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)) &&
215 !isa<GlobalValue>(I->getOperand(0)))
218 // Finally, we have to check to make sure there are no instructions
219 // before the load in its basic block, as we are going to hoist the loop
220 // out to its predecessor.
221 if (PBB->begin() != BasicBlock::iterator(I))
224 case Instruction::Add:
225 case Instruction::Sub:
226 case Instruction::And:
227 case Instruction::Or:
228 case Instruction::Xor:
229 case Instruction::Shl:
230 case Instruction::Shr:
231 break; // These are all cheap and non-trapping instructions.
234 // Okay, we can only really hoist these out if their operands are not
235 // defined in the conditional region.
236 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
237 if (!DominatesMergePoint(I->getOperand(i), BB, false))
239 // Okay, it's safe to do this!
245 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
246 // instructions that compare a value against a constant, return the value being
247 // compared, and stick the constant into the Values vector.
248 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
249 if (Instruction *Inst = dyn_cast<Instruction>(V))
250 if (Inst->getOpcode() == Instruction::SetEQ) {
251 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
253 return Inst->getOperand(0);
254 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
256 return Inst->getOperand(1);
258 } else if (Inst->getOpcode() == Instruction::Or) {
259 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
260 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
267 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
268 // setne instructions that compare a value against a constant, return the value
269 // being compared, and stick the constant into the Values vector.
270 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
271 if (Instruction *Inst = dyn_cast<Instruction>(V))
272 if (Inst->getOpcode() == Instruction::SetNE) {
273 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
275 return Inst->getOperand(0);
276 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
278 return Inst->getOperand(1);
280 } else if (Inst->getOpcode() == Instruction::Cast) {
281 // Cast of X to bool is really a comparison against zero.
282 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
283 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0));
284 return Inst->getOperand(0);
285 } else if (Inst->getOpcode() == Instruction::And) {
286 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
287 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
296 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
297 /// bunch of comparisons of one value against constants, return the value and
298 /// the constants being compared.
299 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
300 std::vector<ConstantInt*> &Values) {
301 if (Cond->getOpcode() == Instruction::Or) {
302 CompVal = GatherConstantSetEQs(Cond, Values);
304 // Return true to indicate that the condition is true if the CompVal is
305 // equal to one of the constants.
307 } else if (Cond->getOpcode() == Instruction::And) {
308 CompVal = GatherConstantSetNEs(Cond, Values);
310 // Return false to indicate that the condition is false if the CompVal is
311 // equal to one of the constants.
317 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
318 /// has no side effects, nuke it. If it uses any instructions that become dead
319 /// because the instruction is now gone, nuke them too.
320 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
321 if (isInstructionTriviallyDead(I)) {
322 std::vector<Value*> Operands(I->op_begin(), I->op_end());
323 I->getParent()->getInstList().erase(I);
324 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
325 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
326 ErasePossiblyDeadInstructionTree(OpI);
330 /// SafeToMergeTerminators - Return true if it is safe to merge these two
331 /// terminator instructions together.
333 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
334 if (SI1 == SI2) return false; // Can't merge with self!
336 // It is not safe to merge these two switch instructions if they have a common
337 // successor, and if that successor has a PHI node, and if *that* PHI node has
338 // conflicting incoming values from the two switch blocks.
339 BasicBlock *SI1BB = SI1->getParent();
340 BasicBlock *SI2BB = SI2->getParent();
341 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
343 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
344 if (SI1Succs.count(*I))
345 for (BasicBlock::iterator BBI = (*I)->begin();
346 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
347 if (PN->getIncomingValueForBlock(SI1BB) !=
348 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();
365 PHINode *PN = dyn_cast<PHINode>(I); ++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.
756 Value *NewRetVal = new SelectInst(BI->getCondition(), TrueValue,
757 FalseValue, "retval", BI);
758 new ReturnInst(NewRetVal, BI);
759 BI->getParent()->getInstList().erase(BI);
765 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
766 // Check to see if the first instruction in this block is just an unwind.
767 // If so, replace any invoke instructions which use this as an exception
768 // destination with call instructions.
770 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
771 while (!Preds.empty()) {
772 BasicBlock *Pred = Preds.back();
773 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
774 if (II->getUnwindDest() == BB) {
775 // Insert a new branch instruction before the invoke, because this
776 // is now a fall through...
777 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
778 Pred->getInstList().remove(II); // Take out of symbol table
780 // Insert the call now...
781 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
782 CallInst *CI = new CallInst(II->getCalledValue(), Args,
784 // If the invoke produced a value, the Call now does instead
785 II->replaceAllUsesWith(CI);
793 // If this block is now dead, remove it.
794 if (pred_begin(BB) == pred_end(BB)) {
795 // We know there are no successors, so just nuke the block.
796 M->getBasicBlockList().erase(BB);
800 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
801 if (isValueEqualityComparison(SI))
802 if (FoldValueComparisonIntoPredecessors(SI))
803 return SimplifyCFG(BB) || 1;
804 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
805 if (BI->isConditional()) {
806 if (Value *CompVal = isValueEqualityComparison(BI)) {
807 // This block must be empty, except for the setcond inst, if it exists.
808 BasicBlock::iterator I = BB->begin();
810 (&*I == cast<Instruction>(BI->getCondition()) &&
812 if (FoldValueComparisonIntoPredecessors(BI))
813 return SimplifyCFG(BB) | true;
816 // If this basic block is ONLY a setcc and a branch, and if a predecessor
817 // branches to us and one of our successors, fold the setcc into the
818 // predecessor and use logical operations to pick the right destination.
819 BasicBlock *TrueDest = BI->getSuccessor(0);
820 BasicBlock *FalseDest = BI->getSuccessor(1);
821 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition()))
822 if (Cond->getParent() == BB && &BB->front() == Cond &&
823 Cond->getNext() == BI && Cond->hasOneUse() &&
824 TrueDest != BB && FalseDest != BB)
825 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
826 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
827 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
828 BasicBlock *PredBlock = *PI;
829 if (PBI->getSuccessor(0) == FalseDest ||
830 PBI->getSuccessor(1) == TrueDest) {
831 // Invert the predecessors condition test (xor it with true),
832 // which allows us to write this code once.
834 BinaryOperator::createNot(PBI->getCondition(),
835 PBI->getCondition()->getName()+".not", PBI);
836 PBI->setCondition(NewCond);
837 BasicBlock *OldTrue = PBI->getSuccessor(0);
838 BasicBlock *OldFalse = PBI->getSuccessor(1);
839 PBI->setSuccessor(0, OldFalse);
840 PBI->setSuccessor(1, OldTrue);
843 if (PBI->getSuccessor(0) == TrueDest ||
844 PBI->getSuccessor(1) == FalseDest) {
845 // Clone Cond into the predecessor basic block, and or/and the
846 // two conditions together.
847 Instruction *New = Cond->clone();
848 New->setName(Cond->getName());
849 Cond->setName(Cond->getName()+".old");
850 PredBlock->getInstList().insert(PBI, New);
851 Instruction::BinaryOps Opcode =
852 PBI->getSuccessor(0) == TrueDest ?
853 Instruction::Or : Instruction::And;
855 BinaryOperator::create(Opcode, PBI->getCondition(),
856 New, "bothcond", PBI);
857 PBI->setCondition(NewCond);
858 if (PBI->getSuccessor(0) == BB) {
859 AddPredecessorToBlock(TrueDest, PredBlock, BB);
860 PBI->setSuccessor(0, TrueDest);
862 if (PBI->getSuccessor(1) == BB) {
863 AddPredecessorToBlock(FalseDest, PredBlock, BB);
864 PBI->setSuccessor(1, FalseDest);
866 return SimplifyCFG(BB) | 1;
870 // If this block ends with a branch instruction, and if there is one
871 // predecessor, see if the previous block ended with a branch on the same
872 // condition, which makes this conditional branch redundant.
873 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
874 BasicBlock *OnlyPred = *PI++;
875 for (; PI != PE; ++PI)// Search all predecessors, see if they are all same
876 if (*PI != OnlyPred) {
877 OnlyPred = 0; // There are multiple different predecessors...
882 if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
883 if (PBI->isConditional() &&
884 PBI->getCondition() == BI->getCondition() &&
885 (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) {
886 // Okay, the outcome of this conditional branch is statically
887 // knowable. Delete the outgoing CFG edge that is impossible to
889 bool CondIsTrue = PBI->getSuccessor(0) == BB;
890 BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
891 new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
892 BB->getInstList().erase(BI);
893 return SimplifyCFG(BB) | true;
898 // Merge basic blocks into their predecessor if there is only one distinct
899 // pred, and if there is only one distinct successor of the predecessor, and
900 // if there are no PHI nodes.
902 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
903 BasicBlock *OnlyPred = *PI++;
904 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
905 if (*PI != OnlyPred) {
906 OnlyPred = 0; // There are multiple different predecessors...
910 BasicBlock *OnlySucc = 0;
911 if (OnlyPred && OnlyPred != BB && // Don't break self loops
912 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
913 // Check to see if there is only one distinct successor...
914 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
916 for (; SI != SE; ++SI)
917 if (*SI != OnlySucc) {
918 OnlySucc = 0; // There are multiple distinct successors!
924 DEBUG(std::cerr << "Merging: " << BB << "into: " << OnlyPred);
925 TerminatorInst *Term = OnlyPred->getTerminator();
927 // Resolve any PHI nodes at the start of the block. They are all
928 // guaranteed to have exactly one entry if they exist, unless there are
929 // multiple duplicate (but guaranteed to be equal) entries for the
930 // incoming edges. This occurs when there are multiple edges from
931 // OnlyPred to OnlySucc.
933 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
934 PN->replaceAllUsesWith(PN->getIncomingValue(0));
935 BB->getInstList().pop_front(); // Delete the phi node...
938 // Delete the unconditional branch from the predecessor...
939 OnlyPred->getInstList().pop_back();
941 // Move all definitions in the successor to the predecessor...
942 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
944 // Make all PHI nodes that referred to BB now refer to Pred as their
946 BB->replaceAllUsesWith(OnlyPred);
948 std::string OldName = BB->getName();
950 // Erase basic block from the function...
951 M->getBasicBlockList().erase(BB);
953 // Inherit predecessors name if it exists...
954 if (!OldName.empty() && !OnlyPred->hasName())
955 OnlyPred->setName(OldName);
960 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
961 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
962 // Change br (X == 0 | X == 1), T, F into a switch instruction.
963 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
964 Instruction *Cond = cast<Instruction>(BI->getCondition());
965 // If this is a bunch of seteq's or'd together, or if it's a bunch of
966 // 'setne's and'ed together, collect them.
968 std::vector<ConstantInt*> Values;
969 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
970 if (CompVal && CompVal->getType()->isInteger()) {
971 // There might be duplicate constants in the list, which the switch
972 // instruction can't handle, remove them now.
973 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
974 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
976 // Figure out which block is which destination.
977 BasicBlock *DefaultBB = BI->getSuccessor(1);
978 BasicBlock *EdgeBB = BI->getSuccessor(0);
979 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
981 // Create the new switch instruction now.
982 SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
984 // Add all of the 'cases' to the switch instruction.
985 for (unsigned i = 0, e = Values.size(); i != e; ++i)
986 New->addCase(Values[i], EdgeBB);
988 // We added edges from PI to the EdgeBB. As such, if there were any
989 // PHI nodes in EdgeBB, they need entries to be added corresponding to
990 // the number of edges added.
991 for (BasicBlock::iterator BBI = EdgeBB->begin();
992 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
993 Value *InVal = PN->getIncomingValueForBlock(*PI);
994 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
995 PN->addIncoming(InVal, *PI);
998 // Erase the old branch instruction.
999 (*PI)->getInstList().erase(BI);
1001 // Erase the potentially condition tree that was used to computed the
1002 // branch condition.
1003 ErasePossiblyDeadInstructionTree(Cond);
1008 // If there is a trivial two-entry PHI node in this basic block, and we can
1009 // eliminate it, do so now.
1010 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1011 if (PN->getNumIncomingValues() == 2) {
1012 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1013 // statement", which has a very simple dominance structure. Basically, we
1014 // are trying to find the condition that is being branched on, which
1015 // subsequently causes this merge to happen. We really want control
1016 // dependence information for this check, but simplifycfg can't keep it up
1017 // to date, and this catches most of the cases we care about anyway.
1019 BasicBlock *IfTrue, *IfFalse;
1020 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
1021 DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
1022 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1024 // Figure out where to insert instructions as necessary.
1025 BasicBlock::iterator AfterPHIIt = BB->begin();
1026 while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
1028 BasicBlock::iterator I = BB->begin();
1029 while (PHINode *PN = dyn_cast<PHINode>(I)) {
1032 // If we can eliminate this PHI by directly computing it based on the
1033 // condition, do so now. We can't eliminate PHI nodes where the
1034 // incoming values are defined in the conditional parts of the branch,
1035 // so check for this.
1037 if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
1038 DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
1040 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1042 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1044 // If one of the incoming values is defined in the conditional
1045 // region, move it into it's predecessor block, which we know is
1047 if (!DominatesMergePoint(TrueVal, BB, false)) {
1048 Instruction *TrueI = cast<Instruction>(TrueVal);
1049 BasicBlock *OldBB = TrueI->getParent();
1050 OldBB->getInstList().remove(TrueI);
1051 BasicBlock *NewBB = *pred_begin(OldBB);
1052 NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
1054 if (!DominatesMergePoint(FalseVal, BB, false)) {
1055 Instruction *FalseI = cast<Instruction>(FalseVal);
1056 BasicBlock *OldBB = FalseI->getParent();
1057 OldBB->getInstList().remove(FalseI);
1058 BasicBlock *NewBB = *pred_begin(OldBB);
1059 NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
1062 // Change the PHI node into a select instruction.
1063 BasicBlock::iterator InsertPos = PN;
1064 while (isa<PHINode>(InsertPos)) ++InsertPos;
1066 std::string Name = PN->getName(); PN->setName("");
1067 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1069 BB->getInstList().erase(PN);