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"
21 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
28 /// SafeToMergeTerminators - Return true if it is safe to merge these two
29 /// terminator instructions together.
31 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
32 if (SI1 == SI2) return false; // Can't merge with self!
34 // It is not safe to merge these two switch instructions if they have a common
35 // successor, and if that successor has a PHI node, and if *that* PHI node has
36 // conflicting incoming values from the two switch blocks.
37 BasicBlock *SI1BB = SI1->getParent();
38 BasicBlock *SI2BB = SI2->getParent();
39 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
41 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
42 if (SI1Succs.count(*I))
43 for (BasicBlock::iterator BBI = (*I)->begin();
44 isa<PHINode>(BBI); ++BBI) {
45 PHINode *PN = cast<PHINode>(BBI);
46 if (PN->getIncomingValueForBlock(SI1BB) !=
47 PN->getIncomingValueForBlock(SI2BB))
54 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
55 /// now be entries in it from the 'NewPred' block. The values that will be
56 /// flowing into the PHI nodes will be the same as those coming in from
57 /// ExistPred, an existing predecessor of Succ.
58 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
59 BasicBlock *ExistPred) {
60 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
61 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
62 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
64 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
65 PHINode *PN = cast<PHINode>(I);
66 Value *V = PN->getIncomingValueForBlock(ExistPred);
67 PN->addIncoming(V, NewPred);
71 // CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
72 // almost-empty BB ending in an unconditional branch to Succ, into succ.
74 // Assumption: Succ is the single successor for BB.
76 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
77 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
79 // Check to see if one of the predecessors of BB is already a predecessor of
80 // Succ. If so, we cannot do the transformation if there are any PHI nodes
81 // with incompatible values coming in from the two edges!
83 if (isa<PHINode>(Succ->front())) {
84 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
85 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);\
87 if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
88 // Loop over all of the PHI nodes checking to see if there are
89 // incompatible values coming in.
90 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
91 PHINode *PN = cast<PHINode>(I);
92 // Loop up the entries in the PHI node for BB and for *PI if the
93 // values coming in are non-equal, we cannot merge these two blocks
94 // (instead we should insert a conditional move or something, then
96 if (PN->getIncomingValueForBlock(BB) !=
97 PN->getIncomingValueForBlock(*PI))
98 return false; // Values are not equal...
103 // Finally, if BB has PHI nodes that are used by things other than the PHIs in
104 // Succ and Succ has predecessors that are not Succ and not Pred, we cannot
105 // fold these blocks, as we don't know whether BB dominates Succ or not to
106 // update the PHI nodes correctly.
107 if (!isa<PHINode>(BB->begin()) || Succ->getSinglePredecessor()) return true;
109 // If the predecessors of Succ are only BB and Succ itself, we can handle this.
111 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
112 if (*PI != Succ && *PI != BB) {
116 if (IsSafe) return true;
118 // If the PHI nodes in BB are only used by instructions in Succ, we are ok.
120 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I) && IsSafe; ++I) {
121 PHINode *PN = cast<PHINode>(I);
122 for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end(); UI != E;
124 if (cast<Instruction>(*UI)->getParent() != Succ) {
133 /// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional
134 /// branch to Succ, and contains no instructions other than PHI nodes and the
135 /// branch. If possible, eliminate BB.
136 static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
138 // If our successor has PHI nodes, then we need to update them to include
139 // entries for BB's predecessors, not for BB itself. Be careful though,
140 // if this transformation fails (returns true) then we cannot do this
143 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
145 DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB);
147 if (isa<PHINode>(Succ->begin())) {
148 // If there is more than one pred of succ, and there are PHI nodes in
149 // the successor, then we need to add incoming edges for the PHI nodes
151 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
153 // Loop over all of the PHI nodes in the successor of BB.
154 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
155 PHINode *PN = cast<PHINode>(I);
156 Value *OldVal = PN->removeIncomingValue(BB, false);
157 assert(OldVal && "No entry in PHI for Pred BB!");
159 // If this incoming value is one of the PHI nodes in BB, the new entries
160 // in the PHI node are the entries from the old PHI.
161 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
162 PHINode *OldValPN = cast<PHINode>(OldVal);
163 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
164 PN->addIncoming(OldValPN->getIncomingValue(i),
165 OldValPN->getIncomingBlock(i));
167 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
168 End = BBPreds.end(); PredI != End; ++PredI) {
169 // Add an incoming value for each of the new incoming values...
170 PN->addIncoming(OldVal, *PredI);
176 if (isa<PHINode>(&BB->front())) {
177 std::vector<BasicBlock*>
178 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
180 // Move all PHI nodes in BB to Succ if they are alive, otherwise
182 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
183 if (PN->use_empty()) {
184 // Just remove the dead phi. This happens if Succ's PHIs were the only
185 // users of the PHI nodes.
186 PN->eraseFromParent();
188 // The instruction is alive, so this means that Succ must have
189 // *ONLY* had BB as a predecessor, and the PHI node is still valid
190 // now. Simply move it into Succ, because we know that BB
191 // strictly dominated Succ.
192 Succ->getInstList().splice(Succ->begin(),
193 BB->getInstList(), BB->begin());
195 // We need to add new entries for the PHI node to account for
196 // predecessors of Succ that the PHI node does not take into
197 // account. At this point, since we know that BB dominated succ,
198 // this means that we should any newly added incoming edges should
199 // use the PHI node as the value for these edges, because they are
201 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
202 if (OldSuccPreds[i] != BB)
203 PN->addIncoming(PN, OldSuccPreds[i]);
207 // Everything that jumped to BB now goes to Succ.
208 std::string OldName = BB->getName();
209 BB->replaceAllUsesWith(Succ);
210 BB->eraseFromParent(); // Delete the old basic block.
212 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
213 Succ->setName(OldName);
217 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
218 /// presumably PHI nodes in it), check to see if the merge at this block is due
219 /// to an "if condition". If so, return the boolean condition that determines
220 /// which entry into BB will be taken. Also, return by references the block
221 /// that will be entered from if the condition is true, and the block that will
222 /// be entered if the condition is false.
225 static Value *GetIfCondition(BasicBlock *BB,
226 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
227 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
228 "Function can only handle blocks with 2 predecessors!");
229 BasicBlock *Pred1 = *pred_begin(BB);
230 BasicBlock *Pred2 = *++pred_begin(BB);
232 // We can only handle branches. Other control flow will be lowered to
233 // branches if possible anyway.
234 if (!isa<BranchInst>(Pred1->getTerminator()) ||
235 !isa<BranchInst>(Pred2->getTerminator()))
237 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
238 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
240 // Eliminate code duplication by ensuring that Pred1Br is conditional if
242 if (Pred2Br->isConditional()) {
243 // If both branches are conditional, we don't have an "if statement". In
244 // reality, we could transform this case, but since the condition will be
245 // required anyway, we stand no chance of eliminating it, so the xform is
246 // probably not profitable.
247 if (Pred1Br->isConditional())
250 std::swap(Pred1, Pred2);
251 std::swap(Pred1Br, Pred2Br);
254 if (Pred1Br->isConditional()) {
255 // If we found a conditional branch predecessor, make sure that it branches
256 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
257 if (Pred1Br->getSuccessor(0) == BB &&
258 Pred1Br->getSuccessor(1) == Pred2) {
261 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
262 Pred1Br->getSuccessor(1) == BB) {
266 // We know that one arm of the conditional goes to BB, so the other must
267 // go somewhere unrelated, and this must not be an "if statement".
271 // The only thing we have to watch out for here is to make sure that Pred2
272 // doesn't have incoming edges from other blocks. If it does, the condition
273 // doesn't dominate BB.
274 if (++pred_begin(Pred2) != pred_end(Pred2))
277 return Pred1Br->getCondition();
280 // Ok, if we got here, both predecessors end with an unconditional branch to
281 // BB. Don't panic! If both blocks only have a single (identical)
282 // predecessor, and THAT is a conditional branch, then we're all ok!
283 if (pred_begin(Pred1) == pred_end(Pred1) ||
284 ++pred_begin(Pred1) != pred_end(Pred1) ||
285 pred_begin(Pred2) == pred_end(Pred2) ||
286 ++pred_begin(Pred2) != pred_end(Pred2) ||
287 *pred_begin(Pred1) != *pred_begin(Pred2))
290 // Otherwise, if this is a conditional branch, then we can use it!
291 BasicBlock *CommonPred = *pred_begin(Pred1);
292 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
293 assert(BI->isConditional() && "Two successors but not conditional?");
294 if (BI->getSuccessor(0) == Pred1) {
301 return BI->getCondition();
307 // If we have a merge point of an "if condition" as accepted above, return true
308 // if the specified value dominates the block. We don't handle the true
309 // generality of domination here, just a special case which works well enough
312 // If AggressiveInsts is non-null, and if V does not dominate BB, we check to
313 // see if V (which must be an instruction) is cheap to compute and is
314 // non-trapping. If both are true, the instruction is inserted into the set and
316 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
317 std::set<Instruction*> *AggressiveInsts) {
318 Instruction *I = dyn_cast<Instruction>(V);
319 if (!I) return true; // Non-instructions all dominate instructions.
320 BasicBlock *PBB = I->getParent();
322 // We don't want to allow weird loops that might have the "if condition" in
323 // the bottom of this block.
324 if (PBB == BB) return false;
326 // If this instruction is defined in a block that contains an unconditional
327 // branch to BB, then it must be in the 'conditional' part of the "if
329 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
330 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
331 if (!AggressiveInsts) return false;
332 // Okay, it looks like the instruction IS in the "condition". Check to
333 // see if its a cheap instruction to unconditionally compute, and if it
334 // only uses stuff defined outside of the condition. If so, hoist it out.
335 switch (I->getOpcode()) {
336 default: return false; // Cannot hoist this out safely.
337 case Instruction::Load:
338 // We can hoist loads that are non-volatile and obviously cannot trap.
339 if (cast<LoadInst>(I)->isVolatile())
341 if (!isa<AllocaInst>(I->getOperand(0)) &&
342 !isa<Constant>(I->getOperand(0)))
345 // Finally, we have to check to make sure there are no instructions
346 // before the load in its basic block, as we are going to hoist the loop
347 // out to its predecessor.
348 if (PBB->begin() != BasicBlock::iterator(I))
351 case Instruction::Add:
352 case Instruction::Sub:
353 case Instruction::And:
354 case Instruction::Or:
355 case Instruction::Xor:
356 case Instruction::Shl:
357 case Instruction::Shr:
358 case Instruction::SetEQ:
359 case Instruction::SetNE:
360 case Instruction::SetLT:
361 case Instruction::SetGT:
362 case Instruction::SetLE:
363 case Instruction::SetGE:
364 break; // These are all cheap and non-trapping instructions.
367 // Okay, we can only really hoist these out if their operands are not
368 // defined in the conditional region.
369 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
370 if (!DominatesMergePoint(I->getOperand(i), BB, 0))
372 // Okay, it's safe to do this! Remember this instruction.
373 AggressiveInsts->insert(I);
379 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
380 // instructions that compare a value against a constant, return the value being
381 // compared, and stick the constant into the Values vector.
382 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
383 if (Instruction *Inst = dyn_cast<Instruction>(V))
384 if (Inst->getOpcode() == Instruction::SetEQ) {
385 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
387 return Inst->getOperand(0);
388 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
390 return Inst->getOperand(1);
392 } else if (Inst->getOpcode() == Instruction::Or) {
393 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
394 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
401 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
402 // setne instructions that compare a value against a constant, return the value
403 // being compared, and stick the constant into the Values vector.
404 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
405 if (Instruction *Inst = dyn_cast<Instruction>(V))
406 if (Inst->getOpcode() == Instruction::SetNE) {
407 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
409 return Inst->getOperand(0);
410 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
412 return Inst->getOperand(1);
414 } else if (Inst->getOpcode() == Instruction::Cast) {
415 // Cast of X to bool is really a comparison against zero.
416 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
417 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0));
418 return Inst->getOperand(0);
419 } else if (Inst->getOpcode() == Instruction::And) {
420 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
421 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
430 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
431 /// bunch of comparisons of one value against constants, return the value and
432 /// the constants being compared.
433 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
434 std::vector<ConstantInt*> &Values) {
435 if (Cond->getOpcode() == Instruction::Or) {
436 CompVal = GatherConstantSetEQs(Cond, Values);
438 // Return true to indicate that the condition is true if the CompVal is
439 // equal to one of the constants.
441 } else if (Cond->getOpcode() == Instruction::And) {
442 CompVal = GatherConstantSetNEs(Cond, Values);
444 // Return false to indicate that the condition is false if the CompVal is
445 // equal to one of the constants.
451 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
452 /// has no side effects, nuke it. If it uses any instructions that become dead
453 /// because the instruction is now gone, nuke them too.
454 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
455 if (isInstructionTriviallyDead(I)) {
456 std::vector<Value*> Operands(I->op_begin(), I->op_end());
457 I->getParent()->getInstList().erase(I);
458 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
459 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
460 ErasePossiblyDeadInstructionTree(OpI);
464 // isValueEqualityComparison - Return true if the specified terminator checks to
465 // see if a value is equal to constant integer value.
466 static Value *isValueEqualityComparison(TerminatorInst *TI) {
467 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
468 // Do not permit merging of large switch instructions into their
469 // predecessors unless there is only one predecessor.
470 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
471 pred_end(SI->getParent())) > 128)
474 return SI->getCondition();
476 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
477 if (BI->isConditional() && BI->getCondition()->hasOneUse())
478 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
479 if ((SCI->getOpcode() == Instruction::SetEQ ||
480 SCI->getOpcode() == Instruction::SetNE) &&
481 isa<ConstantInt>(SCI->getOperand(1)))
482 return SCI->getOperand(0);
486 // Given a value comparison instruction, decode all of the 'cases' that it
487 // represents and return the 'default' block.
489 GetValueEqualityComparisonCases(TerminatorInst *TI,
490 std::vector<std::pair<ConstantInt*,
491 BasicBlock*> > &Cases) {
492 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
493 Cases.reserve(SI->getNumCases());
494 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
495 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
496 return SI->getDefaultDest();
499 BranchInst *BI = cast<BranchInst>(TI);
500 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
501 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
502 BI->getSuccessor(SCI->getOpcode() ==
503 Instruction::SetNE)));
504 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
508 // EliminateBlockCases - Given an vector of bb/value pairs, remove any entries
509 // in the list that match the specified block.
510 static void EliminateBlockCases(BasicBlock *BB,
511 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
512 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
513 if (Cases[i].second == BB) {
514 Cases.erase(Cases.begin()+i);
519 // ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
522 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
523 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
524 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
526 // Make V1 be smaller than V2.
527 if (V1->size() > V2->size())
530 if (V1->size() == 0) return false;
531 if (V1->size() == 1) {
533 ConstantInt *TheVal = (*V1)[0].first;
534 for (unsigned i = 0, e = V2->size(); i != e; ++i)
535 if (TheVal == (*V2)[i].first)
539 // Otherwise, just sort both lists and compare element by element.
540 std::sort(V1->begin(), V1->end());
541 std::sort(V2->begin(), V2->end());
542 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
543 while (i1 != e1 && i2 != e2) {
544 if ((*V1)[i1].first == (*V2)[i2].first)
546 if ((*V1)[i1].first < (*V2)[i2].first)
554 // SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
555 // terminator instruction and its block is known to only have a single
556 // predecessor block, check to see if that predecessor is also a value
557 // comparison with the same value, and if that comparison determines the outcome
558 // of this comparison. If so, simplify TI. This does a very limited form of
560 static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
562 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
563 if (!PredVal) return false; // Not a value comparison in predecessor.
565 Value *ThisVal = isValueEqualityComparison(TI);
566 assert(ThisVal && "This isn't a value comparison!!");
567 if (ThisVal != PredVal) return false; // Different predicates.
569 // Find out information about when control will move from Pred to TI's block.
570 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
571 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
573 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
575 // Find information about how control leaves this block.
576 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
577 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
578 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
580 // If TI's block is the default block from Pred's comparison, potentially
581 // simplify TI based on this knowledge.
582 if (PredDef == TI->getParent()) {
583 // If we are here, we know that the value is none of those cases listed in
584 // PredCases. If there are any cases in ThisCases that are in PredCases, we
586 if (ValuesOverlap(PredCases, ThisCases)) {
587 if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) {
588 // Okay, one of the successors of this condbr is dead. Convert it to a
590 assert(ThisCases.size() == 1 && "Branch can only have one case!");
591 Value *Cond = BTI->getCondition();
592 // Insert the new branch.
593 Instruction *NI = new BranchInst(ThisDef, TI);
595 // Remove PHI node entries for the dead edge.
596 ThisCases[0].second->removePredecessor(TI->getParent());
598 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
599 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
601 TI->eraseFromParent(); // Nuke the old one.
602 // If condition is now dead, nuke it.
603 if (Instruction *CondI = dyn_cast<Instruction>(Cond))
604 ErasePossiblyDeadInstructionTree(CondI);
608 SwitchInst *SI = cast<SwitchInst>(TI);
609 // Okay, TI has cases that are statically dead, prune them away.
610 std::set<Constant*> DeadCases;
611 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
612 DeadCases.insert(PredCases[i].first);
614 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
615 << "Through successor TI: " << *TI);
617 for (unsigned i = SI->getNumCases()-1; i != 0; --i)
618 if (DeadCases.count(SI->getCaseValue(i))) {
619 SI->getSuccessor(i)->removePredecessor(TI->getParent());
623 DEBUG(std::cerr << "Leaving: " << *TI << "\n");
629 // Otherwise, TI's block must correspond to some matched value. Find out
630 // which value (or set of values) this is.
631 ConstantInt *TIV = 0;
632 BasicBlock *TIBB = TI->getParent();
633 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
634 if (PredCases[i].second == TIBB)
636 TIV = PredCases[i].first;
638 return false; // Cannot handle multiple values coming to this block.
639 assert(TIV && "No edge from pred to succ?");
641 // Okay, we found the one constant that our value can be if we get into TI's
642 // BB. Find out which successor will unconditionally be branched to.
643 BasicBlock *TheRealDest = 0;
644 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
645 if (ThisCases[i].first == TIV) {
646 TheRealDest = ThisCases[i].second;
650 // If not handled by any explicit cases, it is handled by the default case.
651 if (TheRealDest == 0) TheRealDest = ThisDef;
653 // Remove PHI node entries for dead edges.
654 BasicBlock *CheckEdge = TheRealDest;
655 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
656 if (*SI != CheckEdge)
657 (*SI)->removePredecessor(TIBB);
661 // Insert the new branch.
662 Instruction *NI = new BranchInst(TheRealDest, TI);
664 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator()
665 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n");
666 Instruction *Cond = 0;
667 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
668 Cond = dyn_cast<Instruction>(BI->getCondition());
669 TI->eraseFromParent(); // Nuke the old one.
671 if (Cond) ErasePossiblyDeadInstructionTree(Cond);
677 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
678 // equality comparison instruction (either a switch or a branch on "X == c").
679 // See if any of the predecessors of the terminator block are value comparisons
680 // on the same value. If so, and if safe to do so, fold them together.
681 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
682 BasicBlock *BB = TI->getParent();
683 Value *CV = isValueEqualityComparison(TI); // CondVal
684 assert(CV && "Not a comparison?");
685 bool Changed = false;
687 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
688 while (!Preds.empty()) {
689 BasicBlock *Pred = Preds.back();
692 // See if the predecessor is a comparison with the same value.
693 TerminatorInst *PTI = Pred->getTerminator();
694 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
696 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
697 // Figure out which 'cases' to copy from SI to PSI.
698 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
699 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
701 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
702 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
704 // Based on whether the default edge from PTI goes to BB or not, fill in
705 // PredCases and PredDefault with the new switch cases we would like to
707 std::vector<BasicBlock*> NewSuccessors;
709 if (PredDefault == BB) {
710 // If this is the default destination from PTI, only the edges in TI
711 // that don't occur in PTI, or that branch to BB will be activated.
712 std::set<ConstantInt*> PTIHandled;
713 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
714 if (PredCases[i].second != BB)
715 PTIHandled.insert(PredCases[i].first);
717 // The default destination is BB, we don't need explicit targets.
718 std::swap(PredCases[i], PredCases.back());
719 PredCases.pop_back();
723 // Reconstruct the new switch statement we will be building.
724 if (PredDefault != BBDefault) {
725 PredDefault->removePredecessor(Pred);
726 PredDefault = BBDefault;
727 NewSuccessors.push_back(BBDefault);
729 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
730 if (!PTIHandled.count(BBCases[i].first) &&
731 BBCases[i].second != BBDefault) {
732 PredCases.push_back(BBCases[i]);
733 NewSuccessors.push_back(BBCases[i].second);
737 // If this is not the default destination from PSI, only the edges
738 // in SI that occur in PSI with a destination of BB will be
740 std::set<ConstantInt*> PTIHandled;
741 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
742 if (PredCases[i].second == BB) {
743 PTIHandled.insert(PredCases[i].first);
744 std::swap(PredCases[i], PredCases.back());
745 PredCases.pop_back();
749 // Okay, now we know which constants were sent to BB from the
750 // predecessor. Figure out where they will all go now.
751 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
752 if (PTIHandled.count(BBCases[i].first)) {
753 // If this is one we are capable of getting...
754 PredCases.push_back(BBCases[i]);
755 NewSuccessors.push_back(BBCases[i].second);
756 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
759 // If there are any constants vectored to BB that TI doesn't handle,
760 // they must go to the default destination of TI.
761 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
762 E = PTIHandled.end(); I != E; ++I) {
763 PredCases.push_back(std::make_pair(*I, BBDefault));
764 NewSuccessors.push_back(BBDefault);
768 // Okay, at this point, we know which new successor Pred will get. Make
769 // sure we update the number of entries in the PHI nodes for these
771 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
772 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
774 // Now that the successors are updated, create the new Switch instruction.
775 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI);
776 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
777 NewSI->addCase(PredCases[i].first, PredCases[i].second);
779 Instruction *DeadCond = 0;
780 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
781 // If PTI is a branch, remember the condition.
782 DeadCond = dyn_cast<Instruction>(BI->getCondition());
783 Pred->getInstList().erase(PTI);
785 // If the condition is dead now, remove the instruction tree.
786 if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond);
788 // Okay, last check. If BB is still a successor of PSI, then we must
789 // have an infinite loop case. If so, add an infinitely looping block
790 // to handle the case to preserve the behavior of the code.
791 BasicBlock *InfLoopBlock = 0;
792 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
793 if (NewSI->getSuccessor(i) == BB) {
794 if (InfLoopBlock == 0) {
795 // Insert it at the end of the loop, because it's either code,
796 // or it won't matter if it's hot. :)
797 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
798 new BranchInst(InfLoopBlock, InfLoopBlock);
800 NewSI->setSuccessor(i, InfLoopBlock);
809 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
810 /// BB2, hoist any common code in the two blocks up into the branch block. The
811 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
812 static bool HoistThenElseCodeToIf(BranchInst *BI) {
813 // This does very trivial matching, with limited scanning, to find identical
814 // instructions in the two blocks. In particular, we don't want to get into
815 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
816 // such, we currently just scan for obviously identical instructions in an
818 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
819 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
821 Instruction *I1 = BB1->begin(), *I2 = BB2->begin();
822 if (I1->getOpcode() != I2->getOpcode() || !I1->isIdenticalTo(I2) ||
826 // If we get here, we can hoist at least one instruction.
827 BasicBlock *BIParent = BI->getParent();
830 // If we are hoisting the terminator instruction, don't move one (making a
831 // broken BB), instead clone it, and remove BI.
832 if (isa<TerminatorInst>(I1))
833 goto HoistTerminator;
835 // For a normal instruction, we just move one to right before the branch,
836 // then replace all uses of the other with the first. Finally, we remove
837 // the now redundant second instruction.
838 BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
839 if (!I2->use_empty())
840 I2->replaceAllUsesWith(I1);
841 BB2->getInstList().erase(I2);
845 } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2));
850 // Okay, it is safe to hoist the terminator.
851 Instruction *NT = I1->clone();
852 BIParent->getInstList().insert(BI, NT);
853 if (NT->getType() != Type::VoidTy) {
854 I1->replaceAllUsesWith(NT);
855 I2->replaceAllUsesWith(NT);
856 NT->setName(I1->getName());
859 // Hoisting one of the terminators from our successor is a great thing.
860 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
861 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
862 // nodes, so we insert select instruction to compute the final result.
863 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
864 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
866 for (BasicBlock::iterator BBI = SI->begin();
867 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
868 Value *BB1V = PN->getIncomingValueForBlock(BB1);
869 Value *BB2V = PN->getIncomingValueForBlock(BB2);
871 // These values do not agree. Insert a select instruction before NT
872 // that determines the right value.
873 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
875 SI = new SelectInst(BI->getCondition(), BB1V, BB2V,
876 BB1V->getName()+"."+BB2V->getName(), NT);
877 // Make the PHI node use the select for all incoming values for BB1/BB2
878 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
879 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
880 PN->setIncomingValue(i, SI);
885 // Update any PHI nodes in our new successors.
886 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
887 AddPredecessorToBlock(*SI, BIParent, BB1);
889 BI->eraseFromParent();
893 /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
894 /// across this block.
895 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
896 BranchInst *BI = cast<BranchInst>(BB->getTerminator());
897 Value *Cond = BI->getCondition();
899 // If this basic block contains anything other than a PHI (which controls the
900 // branch) and branch itself, bail out. FIXME: improve this in the future.
901 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
902 if (!isa<PHINode>(BBI)) return false;
904 if (&*BBI != Cond || !Cond->hasOneUse())
907 // Looks ok, continue checking.
913 /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
914 /// that is defined in the same block as the branch and if any PHI entries are
915 /// constants, thread edges corresponding to that entry to be branches to their
916 /// ultimate destination.
917 static bool FoldCondBranchOnPHI(BranchInst *BI) {
918 BasicBlock *BB = BI->getParent();
919 PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
920 // NOTE: we currently cannot transform this case if the PHI node is used
921 // outside of the block.
922 if (!PN || PN->getParent() != BB || !PN->hasOneUse())
925 // Degenerate case of a single entry PHI.
926 if (PN->getNumIncomingValues() == 1) {
927 if (PN->getIncomingValue(0) != PN)
928 PN->replaceAllUsesWith(PN->getIncomingValue(0));
930 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
931 PN->eraseFromParent();
935 // Now we know that this block has multiple preds and two succs.
936 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
938 // Okay, this is a simple enough basic block. See if any phi values are
940 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
941 if (ConstantBool *CB = dyn_cast<ConstantBool>(PN->getIncomingValue(i))) {
942 // Okay, we now know that all edges from PredBB should be revectored to
943 // branch to RealDest.
944 BasicBlock *PredBB = PN->getIncomingBlock(i);
945 BasicBlock *RealDest = BI->getSuccessor(!CB->getValue());
947 // If there are PHI nodes in the destination block, we have to add an
948 // entry for PredBB. Instead of being smart about this, just split the
949 // critical edge, which will eliminate the PHI-ness.
950 if (isa<PHINode>(RealDest->begin())) {
951 SplitCriticalEdge(BI, !CB->getValue());
952 RealDest = BI->getSuccessor(!CB->getValue());
954 assert(!isa<PHINode>(RealDest->begin()) && "Crit edge split failure!");
956 // Loop over all of the edges from PredBB to BB, changing them to branch
957 // to RealDest instead.
958 TerminatorInst *PredBBTI = PredBB->getTerminator();
959 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
960 if (PredBBTI->getSuccessor(i) == BB) {
961 BB->removePredecessor(PredBB);
962 PredBBTI->setSuccessor(i, RealDest);
965 // Recurse, simplifying any other constants.
966 return FoldCondBranchOnPHI(BI) | true;
974 /// ConstantIntOrdering - This class implements a stable ordering of constant
975 /// integers that does not depend on their address. This is important for
976 /// applications that sort ConstantInt's to ensure uniqueness.
977 struct ConstantIntOrdering {
978 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
979 return LHS->getRawValue() < RHS->getRawValue();
984 // SimplifyCFG - This function is used to do simplification of a CFG. For
985 // example, it adjusts branches to branches to eliminate the extra hop, it
986 // eliminates unreachable basic blocks, and does other "peephole" optimization
987 // of the CFG. It returns true if a modification was made.
989 // WARNING: The entry node of a function may not be simplified.
991 bool llvm::SimplifyCFG(BasicBlock *BB) {
992 bool Changed = false;
993 Function *M = BB->getParent();
995 assert(BB && BB->getParent() && "Block not embedded in function!");
996 assert(BB->getTerminator() && "Degenerate basic block encountered!");
997 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
999 // Remove basic blocks that have no predecessors... which are unreachable.
1000 if (pred_begin(BB) == pred_end(BB) ||
1001 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
1002 DEBUG(std::cerr << "Removing BB: \n" << *BB);
1004 // Loop through all of our successors and make sure they know that one
1005 // of their predecessors is going away.
1006 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
1007 SI->removePredecessor(BB);
1009 while (!BB->empty()) {
1010 Instruction &I = BB->back();
1011 // If this instruction is used, replace uses with an arbitrary
1012 // value. Because control flow can't get here, we don't care
1013 // what we replace the value with. Note that since this block is
1014 // unreachable, and all values contained within it must dominate their
1015 // uses, that all uses will eventually be removed.
1017 // Make all users of this instruction use undef instead
1018 I.replaceAllUsesWith(UndefValue::get(I.getType()));
1020 // Remove the instruction from the basic block
1021 BB->getInstList().pop_back();
1023 M->getBasicBlockList().erase(BB);
1027 // Check to see if we can constant propagate this terminator instruction
1029 Changed |= ConstantFoldTerminator(BB);
1031 // If this is a returning block with only PHI nodes in it, fold the return
1032 // instruction into any unconditional branch predecessors.
1034 // If any predecessor is a conditional branch that just selects among
1035 // different return values, fold the replace the branch/return with a select
1037 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1038 BasicBlock::iterator BBI = BB->getTerminator();
1039 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
1040 // Find predecessors that end with branches.
1041 std::vector<BasicBlock*> UncondBranchPreds;
1042 std::vector<BranchInst*> CondBranchPreds;
1043 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1044 TerminatorInst *PTI = (*PI)->getTerminator();
1045 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
1046 if (BI->isUnconditional())
1047 UncondBranchPreds.push_back(*PI);
1049 CondBranchPreds.push_back(BI);
1052 // If we found some, do the transformation!
1053 if (!UncondBranchPreds.empty()) {
1054 while (!UncondBranchPreds.empty()) {
1055 BasicBlock *Pred = UncondBranchPreds.back();
1056 UncondBranchPreds.pop_back();
1057 Instruction *UncondBranch = Pred->getTerminator();
1058 // Clone the return and add it to the end of the predecessor.
1059 Instruction *NewRet = RI->clone();
1060 Pred->getInstList().push_back(NewRet);
1062 // If the return instruction returns a value, and if the value was a
1063 // PHI node in "BB", propagate the right value into the return.
1064 if (NewRet->getNumOperands() == 1)
1065 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
1066 if (PN->getParent() == BB)
1067 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
1068 // Update any PHI nodes in the returning block to realize that we no
1069 // longer branch to them.
1070 BB->removePredecessor(Pred);
1071 Pred->getInstList().erase(UncondBranch);
1074 // If we eliminated all predecessors of the block, delete the block now.
1075 if (pred_begin(BB) == pred_end(BB))
1076 // We know there are no successors, so just nuke the block.
1077 M->getBasicBlockList().erase(BB);
1082 // Check out all of the conditional branches going to this return
1083 // instruction. If any of them just select between returns, change the
1084 // branch itself into a select/return pair.
1085 while (!CondBranchPreds.empty()) {
1086 BranchInst *BI = CondBranchPreds.back();
1087 CondBranchPreds.pop_back();
1088 BasicBlock *TrueSucc = BI->getSuccessor(0);
1089 BasicBlock *FalseSucc = BI->getSuccessor(1);
1090 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
1092 // Check to see if the non-BB successor is also a return block.
1093 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
1094 // Check to see if there are only PHI instructions in this block.
1095 BasicBlock::iterator OSI = OtherSucc->getTerminator();
1096 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
1097 // Okay, we found a branch that is going to two return nodes. If
1098 // there is no return value for this function, just change the
1099 // branch into a return.
1100 if (RI->getNumOperands() == 0) {
1101 TrueSucc->removePredecessor(BI->getParent());
1102 FalseSucc->removePredecessor(BI->getParent());
1103 new ReturnInst(0, BI);
1104 BI->getParent()->getInstList().erase(BI);
1108 // Otherwise, figure out what the true and false return values are
1109 // so we can insert a new select instruction.
1110 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
1111 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
1113 // Unwrap any PHI nodes in the return blocks.
1114 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
1115 if (TVPN->getParent() == TrueSucc)
1116 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1117 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
1118 if (FVPN->getParent() == FalseSucc)
1119 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1121 TrueSucc->removePredecessor(BI->getParent());
1122 FalseSucc->removePredecessor(BI->getParent());
1124 // Insert a new select instruction.
1126 Value *BrCond = BI->getCondition();
1127 if (TrueValue != FalseValue)
1128 NewRetVal = new SelectInst(BrCond, TrueValue,
1129 FalseValue, "retval", BI);
1131 NewRetVal = TrueValue;
1133 new ReturnInst(NewRetVal, BI);
1134 BI->getParent()->getInstList().erase(BI);
1135 if (BrCond->use_empty())
1136 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond))
1137 BrCondI->getParent()->getInstList().erase(BrCondI);
1143 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
1144 // Check to see if the first instruction in this block is just an unwind.
1145 // If so, replace any invoke instructions which use this as an exception
1146 // destination with call instructions, and any unconditional branch
1147 // predecessor with an unwind.
1149 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1150 while (!Preds.empty()) {
1151 BasicBlock *Pred = Preds.back();
1152 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) {
1153 if (BI->isUnconditional()) {
1154 Pred->getInstList().pop_back(); // nuke uncond branch
1155 new UnwindInst(Pred); // Use unwind.
1158 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
1159 if (II->getUnwindDest() == BB) {
1160 // Insert a new branch instruction before the invoke, because this
1161 // is now a fall through...
1162 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1163 Pred->getInstList().remove(II); // Take out of symbol table
1165 // Insert the call now...
1166 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1167 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1169 CI->setCallingConv(II->getCallingConv());
1170 // If the invoke produced a value, the Call now does instead
1171 II->replaceAllUsesWith(CI);
1179 // If this block is now dead, remove it.
1180 if (pred_begin(BB) == pred_end(BB)) {
1181 // We know there are no successors, so just nuke the block.
1182 M->getBasicBlockList().erase(BB);
1186 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1187 if (isValueEqualityComparison(SI)) {
1188 // If we only have one predecessor, and if it is a branch on this value,
1189 // see if that predecessor totally determines the outcome of this switch.
1190 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1191 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
1192 return SimplifyCFG(BB) || 1;
1194 // If the block only contains the switch, see if we can fold the block
1195 // away into any preds.
1196 if (SI == &BB->front())
1197 if (FoldValueComparisonIntoPredecessors(SI))
1198 return SimplifyCFG(BB) || 1;
1200 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
1201 if (BI->isUnconditional()) {
1202 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
1203 while (isa<PHINode>(*BBI)) ++BBI;
1205 BasicBlock *Succ = BI->getSuccessor(0);
1206 if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
1207 Succ != BB) // Don't hurt infinite loops!
1208 if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ))
1211 } else { // Conditional branch
1212 if (Value *CompVal = isValueEqualityComparison(BI)) {
1213 // If we only have one predecessor, and if it is a branch on this value,
1214 // see if that predecessor totally determines the outcome of this
1216 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1217 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
1218 return SimplifyCFG(BB) || 1;
1220 // This block must be empty, except for the setcond inst, if it exists.
1221 BasicBlock::iterator I = BB->begin();
1223 (&*I == cast<Instruction>(BI->getCondition()) &&
1225 if (FoldValueComparisonIntoPredecessors(BI))
1226 return SimplifyCFG(BB) | true;
1229 // If this is a branch on a phi node in the current block, thread control
1230 // through this block if any PHI node entries are constants.
1231 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
1232 if (PN->getParent() == BI->getParent())
1233 if (FoldCondBranchOnPHI(BI))
1234 return SimplifyCFG(BB) | true;
1237 // If this basic block is ONLY a setcc and a branch, and if a predecessor
1238 // branches to us and one of our successors, fold the setcc into the
1239 // predecessor and use logical operations to pick the right destination.
1240 BasicBlock *TrueDest = BI->getSuccessor(0);
1241 BasicBlock *FalseDest = BI->getSuccessor(1);
1242 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition()))
1243 if (Cond->getParent() == BB && &BB->front() == Cond &&
1244 Cond->getNext() == BI && Cond->hasOneUse() &&
1245 TrueDest != BB && FalseDest != BB)
1246 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
1247 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1248 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
1249 BasicBlock *PredBlock = *PI;
1250 if (PBI->getSuccessor(0) == FalseDest ||
1251 PBI->getSuccessor(1) == TrueDest) {
1252 // Invert the predecessors condition test (xor it with true),
1253 // which allows us to write this code once.
1255 BinaryOperator::createNot(PBI->getCondition(),
1256 PBI->getCondition()->getName()+".not", PBI);
1257 PBI->setCondition(NewCond);
1258 BasicBlock *OldTrue = PBI->getSuccessor(0);
1259 BasicBlock *OldFalse = PBI->getSuccessor(1);
1260 PBI->setSuccessor(0, OldFalse);
1261 PBI->setSuccessor(1, OldTrue);
1264 if (PBI->getSuccessor(0) == TrueDest ||
1265 PBI->getSuccessor(1) == FalseDest) {
1266 // Clone Cond into the predecessor basic block, and or/and the
1267 // two conditions together.
1268 Instruction *New = Cond->clone();
1269 New->setName(Cond->getName());
1270 Cond->setName(Cond->getName()+".old");
1271 PredBlock->getInstList().insert(PBI, New);
1272 Instruction::BinaryOps Opcode =
1273 PBI->getSuccessor(0) == TrueDest ?
1274 Instruction::Or : Instruction::And;
1276 BinaryOperator::create(Opcode, PBI->getCondition(),
1277 New, "bothcond", PBI);
1278 PBI->setCondition(NewCond);
1279 if (PBI->getSuccessor(0) == BB) {
1280 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1281 PBI->setSuccessor(0, TrueDest);
1283 if (PBI->getSuccessor(1) == BB) {
1284 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1285 PBI->setSuccessor(1, FalseDest);
1287 return SimplifyCFG(BB) | 1;
1291 // If this block ends with a branch instruction, and if there is a
1292 // predecessor that ends on a branch of the same condition, make this
1293 // conditional branch redundant.
1294 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1295 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1296 if (PBI != BI && PBI->isConditional() &&
1297 PBI->getCondition() == BI->getCondition() &&
1298 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1299 // Okay, the outcome of this conditional branch is statically
1300 // knowable. If this block had a single pred, handle specially.
1301 if (BB->getSinglePredecessor()) {
1302 // Turn this into a branch on constant.
1303 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1304 BI->setCondition(ConstantBool::get(CondIsTrue));
1305 return SimplifyCFG(BB); // Nuke the branch on constant.
1308 // Otherwise, if there are multiple predecessors, insert a PHI that
1309 // merges in the constant and simplify the block result.
1310 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1311 PHINode *NewPN = new PHINode(Type::BoolTy,
1312 BI->getCondition()->getName()+".pr",
1314 for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1315 if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
1316 PBI != BI && PBI->isConditional() &&
1317 PBI->getCondition() == BI->getCondition() &&
1318 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1319 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1320 NewPN->addIncoming(ConstantBool::get(CondIsTrue), *PI);
1322 NewPN->addIncoming(BI->getCondition(), *PI);
1325 BI->setCondition(NewPN);
1326 // This will thread the branch.
1327 return SimplifyCFG(BB) | true;
1331 } else if (isa<UnreachableInst>(BB->getTerminator())) {
1332 // If there are any instructions immediately before the unreachable that can
1333 // be removed, do so.
1334 Instruction *Unreachable = BB->getTerminator();
1335 while (Unreachable != BB->begin()) {
1336 BasicBlock::iterator BBI = Unreachable;
1338 if (isa<CallInst>(BBI)) break;
1339 // Delete this instruction
1340 BB->getInstList().erase(BBI);
1344 // If the unreachable instruction is the first in the block, take a gander
1345 // at all of the predecessors of this instruction, and simplify them.
1346 if (&BB->front() == Unreachable) {
1347 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1348 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
1349 TerminatorInst *TI = Preds[i]->getTerminator();
1351 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1352 if (BI->isUnconditional()) {
1353 if (BI->getSuccessor(0) == BB) {
1354 new UnreachableInst(TI);
1355 TI->eraseFromParent();
1359 if (BI->getSuccessor(0) == BB) {
1360 new BranchInst(BI->getSuccessor(1), BI);
1361 BI->eraseFromParent();
1362 } else if (BI->getSuccessor(1) == BB) {
1363 new BranchInst(BI->getSuccessor(0), BI);
1364 BI->eraseFromParent();
1368 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
1369 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1370 if (SI->getSuccessor(i) == BB) {
1371 BB->removePredecessor(SI->getParent());
1376 // If the default value is unreachable, figure out the most popular
1377 // destination and make it the default.
1378 if (SI->getSuccessor(0) == BB) {
1379 std::map<BasicBlock*, unsigned> Popularity;
1380 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1381 Popularity[SI->getSuccessor(i)]++;
1383 // Find the most popular block.
1384 unsigned MaxPop = 0;
1385 BasicBlock *MaxBlock = 0;
1386 for (std::map<BasicBlock*, unsigned>::iterator
1387 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
1388 if (I->second > MaxPop) {
1390 MaxBlock = I->first;
1394 // Make this the new default, allowing us to delete any explicit
1396 SI->setSuccessor(0, MaxBlock);
1399 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
1401 if (isa<PHINode>(MaxBlock->begin()))
1402 for (unsigned i = 0; i != MaxPop-1; ++i)
1403 MaxBlock->removePredecessor(SI->getParent());
1405 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1406 if (SI->getSuccessor(i) == MaxBlock) {
1412 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
1413 if (II->getUnwindDest() == BB) {
1414 // Convert the invoke to a call instruction. This would be a good
1415 // place to note that the call does not throw though.
1416 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1417 II->removeFromParent(); // Take out of symbol table
1419 // Insert the call now...
1420 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1421 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1423 CI->setCallingConv(II->getCallingConv());
1424 // If the invoke produced a value, the Call does now instead.
1425 II->replaceAllUsesWith(CI);
1432 // If this block is now dead, remove it.
1433 if (pred_begin(BB) == pred_end(BB)) {
1434 // We know there are no successors, so just nuke the block.
1435 M->getBasicBlockList().erase(BB);
1441 // Merge basic blocks into their predecessor if there is only one distinct
1442 // pred, and if there is only one distinct successor of the predecessor, and
1443 // if there are no PHI nodes.
1445 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
1446 BasicBlock *OnlyPred = *PI++;
1447 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
1448 if (*PI != OnlyPred) {
1449 OnlyPred = 0; // There are multiple different predecessors...
1453 BasicBlock *OnlySucc = 0;
1454 if (OnlyPred && OnlyPred != BB && // Don't break self loops
1455 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
1456 // Check to see if there is only one distinct successor...
1457 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
1459 for (; SI != SE; ++SI)
1460 if (*SI != OnlySucc) {
1461 OnlySucc = 0; // There are multiple distinct successors!
1467 DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred);
1468 TerminatorInst *Term = OnlyPred->getTerminator();
1470 // Resolve any PHI nodes at the start of the block. They are all
1471 // guaranteed to have exactly one entry if they exist, unless there are
1472 // multiple duplicate (but guaranteed to be equal) entries for the
1473 // incoming edges. This occurs when there are multiple edges from
1474 // OnlyPred to OnlySucc.
1476 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
1477 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1478 BB->getInstList().pop_front(); // Delete the phi node...
1481 // Delete the unconditional branch from the predecessor...
1482 OnlyPred->getInstList().pop_back();
1484 // Move all definitions in the successor to the predecessor...
1485 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
1487 // Make all PHI nodes that referred to BB now refer to Pred as their
1489 BB->replaceAllUsesWith(OnlyPred);
1491 std::string OldName = BB->getName();
1493 // Erase basic block from the function...
1494 M->getBasicBlockList().erase(BB);
1496 // Inherit predecessors name if it exists...
1497 if (!OldName.empty() && !OnlyPred->hasName())
1498 OnlyPred->setName(OldName);
1503 // Otherwise, if this block only has a single predecessor, and if that block
1504 // is a conditional branch, see if we can hoist any code from this block up
1505 // into our predecessor.
1507 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
1508 if (BI->isConditional()) {
1509 // Get the other block.
1510 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
1511 PI = pred_begin(OtherBB);
1513 if (PI == pred_end(OtherBB)) {
1514 // We have a conditional branch to two blocks that are only reachable
1515 // from the condbr. We know that the condbr dominates the two blocks,
1516 // so see if there is any identical code in the "then" and "else"
1517 // blocks. If so, we can hoist it up to the branching block.
1518 Changed |= HoistThenElseCodeToIf(BI);
1522 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1523 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1524 // Change br (X == 0 | X == 1), T, F into a switch instruction.
1525 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
1526 Instruction *Cond = cast<Instruction>(BI->getCondition());
1527 // If this is a bunch of seteq's or'd together, or if it's a bunch of
1528 // 'setne's and'ed together, collect them.
1530 std::vector<ConstantInt*> Values;
1531 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
1532 if (CompVal && CompVal->getType()->isInteger()) {
1533 // There might be duplicate constants in the list, which the switch
1534 // instruction can't handle, remove them now.
1535 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
1536 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
1538 // Figure out which block is which destination.
1539 BasicBlock *DefaultBB = BI->getSuccessor(1);
1540 BasicBlock *EdgeBB = BI->getSuccessor(0);
1541 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
1543 // Create the new switch instruction now.
1544 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI);
1546 // Add all of the 'cases' to the switch instruction.
1547 for (unsigned i = 0, e = Values.size(); i != e; ++i)
1548 New->addCase(Values[i], EdgeBB);
1550 // We added edges from PI to the EdgeBB. As such, if there were any
1551 // PHI nodes in EdgeBB, they need entries to be added corresponding to
1552 // the number of edges added.
1553 for (BasicBlock::iterator BBI = EdgeBB->begin();
1554 isa<PHINode>(BBI); ++BBI) {
1555 PHINode *PN = cast<PHINode>(BBI);
1556 Value *InVal = PN->getIncomingValueForBlock(*PI);
1557 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
1558 PN->addIncoming(InVal, *PI);
1561 // Erase the old branch instruction.
1562 (*PI)->getInstList().erase(BI);
1564 // Erase the potentially condition tree that was used to computed the
1565 // branch condition.
1566 ErasePossiblyDeadInstructionTree(Cond);
1571 // If there is a trivial two-entry PHI node in this basic block, and we can
1572 // eliminate it, do so now.
1573 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1574 if (PN->getNumIncomingValues() == 2) {
1575 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1576 // statement", which has a very simple dominance structure. Basically, we
1577 // are trying to find the condition that is being branched on, which
1578 // subsequently causes this merge to happen. We really want control
1579 // dependence information for this check, but simplifycfg can't keep it up
1580 // to date, and this catches most of the cases we care about anyway.
1582 BasicBlock *IfTrue, *IfFalse;
1583 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
1584 DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
1585 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
1587 // Loop over the PHI's seeing if we can promote them all to select
1588 // instructions. While we are at it, keep track of the instructions
1589 // that need to be moved to the dominating block.
1590 std::set<Instruction*> AggressiveInsts;
1591 bool CanPromote = true;
1593 BasicBlock::iterator AfterPHIIt = BB->begin();
1594 while (isa<PHINode>(AfterPHIIt)) {
1595 PHINode *PN = cast<PHINode>(AfterPHIIt++);
1596 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) {
1597 if (PN->getIncomingValue(0) != PN)
1598 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1600 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
1601 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
1602 &AggressiveInsts) ||
1603 !DominatesMergePoint(PN->getIncomingValue(1), BB,
1604 &AggressiveInsts)) {
1610 // Did we eliminate all PHI's?
1611 CanPromote |= AfterPHIIt == BB->begin();
1613 // If we all PHI nodes are promotable, check to make sure that all
1614 // instructions in the predecessor blocks can be promoted as well. If
1615 // not, we won't be able to get rid of the control flow, so it's not
1616 // worth promoting to select instructions.
1617 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0;
1619 PN = cast<PHINode>(BB->begin());
1620 BasicBlock *Pred = PN->getIncomingBlock(0);
1621 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1623 DomBlock = *pred_begin(Pred);
1624 for (BasicBlock::iterator I = Pred->begin();
1625 !isa<TerminatorInst>(I); ++I)
1626 if (!AggressiveInsts.count(I)) {
1627 // This is not an aggressive instruction that we can promote.
1628 // Because of this, we won't be able to get rid of the control
1629 // flow, so the xform is not worth it.
1635 Pred = PN->getIncomingBlock(1);
1637 cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1639 DomBlock = *pred_begin(Pred);
1640 for (BasicBlock::iterator I = Pred->begin();
1641 !isa<TerminatorInst>(I); ++I)
1642 if (!AggressiveInsts.count(I)) {
1643 // This is not an aggressive instruction that we can promote.
1644 // Because of this, we won't be able to get rid of the control
1645 // flow, so the xform is not worth it.
1652 // If we can still promote the PHI nodes after this gauntlet of tests,
1653 // do all of the PHI's now.
1655 // Move all 'aggressive' instructions, which are defined in the
1656 // conditional parts of the if's up to the dominating block.
1658 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1659 IfBlock1->getInstList(),
1661 IfBlock1->getTerminator());
1664 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1665 IfBlock2->getInstList(),
1667 IfBlock2->getTerminator());
1670 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
1671 // Change the PHI node into a select instruction.
1673 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1675 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1677 std::string Name = PN->getName(); PN->setName("");
1678 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1680 BB->getInstList().erase(PN);