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
188 static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
189 Instruction *I = dyn_cast<Instruction>(V);
190 if (!I) return true; // Non-instructions all dominate instructions.
191 BasicBlock *PBB = I->getParent();
193 // We don't want to allow wierd loops that might have the "if condition" in
194 // the bottom of this block.
195 if (PBB == BB) return false;
197 // If this instruction is defined in a block that contains an unconditional
198 // branch to BB, then it must be in the 'conditional' part of the "if
200 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
201 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
202 if (!AllowAggressive) return false;
203 // Okay, it looks like the instruction IS in the "condition". Check to
204 // see if its a cheap instruction to unconditionally compute, and if it
205 // only uses stuff defined outside of the condition. If so, hoist it out.
206 switch (I->getOpcode()) {
207 default: return false; // Cannot hoist this out safely.
208 case Instruction::Load:
209 // We can hoist loads that are non-volatile and obviously cannot trap.
210 if (cast<LoadInst>(I)->isVolatile())
212 if (!isa<AllocaInst>(I->getOperand(0)) &&
213 !isa<Constant>(I->getOperand(0)))
216 // Finally, we have to check to make sure there are no instructions
217 // before the load in its basic block, as we are going to hoist the loop
218 // out to its predecessor.
219 if (PBB->begin() != BasicBlock::iterator(I))
222 case Instruction::Add:
223 case Instruction::Sub:
224 case Instruction::And:
225 case Instruction::Or:
226 case Instruction::Xor:
227 case Instruction::Shl:
228 case Instruction::Shr:
229 break; // These are all cheap and non-trapping instructions.
232 // Okay, we can only really hoist these out if their operands are not
233 // defined in the conditional region.
234 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
235 if (!DominatesMergePoint(I->getOperand(i), BB, false))
237 // Okay, it's safe to do this!
243 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
244 // instructions that compare a value against a constant, return the value being
245 // compared, and stick the constant into the Values vector.
246 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
247 if (Instruction *Inst = dyn_cast<Instruction>(V))
248 if (Inst->getOpcode() == Instruction::SetEQ) {
249 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
251 return Inst->getOperand(0);
252 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
254 return Inst->getOperand(1);
256 } else if (Inst->getOpcode() == Instruction::Or) {
257 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
258 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
265 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
266 // setne instructions that compare a value against a constant, return the value
267 // being compared, and stick the constant into the Values vector.
268 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
269 if (Instruction *Inst = dyn_cast<Instruction>(V))
270 if (Inst->getOpcode() == Instruction::SetNE) {
271 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
273 return Inst->getOperand(0);
274 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
276 return Inst->getOperand(1);
278 } else if (Inst->getOpcode() == Instruction::Cast) {
279 // Cast of X to bool is really a comparison against zero.
280 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
281 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0));
282 return Inst->getOperand(0);
283 } else if (Inst->getOpcode() == Instruction::And) {
284 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
285 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
294 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
295 /// bunch of comparisons of one value against constants, return the value and
296 /// the constants being compared.
297 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
298 std::vector<ConstantInt*> &Values) {
299 if (Cond->getOpcode() == Instruction::Or) {
300 CompVal = GatherConstantSetEQs(Cond, Values);
302 // Return true to indicate that the condition is true if the CompVal is
303 // equal to one of the constants.
305 } else if (Cond->getOpcode() == Instruction::And) {
306 CompVal = GatherConstantSetNEs(Cond, Values);
308 // Return false to indicate that the condition is false if the CompVal is
309 // equal to one of the constants.
315 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
316 /// has no side effects, nuke it. If it uses any instructions that become dead
317 /// because the instruction is now gone, nuke them too.
318 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
319 if (isInstructionTriviallyDead(I)) {
320 std::vector<Value*> Operands(I->op_begin(), I->op_end());
321 I->getParent()->getInstList().erase(I);
322 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
323 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
324 ErasePossiblyDeadInstructionTree(OpI);
328 /// SafeToMergeTerminators - Return true if it is safe to merge these two
329 /// terminator instructions together.
331 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
332 if (SI1 == SI2) return false; // Can't merge with self!
334 // It is not safe to merge these two switch instructions if they have a common
335 // successor, and if that successor has a PHI node, and if *that* PHI node has
336 // conflicting incoming values from the two switch blocks.
337 BasicBlock *SI1BB = SI1->getParent();
338 BasicBlock *SI2BB = SI2->getParent();
339 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
341 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
342 if (SI1Succs.count(*I))
343 for (BasicBlock::iterator BBI = (*I)->begin();
344 isa<PHINode>(BBI); ++BBI) {
345 PHINode *PN = cast<PHINode>(BBI);
346 if (PN->getIncomingValueForBlock(SI1BB) !=
347 PN->getIncomingValueForBlock(SI2BB))
354 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
355 /// now be entries in it from the 'NewPred' block. The values that will be
356 /// flowing into the PHI nodes will be the same as those coming in from
357 /// ExistPred, an existing predecessor of Succ.
358 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
359 BasicBlock *ExistPred) {
360 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
361 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
362 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
364 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
365 PHINode *PN = cast<PHINode>(I);
366 Value *V = PN->getIncomingValueForBlock(ExistPred);
367 PN->addIncoming(V, NewPred);
371 // isValueEqualityComparison - Return true if the specified terminator checks to
372 // see if a value is equal to constant integer value.
373 static Value *isValueEqualityComparison(TerminatorInst *TI) {
374 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
375 // Do not permit merging of large switch instructions into their
376 // predecessors unless there is only one predecessor.
377 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
378 pred_end(SI->getParent())) > 128)
381 return SI->getCondition();
383 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
384 if (BI->isConditional() && BI->getCondition()->hasOneUse())
385 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
386 if ((SCI->getOpcode() == Instruction::SetEQ ||
387 SCI->getOpcode() == Instruction::SetNE) &&
388 isa<ConstantInt>(SCI->getOperand(1)))
389 return SCI->getOperand(0);
393 // Given a value comparison instruction, decode all of the 'cases' that it
394 // represents and return the 'default' block.
396 GetValueEqualityComparisonCases(TerminatorInst *TI,
397 std::vector<std::pair<ConstantInt*,
398 BasicBlock*> > &Cases) {
399 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
400 Cases.reserve(SI->getNumCases());
401 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
402 Cases.push_back(std::make_pair(cast<ConstantInt>(SI->getCaseValue(i)),
403 SI->getSuccessor(i)));
404 return SI->getDefaultDest();
407 BranchInst *BI = cast<BranchInst>(TI);
408 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
409 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
410 BI->getSuccessor(SCI->getOpcode() ==
411 Instruction::SetNE)));
412 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
416 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
417 // equality comparison instruction (either a switch or a branch on "X == c").
418 // See if any of the predecessors of the terminator block are value comparisons
419 // on the same value. If so, and if safe to do so, fold them together.
420 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
421 BasicBlock *BB = TI->getParent();
422 Value *CV = isValueEqualityComparison(TI); // CondVal
423 assert(CV && "Not a comparison?");
424 bool Changed = false;
426 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
427 while (!Preds.empty()) {
428 BasicBlock *Pred = Preds.back();
431 // See if the predecessor is a comparison with the same value.
432 TerminatorInst *PTI = Pred->getTerminator();
433 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
435 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
436 // Figure out which 'cases' to copy from SI to PSI.
437 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
438 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
440 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
441 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
443 // Based on whether the default edge from PTI goes to BB or not, fill in
444 // PredCases and PredDefault with the new switch cases we would like to
446 std::vector<BasicBlock*> NewSuccessors;
448 if (PredDefault == BB) {
449 // If this is the default destination from PTI, only the edges in TI
450 // that don't occur in PTI, or that branch to BB will be activated.
451 std::set<ConstantInt*> PTIHandled;
452 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
453 if (PredCases[i].second != BB)
454 PTIHandled.insert(PredCases[i].first);
456 // The default destination is BB, we don't need explicit targets.
457 std::swap(PredCases[i], PredCases.back());
458 PredCases.pop_back();
462 // Reconstruct the new switch statement we will be building.
463 if (PredDefault != BBDefault) {
464 PredDefault->removePredecessor(Pred);
465 PredDefault = BBDefault;
466 NewSuccessors.push_back(BBDefault);
468 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
469 if (!PTIHandled.count(BBCases[i].first) &&
470 BBCases[i].second != BBDefault) {
471 PredCases.push_back(BBCases[i]);
472 NewSuccessors.push_back(BBCases[i].second);
476 // If this is not the default destination from PSI, only the edges
477 // in SI that occur in PSI with a destination of BB will be
479 std::set<ConstantInt*> PTIHandled;
480 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
481 if (PredCases[i].second == BB) {
482 PTIHandled.insert(PredCases[i].first);
483 std::swap(PredCases[i], PredCases.back());
484 PredCases.pop_back();
488 // Okay, now we know which constants were sent to BB from the
489 // predecessor. Figure out where they will all go now.
490 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
491 if (PTIHandled.count(BBCases[i].first)) {
492 // If this is one we are capable of getting...
493 PredCases.push_back(BBCases[i]);
494 NewSuccessors.push_back(BBCases[i].second);
495 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
498 // If there are any constants vectored to BB that TI doesn't handle,
499 // they must go to the default destination of TI.
500 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
501 E = PTIHandled.end(); I != E; ++I) {
502 PredCases.push_back(std::make_pair(*I, BBDefault));
503 NewSuccessors.push_back(BBDefault);
507 // Okay, at this point, we know which new successor Pred will get. Make
508 // sure we update the number of entries in the PHI nodes for these
510 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
511 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
513 // Now that the successors are updated, create the new Switch instruction.
514 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PTI);
515 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
516 NewSI->addCase(PredCases[i].first, PredCases[i].second);
517 Pred->getInstList().erase(PTI);
519 // Okay, last check. If BB is still a successor of PSI, then we must
520 // have an infinite loop case. If so, add an infinitely looping block
521 // to handle the case to preserve the behavior of the code.
522 BasicBlock *InfLoopBlock = 0;
523 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
524 if (NewSI->getSuccessor(i) == BB) {
525 if (InfLoopBlock == 0) {
526 // Insert it at the end of the loop, because it's either code,
527 // or it won't matter if it's hot. :)
528 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
529 new BranchInst(InfLoopBlock, InfLoopBlock);
531 NewSI->setSuccessor(i, InfLoopBlock);
541 /// ConstantIntOrdering - This class implements a stable ordering of constant
542 /// integers that does not depend on their address. This is important for
543 /// applications that sort ConstantInt's to ensure uniqueness.
544 struct ConstantIntOrdering {
545 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
546 return LHS->getRawValue() < RHS->getRawValue();
552 // SimplifyCFG - This function is used to do simplification of a CFG. For
553 // example, it adjusts branches to branches to eliminate the extra hop, it
554 // eliminates unreachable basic blocks, and does other "peephole" optimization
555 // of the CFG. It returns true if a modification was made.
557 // WARNING: The entry node of a function may not be simplified.
559 bool llvm::SimplifyCFG(BasicBlock *BB) {
560 bool Changed = false;
561 Function *M = BB->getParent();
563 assert(BB && BB->getParent() && "Block not embedded in function!");
564 assert(BB->getTerminator() && "Degenerate basic block encountered!");
565 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
567 // Remove basic blocks that have no predecessors... which are unreachable.
568 if (pred_begin(BB) == pred_end(BB) ||
569 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
570 DEBUG(std::cerr << "Removing BB: \n" << *BB);
572 // Loop through all of our successors and make sure they know that one
573 // of their predecessors is going away.
574 for_each(succ_begin(BB), succ_end(BB),
575 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
577 while (!BB->empty()) {
578 Instruction &I = BB->back();
579 // If this instruction is used, replace uses with an arbitrary
580 // constant value. Because control flow can't get here, we don't care
581 // what we replace the value with. Note that since this block is
582 // unreachable, and all values contained within it must dominate their
583 // uses, that all uses will eventually be removed.
585 // Make all users of this instruction reference the constant instead
586 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
588 // Remove the instruction from the basic block
589 BB->getInstList().pop_back();
591 M->getBasicBlockList().erase(BB);
595 // Check to see if we can constant propagate this terminator instruction
597 Changed |= ConstantFoldTerminator(BB);
599 // Check to see if this block has no non-phi instructions and only a single
600 // successor. If so, replace references to this basic block with references
602 succ_iterator SI(succ_begin(BB));
603 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
605 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
606 while (isa<PHINode>(*BBI)) ++BBI;
608 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
609 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
611 if (Succ != BB) { // Arg, don't hurt infinite loops!
612 // If our successor has PHI nodes, then we need to update them to
613 // include entries for BB's predecessors, not for BB itself.
614 // Be careful though, if this transformation fails (returns true) then
615 // we cannot do this transformation!
617 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
618 DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB);
619 std::string OldName = BB->getName();
621 std::vector<BasicBlock*>
622 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
624 // Move all PHI nodes in BB to Succ if they are alive, otherwise
626 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
628 BB->getInstList().erase(BB->begin()); // Nuke instruction...
630 // The instruction is alive, so this means that Succ must have
631 // *ONLY* had BB as a predecessor, and the PHI node is still valid
632 // now. Simply move it into Succ, because we know that BB
633 // strictly dominated Succ.
634 BB->getInstList().remove(BB->begin());
635 Succ->getInstList().push_front(PN);
637 // We need to add new entries for the PHI node to account for
638 // predecessors of Succ that the PHI node does not take into
639 // account. At this point, since we know that BB dominated succ,
640 // this means that we should any newly added incoming edges should
641 // use the PHI node as the value for these edges, because they are
643 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
644 if (OldSuccPreds[i] != BB)
645 PN->addIncoming(PN, OldSuccPreds[i]);
648 // Everything that jumped to BB now goes to Succ...
649 BB->replaceAllUsesWith(Succ);
651 // Delete the old basic block...
652 M->getBasicBlockList().erase(BB);
654 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
655 Succ->setName(OldName);
662 // If this is a returning block with only PHI nodes in it, fold the return
663 // instruction into any unconditional branch predecessors.
665 // If any predecessor is a conditional branch that just selects among
666 // different return values, fold the replace the branch/return with a select
668 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
669 BasicBlock::iterator BBI = BB->getTerminator();
670 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
671 // Find predecessors that end with branches.
672 std::vector<BasicBlock*> UncondBranchPreds;
673 std::vector<BranchInst*> CondBranchPreds;
674 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
675 TerminatorInst *PTI = (*PI)->getTerminator();
676 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
677 if (BI->isUnconditional())
678 UncondBranchPreds.push_back(*PI);
680 CondBranchPreds.push_back(BI);
683 // If we found some, do the transformation!
684 if (!UncondBranchPreds.empty()) {
685 while (!UncondBranchPreds.empty()) {
686 BasicBlock *Pred = UncondBranchPreds.back();
687 UncondBranchPreds.pop_back();
688 Instruction *UncondBranch = Pred->getTerminator();
689 // Clone the return and add it to the end of the predecessor.
690 Instruction *NewRet = RI->clone();
691 Pred->getInstList().push_back(NewRet);
693 // If the return instruction returns a value, and if the value was a
694 // PHI node in "BB", propagate the right value into the return.
695 if (NewRet->getNumOperands() == 1)
696 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
697 if (PN->getParent() == BB)
698 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
699 // Update any PHI nodes in the returning block to realize that we no
700 // longer branch to them.
701 BB->removePredecessor(Pred);
702 Pred->getInstList().erase(UncondBranch);
705 // If we eliminated all predecessors of the block, delete the block now.
706 if (pred_begin(BB) == pred_end(BB))
707 // We know there are no successors, so just nuke the block.
708 M->getBasicBlockList().erase(BB);
713 // Check out all of the conditional branches going to this return
714 // instruction. If any of them just select between returns, change the
715 // branch itself into a select/return pair.
716 while (!CondBranchPreds.empty()) {
717 BranchInst *BI = CondBranchPreds.back();
718 CondBranchPreds.pop_back();
719 BasicBlock *TrueSucc = BI->getSuccessor(0);
720 BasicBlock *FalseSucc = BI->getSuccessor(1);
721 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
723 // Check to see if the non-BB successor is also a return block.
724 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
725 // Check to see if there are only PHI instructions in this block.
726 BasicBlock::iterator OSI = OtherSucc->getTerminator();
727 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
728 // Okay, we found a branch that is going to two return nodes. If
729 // there is no return value for this function, just change the
730 // branch into a return.
731 if (RI->getNumOperands() == 0) {
732 TrueSucc->removePredecessor(BI->getParent());
733 FalseSucc->removePredecessor(BI->getParent());
734 new ReturnInst(0, BI);
735 BI->getParent()->getInstList().erase(BI);
739 // Otherwise, figure out what the true and false return values are
740 // so we can insert a new select instruction.
741 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
742 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
744 // Unwrap any PHI nodes in the return blocks.
745 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
746 if (TVPN->getParent() == TrueSucc)
747 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
748 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
749 if (FVPN->getParent() == FalseSucc)
750 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
752 TrueSucc->removePredecessor(BI->getParent());
753 FalseSucc->removePredecessor(BI->getParent());
755 // Insert a new select instruction.
757 Value *BrCond = BI->getCondition();
758 if (TrueValue != FalseValue)
759 NewRetVal = new SelectInst(BrCond, TrueValue,
760 FalseValue, "retval", BI);
762 NewRetVal = TrueValue;
764 new ReturnInst(NewRetVal, BI);
765 BI->getParent()->getInstList().erase(BI);
766 if (BrCond->use_empty())
767 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond))
768 BrCondI->getParent()->getInstList().erase(BrCondI);
774 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
775 // Check to see if the first instruction in this block is just an unwind.
776 // If so, replace any invoke instructions which use this as an exception
777 // destination with call instructions, and any unconditional branch
778 // predecessor with an unwind.
780 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
781 while (!Preds.empty()) {
782 BasicBlock *Pred = Preds.back();
783 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) {
784 if (BI->isUnconditional()) {
785 Pred->getInstList().pop_back(); // nuke uncond branch
786 new UnwindInst(Pred); // Use unwind.
789 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
790 if (II->getUnwindDest() == BB) {
791 // Insert a new branch instruction before the invoke, because this
792 // is now a fall through...
793 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
794 Pred->getInstList().remove(II); // Take out of symbol table
796 // Insert the call now...
797 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
798 CallInst *CI = new CallInst(II->getCalledValue(), Args,
800 // If the invoke produced a value, the Call now does instead
801 II->replaceAllUsesWith(CI);
809 // If this block is now dead, remove it.
810 if (pred_begin(BB) == pred_end(BB)) {
811 // We know there are no successors, so just nuke the block.
812 M->getBasicBlockList().erase(BB);
816 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
817 if (isValueEqualityComparison(SI))
818 if (FoldValueComparisonIntoPredecessors(SI))
819 return SimplifyCFG(BB) || 1;
820 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
821 if (BI->isConditional()) {
822 if (Value *CompVal = isValueEqualityComparison(BI)) {
823 // This block must be empty, except for the setcond inst, if it exists.
824 BasicBlock::iterator I = BB->begin();
826 (&*I == cast<Instruction>(BI->getCondition()) &&
828 if (FoldValueComparisonIntoPredecessors(BI))
829 return SimplifyCFG(BB) | true;
832 // If this basic block is ONLY a setcc and a branch, and if a predecessor
833 // branches to us and one of our successors, fold the setcc into the
834 // predecessor and use logical operations to pick the right destination.
835 BasicBlock *TrueDest = BI->getSuccessor(0);
836 BasicBlock *FalseDest = BI->getSuccessor(1);
837 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition()))
838 if (Cond->getParent() == BB && &BB->front() == Cond &&
839 Cond->getNext() == BI && Cond->hasOneUse() &&
840 TrueDest != BB && FalseDest != BB)
841 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
842 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
843 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
844 BasicBlock *PredBlock = *PI;
845 if (PBI->getSuccessor(0) == FalseDest ||
846 PBI->getSuccessor(1) == TrueDest) {
847 // Invert the predecessors condition test (xor it with true),
848 // which allows us to write this code once.
850 BinaryOperator::createNot(PBI->getCondition(),
851 PBI->getCondition()->getName()+".not", PBI);
852 PBI->setCondition(NewCond);
853 BasicBlock *OldTrue = PBI->getSuccessor(0);
854 BasicBlock *OldFalse = PBI->getSuccessor(1);
855 PBI->setSuccessor(0, OldFalse);
856 PBI->setSuccessor(1, OldTrue);
859 if (PBI->getSuccessor(0) == TrueDest ||
860 PBI->getSuccessor(1) == FalseDest) {
861 // Clone Cond into the predecessor basic block, and or/and the
862 // two conditions together.
863 Instruction *New = Cond->clone();
864 New->setName(Cond->getName());
865 Cond->setName(Cond->getName()+".old");
866 PredBlock->getInstList().insert(PBI, New);
867 Instruction::BinaryOps Opcode =
868 PBI->getSuccessor(0) == TrueDest ?
869 Instruction::Or : Instruction::And;
871 BinaryOperator::create(Opcode, PBI->getCondition(),
872 New, "bothcond", PBI);
873 PBI->setCondition(NewCond);
874 if (PBI->getSuccessor(0) == BB) {
875 AddPredecessorToBlock(TrueDest, PredBlock, BB);
876 PBI->setSuccessor(0, TrueDest);
878 if (PBI->getSuccessor(1) == BB) {
879 AddPredecessorToBlock(FalseDest, PredBlock, BB);
880 PBI->setSuccessor(1, FalseDest);
882 return SimplifyCFG(BB) | 1;
886 // If this block ends with a branch instruction, and if there is one
887 // predecessor, see if the previous block ended with a branch on the same
888 // condition, which makes this conditional branch redundant.
889 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
890 BasicBlock *OnlyPred = *PI++;
891 for (; PI != PE; ++PI)// Search all predecessors, see if they are all same
892 if (*PI != OnlyPred) {
893 OnlyPred = 0; // There are multiple different predecessors...
898 if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
899 if (PBI->isConditional() &&
900 PBI->getCondition() == BI->getCondition() &&
901 (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) {
902 // Okay, the outcome of this conditional branch is statically
903 // knowable. Delete the outgoing CFG edge that is impossible to
905 bool CondIsTrue = PBI->getSuccessor(0) == BB;
906 BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
907 new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
908 BB->getInstList().erase(BI);
909 return SimplifyCFG(BB) | true;
914 // Merge basic blocks into their predecessor if there is only one distinct
915 // pred, and if there is only one distinct successor of the predecessor, and
916 // if there are no PHI nodes.
918 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
919 BasicBlock *OnlyPred = *PI++;
920 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
921 if (*PI != OnlyPred) {
922 OnlyPred = 0; // There are multiple different predecessors...
926 BasicBlock *OnlySucc = 0;
927 if (OnlyPred && OnlyPred != BB && // Don't break self loops
928 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
929 // Check to see if there is only one distinct successor...
930 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
932 for (; SI != SE; ++SI)
933 if (*SI != OnlySucc) {
934 OnlySucc = 0; // There are multiple distinct successors!
940 DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred);
941 TerminatorInst *Term = OnlyPred->getTerminator();
943 // Resolve any PHI nodes at the start of the block. They are all
944 // guaranteed to have exactly one entry if they exist, unless there are
945 // multiple duplicate (but guaranteed to be equal) entries for the
946 // incoming edges. This occurs when there are multiple edges from
947 // OnlyPred to OnlySucc.
949 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
950 PN->replaceAllUsesWith(PN->getIncomingValue(0));
951 BB->getInstList().pop_front(); // Delete the phi node...
954 // Delete the unconditional branch from the predecessor...
955 OnlyPred->getInstList().pop_back();
957 // Move all definitions in the successor to the predecessor...
958 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
960 // Make all PHI nodes that referred to BB now refer to Pred as their
962 BB->replaceAllUsesWith(OnlyPred);
964 std::string OldName = BB->getName();
966 // Erase basic block from the function...
967 M->getBasicBlockList().erase(BB);
969 // Inherit predecessors name if it exists...
970 if (!OldName.empty() && !OnlyPred->hasName())
971 OnlyPred->setName(OldName);
976 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
977 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
978 // Change br (X == 0 | X == 1), T, F into a switch instruction.
979 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
980 Instruction *Cond = cast<Instruction>(BI->getCondition());
981 // If this is a bunch of seteq's or'd together, or if it's a bunch of
982 // 'setne's and'ed together, collect them.
984 std::vector<ConstantInt*> Values;
985 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
986 if (CompVal && CompVal->getType()->isInteger()) {
987 // There might be duplicate constants in the list, which the switch
988 // instruction can't handle, remove them now.
989 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
990 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
992 // Figure out which block is which destination.
993 BasicBlock *DefaultBB = BI->getSuccessor(1);
994 BasicBlock *EdgeBB = BI->getSuccessor(0);
995 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
997 // Create the new switch instruction now.
998 SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
1000 // Add all of the 'cases' to the switch instruction.
1001 for (unsigned i = 0, e = Values.size(); i != e; ++i)
1002 New->addCase(Values[i], EdgeBB);
1004 // We added edges from PI to the EdgeBB. As such, if there were any
1005 // PHI nodes in EdgeBB, they need entries to be added corresponding to
1006 // the number of edges added.
1007 for (BasicBlock::iterator BBI = EdgeBB->begin();
1008 isa<PHINode>(BBI); ++BBI) {
1009 PHINode *PN = cast<PHINode>(BBI);
1010 Value *InVal = PN->getIncomingValueForBlock(*PI);
1011 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
1012 PN->addIncoming(InVal, *PI);
1015 // Erase the old branch instruction.
1016 (*PI)->getInstList().erase(BI);
1018 // Erase the potentially condition tree that was used to computed the
1019 // branch condition.
1020 ErasePossiblyDeadInstructionTree(Cond);
1025 // If there is a trivial two-entry PHI node in this basic block, and we can
1026 // eliminate it, do so now.
1027 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1028 if (PN->getNumIncomingValues() == 2) {
1029 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1030 // statement", which has a very simple dominance structure. Basically, we
1031 // are trying to find the condition that is being branched on, which
1032 // subsequently causes this merge to happen. We really want control
1033 // dependence information for this check, but simplifycfg can't keep it up
1034 // to date, and this catches most of the cases we care about anyway.
1036 BasicBlock *IfTrue, *IfFalse;
1037 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
1038 DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
1039 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1041 // Figure out where to insert instructions as necessary.
1042 BasicBlock::iterator AfterPHIIt = BB->begin();
1043 while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
1045 BasicBlock::iterator I = BB->begin();
1046 while (PHINode *PN = dyn_cast<PHINode>(I)) {
1049 // If we can eliminate this PHI by directly computing it based on the
1050 // condition, do so now. We can't eliminate PHI nodes where the
1051 // incoming values are defined in the conditional parts of the branch,
1052 // so check for this.
1054 if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
1055 DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
1057 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1059 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1061 // If one of the incoming values is defined in the conditional
1062 // region, move it into it's predecessor block, which we know is
1064 if (!DominatesMergePoint(TrueVal, BB, false)) {
1065 Instruction *TrueI = cast<Instruction>(TrueVal);
1066 BasicBlock *OldBB = TrueI->getParent();
1067 OldBB->getInstList().remove(TrueI);
1068 BasicBlock *NewBB = *pred_begin(OldBB);
1069 NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
1071 if (!DominatesMergePoint(FalseVal, BB, false)) {
1072 Instruction *FalseI = cast<Instruction>(FalseVal);
1073 BasicBlock *OldBB = FalseI->getParent();
1074 OldBB->getInstList().remove(FalseI);
1075 BasicBlock *NewBB = *pred_begin(OldBB);
1076 NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
1079 // Change the PHI node into a select instruction.
1080 BasicBlock::iterator InsertPos = PN;
1081 while (isa<PHINode>(InsertPos)) ++InsertPos;
1083 std::string Name = PN->getName(); PN->setName("");
1084 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1086 BB->getInstList().erase(PN);