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 #include "llvm/Transforms/Utils/Local.h"
15 #include "llvm/Constants.h"
16 #include "llvm/Instructions.h"
17 #include "llvm/Type.h"
18 #include "llvm/Support/CFG.h"
24 // PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
25 // predecessors from "BB". This is a little tricky because "Succ" has PHI
26 // nodes, which need to have extra slots added to them to hold the merge edges
27 // from BB's predecessors, and BB itself might have had PHI nodes in it. This
28 // function returns true (failure) if the Succ BB already has a predecessor that
29 // is a predecessor of BB and incoming PHI arguments would not be discernible.
31 // Assumption: Succ is the single successor for BB.
33 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
34 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
36 if (!isa<PHINode>(Succ->front()))
37 return false; // We can make the transformation, no problem.
39 // If there is more than one predecessor, and there are PHI nodes in
40 // the successor, then we need to add incoming edges for the PHI nodes
42 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
44 // Check to see if one of the predecessors of BB is already a predecessor of
45 // Succ. If so, we cannot do the transformation if there are any PHI nodes
46 // with incompatible values coming in from the two edges!
48 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
49 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
50 // Loop over all of the PHI nodes checking to see if there are
51 // incompatible values coming in.
52 for (BasicBlock::iterator I = Succ->begin();
53 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
54 // Loop up the entries in the PHI node for BB and for *PI if the values
55 // coming in are non-equal, we cannot merge these two blocks (instead we
56 // should insert a conditional move or something, then merge the
58 int Idx1 = PN->getBasicBlockIndex(BB);
59 int Idx2 = PN->getBasicBlockIndex(*PI);
60 assert(Idx1 != -1 && Idx2 != -1 &&
61 "Didn't have entries for my predecessors??");
62 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
63 return true; // Values are not equal...
67 // Loop over all of the PHI nodes in the successor BB
68 for (BasicBlock::iterator I = Succ->begin();
69 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
70 Value *OldVal = PN->removeIncomingValue(BB, false);
71 assert(OldVal && "No entry in PHI for Pred BB!");
73 // If this incoming value is one of the PHI nodes in BB...
74 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
75 PHINode *OldValPN = cast<PHINode>(OldVal);
76 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
77 End = BBPreds.end(); PredI != End; ++PredI) {
78 PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
81 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
82 End = BBPreds.end(); PredI != End; ++PredI) {
83 // Add an incoming value for each of the new incoming values...
84 PN->addIncoming(OldVal, *PredI);
91 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
92 /// presumably PHI nodes in it), check to see if the merge at this block is due
93 /// to an "if condition". If so, return the boolean condition that determines
94 /// which entry into BB will be taken. Also, return by references the block
95 /// that will be entered from if the condition is true, and the block that will
96 /// be entered if the condition is false.
99 static Value *GetIfCondition(BasicBlock *BB,
100 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
101 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
102 "Function can only handle blocks with 2 predecessors!");
103 BasicBlock *Pred1 = *pred_begin(BB);
104 BasicBlock *Pred2 = *++pred_begin(BB);
106 // We can only handle branches. Other control flow will be lowered to
107 // branches if possible anyway.
108 if (!isa<BranchInst>(Pred1->getTerminator()) ||
109 !isa<BranchInst>(Pred2->getTerminator()))
111 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
112 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
114 // Eliminate code duplication by ensuring that Pred1Br is conditional if
116 if (Pred2Br->isConditional()) {
117 // If both branches are conditional, we don't have an "if statement". In
118 // reality, we could transform this case, but since the condition will be
119 // required anyway, we stand no chance of eliminating it, so the xform is
120 // probably not profitable.
121 if (Pred1Br->isConditional())
124 std::swap(Pred1, Pred2);
125 std::swap(Pred1Br, Pred2Br);
128 if (Pred1Br->isConditional()) {
129 // If we found a conditional branch predecessor, make sure that it branches
130 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
131 if (Pred1Br->getSuccessor(0) == BB &&
132 Pred1Br->getSuccessor(1) == Pred2) {
135 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
136 Pred1Br->getSuccessor(1) == BB) {
140 // We know that one arm of the conditional goes to BB, so the other must
141 // go somewhere unrelated, and this must not be an "if statement".
145 // The only thing we have to watch out for here is to make sure that Pred2
146 // doesn't have incoming edges from other blocks. If it does, the condition
147 // doesn't dominate BB.
148 if (++pred_begin(Pred2) != pred_end(Pred2))
151 return Pred1Br->getCondition();
154 // Ok, if we got here, both predecessors end with an unconditional branch to
155 // BB. Don't panic! If both blocks only have a single (identical)
156 // predecessor, and THAT is a conditional branch, then we're all ok!
157 if (pred_begin(Pred1) == pred_end(Pred1) ||
158 ++pred_begin(Pred1) != pred_end(Pred1) ||
159 pred_begin(Pred2) == pred_end(Pred2) ||
160 ++pred_begin(Pred2) != pred_end(Pred2) ||
161 *pred_begin(Pred1) != *pred_begin(Pred2))
164 // Otherwise, if this is a conditional branch, then we can use it!
165 BasicBlock *CommonPred = *pred_begin(Pred1);
166 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
167 assert(BI->isConditional() && "Two successors but not conditional?");
168 if (BI->getSuccessor(0) == Pred1) {
175 return BI->getCondition();
181 // If we have a merge point of an "if condition" as accepted above, return true
182 // if the specified value dominates the block. We don't handle the true
183 // generality of domination here, just a special case which works well enough
185 static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
186 Instruction *I = dyn_cast<Instruction>(V);
187 if (!I) return true; // Non-instructions all dominate instructions.
188 BasicBlock *PBB = I->getParent();
190 // We don't want to allow wierd loops that might have the "if condition" in
191 // the bottom of this block.
192 if (PBB == BB) return false;
194 // If this instruction is defined in a block that contains an unconditional
195 // branch to BB, then it must be in the 'conditional' part of the "if
197 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
198 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
199 if (!AllowAggressive) return false;
200 // Okay, it looks like the instruction IS in the "condition". Check to
201 // see if its a cheap instruction to unconditionally compute, and if it
202 // only uses stuff defined outside of the condition. If so, hoist it out.
203 switch (I->getOpcode()) {
204 default: return false; // Cannot hoist this out safely.
205 case Instruction::Load:
206 // We can hoist loads that are non-volatile and obviously cannot trap.
207 if (cast<LoadInst>(I)->isVolatile())
209 if (!isa<AllocaInst>(I->getOperand(0)) &&
210 !isa<Constant>(I->getOperand(0)) &&
211 !isa<GlobalValue>(I->getOperand(0)))
214 // Finally, we have to check to make sure there are no instructions
215 // before the load in its basic block, as we are going to hoist the loop
216 // out to its predecessor.
217 if (PBB->begin() != BasicBlock::iterator(I))
220 case Instruction::Add:
221 case Instruction::Sub:
222 case Instruction::And:
223 case Instruction::Or:
224 case Instruction::Xor:
225 case Instruction::Shl:
226 case Instruction::Shr:
227 break; // These are all cheap and non-trapping instructions.
230 // Okay, we can only really hoist these out if their operands are not
231 // defined in the conditional region.
232 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
233 if (!DominatesMergePoint(I->getOperand(i), BB, false))
235 // Okay, it's safe to do this!
241 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
242 // instructions that compare a value against a constant, return the value being
243 // compared, and stick the constant into the Values vector.
244 static Value *GatherConstantSetEQs(Value *V, std::vector<Constant*> &Values) {
245 if (Instruction *Inst = dyn_cast<Instruction>(V))
246 if (Inst->getOpcode() == Instruction::SetEQ) {
247 if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
249 return Inst->getOperand(0);
250 } else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
252 return Inst->getOperand(1);
254 } else if (Inst->getOpcode() == Instruction::Or) {
255 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
256 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
263 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
264 // setne instructions that compare a value against a constant, return the value
265 // being compared, and stick the constant into the Values vector.
266 static Value *GatherConstantSetNEs(Value *V, std::vector<Constant*> &Values) {
267 if (Instruction *Inst = dyn_cast<Instruction>(V))
268 if (Inst->getOpcode() == Instruction::SetNE) {
269 if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
271 return Inst->getOperand(0);
272 } else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
274 return Inst->getOperand(1);
276 } else if (Inst->getOpcode() == Instruction::Cast) {
277 // Cast of X to bool is really a comparison against zero.
278 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
279 Values.push_back(Constant::getNullValue(Inst->getOperand(0)->getType()));
280 return Inst->getOperand(0);
281 } else if (Inst->getOpcode() == Instruction::And) {
282 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
283 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
292 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
293 /// bunch of comparisons of one value against constants, return the value and
294 /// the constants being compared.
295 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
296 std::vector<Constant*> &Values) {
297 if (Cond->getOpcode() == Instruction::Or) {
298 CompVal = GatherConstantSetEQs(Cond, Values);
300 // Return true to indicate that the condition is true if the CompVal is
301 // equal to one of the constants.
303 } else if (Cond->getOpcode() == Instruction::And) {
304 CompVal = GatherConstantSetNEs(Cond, Values);
306 // Return false to indicate that the condition is false if the CompVal is
307 // equal to one of the constants.
313 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
314 /// has no side effects, nuke it. If it uses any instructions that become dead
315 /// because the instruction is now gone, nuke them too.
316 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
317 if (isInstructionTriviallyDead(I)) {
318 std::vector<Value*> Operands(I->op_begin(), I->op_end());
319 I->getParent()->getInstList().erase(I);
320 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
321 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
322 ErasePossiblyDeadInstructionTree(OpI);
326 /// SafeToMergeTerminators - Return true if it is safe to merge these two
327 /// terminator instructions together.
329 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
330 if (SI1 == SI2) return false; // Can't merge with self!
332 // It is not safe to merge these two switch instructions if they have a common
333 // successor, and if that successor has a PHI node, and if that PHI node has
334 // conflicting incoming values from the two switch blocks.
335 BasicBlock *SI1BB = SI1->getParent();
336 BasicBlock *SI2BB = SI2->getParent();
337 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
339 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
340 if (SI1Succs.count(*I))
341 for (BasicBlock::iterator BBI = (*I)->begin();
342 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
343 if (PN->getIncomingValueForBlock(SI1BB) !=
344 PN->getIncomingValueForBlock(SI2BB))
350 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
351 /// now be entries in it from the 'NewPred' block. The values that will be
352 /// flowing into the PHI nodes will be the same as those coming in from
353 /// ExistPred, and existing predecessor of Succ.
354 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
355 BasicBlock *ExistPred) {
356 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
357 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
358 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
360 for (BasicBlock::iterator I = Succ->begin();
361 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
362 Value *V = PN->getIncomingValueForBlock(ExistPred);
363 PN->addIncoming(V, NewPred);
367 // isValueEqualityComparison - Return true if the specified terminator checks to
368 // see if a value is equal to constant integer value.
369 static Value *isValueEqualityComparison(TerminatorInst *TI) {
370 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
371 // Do not permit merging of large switch instructions into their
372 // predecessors unless there is only one predecessor.
373 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
374 pred_end(SI->getParent())) > 128)
377 return SI->getCondition();
379 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
380 if (BI->isConditional() && BI->getCondition()->hasOneUse())
381 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
382 if ((SCI->getOpcode() == Instruction::SetEQ ||
383 SCI->getOpcode() == Instruction::SetNE) &&
384 isa<ConstantInt>(SCI->getOperand(1)))
385 return SCI->getOperand(0);
389 // Given a value comparison instruction, decode all of the 'cases' that it
390 // represents and return the 'default' block.
392 GetValueEqualityComparisonCases(TerminatorInst *TI,
393 std::vector<std::pair<ConstantInt*,
394 BasicBlock*> > &Cases) {
395 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
396 Cases.reserve(SI->getNumCases());
397 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
398 Cases.push_back(std::make_pair(cast<ConstantInt>(SI->getCaseValue(i)),
399 SI->getSuccessor(i)));
400 return SI->getDefaultDest();
403 BranchInst *BI = cast<BranchInst>(TI);
404 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
405 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
406 BI->getSuccessor(SCI->getOpcode() ==
407 Instruction::SetNE)));
408 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
412 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
413 // equality comparison instruction (either a switch or a branch on "X == c").
414 // See if any of the predecessors of the terminator block are value comparisons
415 // on the same value. If so, and if safe to do so, fold them together.
416 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
417 BasicBlock *BB = TI->getParent();
418 Value *CV = isValueEqualityComparison(TI); // CondVal
419 assert(CV && "Not a comparison?");
420 bool Changed = false;
422 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
423 while (!Preds.empty()) {
424 BasicBlock *Pred = Preds.back();
427 // See if the predecessor is a comparison with the same value.
428 TerminatorInst *PTI = Pred->getTerminator();
429 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
431 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
432 // Figure out which 'cases' to copy from SI to PSI.
433 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
434 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
436 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
437 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
439 // Based on whether the default edge from PTI goes to BB or not, fill in
440 // PredCases and PredDefault with the new switch cases we would like to
442 std::vector<BasicBlock*> NewSuccessors;
444 if (PredDefault == BB) {
445 // If this is the default destination from PTI, only the edges in TI
446 // that don't occur in PTI, or that branch to BB will be activated.
447 std::set<ConstantInt*> PTIHandled;
448 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
449 if (PredCases[i].second != BB)
450 PTIHandled.insert(PredCases[i].first);
452 // The default destination is BB, we don't need explicit targets.
453 std::swap(PredCases[i], PredCases.back());
454 PredCases.pop_back();
458 // Reconstruct the new switch statement we will be building.
459 if (PredDefault != BBDefault) {
460 PredDefault->removePredecessor(Pred);
461 PredDefault = BBDefault;
462 NewSuccessors.push_back(BBDefault);
464 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
465 if (!PTIHandled.count(BBCases[i].first) &&
466 BBCases[i].second != BBDefault) {
467 PredCases.push_back(BBCases[i]);
468 NewSuccessors.push_back(BBCases[i].second);
472 // If this is not the default destination from PSI, only the edges
473 // in SI that occur in PSI with a destination of BB will be
475 std::set<ConstantInt*> PTIHandled;
476 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
477 if (PredCases[i].second == BB) {
478 PTIHandled.insert(PredCases[i].first);
479 std::swap(PredCases[i], PredCases.back());
480 PredCases.pop_back();
484 // Okay, now we know which constants were sent to BB from the
485 // predecessor. Figure out where they will all go now.
486 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
487 if (PTIHandled.count(BBCases[i].first)) {
488 // If this is one we are capable of getting...
489 PredCases.push_back(BBCases[i]);
490 NewSuccessors.push_back(BBCases[i].second);
491 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
494 // If there are any constants vectored to BB that TI doesn't handle,
495 // they must go to the default destination of TI.
496 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
497 E = PTIHandled.end(); I != E; ++I) {
498 PredCases.push_back(std::make_pair(*I, BBDefault));
499 NewSuccessors.push_back(BBDefault);
503 // Okay, at this point, we know which new successor Pred will get. Make
504 // sure we update the number of entries in the PHI nodes for these
506 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
507 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
509 // Now that the successors are updated, create the new Switch instruction.
510 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PTI);
511 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
512 NewSI->addCase(PredCases[i].first, PredCases[i].second);
513 Pred->getInstList().erase(PTI);
515 // Okay, last check. If BB is still a successor of PSI, then we must
516 // have an infinite loop case. If so, add an infinitely looping block
517 // to handle the case to preserve the behavior of the code.
518 BasicBlock *InfLoopBlock = 0;
519 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
520 if (NewSI->getSuccessor(i) == BB) {
521 if (InfLoopBlock == 0) {
522 // Insert it at the end of the loop, because it's either code,
523 // or it won't matter if it's hot. :)
524 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
525 new BranchInst(InfLoopBlock, InfLoopBlock);
527 NewSI->setSuccessor(i, InfLoopBlock);
537 // SimplifyCFG - This function is used to do simplification of a CFG. For
538 // example, it adjusts branches to branches to eliminate the extra hop, it
539 // eliminates unreachable basic blocks, and does other "peephole" optimization
540 // of the CFG. It returns true if a modification was made.
542 // WARNING: The entry node of a function may not be simplified.
544 bool llvm::SimplifyCFG(BasicBlock *BB) {
545 bool Changed = false;
546 Function *M = BB->getParent();
548 assert(BB && BB->getParent() && "Block not embedded in function!");
549 assert(BB->getTerminator() && "Degenerate basic block encountered!");
550 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
552 // Remove basic blocks that have no predecessors... which are unreachable.
553 if (pred_begin(BB) == pred_end(BB) ||
554 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
555 //cerr << "Removing BB: \n" << BB;
557 // Loop through all of our successors and make sure they know that one
558 // of their predecessors is going away.
559 for_each(succ_begin(BB), succ_end(BB),
560 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
562 while (!BB->empty()) {
563 Instruction &I = BB->back();
564 // If this instruction is used, replace uses with an arbitrary
565 // constant value. Because control flow can't get here, we don't care
566 // what we replace the value with. Note that since this block is
567 // unreachable, and all values contained within it must dominate their
568 // uses, that all uses will eventually be removed.
570 // Make all users of this instruction reference the constant instead
571 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
573 // Remove the instruction from the basic block
574 BB->getInstList().pop_back();
576 M->getBasicBlockList().erase(BB);
580 // Check to see if we can constant propagate this terminator instruction
582 Changed |= ConstantFoldTerminator(BB);
584 // Check to see if this block has no non-phi instructions and only a single
585 // successor. If so, replace references to this basic block with references
587 succ_iterator SI(succ_begin(BB));
588 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
590 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
591 while (isa<PHINode>(*BBI)) ++BBI;
593 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
594 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
596 if (Succ != BB) { // Arg, don't hurt infinite loops!
597 // If our successor has PHI nodes, then we need to update them to
598 // include entries for BB's predecessors, not for BB itself.
599 // Be careful though, if this transformation fails (returns true) then
600 // we cannot do this transformation!
602 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
603 //cerr << "Killing Trivial BB: \n" << BB;
604 std::string OldName = BB->getName();
606 std::vector<BasicBlock*>
607 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
609 // Move all PHI nodes in BB to Succ if they are alive, otherwise
611 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
613 BB->getInstList().erase(BB->begin()); // Nuke instruction...
615 // The instruction is alive, so this means that Succ must have
616 // *ONLY* had BB as a predecessor, and the PHI node is still valid
617 // now. Simply move it into Succ, because we know that BB
618 // strictly dominated Succ.
619 BB->getInstList().remove(BB->begin());
620 Succ->getInstList().push_front(PN);
622 // We need to add new entries for the PHI node to account for
623 // predecessors of Succ that the PHI node does not take into
624 // account. At this point, since we know that BB dominated succ,
625 // this means that we should any newly added incoming edges should
626 // use the PHI node as the value for these edges, because they are
629 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
630 if (OldSuccPreds[i] != BB)
631 PN->addIncoming(PN, OldSuccPreds[i]);
634 // Everything that jumped to BB now goes to Succ...
635 BB->replaceAllUsesWith(Succ);
637 // Delete the old basic block...
638 M->getBasicBlockList().erase(BB);
640 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
641 Succ->setName(OldName);
643 //cerr << "Function after removal: \n" << M;
650 // If this is a returning block with only PHI nodes in it, fold the return
651 // instruction into any unconditional branch predecessors.
653 // If any predecessor is a conditional branch that just selects among
654 // different return values, fold the replace the branch/return with a select
656 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
657 BasicBlock::iterator BBI = BB->getTerminator();
658 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
659 // Find predecessors that end with branches.
660 std::vector<BasicBlock*> UncondBranchPreds;
661 std::vector<BranchInst*> CondBranchPreds;
662 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
663 TerminatorInst *PTI = (*PI)->getTerminator();
664 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
665 if (BI->isUnconditional())
666 UncondBranchPreds.push_back(*PI);
668 CondBranchPreds.push_back(BI);
671 // If we found some, do the transformation!
672 if (!UncondBranchPreds.empty()) {
673 while (!UncondBranchPreds.empty()) {
674 BasicBlock *Pred = UncondBranchPreds.back();
675 UncondBranchPreds.pop_back();
676 Instruction *UncondBranch = Pred->getTerminator();
677 // Clone the return and add it to the end of the predecessor.
678 Instruction *NewRet = RI->clone();
679 Pred->getInstList().push_back(NewRet);
681 // If the return instruction returns a value, and if the value was a
682 // PHI node in "BB", propagate the right value into the return.
683 if (NewRet->getNumOperands() == 1)
684 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
685 if (PN->getParent() == BB)
686 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
687 // Update any PHI nodes in the returning block to realize that we no
688 // longer branch to them.
689 BB->removePredecessor(Pred);
690 Pred->getInstList().erase(UncondBranch);
693 // If we eliminated all predecessors of the block, delete the block now.
694 if (pred_begin(BB) == pred_end(BB))
695 // We know there are no successors, so just nuke the block.
696 M->getBasicBlockList().erase(BB);
701 // Check out all of the conditional branches going to this return
702 // instruction. If any of them just select between returns, change the
703 // branch itself into a select/return pair.
704 while (!CondBranchPreds.empty()) {
705 BranchInst *BI = CondBranchPreds.back();
706 CondBranchPreds.pop_back();
707 BasicBlock *TrueSucc = BI->getSuccessor(0);
708 BasicBlock *FalseSucc = BI->getSuccessor(1);
709 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
711 // Check to see if the non-BB successor is also a return block.
712 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
713 // Check to see if there are only PHI instructions in this block.
714 BasicBlock::iterator OSI = OtherSucc->getTerminator();
715 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
716 // Okay, we found a branch that is going to two return nodes. If
717 // there is no return value for this function, just change the
718 // branch into a return.
719 if (RI->getNumOperands() == 0) {
720 TrueSucc->removePredecessor(BI->getParent());
721 FalseSucc->removePredecessor(BI->getParent());
722 new ReturnInst(0, BI);
723 BI->getParent()->getInstList().erase(BI);
727 // Otherwise, figure out what the true and false return values are
728 // so we can insert a new select instruction.
729 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
730 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
732 // Unwrap any PHI nodes in the return blocks.
733 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
734 if (TVPN->getParent() == TrueSucc)
735 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
736 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
737 if (FVPN->getParent() == FalseSucc)
738 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
740 TrueSucc->removePredecessor(BI->getParent());
741 FalseSucc->removePredecessor(BI->getParent());
743 // Insert a new select instruction.
744 Value *NewRetVal = new SelectInst(BI->getCondition(), TrueValue,
745 FalseValue, "retval", BI);
746 new ReturnInst(NewRetVal, BI);
747 BI->getParent()->getInstList().erase(BI);
753 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
754 // Check to see if the first instruction in this block is just an unwind.
755 // If so, replace any invoke instructions which use this as an exception
756 // destination with call instructions.
758 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
759 while (!Preds.empty()) {
760 BasicBlock *Pred = Preds.back();
761 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
762 if (II->getUnwindDest() == BB) {
763 // Insert a new branch instruction before the invoke, because this
764 // is now a fall through...
765 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
766 Pred->getInstList().remove(II); // Take out of symbol table
768 // Insert the call now...
769 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
770 CallInst *CI = new CallInst(II->getCalledValue(), Args,
772 // If the invoke produced a value, the Call now does instead
773 II->replaceAllUsesWith(CI);
781 // If this block is now dead, remove it.
782 if (pred_begin(BB) == pred_end(BB)) {
783 // We know there are no successors, so just nuke the block.
784 M->getBasicBlockList().erase(BB);
788 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
789 if (isValueEqualityComparison(SI))
790 if (FoldValueComparisonIntoPredecessors(SI))
791 return SimplifyCFG(BB) || 1;
792 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
793 if (BI->isConditional()) {
794 if (Value *CompVal = isValueEqualityComparison(BI)) {
795 // This block must be empty, except for the setcond inst, if it exists.
796 BasicBlock::iterator I = BB->begin();
798 (&*I == cast<Instruction>(BI->getCondition()) &&
800 if (FoldValueComparisonIntoPredecessors(BI))
801 return SimplifyCFG(BB) | true;
804 // If this basic block is ONLY a setcc and a branch, and if a predecessor
805 // branches to us and one of our successors, fold the setcc into the
806 // predecessor and use logical operations to pick the right destination.
807 if (Instruction *Cond = dyn_cast<Instruction>(BI->getCondition()))
808 if (Cond->getParent() == BB && &BB->front() == Cond &&
809 Cond->getNext() == BI && Cond->hasOneUse()) {
810 BasicBlock *TrueDest = BI->getSuccessor(0);
811 BasicBlock *FalseDest = BI->getSuccessor(1);
813 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
814 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
815 if (PBI->isConditional()) {
816 if (PBI->getSuccessor(0) == FalseDest ||
817 PBI->getSuccessor(1) == TrueDest) {
818 // Invert the predecessors condition test (xor it with true),
819 // which allows us to write this code once.
821 BinaryOperator::createNot(PBI->getCondition(),
822 PBI->getCondition()->getName()+".not", PBI);
823 PBI->setCondition(NewCond);
824 BasicBlock *OldTrue = PBI->getSuccessor(0);
825 BasicBlock *OldFalse = PBI->getSuccessor(1);
826 PBI->setSuccessor(0, OldFalse);
827 PBI->setSuccessor(1, OldTrue);
830 if (PBI->getSuccessor(0) == TrueDest ||
831 PBI->getSuccessor(1) == FalseDest) {
832 // Clone Cond into the predecessor basic block, and and the
833 // two conditions together.
834 Instruction *New = Cond->clone();
835 New->setName(Cond->getName());
836 Cond->setName(Cond->getName()+".old");
837 (*PI)->getInstList().insert(PBI, New);
838 Instruction::BinaryOps Opcode =
839 PBI->getSuccessor(0) == TrueDest ?
840 Instruction::Or : Instruction::And;
842 BinaryOperator::create(Opcode, PBI->getCondition(),
843 New, "bothcond", PBI);
844 PBI->setCondition(NewCond);
845 if (PBI->getSuccessor(0) == BB) {
846 AddPredecessorToBlock(TrueDest, *PI, BB);
847 PBI->setSuccessor(0, TrueDest);
849 if (PBI->getSuccessor(1) == BB) {
850 AddPredecessorToBlock(FalseDest, *PI, BB);
851 PBI->setSuccessor(1, FalseDest);
853 return SimplifyCFG(BB) | 1;
858 // If this block ends with a branch instruction, and if there is one
859 // predecessor, see if the previous block ended with a branch on the same
860 // condition, which makes this conditional branch redundant.
861 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
862 BasicBlock *OnlyPred = *PI++;
863 for (; PI != PE; ++PI)// Search all predecessors, see if they are all same
864 if (*PI != OnlyPred) {
865 OnlyPred = 0; // There are multiple different predecessors...
870 if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
871 if (PBI->isConditional() &&
872 PBI->getCondition() == BI->getCondition() &&
873 (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) {
874 // Okay, the outcome of this conditional branch is statically
875 // knowable. Delete the outgoing CFG edge that is impossible to
877 bool CondIsTrue = PBI->getSuccessor(0) == BB;
878 BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
879 new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
880 BB->getInstList().erase(BI);
881 return SimplifyCFG(BB) | true;
886 // Merge basic blocks into their predecessor if there is only one distinct
887 // pred, and if there is only one distinct successor of the predecessor, and
888 // if there are no PHI nodes.
890 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
891 BasicBlock *OnlyPred = *PI++;
892 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
893 if (*PI != OnlyPred) {
894 OnlyPred = 0; // There are multiple different predecessors...
898 BasicBlock *OnlySucc = 0;
899 if (OnlyPred && OnlyPred != BB && // Don't break self loops
900 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
901 // Check to see if there is only one distinct successor...
902 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
904 for (; SI != SE; ++SI)
905 if (*SI != OnlySucc) {
906 OnlySucc = 0; // There are multiple distinct successors!
912 //cerr << "Merging: " << BB << "into: " << OnlyPred;
913 TerminatorInst *Term = OnlyPred->getTerminator();
915 // Resolve any PHI nodes at the start of the block. They are all
916 // guaranteed to have exactly one entry if they exist, unless there are
917 // multiple duplicate (but guaranteed to be equal) entries for the
918 // incoming edges. This occurs when there are multiple edges from
919 // OnlyPred to OnlySucc.
921 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
922 PN->replaceAllUsesWith(PN->getIncomingValue(0));
923 BB->getInstList().pop_front(); // Delete the phi node...
926 // Delete the unconditional branch from the predecessor...
927 OnlyPred->getInstList().pop_back();
929 // Move all definitions in the successor to the predecessor...
930 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
932 // Make all PHI nodes that referred to BB now refer to Pred as their
934 BB->replaceAllUsesWith(OnlyPred);
936 std::string OldName = BB->getName();
938 // Erase basic block from the function...
939 M->getBasicBlockList().erase(BB);
941 // Inherit predecessors name if it exists...
942 if (!OldName.empty() && !OnlyPred->hasName())
943 OnlyPred->setName(OldName);
948 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
949 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
950 // Change br (X == 0 | X == 1), T, F into a switch instruction.
951 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
952 Instruction *Cond = cast<Instruction>(BI->getCondition());
953 // If this is a bunch of seteq's or'd together, or if it's a bunch of
954 // 'setne's and'ed together, collect them.
956 std::vector<Constant*> Values;
957 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
958 if (CompVal && CompVal->getType()->isInteger()) {
959 // There might be duplicate constants in the list, which the switch
960 // instruction can't handle, remove them now.
961 std::sort(Values.begin(), Values.end());
962 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
964 // Figure out which block is which destination.
965 BasicBlock *DefaultBB = BI->getSuccessor(1);
966 BasicBlock *EdgeBB = BI->getSuccessor(0);
967 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
969 // Create the new switch instruction now.
970 SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
972 // Add all of the 'cases' to the switch instruction.
973 for (unsigned i = 0, e = Values.size(); i != e; ++i)
974 New->addCase(Values[i], EdgeBB);
976 // We added edges from PI to the EdgeBB. As such, if there were any
977 // PHI nodes in EdgeBB, they need entries to be added corresponding to
978 // the number of edges added.
979 for (BasicBlock::iterator BBI = EdgeBB->begin();
980 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
981 Value *InVal = PN->getIncomingValueForBlock(*PI);
982 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
983 PN->addIncoming(InVal, *PI);
986 // Erase the old branch instruction.
987 (*PI)->getInstList().erase(BI);
989 // Erase the potentially condition tree that was used to computed the
991 ErasePossiblyDeadInstructionTree(Cond);
996 // If there is a trivial two-entry PHI node in this basic block, and we can
997 // eliminate it, do so now.
998 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
999 if (PN->getNumIncomingValues() == 2) {
1000 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1001 // statement", which has a very simple dominance structure. Basically, we
1002 // are trying to find the condition that is being branched on, which
1003 // subsequently causes this merge to happen. We really want control
1004 // dependence information for this check, but simplifycfg can't keep it up
1005 // to date, and this catches most of the cases we care about anyway.
1007 BasicBlock *IfTrue, *IfFalse;
1008 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
1009 //std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
1010 // << IfTrue->getName() << " F: " << IfFalse->getName() << "\n";
1012 // Figure out where to insert instructions as necessary.
1013 BasicBlock::iterator AfterPHIIt = BB->begin();
1014 while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
1016 BasicBlock::iterator I = BB->begin();
1017 while (PHINode *PN = dyn_cast<PHINode>(I)) {
1020 // If we can eliminate this PHI by directly computing it based on the
1021 // condition, do so now. We can't eliminate PHI nodes where the
1022 // incoming values are defined in the conditional parts of the branch,
1023 // so check for this.
1025 if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
1026 DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
1028 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1030 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1032 // If one of the incoming values is defined in the conditional
1033 // region, move it into it's predecessor block, which we know is
1035 if (!DominatesMergePoint(TrueVal, BB, false)) {
1036 Instruction *TrueI = cast<Instruction>(TrueVal);
1037 BasicBlock *OldBB = TrueI->getParent();
1038 OldBB->getInstList().remove(TrueI);
1039 BasicBlock *NewBB = *pred_begin(OldBB);
1040 NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
1042 if (!DominatesMergePoint(FalseVal, BB, false)) {
1043 Instruction *FalseI = cast<Instruction>(FalseVal);
1044 BasicBlock *OldBB = FalseI->getParent();
1045 OldBB->getInstList().remove(FalseI);
1046 BasicBlock *NewBB = *pred_begin(OldBB);
1047 NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
1050 // Change the PHI node into a select instruction.
1051 BasicBlock::iterator InsertPos = PN;
1052 while (isa<PHINode>(InsertPos)) ++InsertPos;
1054 std::string Name = PN->getName(); PN->setName("");
1055 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1057 BB->getInstList().erase(PN);