1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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/Type.h"
19 #include "llvm/Support/CFG.h"
20 #include "llvm/Support/Debug.h"
27 // PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
28 // predecessors from "BB". This is a little tricky because "Succ" has PHI
29 // nodes, which need to have extra slots added to them to hold the merge edges
30 // from BB's predecessors, and BB itself might have had PHI nodes in it. This
31 // function returns true (failure) if the Succ BB already has a predecessor that
32 // is a predecessor of BB and incoming PHI arguments would not be discernible.
34 // Assumption: Succ is the single successor for BB.
36 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
37 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
39 if (!isa<PHINode>(Succ->front()))
40 return false; // We can make the transformation, no problem.
42 // If there is more than one predecessor, and there are PHI nodes in
43 // the successor, then we need to add incoming edges for the PHI nodes
45 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
47 // Check to see if one of the predecessors of BB is already a predecessor of
48 // Succ. If so, we cannot do the transformation if there are any PHI nodes
49 // with incompatible values coming in from the two edges!
51 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
52 if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
53 // Loop over all of the PHI nodes checking to see if there are
54 // incompatible values coming in.
55 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
56 PHINode *PN = cast<PHINode>(I);
57 // Loop up the entries in the PHI node for BB and for *PI if the values
58 // coming in are non-equal, we cannot merge these two blocks (instead we
59 // should insert a conditional move or something, then merge the
61 int Idx1 = PN->getBasicBlockIndex(BB);
62 int Idx2 = PN->getBasicBlockIndex(*PI);
63 assert(Idx1 != -1 && Idx2 != -1 &&
64 "Didn't have entries for my predecessors??");
65 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
66 return true; // Values are not equal...
70 // Loop over all of the PHI nodes in the successor BB.
71 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
72 PHINode *PN = cast<PHINode>(I);
73 Value *OldVal = PN->removeIncomingValue(BB, false);
74 assert(OldVal && "No entry in PHI for Pred BB!");
76 // If this incoming value is one of the PHI nodes in BB, the new entries in
77 // the PHI node are the entries from the old PHI.
78 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
79 PHINode *OldValPN = cast<PHINode>(OldVal);
80 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
81 PN->addIncoming(OldValPN->getIncomingValue(i),
82 OldValPN->getIncomingBlock(i));
84 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
85 End = BBPreds.end(); PredI != End; ++PredI) {
86 // Add an incoming value for each of the new incoming values...
87 PN->addIncoming(OldVal, *PredI);
94 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
95 /// presumably PHI nodes in it), check to see if the merge at this block is due
96 /// to an "if condition". If so, return the boolean condition that determines
97 /// which entry into BB will be taken. Also, return by references the block
98 /// that will be entered from if the condition is true, and the block that will
99 /// be entered if the condition is false.
102 static Value *GetIfCondition(BasicBlock *BB,
103 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
104 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
105 "Function can only handle blocks with 2 predecessors!");
106 BasicBlock *Pred1 = *pred_begin(BB);
107 BasicBlock *Pred2 = *++pred_begin(BB);
109 // We can only handle branches. Other control flow will be lowered to
110 // branches if possible anyway.
111 if (!isa<BranchInst>(Pred1->getTerminator()) ||
112 !isa<BranchInst>(Pred2->getTerminator()))
114 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
115 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
117 // Eliminate code duplication by ensuring that Pred1Br is conditional if
119 if (Pred2Br->isConditional()) {
120 // If both branches are conditional, we don't have an "if statement". In
121 // reality, we could transform this case, but since the condition will be
122 // required anyway, we stand no chance of eliminating it, so the xform is
123 // probably not profitable.
124 if (Pred1Br->isConditional())
127 std::swap(Pred1, Pred2);
128 std::swap(Pred1Br, Pred2Br);
131 if (Pred1Br->isConditional()) {
132 // If we found a conditional branch predecessor, make sure that it branches
133 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
134 if (Pred1Br->getSuccessor(0) == BB &&
135 Pred1Br->getSuccessor(1) == Pred2) {
138 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
139 Pred1Br->getSuccessor(1) == BB) {
143 // We know that one arm of the conditional goes to BB, so the other must
144 // go somewhere unrelated, and this must not be an "if statement".
148 // The only thing we have to watch out for here is to make sure that Pred2
149 // doesn't have incoming edges from other blocks. If it does, the condition
150 // doesn't dominate BB.
151 if (++pred_begin(Pred2) != pred_end(Pred2))
154 return Pred1Br->getCondition();
157 // Ok, if we got here, both predecessors end with an unconditional branch to
158 // BB. Don't panic! If both blocks only have a single (identical)
159 // predecessor, and THAT is a conditional branch, then we're all ok!
160 if (pred_begin(Pred1) == pred_end(Pred1) ||
161 ++pred_begin(Pred1) != pred_end(Pred1) ||
162 pred_begin(Pred2) == pred_end(Pred2) ||
163 ++pred_begin(Pred2) != pred_end(Pred2) ||
164 *pred_begin(Pred1) != *pred_begin(Pred2))
167 // Otherwise, if this is a conditional branch, then we can use it!
168 BasicBlock *CommonPred = *pred_begin(Pred1);
169 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
170 assert(BI->isConditional() && "Two successors but not conditional?");
171 if (BI->getSuccessor(0) == Pred1) {
178 return BI->getCondition();
184 // If we have a merge point of an "if condition" as accepted above, return true
185 // if the specified value dominates the block. We don't handle the true
186 // generality of domination here, just a special case which works well enough
189 // If AggressiveInsts is non-null, and if V does not dominate BB, we check to
190 // see if V (which must be an instruction) is cheap to compute and is
191 // non-trapping. If both are true, the instruction is inserted into the set and
193 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
194 std::set<Instruction*> *AggressiveInsts) {
195 Instruction *I = dyn_cast<Instruction>(V);
196 if (!I) return true; // Non-instructions all dominate instructions.
197 BasicBlock *PBB = I->getParent();
199 // We don't want to allow weird loops that might have the "if condition" in
200 // the bottom of this block.
201 if (PBB == BB) return false;
203 // If this instruction is defined in a block that contains an unconditional
204 // branch to BB, then it must be in the 'conditional' part of the "if
206 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
207 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
208 if (!AggressiveInsts) return false;
209 // Okay, it looks like the instruction IS in the "condition". Check to
210 // see if its a cheap instruction to unconditionally compute, and if it
211 // only uses stuff defined outside of the condition. If so, hoist it out.
212 switch (I->getOpcode()) {
213 default: return false; // Cannot hoist this out safely.
214 case Instruction::Load:
215 // We can hoist loads that are non-volatile and obviously cannot trap.
216 if (cast<LoadInst>(I)->isVolatile())
218 if (!isa<AllocaInst>(I->getOperand(0)) &&
219 !isa<Constant>(I->getOperand(0)))
222 // Finally, we have to check to make sure there are no instructions
223 // before the load in its basic block, as we are going to hoist the loop
224 // out to its predecessor.
225 if (PBB->begin() != BasicBlock::iterator(I))
228 case Instruction::Add:
229 case Instruction::Sub:
230 case Instruction::And:
231 case Instruction::Or:
232 case Instruction::Xor:
233 case Instruction::Shl:
234 case Instruction::Shr:
235 case Instruction::SetEQ:
236 case Instruction::SetNE:
237 case Instruction::SetLT:
238 case Instruction::SetGT:
239 case Instruction::SetLE:
240 case Instruction::SetGE:
241 break; // These are all cheap and non-trapping instructions.
244 // Okay, we can only really hoist these out if their operands are not
245 // defined in the conditional region.
246 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
247 if (!DominatesMergePoint(I->getOperand(i), BB, 0))
249 // Okay, it's safe to do this! Remember this instruction.
250 AggressiveInsts->insert(I);
256 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
257 // instructions that compare a value against a constant, return the value being
258 // compared, and stick the constant into the Values vector.
259 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
260 if (Instruction *Inst = dyn_cast<Instruction>(V))
261 if (Inst->getOpcode() == Instruction::SetEQ) {
262 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
264 return Inst->getOperand(0);
265 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
267 return Inst->getOperand(1);
269 } else if (Inst->getOpcode() == Instruction::Or) {
270 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
271 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
278 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
279 // setne instructions that compare a value against a constant, return the value
280 // being compared, and stick the constant into the Values vector.
281 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
282 if (Instruction *Inst = dyn_cast<Instruction>(V))
283 if (Inst->getOpcode() == Instruction::SetNE) {
284 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
286 return Inst->getOperand(0);
287 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
289 return Inst->getOperand(1);
291 } else if (Inst->getOpcode() == Instruction::Cast) {
292 // Cast of X to bool is really a comparison against zero.
293 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
294 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0));
295 return Inst->getOperand(0);
296 } else if (Inst->getOpcode() == Instruction::And) {
297 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
298 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
307 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
308 /// bunch of comparisons of one value against constants, return the value and
309 /// the constants being compared.
310 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
311 std::vector<ConstantInt*> &Values) {
312 if (Cond->getOpcode() == Instruction::Or) {
313 CompVal = GatherConstantSetEQs(Cond, Values);
315 // Return true to indicate that the condition is true if the CompVal is
316 // equal to one of the constants.
318 } else if (Cond->getOpcode() == Instruction::And) {
319 CompVal = GatherConstantSetNEs(Cond, Values);
321 // Return false to indicate that the condition is false if the CompVal is
322 // equal to one of the constants.
328 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
329 /// has no side effects, nuke it. If it uses any instructions that become dead
330 /// because the instruction is now gone, nuke them too.
331 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
332 if (isInstructionTriviallyDead(I)) {
333 std::vector<Value*> Operands(I->op_begin(), I->op_end());
334 I->getParent()->getInstList().erase(I);
335 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
336 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
337 ErasePossiblyDeadInstructionTree(OpI);
341 /// SafeToMergeTerminators - Return true if it is safe to merge these two
342 /// terminator instructions together.
344 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
345 if (SI1 == SI2) return false; // Can't merge with self!
347 // It is not safe to merge these two switch instructions if they have a common
348 // successor, and if that successor has a PHI node, and if *that* PHI node has
349 // conflicting incoming values from the two switch blocks.
350 BasicBlock *SI1BB = SI1->getParent();
351 BasicBlock *SI2BB = SI2->getParent();
352 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
354 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
355 if (SI1Succs.count(*I))
356 for (BasicBlock::iterator BBI = (*I)->begin();
357 isa<PHINode>(BBI); ++BBI) {
358 PHINode *PN = cast<PHINode>(BBI);
359 if (PN->getIncomingValueForBlock(SI1BB) !=
360 PN->getIncomingValueForBlock(SI2BB))
367 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
368 /// now be entries in it from the 'NewPred' block. The values that will be
369 /// flowing into the PHI nodes will be the same as those coming in from
370 /// ExistPred, an existing predecessor of Succ.
371 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
372 BasicBlock *ExistPred) {
373 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
374 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
375 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
377 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
378 PHINode *PN = cast<PHINode>(I);
379 Value *V = PN->getIncomingValueForBlock(ExistPred);
380 PN->addIncoming(V, NewPred);
384 // isValueEqualityComparison - Return true if the specified terminator checks to
385 // see if a value is equal to constant integer value.
386 static Value *isValueEqualityComparison(TerminatorInst *TI) {
387 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
388 // Do not permit merging of large switch instructions into their
389 // predecessors unless there is only one predecessor.
390 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
391 pred_end(SI->getParent())) > 128)
394 return SI->getCondition();
396 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
397 if (BI->isConditional() && BI->getCondition()->hasOneUse())
398 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
399 if ((SCI->getOpcode() == Instruction::SetEQ ||
400 SCI->getOpcode() == Instruction::SetNE) &&
401 isa<ConstantInt>(SCI->getOperand(1)))
402 return SCI->getOperand(0);
406 // Given a value comparison instruction, decode all of the 'cases' that it
407 // represents and return the 'default' block.
409 GetValueEqualityComparisonCases(TerminatorInst *TI,
410 std::vector<std::pair<ConstantInt*,
411 BasicBlock*> > &Cases) {
412 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
413 Cases.reserve(SI->getNumCases());
414 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
415 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
416 return SI->getDefaultDest();
419 BranchInst *BI = cast<BranchInst>(TI);
420 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
421 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
422 BI->getSuccessor(SCI->getOpcode() ==
423 Instruction::SetNE)));
424 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
428 // EliminateBlockCases - Given an vector of bb/value pairs, remove any entries
429 // in the list that match the specified block.
430 static void EliminateBlockCases(BasicBlock *BB,
431 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
432 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
433 if (Cases[i].second == BB) {
434 Cases.erase(Cases.begin()+i);
439 // ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
442 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
443 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
444 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
446 // Make V1 be smaller than V2.
447 if (V1->size() > V2->size())
450 if (V1->size() == 0) return false;
451 if (V1->size() == 1) {
453 ConstantInt *TheVal = (*V1)[0].first;
454 for (unsigned i = 0, e = V2->size(); i != e; ++i)
455 if (TheVal == (*V2)[i].first)
459 // Otherwise, just sort both lists and compare element by element.
460 std::sort(V1->begin(), V1->end());
461 std::sort(V2->begin(), V2->end());
462 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
463 while (i1 != e1 && i2 != e2) {
464 if ((*V1)[i1].first == (*V2)[i2].first)
466 if ((*V1)[i1].first < (*V2)[i2].first)
474 // SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
475 // terminator instruction and its block is known to only have a single
476 // predecessor block, check to see if that predecessor is also a value
477 // comparison with the same value, and if that comparison determines the outcome
478 // of this comparison. If so, simplify TI. This does a very limited form of
480 static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
482 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
483 if (!PredVal) return false; // Not a value comparison in predecessor.
485 Value *ThisVal = isValueEqualityComparison(TI);
486 assert(ThisVal && "This isn't a value comparison!!");
487 if (ThisVal != PredVal) return false; // Different predicates.
489 // Find out information about when control will move from Pred to TI's block.
490 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
491 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
493 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
495 // Find information about how control leaves this block.
496 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
497 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
498 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
500 // If TI's block is the default block from Pred's comparison, potentially
501 // simplify TI based on this knowledge.
502 if (PredDef == TI->getParent()) {
503 // If we are here, we know that the value is none of those cases listed in
504 // PredCases. If there are any cases in ThisCases that are in PredCases, we
506 if (ValuesOverlap(PredCases, ThisCases)) {
507 if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) {
508 // Okay, one of the successors of this condbr is dead. Convert it to a
510 assert(ThisCases.size() == 1 && "Branch can only have one case!");
511 Value *Cond = BTI->getCondition();
512 // Insert the new branch.
513 Instruction *NI = new BranchInst(ThisDef, TI);
515 // Remove PHI node entries for the dead edge.
516 ThisCases[0].second->removePredecessor(TI->getParent());
518 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
519 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
521 TI->eraseFromParent(); // Nuke the old one.
522 // If condition is now dead, nuke it.
523 if (Instruction *CondI = dyn_cast<Instruction>(Cond))
524 ErasePossiblyDeadInstructionTree(CondI);
528 SwitchInst *SI = cast<SwitchInst>(TI);
529 // Okay, TI has cases that are statically dead, prune them away.
530 std::set<Constant*> DeadCases;
531 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
532 DeadCases.insert(PredCases[i].first);
534 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
535 << "Through successor TI: " << *TI);
537 for (unsigned i = SI->getNumCases()-1; i != 0; --i)
538 if (DeadCases.count(SI->getCaseValue(i))) {
539 SI->getSuccessor(i)->removePredecessor(TI->getParent());
543 DEBUG(std::cerr << "Leaving: " << *TI << "\n");
549 // Otherwise, TI's block must correspond to some matched value. Find out
550 // which value (or set of values) this is.
551 ConstantInt *TIV = 0;
552 BasicBlock *TIBB = TI->getParent();
553 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
554 if (PredCases[i].second == TIBB)
556 TIV = PredCases[i].first;
558 return false; // Cannot handle multiple values coming to this block.
559 assert(TIV && "No edge from pred to succ?");
561 // Okay, we found the one constant that our value can be if we get into TI's
562 // BB. Find out which successor will unconditionally be branched to.
563 BasicBlock *TheRealDest = 0;
564 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
565 if (ThisCases[i].first == TIV) {
566 TheRealDest = ThisCases[i].second;
570 // If not handled by any explicit cases, it is handled by the default case.
571 if (TheRealDest == 0) TheRealDest = ThisDef;
573 // Remove PHI node entries for dead edges.
574 BasicBlock *CheckEdge = TheRealDest;
575 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
576 if (*SI != CheckEdge)
577 (*SI)->removePredecessor(TIBB);
581 // Insert the new branch.
582 Instruction *NI = new BranchInst(TheRealDest, TI);
584 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
585 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
586 Instruction *Cond = 0;
587 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
588 Cond = dyn_cast<Instruction>(BI->getCondition());
589 TI->eraseFromParent(); // Nuke the old one.
591 if (Cond) ErasePossiblyDeadInstructionTree(Cond);
597 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
598 // equality comparison instruction (either a switch or a branch on "X == c").
599 // See if any of the predecessors of the terminator block are value comparisons
600 // on the same value. If so, and if safe to do so, fold them together.
601 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
602 BasicBlock *BB = TI->getParent();
603 Value *CV = isValueEqualityComparison(TI); // CondVal
604 assert(CV && "Not a comparison?");
605 bool Changed = false;
607 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
608 while (!Preds.empty()) {
609 BasicBlock *Pred = Preds.back();
612 // See if the predecessor is a comparison with the same value.
613 TerminatorInst *PTI = Pred->getTerminator();
614 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
616 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
617 // Figure out which 'cases' to copy from SI to PSI.
618 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
619 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
621 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
622 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
624 // Based on whether the default edge from PTI goes to BB or not, fill in
625 // PredCases and PredDefault with the new switch cases we would like to
627 std::vector<BasicBlock*> NewSuccessors;
629 if (PredDefault == BB) {
630 // If this is the default destination from PTI, only the edges in TI
631 // that don't occur in PTI, or that branch to BB will be activated.
632 std::set<ConstantInt*> PTIHandled;
633 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
634 if (PredCases[i].second != BB)
635 PTIHandled.insert(PredCases[i].first);
637 // The default destination is BB, we don't need explicit targets.
638 std::swap(PredCases[i], PredCases.back());
639 PredCases.pop_back();
643 // Reconstruct the new switch statement we will be building.
644 if (PredDefault != BBDefault) {
645 PredDefault->removePredecessor(Pred);
646 PredDefault = BBDefault;
647 NewSuccessors.push_back(BBDefault);
649 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
650 if (!PTIHandled.count(BBCases[i].first) &&
651 BBCases[i].second != BBDefault) {
652 PredCases.push_back(BBCases[i]);
653 NewSuccessors.push_back(BBCases[i].second);
657 // If this is not the default destination from PSI, only the edges
658 // in SI that occur in PSI with a destination of BB will be
660 std::set<ConstantInt*> PTIHandled;
661 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
662 if (PredCases[i].second == BB) {
663 PTIHandled.insert(PredCases[i].first);
664 std::swap(PredCases[i], PredCases.back());
665 PredCases.pop_back();
669 // Okay, now we know which constants were sent to BB from the
670 // predecessor. Figure out where they will all go now.
671 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
672 if (PTIHandled.count(BBCases[i].first)) {
673 // If this is one we are capable of getting...
674 PredCases.push_back(BBCases[i]);
675 NewSuccessors.push_back(BBCases[i].second);
676 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
679 // If there are any constants vectored to BB that TI doesn't handle,
680 // they must go to the default destination of TI.
681 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
682 E = PTIHandled.end(); I != E; ++I) {
683 PredCases.push_back(std::make_pair(*I, BBDefault));
684 NewSuccessors.push_back(BBDefault);
688 // Okay, at this point, we know which new successor Pred will get. Make
689 // sure we update the number of entries in the PHI nodes for these
691 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
692 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
694 // Now that the successors are updated, create the new Switch instruction.
695 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI);
696 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
697 NewSI->addCase(PredCases[i].first, PredCases[i].second);
699 Instruction *DeadCond = 0;
700 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
701 // If PTI is a branch, remember the condition.
702 DeadCond = dyn_cast<Instruction>(BI->getCondition());
703 Pred->getInstList().erase(PTI);
705 // If the condition is dead now, remove the instruction tree.
706 if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond);
708 // Okay, last check. If BB is still a successor of PSI, then we must
709 // have an infinite loop case. If so, add an infinitely looping block
710 // to handle the case to preserve the behavior of the code.
711 BasicBlock *InfLoopBlock = 0;
712 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
713 if (NewSI->getSuccessor(i) == BB) {
714 if (InfLoopBlock == 0) {
715 // Insert it at the end of the loop, because it's either code,
716 // or it won't matter if it's hot. :)
717 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
718 new BranchInst(InfLoopBlock, InfLoopBlock);
720 NewSI->setSuccessor(i, InfLoopBlock);
729 /// HoistThenElseCodeToIf - Given a conditional branch that codes to BB1 and
730 /// BB2, hoist any common code in the two blocks up into the branch block. The
731 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
732 static bool HoistThenElseCodeToIf(BranchInst *BI) {
733 // This does very trivial matching, with limited scanning, to find identical
734 // instructions in the two blocks. In particular, we don't want to get into
735 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
736 // such, we currently just scan for obviously identical instructions in an
738 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
739 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
741 Instruction *I1 = BB1->begin(), *I2 = BB2->begin();
742 if (I1->getOpcode() != I2->getOpcode() || !I1->isIdenticalTo(I2))
745 // If we get here, we can hoist at least one instruction.
746 BasicBlock *BIParent = BI->getParent();
749 // If we are hoisting the terminator instruction, don't move one (making a
750 // broken BB), instead clone it, and remove BI.
751 if (isa<TerminatorInst>(I1))
752 goto HoistTerminator;
754 // For a normal instruction, we just move one to right before the branch,
755 // then replace all uses of the other with the first. Finally, we remove
756 // the now redundant second instruction.
757 BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
758 if (!I2->use_empty())
759 I2->replaceAllUsesWith(I1);
760 BB2->getInstList().erase(I2);
764 } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2));
769 // Okay, it is safe to hoist the terminator.
770 Instruction *NT = I1->clone();
771 BIParent->getInstList().insert(BI, NT);
772 if (NT->getType() != Type::VoidTy) {
773 I1->replaceAllUsesWith(NT);
774 I2->replaceAllUsesWith(NT);
775 NT->setName(I1->getName());
778 // Hoisting one of the terminators from our successor is a great thing.
779 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
780 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
781 // nodes, so we insert select instruction to compute the final result.
782 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
783 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
785 for (BasicBlock::iterator BBI = SI->begin();
786 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
787 Value *BB1V = PN->getIncomingValueForBlock(BB1);
788 Value *BB2V = PN->getIncomingValueForBlock(BB2);
790 // These values do not agree. Insert a select instruction before NT
791 // that determines the right value.
792 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
794 SI = new SelectInst(BI->getCondition(), BB1V, BB2V,
795 BB1V->getName()+"."+BB2V->getName(), NT);
796 // Make the PHI node use the select for all incoming values for BB1/BB2
797 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
798 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
799 PN->setIncomingValue(i, SI);
804 // Update any PHI nodes in our new successors.
805 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
806 AddPredecessorToBlock(*SI, BIParent, BB1);
808 BI->eraseFromParent();
813 /// ConstantIntOrdering - This class implements a stable ordering of constant
814 /// integers that does not depend on their address. This is important for
815 /// applications that sort ConstantInt's to ensure uniqueness.
816 struct ConstantIntOrdering {
817 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
818 return LHS->getRawValue() < RHS->getRawValue();
824 // SimplifyCFG - This function is used to do simplification of a CFG. For
825 // example, it adjusts branches to branches to eliminate the extra hop, it
826 // eliminates unreachable basic blocks, and does other "peephole" optimization
827 // of the CFG. It returns true if a modification was made.
829 // WARNING: The entry node of a function may not be simplified.
831 bool llvm::SimplifyCFG(BasicBlock *BB) {
832 bool Changed = false;
833 Function *M = BB->getParent();
835 assert(BB && BB->getParent() && "Block not embedded in function!");
836 assert(BB->getTerminator() && "Degenerate basic block encountered!");
837 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
839 // Remove basic blocks that have no predecessors... which are unreachable.
840 if (pred_begin(BB) == pred_end(BB) ||
841 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
842 DEBUG(std::cerr << "Removing BB: \n" << *BB);
844 // Loop through all of our successors and make sure they know that one
845 // of their predecessors is going away.
846 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
847 SI->removePredecessor(BB);
849 while (!BB->empty()) {
850 Instruction &I = BB->back();
851 // If this instruction is used, replace uses with an arbitrary
852 // constant value. Because control flow can't get here, we don't care
853 // what we replace the value with. Note that since this block is
854 // unreachable, and all values contained within it must dominate their
855 // uses, that all uses will eventually be removed.
857 // Make all users of this instruction reference the constant instead
858 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
860 // Remove the instruction from the basic block
861 BB->getInstList().pop_back();
863 M->getBasicBlockList().erase(BB);
867 // Check to see if we can constant propagate this terminator instruction
869 Changed |= ConstantFoldTerminator(BB);
871 // Check to see if this block has no non-phi instructions and only a single
872 // successor. If so, replace references to this basic block with references
874 succ_iterator SI(succ_begin(BB));
875 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
876 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
877 while (isa<PHINode>(*BBI)) ++BBI;
879 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor.
880 if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
881 Succ != BB) { // Don't hurt infinite loops!
882 // If our successor has PHI nodes, then we need to update them to include
883 // entries for BB's predecessors, not for BB itself. Be careful though,
884 // if this transformation fails (returns true) then we cannot do this
887 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
888 DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB);
890 if (isa<PHINode>(&BB->front())) {
891 std::vector<BasicBlock*>
892 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
894 // Move all PHI nodes in BB to Succ if they are alive, otherwise
896 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
898 BB->getInstList().erase(BB->begin()); // Nuke instruction.
900 // The instruction is alive, so this means that Succ must have
901 // *ONLY* had BB as a predecessor, and the PHI node is still valid
902 // now. Simply move it into Succ, because we know that BB
903 // strictly dominated Succ.
904 BB->getInstList().remove(BB->begin());
905 Succ->getInstList().push_front(PN);
907 // We need to add new entries for the PHI node to account for
908 // predecessors of Succ that the PHI node does not take into
909 // account. At this point, since we know that BB dominated succ,
910 // this means that we should any newly added incoming edges should
911 // use the PHI node as the value for these edges, because they are
913 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
914 if (OldSuccPreds[i] != BB)
915 PN->addIncoming(PN, OldSuccPreds[i]);
919 // Everything that jumped to BB now goes to Succ.
920 std::string OldName = BB->getName();
921 BB->replaceAllUsesWith(Succ);
922 BB->eraseFromParent(); // Delete the old basic block.
924 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
925 Succ->setName(OldName);
931 // If this is a returning block with only PHI nodes in it, fold the return
932 // instruction into any unconditional branch predecessors.
934 // If any predecessor is a conditional branch that just selects among
935 // different return values, fold the replace the branch/return with a select
937 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
938 BasicBlock::iterator BBI = BB->getTerminator();
939 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
940 // Find predecessors that end with branches.
941 std::vector<BasicBlock*> UncondBranchPreds;
942 std::vector<BranchInst*> CondBranchPreds;
943 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
944 TerminatorInst *PTI = (*PI)->getTerminator();
945 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
946 if (BI->isUnconditional())
947 UncondBranchPreds.push_back(*PI);
949 CondBranchPreds.push_back(BI);
952 // If we found some, do the transformation!
953 if (!UncondBranchPreds.empty()) {
954 while (!UncondBranchPreds.empty()) {
955 BasicBlock *Pred = UncondBranchPreds.back();
956 UncondBranchPreds.pop_back();
957 Instruction *UncondBranch = Pred->getTerminator();
958 // Clone the return and add it to the end of the predecessor.
959 Instruction *NewRet = RI->clone();
960 Pred->getInstList().push_back(NewRet);
962 // If the return instruction returns a value, and if the value was a
963 // PHI node in "BB", propagate the right value into the return.
964 if (NewRet->getNumOperands() == 1)
965 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
966 if (PN->getParent() == BB)
967 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
968 // Update any PHI nodes in the returning block to realize that we no
969 // longer branch to them.
970 BB->removePredecessor(Pred);
971 Pred->getInstList().erase(UncondBranch);
974 // If we eliminated all predecessors of the block, delete the block now.
975 if (pred_begin(BB) == pred_end(BB))
976 // We know there are no successors, so just nuke the block.
977 M->getBasicBlockList().erase(BB);
982 // Check out all of the conditional branches going to this return
983 // instruction. If any of them just select between returns, change the
984 // branch itself into a select/return pair.
985 while (!CondBranchPreds.empty()) {
986 BranchInst *BI = CondBranchPreds.back();
987 CondBranchPreds.pop_back();
988 BasicBlock *TrueSucc = BI->getSuccessor(0);
989 BasicBlock *FalseSucc = BI->getSuccessor(1);
990 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
992 // Check to see if the non-BB successor is also a return block.
993 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
994 // Check to see if there are only PHI instructions in this block.
995 BasicBlock::iterator OSI = OtherSucc->getTerminator();
996 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
997 // Okay, we found a branch that is going to two return nodes. If
998 // there is no return value for this function, just change the
999 // branch into a return.
1000 if (RI->getNumOperands() == 0) {
1001 TrueSucc->removePredecessor(BI->getParent());
1002 FalseSucc->removePredecessor(BI->getParent());
1003 new ReturnInst(0, BI);
1004 BI->getParent()->getInstList().erase(BI);
1008 // Otherwise, figure out what the true and false return values are
1009 // so we can insert a new select instruction.
1010 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
1011 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
1013 // Unwrap any PHI nodes in the return blocks.
1014 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
1015 if (TVPN->getParent() == TrueSucc)
1016 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1017 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
1018 if (FVPN->getParent() == FalseSucc)
1019 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1021 TrueSucc->removePredecessor(BI->getParent());
1022 FalseSucc->removePredecessor(BI->getParent());
1024 // Insert a new select instruction.
1026 Value *BrCond = BI->getCondition();
1027 if (TrueValue != FalseValue)
1028 NewRetVal = new SelectInst(BrCond, TrueValue,
1029 FalseValue, "retval", BI);
1031 NewRetVal = TrueValue;
1033 new ReturnInst(NewRetVal, BI);
1034 BI->getParent()->getInstList().erase(BI);
1035 if (BrCond->use_empty())
1036 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond))
1037 BrCondI->getParent()->getInstList().erase(BrCondI);
1043 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
1044 // Check to see if the first instruction in this block is just an unwind.
1045 // If so, replace any invoke instructions which use this as an exception
1046 // destination with call instructions, and any unconditional branch
1047 // predecessor with an unwind.
1049 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1050 while (!Preds.empty()) {
1051 BasicBlock *Pred = Preds.back();
1052 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) {
1053 if (BI->isUnconditional()) {
1054 Pred->getInstList().pop_back(); // nuke uncond branch
1055 new UnwindInst(Pred); // Use unwind.
1058 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
1059 if (II->getUnwindDest() == BB) {
1060 // Insert a new branch instruction before the invoke, because this
1061 // is now a fall through...
1062 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1063 Pred->getInstList().remove(II); // Take out of symbol table
1065 // Insert the call now...
1066 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1067 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1069 CI->setCallingConv(II->getCallingConv());
1070 // If the invoke produced a value, the Call now does instead
1071 II->replaceAllUsesWith(CI);
1079 // If this block is now dead, remove it.
1080 if (pred_begin(BB) == pred_end(BB)) {
1081 // We know there are no successors, so just nuke the block.
1082 M->getBasicBlockList().erase(BB);
1086 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1087 if (isValueEqualityComparison(SI)) {
1088 // If we only have one predecessor, and if it is a branch on this value,
1089 // see if that predecessor totally determines the outcome of this switch.
1090 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1091 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
1092 return SimplifyCFG(BB) || 1;
1094 // If the block only contains the switch, see if we can fold the block
1095 // away into any preds.
1096 if (SI == &BB->front())
1097 if (FoldValueComparisonIntoPredecessors(SI))
1098 return SimplifyCFG(BB) || 1;
1100 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
1101 if (BI->isConditional()) {
1102 if (Value *CompVal = isValueEqualityComparison(BI)) {
1103 // If we only have one predecessor, and if it is a branch on this value,
1104 // see if that predecessor totally determines the outcome of this
1106 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1107 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
1108 return SimplifyCFG(BB) || 1;
1110 // This block must be empty, except for the setcond inst, if it exists.
1111 BasicBlock::iterator I = BB->begin();
1113 (&*I == cast<Instruction>(BI->getCondition()) &&
1115 if (FoldValueComparisonIntoPredecessors(BI))
1116 return SimplifyCFG(BB) | true;
1119 // If this basic block is ONLY a setcc and a branch, and if a predecessor
1120 // branches to us and one of our successors, fold the setcc into the
1121 // predecessor and use logical operations to pick the right destination.
1122 BasicBlock *TrueDest = BI->getSuccessor(0);
1123 BasicBlock *FalseDest = BI->getSuccessor(1);
1124 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition()))
1125 if (Cond->getParent() == BB && &BB->front() == Cond &&
1126 Cond->getNext() == BI && Cond->hasOneUse() &&
1127 TrueDest != BB && FalseDest != BB)
1128 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
1129 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1130 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
1131 BasicBlock *PredBlock = *PI;
1132 if (PBI->getSuccessor(0) == FalseDest ||
1133 PBI->getSuccessor(1) == TrueDest) {
1134 // Invert the predecessors condition test (xor it with true),
1135 // which allows us to write this code once.
1137 BinaryOperator::createNot(PBI->getCondition(),
1138 PBI->getCondition()->getName()+".not", PBI);
1139 PBI->setCondition(NewCond);
1140 BasicBlock *OldTrue = PBI->getSuccessor(0);
1141 BasicBlock *OldFalse = PBI->getSuccessor(1);
1142 PBI->setSuccessor(0, OldFalse);
1143 PBI->setSuccessor(1, OldTrue);
1146 if (PBI->getSuccessor(0) == TrueDest ||
1147 PBI->getSuccessor(1) == FalseDest) {
1148 // Clone Cond into the predecessor basic block, and or/and the
1149 // two conditions together.
1150 Instruction *New = Cond->clone();
1151 New->setName(Cond->getName());
1152 Cond->setName(Cond->getName()+".old");
1153 PredBlock->getInstList().insert(PBI, New);
1154 Instruction::BinaryOps Opcode =
1155 PBI->getSuccessor(0) == TrueDest ?
1156 Instruction::Or : Instruction::And;
1158 BinaryOperator::create(Opcode, PBI->getCondition(),
1159 New, "bothcond", PBI);
1160 PBI->setCondition(NewCond);
1161 if (PBI->getSuccessor(0) == BB) {
1162 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1163 PBI->setSuccessor(0, TrueDest);
1165 if (PBI->getSuccessor(1) == BB) {
1166 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1167 PBI->setSuccessor(1, FalseDest);
1169 return SimplifyCFG(BB) | 1;
1173 // If this block ends with a branch instruction, and if there is one
1174 // predecessor, see if the previous block ended with a branch on the same
1175 // condition, which makes this conditional branch redundant.
1176 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
1177 BasicBlock *OnlyPred = *PI++;
1178 for (; PI != PE; ++PI)// Search all predecessors, see if they are all same
1179 if (*PI != OnlyPred) {
1180 OnlyPred = 0; // There are multiple different predecessors...
1185 if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
1186 if (PBI->isConditional() &&
1187 PBI->getCondition() == BI->getCondition() &&
1188 (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) {
1189 // Okay, the outcome of this conditional branch is statically
1190 // knowable. Delete the outgoing CFG edge that is impossible to
1192 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1193 BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
1194 new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
1195 BB->getInstList().erase(BI);
1196 return SimplifyCFG(BB) | true;
1199 } else if (isa<UnreachableInst>(BB->getTerminator())) {
1200 // If there are any instructions immediately before the unreachable that can
1201 // be removed, do so.
1202 Instruction *Unreachable = BB->getTerminator();
1203 while (Unreachable != BB->begin()) {
1204 BasicBlock::iterator BBI = Unreachable;
1206 if (isa<CallInst>(BBI)) break;
1207 // Delete this instruction
1208 BB->getInstList().erase(BBI);
1212 // If the unreachable instruction is the first in the block, take a gander
1213 // at all of the predecessors of this instruction, and simplify them.
1214 if (&BB->front() == Unreachable) {
1215 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1216 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
1217 TerminatorInst *TI = Preds[i]->getTerminator();
1219 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1220 if (BI->isUnconditional()) {
1221 if (BI->getSuccessor(0) == BB) {
1222 new UnreachableInst(TI);
1223 TI->eraseFromParent();
1227 if (BI->getSuccessor(0) == BB) {
1228 new BranchInst(BI->getSuccessor(1), BI);
1229 BI->eraseFromParent();
1230 } else if (BI->getSuccessor(1) == BB) {
1231 new BranchInst(BI->getSuccessor(0), BI);
1232 BI->eraseFromParent();
1236 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
1237 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1238 if (SI->getSuccessor(i) == BB) {
1239 BB->removePredecessor(SI->getParent());
1244 // If the default value is unreachable, figure out the most popular
1245 // destination and make it the default.
1246 if (SI->getSuccessor(0) == BB) {
1247 std::map<BasicBlock*, unsigned> Popularity;
1248 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1249 Popularity[SI->getSuccessor(i)]++;
1251 // Find the most popular block.
1252 unsigned MaxPop = 0;
1253 BasicBlock *MaxBlock = 0;
1254 for (std::map<BasicBlock*, unsigned>::iterator
1255 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
1256 if (I->second > MaxPop) {
1258 MaxBlock = I->first;
1262 // Make this the new default, allowing us to delete any explicit
1264 SI->setSuccessor(0, MaxBlock);
1267 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
1269 if (isa<PHINode>(MaxBlock->begin()))
1270 for (unsigned i = 0; i != MaxPop-1; ++i)
1271 MaxBlock->removePredecessor(SI->getParent());
1273 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1274 if (SI->getSuccessor(i) == MaxBlock) {
1280 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
1281 if (II->getUnwindDest() == BB) {
1282 // Convert the invoke to a call instruction. This would be a good
1283 // place to note that the call does not throw though.
1284 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1285 II->removeFromParent(); // Take out of symbol table
1287 // Insert the call now...
1288 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1289 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1291 CI->setCallingConv(II->getCallingConv());
1292 // If the invoke produced a value, the Call does now instead.
1293 II->replaceAllUsesWith(CI);
1300 // If this block is now dead, remove it.
1301 if (pred_begin(BB) == pred_end(BB)) {
1302 // We know there are no successors, so just nuke the block.
1303 M->getBasicBlockList().erase(BB);
1309 // Merge basic blocks into their predecessor if there is only one distinct
1310 // pred, and if there is only one distinct successor of the predecessor, and
1311 // if there are no PHI nodes.
1313 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
1314 BasicBlock *OnlyPred = *PI++;
1315 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
1316 if (*PI != OnlyPred) {
1317 OnlyPred = 0; // There are multiple different predecessors...
1321 BasicBlock *OnlySucc = 0;
1322 if (OnlyPred && OnlyPred != BB && // Don't break self loops
1323 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
1324 // Check to see if there is only one distinct successor...
1325 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
1327 for (; SI != SE; ++SI)
1328 if (*SI != OnlySucc) {
1329 OnlySucc = 0; // There are multiple distinct successors!
1335 DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred);
1336 TerminatorInst *Term = OnlyPred->getTerminator();
1338 // Resolve any PHI nodes at the start of the block. They are all
1339 // guaranteed to have exactly one entry if they exist, unless there are
1340 // multiple duplicate (but guaranteed to be equal) entries for the
1341 // incoming edges. This occurs when there are multiple edges from
1342 // OnlyPred to OnlySucc.
1344 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
1345 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1346 BB->getInstList().pop_front(); // Delete the phi node...
1349 // Delete the unconditional branch from the predecessor...
1350 OnlyPred->getInstList().pop_back();
1352 // Move all definitions in the successor to the predecessor...
1353 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
1355 // Make all PHI nodes that referred to BB now refer to Pred as their
1357 BB->replaceAllUsesWith(OnlyPred);
1359 std::string OldName = BB->getName();
1361 // Erase basic block from the function...
1362 M->getBasicBlockList().erase(BB);
1364 // Inherit predecessors name if it exists...
1365 if (!OldName.empty() && !OnlyPred->hasName())
1366 OnlyPred->setName(OldName);
1371 // Otherwise, if this block only has a single predecessor, and if that block
1372 // is a conditional branch, see if we can hoist any code from this block up
1373 // into our predecessor.
1375 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
1376 if (BI->isConditional()) {
1377 // Get the other block.
1378 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
1379 PI = pred_begin(OtherBB);
1381 if (PI == pred_end(OtherBB)) {
1382 // We have a conditional branch to two blocks that are only reachable
1383 // from the condbr. We know that the condbr dominates the two blocks,
1384 // so see if there is any identical code in the "then" and "else"
1385 // blocks. If so, we can hoist it up to the branching block.
1386 Changed |= HoistThenElseCodeToIf(BI);
1390 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1391 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1392 // Change br (X == 0 | X == 1), T, F into a switch instruction.
1393 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
1394 Instruction *Cond = cast<Instruction>(BI->getCondition());
1395 // If this is a bunch of seteq's or'd together, or if it's a bunch of
1396 // 'setne's and'ed together, collect them.
1398 std::vector<ConstantInt*> Values;
1399 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
1400 if (CompVal && CompVal->getType()->isInteger()) {
1401 // There might be duplicate constants in the list, which the switch
1402 // instruction can't handle, remove them now.
1403 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
1404 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
1406 // Figure out which block is which destination.
1407 BasicBlock *DefaultBB = BI->getSuccessor(1);
1408 BasicBlock *EdgeBB = BI->getSuccessor(0);
1409 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
1411 // Create the new switch instruction now.
1412 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI);
1414 // Add all of the 'cases' to the switch instruction.
1415 for (unsigned i = 0, e = Values.size(); i != e; ++i)
1416 New->addCase(Values[i], EdgeBB);
1418 // We added edges from PI to the EdgeBB. As such, if there were any
1419 // PHI nodes in EdgeBB, they need entries to be added corresponding to
1420 // the number of edges added.
1421 for (BasicBlock::iterator BBI = EdgeBB->begin();
1422 isa<PHINode>(BBI); ++BBI) {
1423 PHINode *PN = cast<PHINode>(BBI);
1424 Value *InVal = PN->getIncomingValueForBlock(*PI);
1425 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
1426 PN->addIncoming(InVal, *PI);
1429 // Erase the old branch instruction.
1430 (*PI)->getInstList().erase(BI);
1432 // Erase the potentially condition tree that was used to computed the
1433 // branch condition.
1434 ErasePossiblyDeadInstructionTree(Cond);
1439 // If there is a trivial two-entry PHI node in this basic block, and we can
1440 // eliminate it, do so now.
1441 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1442 if (PN->getNumIncomingValues() == 2) {
1443 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1444 // statement", which has a very simple dominance structure. Basically, we
1445 // are trying to find the condition that is being branched on, which
1446 // subsequently causes this merge to happen. We really want control
1447 // dependence information for this check, but simplifycfg can't keep it up
1448 // to date, and this catches most of the cases we care about anyway.
1450 BasicBlock *IfTrue, *IfFalse;
1451 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
1452 DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
1453 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1455 // Loop over the PHI's seeing if we can promote them all to select
1456 // instructions. While we are at it, keep track of the instructions
1457 // that need to be moved to the dominating block.
1458 std::set<Instruction*> AggressiveInsts;
1459 bool CanPromote = true;
1461 BasicBlock::iterator AfterPHIIt = BB->begin();
1462 while (isa<PHINode>(AfterPHIIt)) {
1463 PHINode *PN = cast<PHINode>(AfterPHIIt++);
1464 if (PN->getIncomingValue(0) == PN->getIncomingValue(1))
1465 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1466 else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
1467 &AggressiveInsts) ||
1468 !DominatesMergePoint(PN->getIncomingValue(1), BB,
1469 &AggressiveInsts)) {
1475 // Did we eliminate all PHI's?
1476 CanPromote |= AfterPHIIt == BB->begin();
1478 // If we all PHI nodes are promotable, check to make sure that all
1479 // instructions in the predecessor blocks can be promoted as well. If
1480 // not, we won't be able to get rid of the control flow, so it's not
1481 // worth promoting to select instructions.
1482 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0;
1484 PN = cast<PHINode>(BB->begin());
1485 BasicBlock *Pred = PN->getIncomingBlock(0);
1486 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1488 DomBlock = *pred_begin(Pred);
1489 for (BasicBlock::iterator I = Pred->begin();
1490 !isa<TerminatorInst>(I); ++I)
1491 if (!AggressiveInsts.count(I)) {
1492 // This is not an aggressive instruction that we can promote.
1493 // Because of this, we won't be able to get rid of the control
1494 // flow, so the xform is not worth it.
1500 Pred = PN->getIncomingBlock(1);
1502 cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1504 DomBlock = *pred_begin(Pred);
1505 for (BasicBlock::iterator I = Pred->begin();
1506 !isa<TerminatorInst>(I); ++I)
1507 if (!AggressiveInsts.count(I)) {
1508 // This is not an aggressive instruction that we can promote.
1509 // Because of this, we won't be able to get rid of the control
1510 // flow, so the xform is not worth it.
1517 // If we can still promote the PHI nodes after this gauntlet of tests,
1518 // do all of the PHI's now.
1520 // Move all 'aggressive' instructions, which are defined in the
1521 // conditional parts of the if's up to the dominating block.
1523 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1524 IfBlock1->getInstList(),
1526 IfBlock1->getTerminator());
1529 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1530 IfBlock2->getInstList(),
1532 IfBlock2->getTerminator());
1535 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
1536 // Change the PHI node into a select instruction.
1538 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1540 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1542 std::string Name = PN->getName(); PN->setName("");
1543 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1545 BB->getInstList().erase(PN);