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/DerivedTypes.h"
20 #include "llvm/Support/CFG.h"
21 #include "llvm/Support/Debug.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
29 /// SafeToMergeTerminators - Return true if it is safe to merge these two
30 /// terminator instructions together.
32 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
33 if (SI1 == SI2) return false; // Can't merge with self!
35 // It is not safe to merge these two switch instructions if they have a common
36 // successor, and if that successor has a PHI node, and if *that* PHI node has
37 // conflicting incoming values from the two switch blocks.
38 BasicBlock *SI1BB = SI1->getParent();
39 BasicBlock *SI2BB = SI2->getParent();
40 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
42 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
43 if (SI1Succs.count(*I))
44 for (BasicBlock::iterator BBI = (*I)->begin();
45 isa<PHINode>(BBI); ++BBI) {
46 PHINode *PN = cast<PHINode>(BBI);
47 if (PN->getIncomingValueForBlock(SI1BB) !=
48 PN->getIncomingValueForBlock(SI2BB))
55 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
56 /// now be entries in it from the 'NewPred' block. The values that will be
57 /// flowing into the PHI nodes will be the same as those coming in from
58 /// ExistPred, an existing predecessor of Succ.
59 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
60 BasicBlock *ExistPred) {
61 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
62 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
63 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
65 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
66 PHINode *PN = cast<PHINode>(I);
67 Value *V = PN->getIncomingValueForBlock(ExistPred);
68 PN->addIncoming(V, NewPred);
72 // CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
73 // almost-empty BB ending in an unconditional branch to Succ, into succ.
75 // Assumption: Succ is the single successor for BB.
77 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
78 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
80 // Check to see if one of the predecessors of BB is already a predecessor of
81 // Succ. If so, we cannot do the transformation if there are any PHI nodes
82 // with incompatible values coming in from the two edges!
84 if (isa<PHINode>(Succ->front())) {
85 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
86 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
88 if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
89 // Loop over all of the PHI nodes checking to see if there are
90 // incompatible values coming in.
91 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
92 PHINode *PN = cast<PHINode>(I);
93 // Loop up the entries in the PHI node for BB and for *PI if the
94 // values coming in are non-equal, we cannot merge these two blocks
95 // (instead we should insert a conditional move or something, then
97 if (PN->getIncomingValueForBlock(BB) !=
98 PN->getIncomingValueForBlock(*PI))
99 return false; // Values are not equal...
104 // Finally, if BB has PHI nodes that are used by things other than the PHIs in
105 // Succ and Succ has predecessors that are not Succ and not Pred, we cannot
106 // fold these blocks, as we don't know whether BB dominates Succ or not to
107 // update the PHI nodes correctly.
108 if (!isa<PHINode>(BB->begin()) || Succ->getSinglePredecessor()) return true;
110 // If the predecessors of Succ are only BB and Succ itself, we can handle this.
112 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
113 if (*PI != Succ && *PI != BB) {
117 if (IsSafe) return true;
119 // If the PHI nodes in BB are only used by instructions in Succ, we are ok if
120 // BB and Succ have no common predecessors.
121 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) {
122 PHINode *PN = cast<PHINode>(I);
123 for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end(); UI != E;
125 if (cast<Instruction>(*UI)->getParent() != Succ)
129 // Scan the predecessor sets of BB and Succ, making sure there are no common
130 // predecessors. Common predecessors would cause us to build a phi node with
131 // differing incoming values, which is not legal.
132 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
133 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
134 if (BBPreds.count(*PI))
140 /// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional
141 /// branch to Succ, and contains no instructions other than PHI nodes and the
142 /// branch. If possible, eliminate BB.
143 static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
145 // If our successor has PHI nodes, then we need to update them to include
146 // entries for BB's predecessors, not for BB itself. Be careful though,
147 // if this transformation fails (returns true) then we cannot do this
150 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
152 DOUT << "Killing Trivial BB: \n" << *BB;
154 if (isa<PHINode>(Succ->begin())) {
155 // If there is more than one pred of succ, and there are PHI nodes in
156 // the successor, then we need to add incoming edges for the PHI nodes
158 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
160 // Loop over all of the PHI nodes in the successor of BB.
161 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
162 PHINode *PN = cast<PHINode>(I);
163 Value *OldVal = PN->removeIncomingValue(BB, false);
164 assert(OldVal && "No entry in PHI for Pred BB!");
166 // If this incoming value is one of the PHI nodes in BB, the new entries
167 // in the PHI node are the entries from the old PHI.
168 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
169 PHINode *OldValPN = cast<PHINode>(OldVal);
170 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
171 PN->addIncoming(OldValPN->getIncomingValue(i),
172 OldValPN->getIncomingBlock(i));
174 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
175 End = BBPreds.end(); PredI != End; ++PredI) {
176 // Add an incoming value for each of the new incoming values...
177 PN->addIncoming(OldVal, *PredI);
183 if (isa<PHINode>(&BB->front())) {
184 std::vector<BasicBlock*>
185 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
187 // Move all PHI nodes in BB to Succ if they are alive, otherwise
189 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
190 if (PN->use_empty()) {
191 // Just remove the dead phi. This happens if Succ's PHIs were the only
192 // users of the PHI nodes.
193 PN->eraseFromParent();
195 // The instruction is alive, so this means that Succ must have
196 // *ONLY* had BB as a predecessor, and the PHI node is still valid
197 // now. Simply move it into Succ, because we know that BB
198 // strictly dominated Succ.
199 Succ->getInstList().splice(Succ->begin(),
200 BB->getInstList(), BB->begin());
202 // We need to add new entries for the PHI node to account for
203 // predecessors of Succ that the PHI node does not take into
204 // account. At this point, since we know that BB dominated succ,
205 // this means that we should any newly added incoming edges should
206 // use the PHI node as the value for these edges, because they are
208 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
209 if (OldSuccPreds[i] != BB)
210 PN->addIncoming(PN, OldSuccPreds[i]);
214 // Everything that jumped to BB now goes to Succ.
215 std::string OldName = BB->getName();
216 BB->replaceAllUsesWith(Succ);
217 BB->eraseFromParent(); // Delete the old basic block.
219 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
220 Succ->setName(OldName);
224 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
225 /// presumably PHI nodes in it), check to see if the merge at this block is due
226 /// to an "if condition". If so, return the boolean condition that determines
227 /// which entry into BB will be taken. Also, return by references the block
228 /// that will be entered from if the condition is true, and the block that will
229 /// be entered if the condition is false.
232 static Value *GetIfCondition(BasicBlock *BB,
233 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
234 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
235 "Function can only handle blocks with 2 predecessors!");
236 BasicBlock *Pred1 = *pred_begin(BB);
237 BasicBlock *Pred2 = *++pred_begin(BB);
239 // We can only handle branches. Other control flow will be lowered to
240 // branches if possible anyway.
241 if (!isa<BranchInst>(Pred1->getTerminator()) ||
242 !isa<BranchInst>(Pred2->getTerminator()))
244 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
245 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
247 // Eliminate code duplication by ensuring that Pred1Br is conditional if
249 if (Pred2Br->isConditional()) {
250 // If both branches are conditional, we don't have an "if statement". In
251 // reality, we could transform this case, but since the condition will be
252 // required anyway, we stand no chance of eliminating it, so the xform is
253 // probably not profitable.
254 if (Pred1Br->isConditional())
257 std::swap(Pred1, Pred2);
258 std::swap(Pred1Br, Pred2Br);
261 if (Pred1Br->isConditional()) {
262 // If we found a conditional branch predecessor, make sure that it branches
263 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
264 if (Pred1Br->getSuccessor(0) == BB &&
265 Pred1Br->getSuccessor(1) == Pred2) {
268 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
269 Pred1Br->getSuccessor(1) == BB) {
273 // We know that one arm of the conditional goes to BB, so the other must
274 // go somewhere unrelated, and this must not be an "if statement".
278 // The only thing we have to watch out for here is to make sure that Pred2
279 // doesn't have incoming edges from other blocks. If it does, the condition
280 // doesn't dominate BB.
281 if (++pred_begin(Pred2) != pred_end(Pred2))
284 return Pred1Br->getCondition();
287 // Ok, if we got here, both predecessors end with an unconditional branch to
288 // BB. Don't panic! If both blocks only have a single (identical)
289 // predecessor, and THAT is a conditional branch, then we're all ok!
290 if (pred_begin(Pred1) == pred_end(Pred1) ||
291 ++pred_begin(Pred1) != pred_end(Pred1) ||
292 pred_begin(Pred2) == pred_end(Pred2) ||
293 ++pred_begin(Pred2) != pred_end(Pred2) ||
294 *pred_begin(Pred1) != *pred_begin(Pred2))
297 // Otherwise, if this is a conditional branch, then we can use it!
298 BasicBlock *CommonPred = *pred_begin(Pred1);
299 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
300 assert(BI->isConditional() && "Two successors but not conditional?");
301 if (BI->getSuccessor(0) == Pred1) {
308 return BI->getCondition();
314 // If we have a merge point of an "if condition" as accepted above, return true
315 // if the specified value dominates the block. We don't handle the true
316 // generality of domination here, just a special case which works well enough
319 // If AggressiveInsts is non-null, and if V does not dominate BB, we check to
320 // see if V (which must be an instruction) is cheap to compute and is
321 // non-trapping. If both are true, the instruction is inserted into the set and
323 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
324 std::set<Instruction*> *AggressiveInsts) {
325 Instruction *I = dyn_cast<Instruction>(V);
327 // Non-instructions all dominate instructions, but not all constantexprs
328 // can be executed unconditionally.
329 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
334 BasicBlock *PBB = I->getParent();
336 // We don't want to allow weird loops that might have the "if condition" in
337 // the bottom of this block.
338 if (PBB == BB) return false;
340 // If this instruction is defined in a block that contains an unconditional
341 // branch to BB, then it must be in the 'conditional' part of the "if
343 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
344 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
345 if (!AggressiveInsts) return false;
346 // Okay, it looks like the instruction IS in the "condition". Check to
347 // see if its a cheap instruction to unconditionally compute, and if it
348 // only uses stuff defined outside of the condition. If so, hoist it out.
349 switch (I->getOpcode()) {
350 default: return false; // Cannot hoist this out safely.
351 case Instruction::Load:
352 // We can hoist loads that are non-volatile and obviously cannot trap.
353 if (cast<LoadInst>(I)->isVolatile())
355 if (!isa<AllocaInst>(I->getOperand(0)) &&
356 !isa<Constant>(I->getOperand(0)))
359 // Finally, we have to check to make sure there are no instructions
360 // before the load in its basic block, as we are going to hoist the loop
361 // out to its predecessor.
362 if (PBB->begin() != BasicBlock::iterator(I))
365 case Instruction::Add:
366 case Instruction::Sub:
367 case Instruction::And:
368 case Instruction::Or:
369 case Instruction::Xor:
370 case Instruction::Shl:
371 case Instruction::LShr:
372 case Instruction::AShr:
373 case Instruction::ICmp:
374 case Instruction::FCmp:
375 break; // These are all cheap and non-trapping instructions.
378 // Okay, we can only really hoist these out if their operands are not
379 // defined in the conditional region.
380 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
381 if (!DominatesMergePoint(I->getOperand(i), BB, 0))
383 // Okay, it's safe to do this! Remember this instruction.
384 AggressiveInsts->insert(I);
390 // GatherConstantSetEQs - Given a potentially 'or'd together collection of
391 // icmp_eq instructions that compare a value against a constant, return the
392 // value being compared, and stick the constant into the Values vector.
393 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
394 if (Instruction *Inst = dyn_cast<Instruction>(V))
395 if (Inst->getOpcode() == Instruction::ICmp &&
396 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_EQ) {
397 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
399 return Inst->getOperand(0);
400 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
402 return Inst->getOperand(1);
404 } else if (Inst->getOpcode() == Instruction::Or) {
405 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
406 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
413 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
414 // setne instructions that compare a value against a constant, return the value
415 // being compared, and stick the constant into the Values vector.
416 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
417 if (Instruction *Inst = dyn_cast<Instruction>(V))
418 if (Inst->getOpcode() == Instruction::ICmp &&
419 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_NE) {
420 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
422 return Inst->getOperand(0);
423 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
425 return Inst->getOperand(1);
427 } else if (Inst->getOpcode() == Instruction::And) {
428 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
429 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
438 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
439 /// bunch of comparisons of one value against constants, return the value and
440 /// the constants being compared.
441 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
442 std::vector<ConstantInt*> &Values) {
443 if (Cond->getOpcode() == Instruction::Or) {
444 CompVal = GatherConstantSetEQs(Cond, Values);
446 // Return true to indicate that the condition is true if the CompVal is
447 // equal to one of the constants.
449 } else if (Cond->getOpcode() == Instruction::And) {
450 CompVal = GatherConstantSetNEs(Cond, Values);
452 // Return false to indicate that the condition is false if the CompVal is
453 // equal to one of the constants.
459 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
460 /// has no side effects, nuke it. If it uses any instructions that become dead
461 /// because the instruction is now gone, nuke them too.
462 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
463 if (!isInstructionTriviallyDead(I)) return;
465 std::vector<Instruction*> InstrsToInspect;
466 InstrsToInspect.push_back(I);
468 while (!InstrsToInspect.empty()) {
469 I = InstrsToInspect.back();
470 InstrsToInspect.pop_back();
472 if (!isInstructionTriviallyDead(I)) continue;
474 // If I is in the work list multiple times, remove previous instances.
475 for (unsigned i = 0, e = InstrsToInspect.size(); i != e; ++i)
476 if (InstrsToInspect[i] == I) {
477 InstrsToInspect.erase(InstrsToInspect.begin()+i);
481 // Add operands of dead instruction to worklist.
482 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
483 if (Instruction *OpI = dyn_cast<Instruction>(I->getOperand(i)))
484 InstrsToInspect.push_back(OpI);
486 // Remove dead instruction.
487 I->eraseFromParent();
491 // isValueEqualityComparison - Return true if the specified terminator checks to
492 // see if a value is equal to constant integer value.
493 static Value *isValueEqualityComparison(TerminatorInst *TI) {
494 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
495 // Do not permit merging of large switch instructions into their
496 // predecessors unless there is only one predecessor.
497 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
498 pred_end(SI->getParent())) > 128)
501 return SI->getCondition();
503 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
504 if (BI->isConditional() && BI->getCondition()->hasOneUse())
505 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition()))
506 if ((ICI->getPredicate() == ICmpInst::ICMP_EQ ||
507 ICI->getPredicate() == ICmpInst::ICMP_NE) &&
508 isa<ConstantInt>(ICI->getOperand(1)))
509 return ICI->getOperand(0);
513 // Given a value comparison instruction, decode all of the 'cases' that it
514 // represents and return the 'default' block.
516 GetValueEqualityComparisonCases(TerminatorInst *TI,
517 std::vector<std::pair<ConstantInt*,
518 BasicBlock*> > &Cases) {
519 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
520 Cases.reserve(SI->getNumCases());
521 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
522 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
523 return SI->getDefaultDest();
526 BranchInst *BI = cast<BranchInst>(TI);
527 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
528 Cases.push_back(std::make_pair(cast<ConstantInt>(ICI->getOperand(1)),
529 BI->getSuccessor(ICI->getPredicate() ==
530 ICmpInst::ICMP_NE)));
531 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
535 // EliminateBlockCases - Given an vector of bb/value pairs, remove any entries
536 // in the list that match the specified block.
537 static void EliminateBlockCases(BasicBlock *BB,
538 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
539 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
540 if (Cases[i].second == BB) {
541 Cases.erase(Cases.begin()+i);
546 // ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
549 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
550 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
551 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
553 // Make V1 be smaller than V2.
554 if (V1->size() > V2->size())
557 if (V1->size() == 0) return false;
558 if (V1->size() == 1) {
560 ConstantInt *TheVal = (*V1)[0].first;
561 for (unsigned i = 0, e = V2->size(); i != e; ++i)
562 if (TheVal == (*V2)[i].first)
566 // Otherwise, just sort both lists and compare element by element.
567 std::sort(V1->begin(), V1->end());
568 std::sort(V2->begin(), V2->end());
569 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
570 while (i1 != e1 && i2 != e2) {
571 if ((*V1)[i1].first == (*V2)[i2].first)
573 if ((*V1)[i1].first < (*V2)[i2].first)
581 // SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
582 // terminator instruction and its block is known to only have a single
583 // predecessor block, check to see if that predecessor is also a value
584 // comparison with the same value, and if that comparison determines the outcome
585 // of this comparison. If so, simplify TI. This does a very limited form of
587 static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
589 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
590 if (!PredVal) return false; // Not a value comparison in predecessor.
592 Value *ThisVal = isValueEqualityComparison(TI);
593 assert(ThisVal && "This isn't a value comparison!!");
594 if (ThisVal != PredVal) return false; // Different predicates.
596 // Find out information about when control will move from Pred to TI's block.
597 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
598 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
600 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
602 // Find information about how control leaves this block.
603 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
604 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
605 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
607 // If TI's block is the default block from Pred's comparison, potentially
608 // simplify TI based on this knowledge.
609 if (PredDef == TI->getParent()) {
610 // If we are here, we know that the value is none of those cases listed in
611 // PredCases. If there are any cases in ThisCases that are in PredCases, we
613 if (ValuesOverlap(PredCases, ThisCases)) {
614 if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) {
615 // Okay, one of the successors of this condbr is dead. Convert it to a
617 assert(ThisCases.size() == 1 && "Branch can only have one case!");
618 Value *Cond = BTI->getCondition();
619 // Insert the new branch.
620 Instruction *NI = new BranchInst(ThisDef, TI);
622 // Remove PHI node entries for the dead edge.
623 ThisCases[0].second->removePredecessor(TI->getParent());
625 DOUT << "Threading pred instr: " << *Pred->getTerminator()
626 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n";
628 TI->eraseFromParent(); // Nuke the old one.
629 // If condition is now dead, nuke it.
630 if (Instruction *CondI = dyn_cast<Instruction>(Cond))
631 ErasePossiblyDeadInstructionTree(CondI);
635 SwitchInst *SI = cast<SwitchInst>(TI);
636 // Okay, TI has cases that are statically dead, prune them away.
637 std::set<Constant*> DeadCases;
638 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
639 DeadCases.insert(PredCases[i].first);
641 DOUT << "Threading pred instr: " << *Pred->getTerminator()
642 << "Through successor TI: " << *TI;
644 for (unsigned i = SI->getNumCases()-1; i != 0; --i)
645 if (DeadCases.count(SI->getCaseValue(i))) {
646 SI->getSuccessor(i)->removePredecessor(TI->getParent());
650 DOUT << "Leaving: " << *TI << "\n";
656 // Otherwise, TI's block must correspond to some matched value. Find out
657 // which value (or set of values) this is.
658 ConstantInt *TIV = 0;
659 BasicBlock *TIBB = TI->getParent();
660 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
661 if (PredCases[i].second == TIBB)
663 TIV = PredCases[i].first;
665 return false; // Cannot handle multiple values coming to this block.
666 assert(TIV && "No edge from pred to succ?");
668 // Okay, we found the one constant that our value can be if we get into TI's
669 // BB. Find out which successor will unconditionally be branched to.
670 BasicBlock *TheRealDest = 0;
671 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
672 if (ThisCases[i].first == TIV) {
673 TheRealDest = ThisCases[i].second;
677 // If not handled by any explicit cases, it is handled by the default case.
678 if (TheRealDest == 0) TheRealDest = ThisDef;
680 // Remove PHI node entries for dead edges.
681 BasicBlock *CheckEdge = TheRealDest;
682 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
683 if (*SI != CheckEdge)
684 (*SI)->removePredecessor(TIBB);
688 // Insert the new branch.
689 Instruction *NI = new BranchInst(TheRealDest, TI);
691 DOUT << "Threading pred instr: " << *Pred->getTerminator()
692 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n";
693 Instruction *Cond = 0;
694 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
695 Cond = dyn_cast<Instruction>(BI->getCondition());
696 TI->eraseFromParent(); // Nuke the old one.
698 if (Cond) ErasePossiblyDeadInstructionTree(Cond);
704 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
705 // equality comparison instruction (either a switch or a branch on "X == c").
706 // See if any of the predecessors of the terminator block are value comparisons
707 // on the same value. If so, and if safe to do so, fold them together.
708 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
709 BasicBlock *BB = TI->getParent();
710 Value *CV = isValueEqualityComparison(TI); // CondVal
711 assert(CV && "Not a comparison?");
712 bool Changed = false;
714 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
715 while (!Preds.empty()) {
716 BasicBlock *Pred = Preds.back();
719 // See if the predecessor is a comparison with the same value.
720 TerminatorInst *PTI = Pred->getTerminator();
721 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
723 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
724 // Figure out which 'cases' to copy from SI to PSI.
725 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
726 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
728 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
729 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
731 // Based on whether the default edge from PTI goes to BB or not, fill in
732 // PredCases and PredDefault with the new switch cases we would like to
734 std::vector<BasicBlock*> NewSuccessors;
736 if (PredDefault == BB) {
737 // If this is the default destination from PTI, only the edges in TI
738 // that don't occur in PTI, or that branch to BB will be activated.
739 std::set<ConstantInt*> PTIHandled;
740 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
741 if (PredCases[i].second != BB)
742 PTIHandled.insert(PredCases[i].first);
744 // The default destination is BB, we don't need explicit targets.
745 std::swap(PredCases[i], PredCases.back());
746 PredCases.pop_back();
750 // Reconstruct the new switch statement we will be building.
751 if (PredDefault != BBDefault) {
752 PredDefault->removePredecessor(Pred);
753 PredDefault = BBDefault;
754 NewSuccessors.push_back(BBDefault);
756 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
757 if (!PTIHandled.count(BBCases[i].first) &&
758 BBCases[i].second != BBDefault) {
759 PredCases.push_back(BBCases[i]);
760 NewSuccessors.push_back(BBCases[i].second);
764 // If this is not the default destination from PSI, only the edges
765 // in SI that occur in PSI with a destination of BB will be
767 std::set<ConstantInt*> PTIHandled;
768 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
769 if (PredCases[i].second == BB) {
770 PTIHandled.insert(PredCases[i].first);
771 std::swap(PredCases[i], PredCases.back());
772 PredCases.pop_back();
776 // Okay, now we know which constants were sent to BB from the
777 // predecessor. Figure out where they will all go now.
778 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
779 if (PTIHandled.count(BBCases[i].first)) {
780 // If this is one we are capable of getting...
781 PredCases.push_back(BBCases[i]);
782 NewSuccessors.push_back(BBCases[i].second);
783 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
786 // If there are any constants vectored to BB that TI doesn't handle,
787 // they must go to the default destination of TI.
788 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
789 E = PTIHandled.end(); I != E; ++I) {
790 PredCases.push_back(std::make_pair(*I, BBDefault));
791 NewSuccessors.push_back(BBDefault);
795 // Okay, at this point, we know which new successor Pred will get. Make
796 // sure we update the number of entries in the PHI nodes for these
798 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
799 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
801 // Now that the successors are updated, create the new Switch instruction.
802 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI);
803 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
804 NewSI->addCase(PredCases[i].first, PredCases[i].second);
806 Instruction *DeadCond = 0;
807 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
808 // If PTI is a branch, remember the condition.
809 DeadCond = dyn_cast<Instruction>(BI->getCondition());
810 Pred->getInstList().erase(PTI);
812 // If the condition is dead now, remove the instruction tree.
813 if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond);
815 // Okay, last check. If BB is still a successor of PSI, then we must
816 // have an infinite loop case. If so, add an infinitely looping block
817 // to handle the case to preserve the behavior of the code.
818 BasicBlock *InfLoopBlock = 0;
819 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
820 if (NewSI->getSuccessor(i) == BB) {
821 if (InfLoopBlock == 0) {
822 // Insert it at the end of the loop, because it's either code,
823 // or it won't matter if it's hot. :)
824 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
825 new BranchInst(InfLoopBlock, InfLoopBlock);
827 NewSI->setSuccessor(i, InfLoopBlock);
836 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
837 /// BB2, hoist any common code in the two blocks up into the branch block. The
838 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
839 static bool HoistThenElseCodeToIf(BranchInst *BI) {
840 // This does very trivial matching, with limited scanning, to find identical
841 // instructions in the two blocks. In particular, we don't want to get into
842 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
843 // such, we currently just scan for obviously identical instructions in an
845 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
846 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
848 Instruction *I1 = BB1->begin(), *I2 = BB2->begin();
849 if (I1->getOpcode() != I2->getOpcode() || isa<PHINode>(I1) ||
850 isa<InvokeInst>(I1) || !I1->isIdenticalTo(I2))
853 // If we get here, we can hoist at least one instruction.
854 BasicBlock *BIParent = BI->getParent();
857 // If we are hoisting the terminator instruction, don't move one (making a
858 // broken BB), instead clone it, and remove BI.
859 if (isa<TerminatorInst>(I1))
860 goto HoistTerminator;
862 // For a normal instruction, we just move one to right before the branch,
863 // then replace all uses of the other with the first. Finally, we remove
864 // the now redundant second instruction.
865 BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
866 if (!I2->use_empty())
867 I2->replaceAllUsesWith(I1);
868 BB2->getInstList().erase(I2);
872 } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2));
877 // Okay, it is safe to hoist the terminator.
878 Instruction *NT = I1->clone();
879 BIParent->getInstList().insert(BI, NT);
880 if (NT->getType() != Type::VoidTy) {
881 I1->replaceAllUsesWith(NT);
882 I2->replaceAllUsesWith(NT);
883 NT->setName(I1->getName());
886 // Hoisting one of the terminators from our successor is a great thing.
887 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
888 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
889 // nodes, so we insert select instruction to compute the final result.
890 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
891 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
893 for (BasicBlock::iterator BBI = SI->begin();
894 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
895 Value *BB1V = PN->getIncomingValueForBlock(BB1);
896 Value *BB2V = PN->getIncomingValueForBlock(BB2);
898 // These values do not agree. Insert a select instruction before NT
899 // that determines the right value.
900 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
902 SI = new SelectInst(BI->getCondition(), BB1V, BB2V,
903 BB1V->getName()+"."+BB2V->getName(), NT);
904 // Make the PHI node use the select for all incoming values for BB1/BB2
905 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
906 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
907 PN->setIncomingValue(i, SI);
912 // Update any PHI nodes in our new successors.
913 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
914 AddPredecessorToBlock(*SI, BIParent, BB1);
916 BI->eraseFromParent();
920 /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
921 /// across this block.
922 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
923 BranchInst *BI = cast<BranchInst>(BB->getTerminator());
926 // If this basic block contains anything other than a PHI (which controls the
927 // branch) and branch itself, bail out. FIXME: improve this in the future.
928 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI, ++Size) {
929 if (Size > 10) return false; // Don't clone large BB's.
931 // We can only support instructions that are do not define values that are
932 // live outside of the current basic block.
933 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
935 Instruction *U = cast<Instruction>(*UI);
936 if (U->getParent() != BB || isa<PHINode>(U)) return false;
939 // Looks ok, continue checking.
945 /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
946 /// that is defined in the same block as the branch and if any PHI entries are
947 /// constants, thread edges corresponding to that entry to be branches to their
948 /// ultimate destination.
949 static bool FoldCondBranchOnPHI(BranchInst *BI) {
950 BasicBlock *BB = BI->getParent();
951 PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
952 // NOTE: we currently cannot transform this case if the PHI node is used
953 // outside of the block.
954 if (!PN || PN->getParent() != BB || !PN->hasOneUse())
957 // Degenerate case of a single entry PHI.
958 if (PN->getNumIncomingValues() == 1) {
959 if (PN->getIncomingValue(0) != PN)
960 PN->replaceAllUsesWith(PN->getIncomingValue(0));
962 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
963 PN->eraseFromParent();
967 // Now we know that this block has multiple preds and two succs.
968 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
970 // Okay, this is a simple enough basic block. See if any phi values are
972 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
974 if ((CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i))) &&
975 CB->getType() == Type::Int1Ty) {
976 // Okay, we now know that all edges from PredBB should be revectored to
977 // branch to RealDest.
978 BasicBlock *PredBB = PN->getIncomingBlock(i);
979 BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
981 if (RealDest == BB) continue; // Skip self loops.
983 // The dest block might have PHI nodes, other predecessors and other
984 // difficult cases. Instead of being smart about this, just insert a new
985 // block that jumps to the destination block, effectively splitting
986 // the edge we are about to create.
987 BasicBlock *EdgeBB = new BasicBlock(RealDest->getName()+".critedge",
988 RealDest->getParent(), RealDest);
989 new BranchInst(RealDest, EdgeBB);
991 for (BasicBlock::iterator BBI = RealDest->begin();
992 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
993 Value *V = PN->getIncomingValueForBlock(BB);
994 PN->addIncoming(V, EdgeBB);
997 // BB may have instructions that are being threaded over. Clone these
998 // instructions into EdgeBB. We know that there will be no uses of the
999 // cloned instructions outside of EdgeBB.
1000 BasicBlock::iterator InsertPt = EdgeBB->begin();
1001 std::map<Value*, Value*> TranslateMap; // Track translated values.
1002 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
1003 if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
1004 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
1006 // Clone the instruction.
1007 Instruction *N = BBI->clone();
1008 if (BBI->hasName()) N->setName(BBI->getName()+".c");
1010 // Update operands due to translation.
1011 for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i) {
1012 std::map<Value*, Value*>::iterator PI =
1013 TranslateMap.find(N->getOperand(i));
1014 if (PI != TranslateMap.end())
1015 N->setOperand(i, PI->second);
1018 // Check for trivial simplification.
1019 if (Constant *C = ConstantFoldInstruction(N)) {
1020 TranslateMap[BBI] = C;
1021 delete N; // Constant folded away, don't need actual inst
1023 // Insert the new instruction into its new home.
1024 EdgeBB->getInstList().insert(InsertPt, N);
1025 if (!BBI->use_empty())
1026 TranslateMap[BBI] = N;
1031 // Loop over all of the edges from PredBB to BB, changing them to branch
1032 // to EdgeBB instead.
1033 TerminatorInst *PredBBTI = PredBB->getTerminator();
1034 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
1035 if (PredBBTI->getSuccessor(i) == BB) {
1036 BB->removePredecessor(PredBB);
1037 PredBBTI->setSuccessor(i, EdgeBB);
1040 // Recurse, simplifying any other constants.
1041 return FoldCondBranchOnPHI(BI) | true;
1048 /// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry
1049 /// PHI node, see if we can eliminate it.
1050 static bool FoldTwoEntryPHINode(PHINode *PN) {
1051 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1052 // statement", which has a very simple dominance structure. Basically, we
1053 // are trying to find the condition that is being branched on, which
1054 // subsequently causes this merge to happen. We really want control
1055 // dependence information for this check, but simplifycfg can't keep it up
1056 // to date, and this catches most of the cases we care about anyway.
1058 BasicBlock *BB = PN->getParent();
1059 BasicBlock *IfTrue, *IfFalse;
1060 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
1061 if (!IfCond) return false;
1063 // Okay, we found that we can merge this two-entry phi node into a select.
1064 // Doing so would require us to fold *all* two entry phi nodes in this block.
1065 // At some point this becomes non-profitable (particularly if the target
1066 // doesn't support cmov's). Only do this transformation if there are two or
1067 // fewer PHI nodes in this block.
1068 unsigned NumPhis = 0;
1069 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
1073 DOUT << "FOUND IF CONDITION! " << *IfCond << " T: "
1074 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n";
1076 // Loop over the PHI's seeing if we can promote them all to select
1077 // instructions. While we are at it, keep track of the instructions
1078 // that need to be moved to the dominating block.
1079 std::set<Instruction*> AggressiveInsts;
1081 BasicBlock::iterator AfterPHIIt = BB->begin();
1082 while (isa<PHINode>(AfterPHIIt)) {
1083 PHINode *PN = cast<PHINode>(AfterPHIIt++);
1084 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) {
1085 if (PN->getIncomingValue(0) != PN)
1086 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1088 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
1089 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
1090 &AggressiveInsts) ||
1091 !DominatesMergePoint(PN->getIncomingValue(1), BB,
1092 &AggressiveInsts)) {
1097 // If we all PHI nodes are promotable, check to make sure that all
1098 // instructions in the predecessor blocks can be promoted as well. If
1099 // not, we won't be able to get rid of the control flow, so it's not
1100 // worth promoting to select instructions.
1101 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0;
1102 PN = cast<PHINode>(BB->begin());
1103 BasicBlock *Pred = PN->getIncomingBlock(0);
1104 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1106 DomBlock = *pred_begin(Pred);
1107 for (BasicBlock::iterator I = Pred->begin();
1108 !isa<TerminatorInst>(I); ++I)
1109 if (!AggressiveInsts.count(I)) {
1110 // This is not an aggressive instruction that we can promote.
1111 // Because of this, we won't be able to get rid of the control
1112 // flow, so the xform is not worth it.
1117 Pred = PN->getIncomingBlock(1);
1118 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1120 DomBlock = *pred_begin(Pred);
1121 for (BasicBlock::iterator I = Pred->begin();
1122 !isa<TerminatorInst>(I); ++I)
1123 if (!AggressiveInsts.count(I)) {
1124 // This is not an aggressive instruction that we can promote.
1125 // Because of this, we won't be able to get rid of the control
1126 // flow, so the xform is not worth it.
1131 // If we can still promote the PHI nodes after this gauntlet of tests,
1132 // do all of the PHI's now.
1134 // Move all 'aggressive' instructions, which are defined in the
1135 // conditional parts of the if's up to the dominating block.
1137 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1138 IfBlock1->getInstList(),
1140 IfBlock1->getTerminator());
1143 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1144 IfBlock2->getInstList(),
1146 IfBlock2->getTerminator());
1149 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
1150 // Change the PHI node into a select instruction.
1152 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1154 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1156 std::string Name = PN->getName(); PN->setName("");
1157 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
1159 BB->getInstList().erase(PN);
1165 /// ConstantIntOrdering - This class implements a stable ordering of constant
1166 /// integers that does not depend on their address. This is important for
1167 /// applications that sort ConstantInt's to ensure uniqueness.
1168 struct ConstantIntOrdering {
1169 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
1170 return LHS->getZExtValue() < RHS->getZExtValue();
1175 // SimplifyCFG - This function is used to do simplification of a CFG. For
1176 // example, it adjusts branches to branches to eliminate the extra hop, it
1177 // eliminates unreachable basic blocks, and does other "peephole" optimization
1178 // of the CFG. It returns true if a modification was made.
1180 // WARNING: The entry node of a function may not be simplified.
1182 bool llvm::SimplifyCFG(BasicBlock *BB) {
1183 bool Changed = false;
1184 Function *M = BB->getParent();
1186 assert(BB && BB->getParent() && "Block not embedded in function!");
1187 assert(BB->getTerminator() && "Degenerate basic block encountered!");
1188 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
1190 // Remove basic blocks that have no predecessors... which are unreachable.
1191 if (pred_begin(BB) == pred_end(BB) ||
1192 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
1193 DOUT << "Removing BB: \n" << *BB;
1195 // Loop through all of our successors and make sure they know that one
1196 // of their predecessors is going away.
1197 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
1198 SI->removePredecessor(BB);
1200 while (!BB->empty()) {
1201 Instruction &I = BB->back();
1202 // If this instruction is used, replace uses with an arbitrary
1203 // value. Because control flow can't get here, we don't care
1204 // what we replace the value with. Note that since this block is
1205 // unreachable, and all values contained within it must dominate their
1206 // uses, that all uses will eventually be removed.
1208 // Make all users of this instruction use undef instead
1209 I.replaceAllUsesWith(UndefValue::get(I.getType()));
1211 // Remove the instruction from the basic block
1212 BB->getInstList().pop_back();
1214 M->getBasicBlockList().erase(BB);
1218 // Check to see if we can constant propagate this terminator instruction
1220 Changed |= ConstantFoldTerminator(BB);
1222 // If this is a returning block with only PHI nodes in it, fold the return
1223 // instruction into any unconditional branch predecessors.
1225 // If any predecessor is a conditional branch that just selects among
1226 // different return values, fold the replace the branch/return with a select
1228 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1229 BasicBlock::iterator BBI = BB->getTerminator();
1230 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
1231 // Find predecessors that end with branches.
1232 std::vector<BasicBlock*> UncondBranchPreds;
1233 std::vector<BranchInst*> CondBranchPreds;
1234 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1235 TerminatorInst *PTI = (*PI)->getTerminator();
1236 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
1237 if (BI->isUnconditional())
1238 UncondBranchPreds.push_back(*PI);
1240 CondBranchPreds.push_back(BI);
1243 // If we found some, do the transformation!
1244 if (!UncondBranchPreds.empty()) {
1245 while (!UncondBranchPreds.empty()) {
1246 BasicBlock *Pred = UncondBranchPreds.back();
1247 DOUT << "FOLDING: " << *BB
1248 << "INTO UNCOND BRANCH PRED: " << *Pred;
1249 UncondBranchPreds.pop_back();
1250 Instruction *UncondBranch = Pred->getTerminator();
1251 // Clone the return and add it to the end of the predecessor.
1252 Instruction *NewRet = RI->clone();
1253 Pred->getInstList().push_back(NewRet);
1255 // If the return instruction returns a value, and if the value was a
1256 // PHI node in "BB", propagate the right value into the return.
1257 if (NewRet->getNumOperands() == 1)
1258 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
1259 if (PN->getParent() == BB)
1260 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
1261 // Update any PHI nodes in the returning block to realize that we no
1262 // longer branch to them.
1263 BB->removePredecessor(Pred);
1264 Pred->getInstList().erase(UncondBranch);
1267 // If we eliminated all predecessors of the block, delete the block now.
1268 if (pred_begin(BB) == pred_end(BB))
1269 // We know there are no successors, so just nuke the block.
1270 M->getBasicBlockList().erase(BB);
1275 // Check out all of the conditional branches going to this return
1276 // instruction. If any of them just select between returns, change the
1277 // branch itself into a select/return pair.
1278 while (!CondBranchPreds.empty()) {
1279 BranchInst *BI = CondBranchPreds.back();
1280 CondBranchPreds.pop_back();
1281 BasicBlock *TrueSucc = BI->getSuccessor(0);
1282 BasicBlock *FalseSucc = BI->getSuccessor(1);
1283 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
1285 // Check to see if the non-BB successor is also a return block.
1286 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
1287 // Check to see if there are only PHI instructions in this block.
1288 BasicBlock::iterator OSI = OtherSucc->getTerminator();
1289 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
1290 // Okay, we found a branch that is going to two return nodes. If
1291 // there is no return value for this function, just change the
1292 // branch into a return.
1293 if (RI->getNumOperands() == 0) {
1294 TrueSucc->removePredecessor(BI->getParent());
1295 FalseSucc->removePredecessor(BI->getParent());
1296 new ReturnInst(0, BI);
1297 BI->getParent()->getInstList().erase(BI);
1301 // Otherwise, figure out what the true and false return values are
1302 // so we can insert a new select instruction.
1303 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
1304 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
1306 // Unwrap any PHI nodes in the return blocks.
1307 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
1308 if (TVPN->getParent() == TrueSucc)
1309 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1310 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
1311 if (FVPN->getParent() == FalseSucc)
1312 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1314 // In order for this transformation to be safe, we must be able to
1315 // unconditionally execute both operands to the return. This is
1316 // normally the case, but we could have a potentially-trapping
1317 // constant expression that prevents this transformation from being
1319 if ((!isa<ConstantExpr>(TrueValue) ||
1320 !cast<ConstantExpr>(TrueValue)->canTrap()) &&
1321 (!isa<ConstantExpr>(TrueValue) ||
1322 !cast<ConstantExpr>(TrueValue)->canTrap())) {
1323 TrueSucc->removePredecessor(BI->getParent());
1324 FalseSucc->removePredecessor(BI->getParent());
1326 // Insert a new select instruction.
1328 Value *BrCond = BI->getCondition();
1329 if (TrueValue != FalseValue)
1330 NewRetVal = new SelectInst(BrCond, TrueValue,
1331 FalseValue, "retval", BI);
1333 NewRetVal = TrueValue;
1335 DOUT << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
1336 << "\n " << *BI << "Select = " << *NewRetVal
1337 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc;
1339 new ReturnInst(NewRetVal, BI);
1340 BI->eraseFromParent();
1341 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond))
1342 if (isInstructionTriviallyDead(BrCondI))
1343 BrCondI->eraseFromParent();
1350 } else if (isa<UnwindInst>(BB->begin())) {
1351 // Check to see if the first instruction in this block is just an unwind.
1352 // If so, replace any invoke instructions which use this as an exception
1353 // destination with call instructions, and any unconditional branch
1354 // predecessor with an unwind.
1356 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1357 while (!Preds.empty()) {
1358 BasicBlock *Pred = Preds.back();
1359 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) {
1360 if (BI->isUnconditional()) {
1361 Pred->getInstList().pop_back(); // nuke uncond branch
1362 new UnwindInst(Pred); // Use unwind.
1365 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
1366 if (II->getUnwindDest() == BB) {
1367 // Insert a new branch instruction before the invoke, because this
1368 // is now a fall through...
1369 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1370 Pred->getInstList().remove(II); // Take out of symbol table
1372 // Insert the call now...
1373 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1374 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1376 CI->setCallingConv(II->getCallingConv());
1377 // If the invoke produced a value, the Call now does instead
1378 II->replaceAllUsesWith(CI);
1386 // If this block is now dead, remove it.
1387 if (pred_begin(BB) == pred_end(BB)) {
1388 // We know there are no successors, so just nuke the block.
1389 M->getBasicBlockList().erase(BB);
1393 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1394 if (isValueEqualityComparison(SI)) {
1395 // If we only have one predecessor, and if it is a branch on this value,
1396 // see if that predecessor totally determines the outcome of this switch.
1397 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1398 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
1399 return SimplifyCFG(BB) || 1;
1401 // If the block only contains the switch, see if we can fold the block
1402 // away into any preds.
1403 if (SI == &BB->front())
1404 if (FoldValueComparisonIntoPredecessors(SI))
1405 return SimplifyCFG(BB) || 1;
1407 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
1408 if (BI->isUnconditional()) {
1409 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
1410 while (isa<PHINode>(*BBI)) ++BBI;
1412 BasicBlock *Succ = BI->getSuccessor(0);
1413 if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
1414 Succ != BB) // Don't hurt infinite loops!
1415 if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ))
1418 } else { // Conditional branch
1419 if (isValueEqualityComparison(BI)) {
1420 // If we only have one predecessor, and if it is a branch on this value,
1421 // see if that predecessor totally determines the outcome of this
1423 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1424 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
1425 return SimplifyCFG(BB) || 1;
1427 // This block must be empty, except for the setcond inst, if it exists.
1428 BasicBlock::iterator I = BB->begin();
1430 (&*I == cast<Instruction>(BI->getCondition()) &&
1432 if (FoldValueComparisonIntoPredecessors(BI))
1433 return SimplifyCFG(BB) | true;
1436 // If this is a branch on a phi node in the current block, thread control
1437 // through this block if any PHI node entries are constants.
1438 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
1439 if (PN->getParent() == BI->getParent())
1440 if (FoldCondBranchOnPHI(BI))
1441 return SimplifyCFG(BB) | true;
1443 // If this basic block is ONLY a setcc and a branch, and if a predecessor
1444 // branches to us and one of our successors, fold the setcc into the
1445 // predecessor and use logical operations to pick the right destination.
1446 BasicBlock *TrueDest = BI->getSuccessor(0);
1447 BasicBlock *FalseDest = BI->getSuccessor(1);
1448 if (Instruction *Cond = dyn_cast<Instruction>(BI->getCondition()))
1449 if ((isa<CmpInst>(Cond) || isa<BinaryOperator>(Cond)) &&
1450 Cond->getParent() == BB && &BB->front() == Cond &&
1451 Cond->getNext() == BI && Cond->hasOneUse() &&
1452 TrueDest != BB && FalseDest != BB)
1453 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
1454 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1455 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
1456 BasicBlock *PredBlock = *PI;
1457 if (PBI->getSuccessor(0) == FalseDest ||
1458 PBI->getSuccessor(1) == TrueDest) {
1459 // Invert the predecessors condition test (xor it with true),
1460 // which allows us to write this code once.
1462 BinaryOperator::createNot(PBI->getCondition(),
1463 PBI->getCondition()->getName()+".not", PBI);
1464 PBI->setCondition(NewCond);
1465 BasicBlock *OldTrue = PBI->getSuccessor(0);
1466 BasicBlock *OldFalse = PBI->getSuccessor(1);
1467 PBI->setSuccessor(0, OldFalse);
1468 PBI->setSuccessor(1, OldTrue);
1471 if ((PBI->getSuccessor(0) == TrueDest && FalseDest != BB) ||
1472 (PBI->getSuccessor(1) == FalseDest && TrueDest != BB)) {
1473 // Clone Cond into the predecessor basic block, and or/and the
1474 // two conditions together.
1475 Instruction *New = Cond->clone();
1476 New->setName(Cond->getName());
1477 Cond->setName(Cond->getName()+".old");
1478 PredBlock->getInstList().insert(PBI, New);
1479 Instruction::BinaryOps Opcode =
1480 PBI->getSuccessor(0) == TrueDest ?
1481 Instruction::Or : Instruction::And;
1483 BinaryOperator::create(Opcode, PBI->getCondition(),
1484 New, "bothcond", PBI);
1485 PBI->setCondition(NewCond);
1486 if (PBI->getSuccessor(0) == BB) {
1487 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1488 PBI->setSuccessor(0, TrueDest);
1490 if (PBI->getSuccessor(1) == BB) {
1491 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1492 PBI->setSuccessor(1, FalseDest);
1494 return SimplifyCFG(BB) | 1;
1498 // Scan predessor blocks for conditional branchs.
1499 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1500 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1501 if (PBI != BI && PBI->isConditional()) {
1503 // If this block ends with a branch instruction, and if there is a
1504 // predecessor that ends on a branch of the same condition, make
1505 // this conditional branch redundant.
1506 if (PBI->getCondition() == BI->getCondition() &&
1507 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1508 // Okay, the outcome of this conditional branch is statically
1509 // knowable. If this block had a single pred, handle specially.
1510 if (BB->getSinglePredecessor()) {
1511 // Turn this into a branch on constant.
1512 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1513 BI->setCondition(ConstantInt::get(Type::Int1Ty, CondIsTrue));
1514 return SimplifyCFG(BB); // Nuke the branch on constant.
1517 // Otherwise, if there are multiple predecessors, insert a PHI
1518 // that merges in the constant and simplify the block result.
1519 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1520 PHINode *NewPN = new PHINode(Type::Int1Ty,
1521 BI->getCondition()->getName()+".pr",
1523 for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1524 if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
1525 PBI != BI && PBI->isConditional() &&
1526 PBI->getCondition() == BI->getCondition() &&
1527 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1528 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1529 NewPN->addIncoming(ConstantInt::get(Type::Int1Ty,
1532 NewPN->addIncoming(BI->getCondition(), *PI);
1535 BI->setCondition(NewPN);
1536 // This will thread the branch.
1537 return SimplifyCFG(BB) | true;
1541 // If this is a conditional branch in an empty block, and if any
1542 // predecessors is a conditional branch to one of our destinations,
1543 // fold the conditions into logical ops and one cond br.
1544 if (&BB->front() == BI) {
1546 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
1548 } else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
1549 PBIOp = 0; BIOp = 1;
1550 } else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
1551 PBIOp = 1; BIOp = 0;
1552 } else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
1558 // Check to make sure that the other destination of this branch
1559 // isn't BB itself. If so, this is an infinite loop that will
1560 // keep getting unwound.
1561 if (PBIOp != -1 && PBI->getSuccessor(PBIOp) == BB)
1564 // Do not perform this transformation if it would require
1565 // insertion of a large number of select instructions. For targets
1566 // without predication/cmovs, this is a big pessimization.
1568 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1570 unsigned NumPhis = 0;
1571 for (BasicBlock::iterator II = CommonDest->begin();
1572 isa<PHINode>(II); ++II, ++NumPhis) {
1574 // Disable this xform.
1581 // Finally, if everything is ok, fold the branches to logical ops.
1583 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1584 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
1586 // If OtherDest *is* BB, then this is a basic block with just
1587 // a conditional branch in it, where one edge (OtherDesg) goes
1588 // back to the block. We know that the program doesn't get
1589 // stuck in the infinite loop, so the condition must be such
1590 // that OtherDest isn't branched through. Forward to CommonDest,
1591 // and avoid an infinite loop at optimizer time.
1592 if (OtherDest == BB)
1593 OtherDest = CommonDest;
1595 DOUT << "FOLDING BRs:" << *PBI->getParent()
1596 << "AND: " << *BI->getParent();
1598 // BI may have other predecessors. Because of this, we leave
1599 // it alone, but modify PBI.
1601 // Make sure we get to CommonDest on True&True directions.
1602 Value *PBICond = PBI->getCondition();
1604 PBICond = BinaryOperator::createNot(PBICond,
1605 PBICond->getName()+".not",
1607 Value *BICond = BI->getCondition();
1609 BICond = BinaryOperator::createNot(BICond,
1610 BICond->getName()+".not",
1612 // Merge the conditions.
1614 BinaryOperator::createOr(PBICond, BICond, "brmerge", PBI);
1616 // Modify PBI to branch on the new condition to the new dests.
1617 PBI->setCondition(Cond);
1618 PBI->setSuccessor(0, CommonDest);
1619 PBI->setSuccessor(1, OtherDest);
1621 // OtherDest may have phi nodes. If so, add an entry from PBI's
1622 // block that are identical to the entries for BI's block.
1624 for (BasicBlock::iterator II = OtherDest->begin();
1625 (PN = dyn_cast<PHINode>(II)); ++II) {
1626 Value *V = PN->getIncomingValueForBlock(BB);
1627 PN->addIncoming(V, PBI->getParent());
1630 // We know that the CommonDest already had an edge from PBI to
1631 // it. If it has PHIs though, the PHIs may have different
1632 // entries for BB and PBI's BB. If so, insert a select to make
1634 for (BasicBlock::iterator II = CommonDest->begin();
1635 (PN = dyn_cast<PHINode>(II)); ++II) {
1636 Value * BIV = PN->getIncomingValueForBlock(BB);
1637 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
1638 Value *PBIV = PN->getIncomingValue(PBBIdx);
1640 // Insert a select in PBI to pick the right value.
1641 Value *NV = new SelectInst(PBICond, PBIV, BIV,
1642 PBIV->getName()+".mux", PBI);
1643 PN->setIncomingValue(PBBIdx, NV);
1647 DOUT << "INTO: " << *PBI->getParent();
1649 // This basic block is probably dead. We know it has at least
1650 // one fewer predecessor.
1651 return SimplifyCFG(BB) | true;
1656 } else if (isa<UnreachableInst>(BB->getTerminator())) {
1657 // If there are any instructions immediately before the unreachable that can
1658 // be removed, do so.
1659 Instruction *Unreachable = BB->getTerminator();
1660 while (Unreachable != BB->begin()) {
1661 BasicBlock::iterator BBI = Unreachable;
1663 if (isa<CallInst>(BBI)) break;
1664 // Delete this instruction
1665 BB->getInstList().erase(BBI);
1669 // If the unreachable instruction is the first in the block, take a gander
1670 // at all of the predecessors of this instruction, and simplify them.
1671 if (&BB->front() == Unreachable) {
1672 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1673 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
1674 TerminatorInst *TI = Preds[i]->getTerminator();
1676 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1677 if (BI->isUnconditional()) {
1678 if (BI->getSuccessor(0) == BB) {
1679 new UnreachableInst(TI);
1680 TI->eraseFromParent();
1684 if (BI->getSuccessor(0) == BB) {
1685 new BranchInst(BI->getSuccessor(1), BI);
1686 BI->eraseFromParent();
1687 } else if (BI->getSuccessor(1) == BB) {
1688 new BranchInst(BI->getSuccessor(0), BI);
1689 BI->eraseFromParent();
1693 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
1694 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1695 if (SI->getSuccessor(i) == BB) {
1696 BB->removePredecessor(SI->getParent());
1701 // If the default value is unreachable, figure out the most popular
1702 // destination and make it the default.
1703 if (SI->getSuccessor(0) == BB) {
1704 std::map<BasicBlock*, unsigned> Popularity;
1705 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1706 Popularity[SI->getSuccessor(i)]++;
1708 // Find the most popular block.
1709 unsigned MaxPop = 0;
1710 BasicBlock *MaxBlock = 0;
1711 for (std::map<BasicBlock*, unsigned>::iterator
1712 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
1713 if (I->second > MaxPop) {
1715 MaxBlock = I->first;
1719 // Make this the new default, allowing us to delete any explicit
1721 SI->setSuccessor(0, MaxBlock);
1724 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
1726 if (isa<PHINode>(MaxBlock->begin()))
1727 for (unsigned i = 0; i != MaxPop-1; ++i)
1728 MaxBlock->removePredecessor(SI->getParent());
1730 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1731 if (SI->getSuccessor(i) == MaxBlock) {
1737 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
1738 if (II->getUnwindDest() == BB) {
1739 // Convert the invoke to a call instruction. This would be a good
1740 // place to note that the call does not throw though.
1741 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1742 II->removeFromParent(); // Take out of symbol table
1744 // Insert the call now...
1745 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1746 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1748 CI->setCallingConv(II->getCallingConv());
1749 // If the invoke produced a value, the Call does now instead.
1750 II->replaceAllUsesWith(CI);
1757 // If this block is now dead, remove it.
1758 if (pred_begin(BB) == pred_end(BB)) {
1759 // We know there are no successors, so just nuke the block.
1760 M->getBasicBlockList().erase(BB);
1766 // Merge basic blocks into their predecessor if there is only one distinct
1767 // pred, and if there is only one distinct successor of the predecessor, and
1768 // if there are no PHI nodes.
1770 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
1771 BasicBlock *OnlyPred = *PI++;
1772 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
1773 if (*PI != OnlyPred) {
1774 OnlyPred = 0; // There are multiple different predecessors...
1778 BasicBlock *OnlySucc = 0;
1779 if (OnlyPred && OnlyPred != BB && // Don't break self loops
1780 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
1781 // Check to see if there is only one distinct successor...
1782 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
1784 for (; SI != SE; ++SI)
1785 if (*SI != OnlySucc) {
1786 OnlySucc = 0; // There are multiple distinct successors!
1792 DOUT << "Merging: " << *BB << "into: " << *OnlyPred;
1794 // Resolve any PHI nodes at the start of the block. They are all
1795 // guaranteed to have exactly one entry if they exist, unless there are
1796 // multiple duplicate (but guaranteed to be equal) entries for the
1797 // incoming edges. This occurs when there are multiple edges from
1798 // OnlyPred to OnlySucc.
1800 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
1801 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1802 BB->getInstList().pop_front(); // Delete the phi node...
1805 // Delete the unconditional branch from the predecessor...
1806 OnlyPred->getInstList().pop_back();
1808 // Move all definitions in the successor to the predecessor...
1809 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
1811 // Make all PHI nodes that referred to BB now refer to Pred as their
1813 BB->replaceAllUsesWith(OnlyPred);
1815 std::string OldName = BB->getName();
1817 // Erase basic block from the function...
1818 M->getBasicBlockList().erase(BB);
1820 // Inherit predecessors name if it exists...
1821 if (!OldName.empty() && !OnlyPred->hasName())
1822 OnlyPred->setName(OldName);
1827 // Otherwise, if this block only has a single predecessor, and if that block
1828 // is a conditional branch, see if we can hoist any code from this block up
1829 // into our predecessor.
1831 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
1832 if (BI->isConditional()) {
1833 // Get the other block.
1834 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
1835 PI = pred_begin(OtherBB);
1837 if (PI == pred_end(OtherBB)) {
1838 // We have a conditional branch to two blocks that are only reachable
1839 // from the condbr. We know that the condbr dominates the two blocks,
1840 // so see if there is any identical code in the "then" and "else"
1841 // blocks. If so, we can hoist it up to the branching block.
1842 Changed |= HoistThenElseCodeToIf(BI);
1846 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1847 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1848 // Change br (X == 0 | X == 1), T, F into a switch instruction.
1849 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
1850 Instruction *Cond = cast<Instruction>(BI->getCondition());
1851 // If this is a bunch of seteq's or'd together, or if it's a bunch of
1852 // 'setne's and'ed together, collect them.
1854 std::vector<ConstantInt*> Values;
1855 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
1856 if (CompVal && CompVal->getType()->isInteger()) {
1857 // There might be duplicate constants in the list, which the switch
1858 // instruction can't handle, remove them now.
1859 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
1860 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
1862 // Figure out which block is which destination.
1863 BasicBlock *DefaultBB = BI->getSuccessor(1);
1864 BasicBlock *EdgeBB = BI->getSuccessor(0);
1865 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
1867 // Create the new switch instruction now.
1868 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI);
1870 // Add all of the 'cases' to the switch instruction.
1871 for (unsigned i = 0, e = Values.size(); i != e; ++i)
1872 New->addCase(Values[i], EdgeBB);
1874 // We added edges from PI to the EdgeBB. As such, if there were any
1875 // PHI nodes in EdgeBB, they need entries to be added corresponding to
1876 // the number of edges added.
1877 for (BasicBlock::iterator BBI = EdgeBB->begin();
1878 isa<PHINode>(BBI); ++BBI) {
1879 PHINode *PN = cast<PHINode>(BBI);
1880 Value *InVal = PN->getIncomingValueForBlock(*PI);
1881 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
1882 PN->addIncoming(InVal, *PI);
1885 // Erase the old branch instruction.
1886 (*PI)->getInstList().erase(BI);
1888 // Erase the potentially condition tree that was used to computed the
1889 // branch condition.
1890 ErasePossiblyDeadInstructionTree(Cond);
1895 // If there is a trivial two-entry PHI node in this basic block, and we can
1896 // eliminate it, do so now.
1897 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1898 if (PN->getNumIncomingValues() == 2)
1899 Changed |= FoldTwoEntryPHINode(PN);