001 BasicBlock::iterator BBI = BB->begin();
2002 // Ignore dbg intrinsics.
2003 while (isa<DbgInfoIntrinsic>(BBI))
2006 if (FoldValueComparisonIntoPredecessors(SI))
2007 return SimplifyCFG(BB) || 1;
2009 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
2010 if (BI->isUnconditional()) {
2011 // If the Terminator is the only non-phi instruction, simplify the block.
2012 BasicBlock::iterator I = BB->getFirstNonPHIOrDbg();
2013 if (I->isTerminator() && BB != &Fn->getEntryBlock() &&
2014 TryToSimplifyUncondBranchFromEmptyBlock(BB))
2017 // If the only instruction in the block is a seteq/setne comparison
2018 // against a constant, try to simplify the block.
2019 if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
2020 if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
2021 for (++I; isa<DbgInfoIntrinsic>(I); ++I)
2023 if (I->isTerminator() &&
2024 TryToSimplifyUncondBranchWithICmpInIt(ICI))
2028 } else { // Conditional branch
2029 if (isValueEqualityComparison(BI)) {
2030 // If we only have one predecessor, and if it is a branch on this value,
2031 // see if that predecessor totally determines the outcome of this
2033 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
2034 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
2035 return SimplifyCFG(BB) | true;
2037 // This block must be empty, except for the setcond inst, if it exists.
2038 // Ignore dbg intrinsics.
2039 BasicBlock::iterator I = BB->begin();
2040 // Ignore dbg intrinsics.
2041 while (isa<DbgInfoIntrinsic>(I))
2044 if (FoldValueComparisonIntoPredecessors(BI))
2045 return SimplifyCFG(BB) | true;
2046 } else if (&*I == cast<Instruction>(BI->getCondition())){
2048 // Ignore dbg intrinsics.
2049 while (isa<DbgInfoIntrinsic>(I))
2051 if (&*I == BI && FoldValueComparisonIntoPredecessors(BI))
2052 return SimplifyCFG(BB) | true;
2056 // If this is a branch on a phi node in the current block, thread control
2057 // through this block if any PHI node entries are constants.
2058 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
2059 if (PN->getParent() == BI->getParent())
2060 if (FoldCondBranchOnPHI(BI))
2061 return SimplifyCFG(BB) | true;
2063 // If this basic block is ONLY a setcc and a branch, and if a predecessor
2064 // branches to us and one of our successors, fold the setcc into the
2065 // predecessor and use logical operations to pick the right destination.
2066 if (FoldBranchToCommonDest(BI))
2067 return SimplifyCFG(BB) | true;
2070 // Scan predecessor blocks for conditional branches.
2071 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
2072 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
2073 if (PBI != BI && PBI->isConditional())
2074 if (SimplifyCondBranchToCondBranch(PBI, BI))
2075 return SimplifyCFG(BB) | true;
2078 // Change br (X == 0 | X == 1), T, F into a switch instruction.
2079 // If this is a bunch of seteq's or'd together, or if it's a bunch of
2080 // 'setne's and'ed together, collect them.
2082 std::vector<ConstantInt*> Values;
2083 bool TrueWhenEqual = GatherValueComparisons(BI->getCondition(), CompVal,
2086 // There might be duplicate constants in the list, which the switch
2087 // instruction can't handle, remove them now.
2088 array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate);
2089 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
2091 // Figure out which block is which destination.
2092 BasicBlock *DefaultBB = BI->getSuccessor(1);
2093 BasicBlock *EdgeBB = BI->getSuccessor(0);
2094 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
2096 // Convert pointer to int before we switch.
2097 if (CompVal->getType()->isPointerTy()) {
2098 assert(TD && "Cannot switch on pointer without TargetData");
2099 CompVal = new PtrToIntInst(CompVal,
2100 TD->getIntPtrType(CompVal->getContext()),
2104 // Create the new switch instruction now.
2105 SwitchInst *New = SwitchInst::Create(CompVal, DefaultBB,
2108 // Add all of the 'cases' to the switch instruction.
2109 for (unsigned i = 0, e = Values.size(); i != e; ++i)
2110 New->addCase(Values[i], EdgeBB);
2112 // We added edges from PI to the EdgeBB. As such, if there were any
2113 // PHI nodes in EdgeBB, they need entries to be added corresponding to
2114 // the number of edges added.
2115 for (BasicBlock::iterator BBI = EdgeBB->begin();
2116 isa<PHINode>(BBI); ++BBI) {
2117 PHINode *PN = cast<PHINode>(BBI);
2118 Value *InVal = PN->getIncomingValueForBlock(BB);
2119 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
2120 PN->addIncoming(InVal, BB);
2123 // Erase the old branch instruction.
2124 EraseTerminatorInstAndDCECond(BI);
2128 } else if (isa<UnreachableInst>(BB->getTerminator())) {
2129 // If there are any instructions immediately before the unreachable that can
2130 // be removed, do so.
2131 Instruction *Unreachable = BB->getTerminator();
2132 while (Unreachable != BB->begin()) {
2133 BasicBlock::iterator BBI = Unreachable;
2135 // Do not delete instructions that can have side effects, like calls
2136 // (which may never return) and volatile loads and stores.
2137 if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI)) break;
2139 if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
2140 if (SI->isVolatile())
2143 if (LoadInst *LI = dyn_cast<LoadInst>(BBI))
2144 if (LI->isVolatile())
2147 // Delete this instruction
2148 BB->getInstList().erase(BBI);
2152 // If the unreachable instruction is the first in the block, take a gander
2153 // at all of the predecessors of this instruction, and simplify them.
2154 if (&BB->front() == Unreachable) {
2155 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
2156 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
2157 TerminatorInst *TI = Preds[i]->getTerminator();
2159 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
2160 if (BI->isUnconditional()) {
2161 if (BI->getSuccessor(0) == BB) {
2162 new UnreachableInst(TI->getContext(), TI);
2163 TI->eraseFromParent();
2167 if (BI->getSuccessor(0) == BB) {
2168 BranchInst::Create(BI->getSuccessor(1), BI);
2169 EraseTerminatorInstAndDCECond(BI);
2170 } else if (BI->getSuccessor(1) == BB) {
2171 BranchInst::Create(BI->getSuccessor(0), BI);
2172 EraseTerminatorInstAndDCECond(BI);
2176 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
2177 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
2178 if (SI->getSuccessor(i) == BB) {
2179 BB->removePredecessor(SI->getParent());
2184 // If the default value is unreachable, figure out the most popular
2185 // destination and make it the default.
2186 if (SI->getSuccessor(0) == BB) {
2187 std::map<BasicBlock*, unsigned> Popularity;
2188 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
2189 Popularity[SI->getSuccessor(i)]++;
2191 // Find the most popular block.
2192 unsigned MaxPop = 0;
2193 BasicBlock *MaxBlock = 0;
2194 for (std::map<BasicBlock*, unsigned>::iterator
2195 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
2196 if (I->second > MaxPop) {
2198 MaxBlock = I->first;
2202 // Make this the new default, allowing us to delete any explicit
2204 SI->setSuccessor(0, MaxBlock);
2207 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
2209 if (isa<PHINode>(MaxBlock->begin()))
2210 for (unsigned i = 0; i != MaxPop-1; ++i)
2211 MaxBlock->removePredecessor(SI->getParent());
2213 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
2214 if (SI->getSuccessor(i) == MaxBlock) {
2220 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
2221 if (II->getUnwindDest() == BB) {
2222 // Convert the invoke to a call instruction. This would be a good
2223 // place to note that the call does not throw though.
2224 BranchInst *BI = BranchInst::Create(II->getNormalDest(), II);
2225 II->removeFromParent(); // Take out of symbol table
2227 // Insert the call now...
2228 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end()-3);
2229 CallInst *CI = CallInst::Create(II->getCalledValue(),
2230 Args.begin(), Args.end(),
2232 CI->setCallingConv(II->getCallingConv());
2233 CI->setAttributes(II->getAttributes());
2234 // If the invoke produced a value, the call does now instead.
2235 II->replaceAllUsesWith(CI);
2242 // If this block is now dead, remove it.
2243 if (pred_begin(BB) == pred_end(BB) && BB != &Fn->getEntryBlock()) {
2244 // We know there are no successors, so just nuke the block.
2245 Fn->getBasicBlockList().erase(BB);
2249 } else if (IndirectBrInst *IBI =
2250 dyn_cast<IndirectBrInst>(BB->getTerminator())) {
2251 // Eliminate redundant destinations.
2252 SmallPtrSet<Value *, 8> Succs;
2253 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
2254 BasicBlock *Dest = IBI->getDestination(i);
2255 if (!Dest->hasAddressTaken() || !Succs.insert(Dest)) {
2256 Dest->removePredecessor(BB);
2257 IBI->removeDestination(i);
2263 if (IBI->getNumDestinations() == 0) {
2264 // If the indirectbr has no successors, change it to unreachable.
2265 new UnreachableInst(IBI->getContext(), IBI);
2266 EraseTerminatorInstAndDCECond(IBI);
2268 } else if (IBI->getNumDestinations() == 1) {
2269 // If the indirectbr has one successor, change it to a direct branch.
2270 BranchInst::Create(IBI->getDestination(0), IBI);
2271 EraseTerminatorInstAndDCECond(IBI);
2273 } else if (SelectInst *SI = dyn_cast<SelectInst>(IBI->getAddress())) {
2274 if (SimplifyIndirectBrOnSelect(IBI, SI))
2275 return SimplifyCFG(BB) | true;
2279 // Merge basic blocks into their predecessor if there is only one distinct
2280 // pred, and if there is only one distinct successor of the predecessor, and
2281 // if there are no PHI nodes.
2283 if (MergeBlockIntoPredecessor(BB))
2286 // Otherwise, if this block only has a single predecessor, and if that block
2287 // is a conditional branch, see if we can hoist any code from this block up
2288 // into our predecessor.
2289 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
2290 BasicBlock *OnlyPred = 0;
2291 for (; PI != PE; ++PI) { // Search all predecessors, see if they are all same
2294 else if (*PI != OnlyPred) {
2295 OnlyPred = 0; // There are multiple different predecessors...
2301 BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator());
2302 if (BI && BI->isConditional()) {
2303 // Get the other block.
2304 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
2305 PI = pred_begin(OtherBB);
2308 if (PI == pred_end(OtherBB)) {
2309 // We have a conditional branch to two blocks that are only reachable
2310 // from the condbr. We know that the condbr dominates the two blocks,
2311 // so see if there is any identical code in the "then" and "else"
2312 // blocks. If so, we can hoist it up to the branching block.
2313 Changed |= HoistThenElseCodeToIf(BI);
2315 BasicBlock* OnlySucc = NULL;
2316 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
2320 else if (*SI != OnlySucc) {
2321 OnlySucc = 0; // There are multiple distinct successors!
2326 if (OnlySucc == OtherBB) {
2327 // If BB's only successor is the other successor of the predecessor,
2328 // i.e. a triangle, see if we can hoist any code from this block up
2329 // to the "if" block.
2330 Changed |= SpeculativelyExecuteBB(BI, BB);
2339 /// SimplifyCFG - This function is used to do simplification of a CFG. For
2340 /// example, it adjusts branches to branches to eliminate the extra hop, it
2341 /// eliminates unreachable basic blocks, and does other "peephole" optimization
2342 /// of the CFG. It returns true if a modification was made.
2344 bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) {
2345 return SimplifyCFGOpt(TD).run(BB);