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 "Support/Debug.h"
28 // PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
29 // predecessors from "BB". This is a little tricky because "Succ" has PHI
30 // nodes, which need to have extra slots added to them to hold the merge edges
31 // from BB's predecessors, and BB itself might have had PHI nodes in it. This
32 // function returns true (failure) if the Succ BB already has a predecessor that
33 // is a predecessor of BB and incoming PHI arguments would not be discernible.
35 // Assumption: Succ is the single successor for BB.
37 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
38 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
40 if (!isa<PHINode>(Succ->front()))
41 return false; // We can make the transformation, no problem.
43 // If there is more than one predecessor, and there are PHI nodes in
44 // the successor, then we need to add incoming edges for the PHI nodes
46 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
48 // Check to see if one of the predecessors of BB is already a predecessor of
49 // Succ. If so, we cannot do the transformation if there are any PHI nodes
50 // with incompatible values coming in from the two edges!
52 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
53 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
54 // Loop over all of the PHI nodes checking to see if there are
55 // incompatible values coming in.
56 for (BasicBlock::iterator I = Succ->begin();
57 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
58 // Loop up the entries in the PHI node for BB and for *PI if the values
59 // coming in are non-equal, we cannot merge these two blocks (instead we
60 // should insert a conditional move or something, then merge the
62 int Idx1 = PN->getBasicBlockIndex(BB);
63 int Idx2 = PN->getBasicBlockIndex(*PI);
64 assert(Idx1 != -1 && Idx2 != -1 &&
65 "Didn't have entries for my predecessors??");
66 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
67 return true; // Values are not equal...
71 // Loop over all of the PHI nodes in the successor BB.
72 for (BasicBlock::iterator I = Succ->begin();
73 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
74 Value *OldVal = PN->removeIncomingValue(BB, false);
75 assert(OldVal && "No entry in PHI for Pred BB!");
77 // If this incoming value is one of the PHI nodes in BB, the new entries in
78 // the PHI node are the entries from the old PHI.
79 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
80 PHINode *OldValPN = cast<PHINode>(OldVal);
81 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
82 PN->addIncoming(OldValPN->getIncomingValue(i),
83 OldValPN->getIncomingBlock(i));
85 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
86 End = BBPreds.end(); PredI != End; ++PredI) {
87 // Add an incoming value for each of the new incoming values...
88 PN->addIncoming(OldVal, *PredI);
95 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
96 /// presumably PHI nodes in it), check to see if the merge at this block is due
97 /// to an "if condition". If so, return the boolean condition that determines
98 /// which entry into BB will be taken. Also, return by references the block
99 /// that will be entered from if the condition is true, and the block that will
100 /// be entered if the condition is false.
103 static Value *GetIfCondition(BasicBlock *BB,
104 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
105 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
106 "Function can only handle blocks with 2 predecessors!");
107 BasicBlock *Pred1 = *pred_begin(BB);
108 BasicBlock *Pred2 = *++pred_begin(BB);
110 // We can only handle branches. Other control flow will be lowered to
111 // branches if possible anyway.
112 if (!isa<BranchInst>(Pred1->getTerminator()) ||
113 !isa<BranchInst>(Pred2->getTerminator()))
115 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
116 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
118 // Eliminate code duplication by ensuring that Pred1Br is conditional if
120 if (Pred2Br->isConditional()) {
121 // If both branches are conditional, we don't have an "if statement". In
122 // reality, we could transform this case, but since the condition will be
123 // required anyway, we stand no chance of eliminating it, so the xform is
124 // probably not profitable.
125 if (Pred1Br->isConditional())
128 std::swap(Pred1, Pred2);
129 std::swap(Pred1Br, Pred2Br);
132 if (Pred1Br->isConditional()) {
133 // If we found a conditional branch predecessor, make sure that it branches
134 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
135 if (Pred1Br->getSuccessor(0) == BB &&
136 Pred1Br->getSuccessor(1) == Pred2) {
139 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
140 Pred1Br->getSuccessor(1) == BB) {
144 // We know that one arm of the conditional goes to BB, so the other must
145 // go somewhere unrelated, and this must not be an "if statement".
149 // The only thing we have to watch out for here is to make sure that Pred2
150 // doesn't have incoming edges from other blocks. If it does, the condition
151 // doesn't dominate BB.
152 if (++pred_begin(Pred2) != pred_end(Pred2))
155 return Pred1Br->getCondition();
158 // Ok, if we got here, both predecessors end with an unconditional branch to
159 // BB. Don't panic! If both blocks only have a single (identical)
160 // predecessor, and THAT is a conditional branch, then we're all ok!
161 if (pred_begin(Pred1) == pred_end(Pred1) ||
162 ++pred_begin(Pred1) != pred_end(Pred1) ||
163 pred_begin(Pred2) == pred_end(Pred2) ||
164 ++pred_begin(Pred2) != pred_end(Pred2) ||
165 *pred_begin(Pred1) != *pred_begin(Pred2))
168 // Otherwise, if this is a conditional branch, then we can use it!
169 BasicBlock *CommonPred = *pred_begin(Pred1);
170 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
171 assert(BI->isConditional() && "Two successors but not conditional?");
172 if (BI->getSuccessor(0) == Pred1) {
179 return BI->getCondition();
185 // If we have a merge point of an "if condition" as accepted above, return true
186 // if the specified value dominates the block. We don't handle the true
187 // generality of domination here, just a special case which works well enough
189 static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
190 Instruction *I = dyn_cast<Instruction>(V);
191 if (!I) return true; // Non-instructions all dominate instructions.
192 BasicBlock *PBB = I->getParent();
194 // We don't want to allow wierd loops that might have the "if condition" in
195 // the bottom of this block.
196 if (PBB == BB) return false;
198 // If this instruction is defined in a block that contains an unconditional
199 // branch to BB, then it must be in the 'conditional' part of the "if
201 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
202 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
203 if (!AllowAggressive) return false;
204 // Okay, it looks like the instruction IS in the "condition". Check to
205 // see if its a cheap instruction to unconditionally compute, and if it
206 // only uses stuff defined outside of the condition. If so, hoist it out.
207 switch (I->getOpcode()) {
208 default: return false; // Cannot hoist this out safely.
209 case Instruction::Load:
210 // We can hoist loads that are non-volatile and obviously cannot trap.
211 if (cast<LoadInst>(I)->isVolatile())
213 if (!isa<AllocaInst>(I->getOperand(0)) &&
214 !isa<Constant>(I->getOperand(0)))
217 // Finally, we have to check to make sure there are no instructions
218 // before the load in its basic block, as we are going to hoist the loop
219 // out to its predecessor.
220 if (PBB->begin() != BasicBlock::iterator(I))
223 case Instruction::Add:
224 case Instruction::Sub:
225 case Instruction::And:
226 case Instruction::Or:
227 case Instruction::Xor:
228 case Instruction::Shl:
229 case Instruction::Shr:
230 break; // These are all cheap and non-trapping instructions.
233 // Okay, we can only really hoist these out if their operands are not
234 // defined in the conditional region.
235 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
236 if (!DominatesMergePoint(I->getOperand(i), BB, false))
238 // Okay, it's safe to do this!
244 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
245 // instructions that compare a value against a constant, return the value being
246 // compared, and stick the constant into the Values vector.
247 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
248 if (Instruction *Inst = dyn_cast<Instruction>(V))
249 if (Inst->getOpcode() == Instruction::SetEQ) {
250 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
252 return Inst->getOperand(0);
253 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
255 return Inst->getOperand(1);
257 } else if (Inst->getOpcode() == Instruction::Or) {
258 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
259 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
266 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
267 // setne instructions that compare a value against a constant, return the value
268 // being compared, and stick the constant into the Values vector.
269 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
270 if (Instruction *Inst = dyn_cast<Instruction>(V))
271 if (Inst->getOpcode() == Instruction::SetNE) {
272 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
274 return Inst->getOperand(0);
275 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
277 return Inst->getOperand(1);
279 } else if (Inst->getOpcode() == Instruction::Cast) {
280 // Cast of X to bool is really a comparison against zero.
281 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
282 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0));
283 return Inst->getOperand(0);
284 } else if (Inst->getOpcode() == Instruction::And) {
285 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
286 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
295 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
296 /// bunch of comparisons of one value against constants, return the value and
297 /// the constants being compared.
298 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
299 std::vector<ConstantInt*> &Values) {
300 if (Cond->getOpcode() == Instruction::Or) {
301 CompVal = GatherConstantSetEQs(Cond, Values);
303 // Return true to indicate that the condition is true if the CompVal is
304 // equal to one of the constants.
306 } else if (Cond->getOpcode() == Instruction::And) {
307 CompVal = GatherConstantSetNEs(Cond, Values);
309 // Return false to indicate that the condition is false if the CompVal is
310 // equal to one of the constants.
316 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
317 /// has no side effects, nuke it. If it uses any instructions that become dead
318 /// because the instruction is now gone, nuke them too.
319 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
320 if (isInstructionTriviallyDead(I)) {
321 std::vector<Value*> Operands(I->op_begin(), I->op_end());
322 I->getParent()->getInstList().erase(I);
323 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
324 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
325 ErasePossiblyDeadInstructionTree(OpI);
329 /// SafeToMergeTerminators - Return true if it is safe to merge these two
330 /// terminator instructions together.
332 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
333 if (SI1 == SI2) return false; // Can't merge with self!
335 // It is not safe to merge these two switch instructions if they have a common
336 // successor, and if that successor has a PHI node, and if *that* PHI node has
337 // conflicting incoming values from the two switch blocks.
338 BasicBlock *SI1BB = SI1->getParent();
339 BasicBlock *SI2BB = SI2->getParent();
340 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
342 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
343 if (SI1Succs.count(*I))
344 for (BasicBlock::iterator BBI = (*I)->begin();
345 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
346 if (PN->getIncomingValueForBlock(SI1BB) !=
347 PN->getIncomingValueForBlock(SI2BB))
353 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
354 /// now be entries in it from the 'NewPred' block. The values that will be
355 /// flowing into the PHI nodes will be the same as those coming in from
356 /// ExistPred, an existing predecessor of Succ.
357 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
358 BasicBlock *ExistPred) {
359 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
360 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
361 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
363 for (BasicBlock::iterator I = Succ->begin();
364 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
365 Value *V = PN->getIncomingValueForBlock(ExistPred);
366 PN->addIncoming(V, NewPred);
370 // isValueEqualityComparison - Return true if the specified terminator checks to
371 // see if a value is equal to constant integer value.
372 static Value *isValueEqualityComparison(TerminatorInst *TI) {
373 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
374 // Do not permit merging of large switch instructions into their
375 // predecessors unless there is only one predecessor.
376 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
377 pred_end(SI->getParent())) > 128)
380 return SI->getCondition();
382 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
383 if (BI->isConditional() && BI->getCondition()->hasOneUse())
384 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
385 if ((SCI->getOpcode() == Instruction::SetEQ ||
386 SCI->getOpcode() == Instruction::SetNE) &&
387 isa<ConstantInt>(SCI->getOperand(1)))
388 return SCI->getOperand(0);
392 // Given a value comparison instruction, decode all of the 'cases' that it
393 // represents and return the 'default' block.
395 GetValueEqualityComparisonCases(TerminatorInst *TI,
396 std::vector<std::pair<ConstantInt*,
397 BasicBlock*> > &Cases) {
398 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
399 Cases.reserve(SI->getNumCases());
400 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
401 Cases.push_back(std::make_pair(cast<ConstantInt>(SI->getCaseValue(i)),
402 SI->getSuccessor(i)));
403 return SI->getDefaultDest();
406 BranchInst *BI = cast<BranchInst>(TI);
407 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
408 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
409 BI->getSuccessor(SCI->getOpcode() ==
410 Instruction::SetNE)));
411 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
415 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
416 // equality comparison instruction (either a switch or a branch on "X == c").
417 // See if any of the predecessors of the terminator block are value comparisons
418 // on the same value. If so, and if safe to do so, fold them together.
419 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
420 BasicBlock *BB = TI->getParent();
421 Value *CV = isValueEqualityComparison(TI); // CondVal
422 assert(CV && "Not a comparison?");
423 bool Changed = false;
425 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
426 while (!Preds.empty()) {
427 BasicBlock *Pred = Preds.back();
430 // See if the predecessor is a comparison with the same value.
431 TerminatorInst *PTI = Pred->getTerminator();
432 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
434 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
435 // Figure out which 'cases' to copy from SI to PSI.
436 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
437 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
439 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
440 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
442 // Based on whether the default edge from PTI goes to BB or not, fill in
443 // PredCases and PredDefault with the new switch cases we would like to
445 std::vector<BasicBlock*> NewSuccessors;
447 if (PredDefault == BB) {
448 // If this is the default destination from PTI, only the edges in TI
449 // that don't occur in PTI, or that branch to BB will be activated.
450 std::set<ConstantInt*> PTIHandled;
451 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
452 if (PredCases[i].second != BB)
453 PTIHandled.insert(PredCases[i].first);
455 // The default destination is BB, we don't need explicit targets.
456 std::swap(PredCases[i], PredCases.back());
457 PredCases.pop_back();
461 // Reconstruct the new switch statement we will be building.
462 if (PredDefault != BBDefault) {
463 PredDefault->removePredecessor(Pred);
464 PredDefault = BBDefault;
465 NewSuccessors.push_back(BBDefault);
467 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
468 if (!PTIHandled.count(BBCases[i].first) &&
469 BBCases[i].second != BBDefault) {
470 PredCases.push_back(BBCases[i]);
471 NewSuccessors.push_back(BBCases[i].second);
475 // If this is not the default destination from PSI, only the edges
476 // in SI that occur in PSI with a destination of BB will be
478 std::set<ConstantInt*> PTIHandled;
479 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
480 if (PredCases[i].second == BB) {
481 PTIHandled.insert(PredCases[i].first);
482 std::swap(PredCases[i], PredCases.back());
483 PredCases.pop_back();
487 // Okay, now we know which constants were sent to BB from the
488 // predecessor. Figure out where they will all go now.
489 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
490 if (PTIHandled.count(BBCases[i].first)) {
491 // If this is one we are capable of getting...
492 PredCases.push_back(BBCases[i]);
493 NewSuccessors.push_back(BBCases[i].second);
494 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
497 // If there are any constants vectored to BB that TI doesn't handle,
498 // they must go to the default destination of TI.
499 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
500 E = PTIHandled.end(); I != E; ++I) {
501 PredCases.push_back(std::make_pair(*I, BBDefault));
502 NewSuccessors.push_back(BBDefault);
506 // Okay, at this point, we know which new successor Pred will get. Make
507 // sure we update the number of entries in the PHI nodes for these
509 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
510 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
512 // Now that the successors are updated, create the new Switch instruction.
513 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PTI);
514 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
515 NewSI->addCase(PredCases[i].first, PredCases[i].second);
516 Pred->getInstList().erase(PTI);
518 // Okay, last check. If BB is still a successor of PSI, then we must
519 // have an infinite loop case. If so, add an infinitely looping block
520 // to handle the case to preserve the behavior of the code.
521 BasicBlock *InfLoopBlock = 0;
522 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
523 if (NewSI->getSuccessor(i) == BB) {
524 if (InfLoopBlock == 0) {
525 // Insert it at the end of the loop, because it's either code,
526 // or it won't matter if it's hot. :)
527 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
528 new BranchInst(InfLoopBlock, InfLoopBlock);
530 NewSI->setSuccessor(i, InfLoopBlock);
540 /// ConstantIntOrdering - This class implements a stable ordering of constant
541 /// integers that does not depend on their address. This is important for
542 /// applications that sort ConstantInt's to ensure uniqueness.
543 struct ConstantIntOrdering {
544 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
545 return LHS->getRawValue() < RHS->getRawValue();
551 // SimplifyCFG - This function is used to do simplification of a CFG. For
552 // example, it adjusts branches to branches to eliminate the extra hop, it
553 // eliminates unreachable basic blocks, and does other "peephole" optimization
554 // of the CFG. It returns true if a modification was made.
556 // WARNING: The entry node of a function may not be simplified.
558 bool llvm::SimplifyCFG(BasicBlock *BB) {
559 bool Changed = false;
560 Function *M = BB->getParent();
562 assert(BB && BB->getParent() && "Block not embedded in function!");
563 assert(BB->getTerminator() && "Degenerate basic block encountered!");
564 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
566 // Remove basic blocks that have no predecessors... which are unreachable.
567 if (pred_begin(BB) == pred_end(BB) ||
568 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
569 DEBUG(std::cerr << "Removing BB: \n" << *BB);
571 // Loop through all of our successors and make sure they know that one
572 // of their predecessors is going away.
573 for_each(succ_begin(BB), succ_end(BB),
574 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
576 while (!BB->empty()) {
577 Instruction &I = BB->back();
578 // If this instruction is used, replace uses with an arbitrary
579 // constant value. Because control flow can't get here, we don't care
580 // what we replace the value with. Note that since this block is
581 // unreachable, and all values contained within it must dominate their
582 // uses, that all uses will eventually be removed.
584 // Make all users of this instruction reference the constant instead
585 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
587 // Remove the instruction from the basic block
588 BB->getInstList().pop_back();
590 M->getBasicBlockList().erase(BB);
594 // Check to see if we can constant propagate this terminator instruction
596 Changed |= ConstantFoldTerminator(BB);
598 // Check to see if this block has no non-phi instructions and only a single
599 // successor. If so, replace references to this basic block with references
601 succ_iterator SI(succ_begin(BB));
602 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
604 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
605 while (isa<PHINode>(*BBI)) ++BBI;
607 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
608 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
610 if (Succ != BB) { // Arg, don't hurt infinite loops!
611 // If our successor has PHI nodes, then we need to update them to
612 // include entries for BB's predecessors, not for BB itself.
613 // Be careful though, if this transformation fails (returns true) then
614 // we cannot do this transformation!
616 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
617 DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB);
618 std::string OldName = BB->getName();
620 std::vector<BasicBlock*>
621 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
623 // Move all PHI nodes in BB to Succ if they are alive, otherwise
625 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
627 BB->getInstList().erase(BB->begin()); // Nuke instruction...
629 // The instruction is alive, so this means that Succ must have
630 // *ONLY* had BB as a predecessor, and the PHI node is still valid
631 // now. Simply move it into Succ, because we know that BB
632 // strictly dominated Succ.
633 BB->getInstList().remove(BB->begin());
634 Succ->getInstList().push_front(PN);
636 // We need to add new entries for the PHI node to account for
637 // predecessors of Succ that the PHI node does not take into
638 // account. At this point, since we know that BB dominated succ,
639 // this means that we should any newly added incoming edges should
640 // use the PHI node as the value for these edges, because they are
642 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
643 if (OldSuccPreds[i] != BB)
644 PN->addIncoming(PN, OldSuccPreds[i]);
647 // Everything that jumped to BB now goes to Succ...
648 BB->replaceAllUsesWith(Succ);
650 // Delete the old basic block...
651 M->getBasicBlockList().erase(BB);
653 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
654 Succ->setName(OldName);
661 // If this is a returning block with only PHI nodes in it, fold the return
662 // instruction into any unconditional branch predecessors.
664 // If any predecessor is a conditional branch that just selects among
665 // different return values, fold the replace the branch/return with a select
667 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
668 BasicBlock::iterator BBI = BB->getTerminator();
669 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
670 // Find predecessors that end with branches.
671 std::vector<BasicBlock*> UncondBranchPreds;
672 std::vector<BranchInst*> CondBranchPreds;
673 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
674 TerminatorInst *PTI = (*PI)->getTerminator();
675 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
676 if (BI->isUnconditional())
677 UncondBranchPreds.push_back(*PI);
679 CondBranchPreds.push_back(BI);
682 // If we found some, do the transformation!
683 if (!UncondBranchPreds.empty()) {
684 while (!UncondBranchPreds.empty()) {
685 BasicBlock *Pred = UncondBranchPreds.back();
686 UncondBranchPreds.pop_back();
687 Instruction *UncondBranch = Pred->getTerminator();
688 // Clone the return and add it to the end of the predecessor.
689 Instruction *NewRet = RI->clone();
690 Pred->getInstList().push_back(NewRet);
692 // If the return instruction returns a value, and if the value was a
693 // PHI node in "BB", propagate the right value into the return.
694 if (NewRet->getNumOperands() == 1)
695 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
696 if (PN->getParent() == BB)
697 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
698 // Update any PHI nodes in the returning block to realize that we no
699 // longer branch to them.
700 BB->removePredecessor(Pred);
701 Pred->getInstList().erase(UncondBranch);
704 // If we eliminated all predecessors of the block, delete the block now.
705 if (pred_begin(BB) == pred_end(BB))
706 // We know there are no successors, so just nuke the block.
707 M->getBasicBlockList().erase(BB);
712 // Check out all of the conditional branches going to this return
713 // instruction. If any of them just select between returns, change the
714 // branch itself into a select/return pair.
715 while (!CondBranchPreds.empty()) {
716 BranchInst *BI = CondBranchPreds.back();
717 CondBranchPreds.pop_back();
718 BasicBlock *TrueSucc = BI->getSuccessor(0);
719 BasicBlock *FalseSucc = BI->getSuccessor(1);
720 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
722 // Check to see if the non-BB successor is also a return block.
723 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
724 // Check to see if there are only PHI instructions in this block.
725 BasicBlock::iterator OSI = OtherSucc->getTerminator();
726 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
727 // Okay, we found a branch that is going to two return nodes. If
728 // there is no return value for this function, just change the
729 // branch into a return.
730 if (RI->getNumOperands() == 0) {
731 TrueSucc->removePredecessor(BI->getParent());
732 FalseSucc->removePredecessor(BI->getParent());
733 new ReturnInst(0, BI);
734 BI->getParent()->getInstList().erase(BI);
738 // Otherwise, figure out what the true and false return values are
739 // so we can insert a new select instruction.
740 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
741 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
743 // Unwrap any PHI nodes in the return blocks.
744 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
745 if (TVPN->getParent() == TrueSucc)
746 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
747 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
748 if (FVPN->getParent() == FalseSucc)
749 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
751 TrueSucc->removePredecessor(BI->getParent());
752 FalseSucc->removePredecessor(BI->getParent());
754 // Insert a new select instruction.
755 Value *NewRetVal = new SelectInst(BI->getCondition(), TrueValue,
756 FalseValue, "retval", BI);
757 new ReturnInst(NewRetVal, BI);
758 BI->getParent()->getInstList().erase(BI);
764 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
765 // Check to see if the first instruction in this block is just an unwind.
766 // If so, replace any invoke instructions which use this as an exception
767 // destination with call instructions.
769 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
770 while (!Preds.empty()) {
771 BasicBlock *Pred = Preds.back();
772 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
773 if (II->getUnwindDest() == BB) {
774 // Insert a new branch instruction before the invoke, because this
775 // is now a fall through...
776 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
777 Pred->getInstList().remove(II); // Take out of symbol table
779 // Insert the call now...
780 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
781 CallInst *CI = new CallInst(II->getCalledValue(), Args,
783 // If the invoke produced a value, the Call now does instead
784 II->replaceAllUsesWith(CI);
792 // If this block is now dead, remove it.
793 if (pred_begin(BB) == pred_end(BB)) {
794 // We know there are no successors, so just nuke the block.
795 M->getBasicBlockList().erase(BB);
799 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
800 if (isValueEqualityComparison(SI))
801 if (FoldValueComparisonIntoPredecessors(SI))
802 return SimplifyCFG(BB) || 1;
803 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
804 if (BI->isConditional()) {
805 if (Value *CompVal = isValueEqualityComparison(BI)) {
806 // This block must be empty, except for the setcond inst, if it exists.
807 BasicBlock::iterator I = BB->begin();
809 (&*I == cast<Instruction>(BI->getCondition()) &&
811 if (FoldValueComparisonIntoPredecessors(BI))
812 return SimplifyCFG(BB) | true;
815 // If this basic block is ONLY a setcc and a branch, and if a predecessor
816 // branches to us and one of our successors, fold the setcc into the
817 // predecessor and use logical operations to pick the right destination.
818 BasicBlock *TrueDest = BI->getSuccessor(0);
819 BasicBlock *FalseDest = BI->getSuccessor(1);
820 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition()))
821 if (Cond->getParent() == BB && &BB->front() == Cond &&
822 Cond->getNext() == BI && Cond->hasOneUse() &&
823 TrueDest != BB && FalseDest != BB)
824 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
825 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
826 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
827 BasicBlock *PredBlock = *PI;
828 if (PBI->getSuccessor(0) == FalseDest ||
829 PBI->getSuccessor(1) == TrueDest) {
830 // Invert the predecessors condition test (xor it with true),
831 // which allows us to write this code once.
833 BinaryOperator::createNot(PBI->getCondition(),
834 PBI->getCondition()->getName()+".not", PBI);
835 PBI->setCondition(NewCond);
836 BasicBlock *OldTrue = PBI->getSuccessor(0);
837 BasicBlock *OldFalse = PBI->getSuccessor(1);
838 PBI->setSuccessor(0, OldFalse);
839 PBI->setSuccessor(1, OldTrue);
842 if (PBI->getSuccessor(0) == TrueDest ||
843 PBI->getSuccessor(1) == FalseDest) {
844 // Clone Cond into the predecessor basic block, and or/and the
845 // two conditions together.
846 Instruction *New = Cond->clone();
847 New->setName(Cond->getName());
848 Cond->setName(Cond->getName()+".old");
849 PredBlock->getInstList().insert(PBI, New);
850 Instruction::BinaryOps Opcode =
851 PBI->getSuccessor(0) == TrueDest ?
852 Instruction::Or : Instruction::And;
854 BinaryOperator::create(Opcode, PBI->getCondition(),
855 New, "bothcond", PBI);
856 PBI->setCondition(NewCond);
857 if (PBI->getSuccessor(0) == BB) {
858 AddPredecessorToBlock(TrueDest, PredBlock, BB);
859 PBI->setSuccessor(0, TrueDest);
861 if (PBI->getSuccessor(1) == BB) {
862 AddPredecessorToBlock(FalseDest, PredBlock, BB);
863 PBI->setSuccessor(1, FalseDest);
865 return SimplifyCFG(BB) | 1;
869 // If this block ends with a branch instruction, and if there is one
870 // predecessor, see if the previous block ended with a branch on the same
871 // condition, which makes this conditional branch redundant.
872 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
873 BasicBlock *OnlyPred = *PI++;
874 for (; PI != PE; ++PI)// Search all predecessors, see if they are all same
875 if (*PI != OnlyPred) {
876 OnlyPred = 0; // There are multiple different predecessors...
881 if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
882 if (PBI->isConditional() &&
883 PBI->getCondition() == BI->getCondition() &&
884 (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) {
885 // Okay, the outcome of this conditional branch is statically
886 // knowable. Delete the outgoing CFG edge that is impossible to
888 bool CondIsTrue = PBI->getSuccessor(0) == BB;
889 BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
890 new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
891 BB->getInstList().erase(BI);
892 return SimplifyCFG(BB) | true;
897 // Merge basic blocks into their predecessor if there is only one distinct
898 // pred, and if there is only one distinct successor of the predecessor, and
899 // if there are no PHI nodes.
901 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
902 BasicBlock *OnlyPred = *PI++;
903 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
904 if (*PI != OnlyPred) {
905 OnlyPred = 0; // There are multiple different predecessors...
909 BasicBlock *OnlySucc = 0;
910 if (OnlyPred && OnlyPred != BB && // Don't break self loops
911 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
912 // Check to see if there is only one distinct successor...
913 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
915 for (; SI != SE; ++SI)
916 if (*SI != OnlySucc) {
917 OnlySucc = 0; // There are multiple distinct successors!
923 DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred);
924 TerminatorInst *Term = OnlyPred->getTerminator();
926 // Resolve any PHI nodes at the start of the block. They are all
927 // guaranteed to have exactly one entry if they exist, unless there are
928 // multiple duplicate (but guaranteed to be equal) entries for the
929 // incoming edges. This occurs when there are multiple edges from
930 // OnlyPred to OnlySucc.
932 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
933 PN->replaceAllUsesWith(PN->getIncomingValue(0));
934 BB->getInstList().pop_front(); // Delete the phi node...
937 // Delete the unconditional branch from the predecessor...
938 OnlyPred->getInstList().pop_back();
940 // Move all definitions in the successor to the predecessor...
941 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
943 // Make all PHI nodes that referred to BB now refer to Pred as their
945 BB->replaceAllUsesWith(OnlyPred);
947 std::string OldName = BB->getName();
949 // Erase basic block from the function...
950 M->getBasicBlockList().erase(BB);
952 // Inherit predecessors name if it exists...
953 if (!OldName.empty() && !OnlyPred->hasName())
954 OnlyPred->setName(OldName);
959 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
960 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
961 // Change br (X == 0 | X == 1), T, F into a switch instruction.
962 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
963 Instruction *Cond = cast<Instruction>(BI->getCondition());
964 // If this is a bunch of seteq's or'd together, or if it's a bunch of
965 // 'setne's and'ed together, collect them.
967 std::vector<ConstantInt*> Values;
968 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
969 if (CompVal && CompVal->getType()->isInteger()) {
970 // There might be duplicate constants in the list, which the switch
971 // instruction can't handle, remove them now.
972 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
973 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
975 // Figure out which block is which destination.
976 BasicBlock *DefaultBB = BI->getSuccessor(1);
977 BasicBlock *EdgeBB = BI->getSuccessor(0);
978 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
980 // Create the new switch instruction now.
981 SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
983 // Add all of the 'cases' to the switch instruction.
984 for (unsigned i = 0, e = Values.size(); i != e; ++i)
985 New->addCase(Values[i], EdgeBB);
987 // We added edges from PI to the EdgeBB. As such, if there were any
988 // PHI nodes in EdgeBB, they need entries to be added corresponding to
989 // the number of edges added.
990 for (BasicBlock::iterator BBI = EdgeBB->begin();
991 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
992 Value *InVal = PN->getIncomingValueForBlock(*PI);
993 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
994 PN->addIncoming(InVal, *PI);
997 // Erase the old branch instruction.
998 (*PI)->getInstList().erase(BI);
1000 // Erase the potentially condition tree that was used to computed the
1001 // branch condition.
1002 ErasePossiblyDeadInstructionTree(Cond);
1007 // If there is a trivial two-entry PHI node in this basic block, and we can
1008 // eliminate it, do so now.
1009 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1010 if (PN->getNumIncomingValues() == 2) {
1011 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1012 // statement", which has a very simple dominance structure. Basically, we
1013 // are trying to find the condition that is being branched on, which
1014 // subsequently causes this merge to happen. We really want control
1015 // dependence information for this check, but simplifycfg can't keep it up
1016 // to date, and this catches most of the cases we care about anyway.
1018 BasicBlock *IfTrue, *IfFalse;
1019 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
1020 DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
1021 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1023 // Figure out where to insert instructions as necessary.
1024 BasicBlock::iterator AfterPHIIt = BB->begin();
1025 while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
1027 BasicBlock::iterator I = BB->begin();
1028 while (PHINode *PN = dyn_cast<PHINode>(I)) {
1031 // If we can eliminate this PHI by directly computing it based on the
1032 // condition, do so now. We can't eliminate PHI nodes where the
1033 // incoming values are defined in the conditional parts of the branch,
1034 // so check for this.
1036 if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
1037 DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
1039 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1041 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1043 // If one of the incoming values is defined in the conditional
1044 // region, move it into it's predecessor block, which we know is
1046 if (!DominatesMergePoint(TrueVal, BB, false)) {
1047 Instruction *TrueI = cast<Instruction>(TrueVal);
1048 BasicBlock *OldBB = TrueI->getParent();
1049 OldBB->getInstList().remove(TrueI);
1050 BasicBlock *NewBB = *pred_begin(OldBB);
1051 NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
1053 if (!DominatesMergePoint(FalseVal, BB, false)) {
1054 Instruction *FalseI = cast<Instruction>(FalseVal);
1055 BasicBlock *OldBB = FalseI->getParent();
1056 OldBB->getInstList().remove(FalseI);
1057 BasicBlock *NewBB = *pred_begin(OldBB);
1058 NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
1061 // Change the PHI node into a select instruction.
1062 BasicBlock::iterator InsertPos = PN;
1063 while (isa<PHINode>(InsertPos)) ++InsertPos;
1065 std::string Name = PN->getName(); PN->setName("");
1066 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1068 BB->getInstList().erase(PN);