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/Analysis/ConstantFolding.h"
23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
30 /// SafeToMergeTerminators - Return true if it is safe to merge these two
31 /// terminator instructions together.
33 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
34 if (SI1 == SI2) return false; // Can't merge with self!
36 // It is not safe to merge these two switch instructions if they have a common
37 // successor, and if that successor has a PHI node, and if *that* PHI node has
38 // conflicting incoming values from the two switch blocks.
39 BasicBlock *SI1BB = SI1->getParent();
40 BasicBlock *SI2BB = SI2->getParent();
41 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
43 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
44 if (SI1Succs.count(*I))
45 for (BasicBlock::iterator BBI = (*I)->begin();
46 isa<PHINode>(BBI); ++BBI) {
47 PHINode *PN = cast<PHINode>(BBI);
48 if (PN->getIncomingValueForBlock(SI1BB) !=
49 PN->getIncomingValueForBlock(SI2BB))
56 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
57 /// now be entries in it from the 'NewPred' block. The values that will be
58 /// flowing into the PHI nodes will be the same as those coming in from
59 /// ExistPred, an existing predecessor of Succ.
60 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
61 BasicBlock *ExistPred) {
62 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
63 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
64 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
66 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
67 PHINode *PN = cast<PHINode>(I);
68 Value *V = PN->getIncomingValueForBlock(ExistPred);
69 PN->addIncoming(V, NewPred);
73 // CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
74 // almost-empty BB ending in an unconditional branch to Succ, into succ.
76 // Assumption: Succ is the single successor for BB.
78 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
79 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
81 // Check to see if one of the predecessors of BB is already a predecessor of
82 // Succ. If so, we cannot do the transformation if there are any PHI nodes
83 // with incompatible values coming in from the two edges!
85 if (isa<PHINode>(Succ->front())) {
86 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
87 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
89 if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
90 // Loop over all of the PHI nodes checking to see if there are
91 // incompatible values coming in.
92 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
93 PHINode *PN = cast<PHINode>(I);
94 // Loop up the entries in the PHI node for BB and for *PI if the
95 // values coming in are non-equal, we cannot merge these two blocks
96 // (instead we should insert a conditional move or something, then
98 if (PN->getIncomingValueForBlock(BB) !=
99 PN->getIncomingValueForBlock(*PI))
100 return false; // Values are not equal...
105 // Finally, if BB has PHI nodes that are used by things other than the PHIs in
106 // Succ and Succ has predecessors that are not Succ and not Pred, we cannot
107 // fold these blocks, as we don't know whether BB dominates Succ or not to
108 // update the PHI nodes correctly.
109 if (!isa<PHINode>(BB->begin()) || Succ->getSinglePredecessor()) return true;
111 // If the predecessors of Succ are only BB and Succ itself, we can handle this.
113 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
114 if (*PI != Succ && *PI != BB) {
118 if (IsSafe) return true;
120 // If the PHI nodes in BB are only used by instructions in Succ, we are ok if
121 // BB and Succ have no common predecessors.
122 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) {
123 PHINode *PN = cast<PHINode>(I);
124 for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end(); UI != E;
126 if (cast<Instruction>(*UI)->getParent() != Succ)
130 // Scan the predecessor sets of BB and Succ, making sure there are no common
131 // predecessors. Common predecessors would cause us to build a phi node with
132 // differing incoming values, which is not legal.
133 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
134 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
135 if (BBPreds.count(*PI))
141 /// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional
142 /// branch to Succ, and contains no instructions other than PHI nodes and the
143 /// branch. If possible, eliminate BB.
144 static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
146 // If our successor has PHI nodes, then we need to update them to include
147 // entries for BB's predecessors, not for BB itself. Be careful though,
148 // if this transformation fails (returns true) then we cannot do this
151 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
153 DOUT << "Killing Trivial BB: \n" << *BB;
155 if (isa<PHINode>(Succ->begin())) {
156 // If there is more than one pred of succ, and there are PHI nodes in
157 // the successor, then we need to add incoming edges for the PHI nodes
159 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
161 // Loop over all of the PHI nodes in the successor of BB.
162 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
163 PHINode *PN = cast<PHINode>(I);
164 Value *OldVal = PN->removeIncomingValue(BB, false);
165 assert(OldVal && "No entry in PHI for Pred BB!");
167 // If this incoming value is one of the PHI nodes in BB, the new entries
168 // in the PHI node are the entries from the old PHI.
169 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
170 PHINode *OldValPN = cast<PHINode>(OldVal);
171 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
172 PN->addIncoming(OldValPN->getIncomingValue(i),
173 OldValPN->getIncomingBlock(i));
175 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
176 End = BBPreds.end(); PredI != End; ++PredI) {
177 // Add an incoming value for each of the new incoming values...
178 PN->addIncoming(OldVal, *PredI);
184 if (isa<PHINode>(&BB->front())) {
185 std::vector<BasicBlock*>
186 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
188 // Move all PHI nodes in BB to Succ if they are alive, otherwise
190 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
191 if (PN->use_empty()) {
192 // Just remove the dead phi. This happens if Succ's PHIs were the only
193 // users of the PHI nodes.
194 PN->eraseFromParent();
196 // The instruction is alive, so this means that Succ must have
197 // *ONLY* had BB as a predecessor, and the PHI node is still valid
198 // now. Simply move it into Succ, because we know that BB
199 // strictly dominated Succ.
200 Succ->getInstList().splice(Succ->begin(),
201 BB->getInstList(), BB->begin());
203 // We need to add new entries for the PHI node to account for
204 // predecessors of Succ that the PHI node does not take into
205 // account. At this point, since we know that BB dominated succ,
206 // this means that we should any newly added incoming edges should
207 // use the PHI node as the value for these edges, because they are
209 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
210 if (OldSuccPreds[i] != BB)
211 PN->addIncoming(PN, OldSuccPreds[i]);
215 // Everything that jumped to BB now goes to Succ.
216 BB->replaceAllUsesWith(Succ);
217 if (!Succ->hasName()) Succ->takeName(BB);
218 BB->eraseFromParent(); // Delete the old basic block.
222 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
223 /// presumably PHI nodes in it), check to see if the merge at this block is due
224 /// to an "if condition". If so, return the boolean condition that determines
225 /// which entry into BB will be taken. Also, return by references the block
226 /// that will be entered from if the condition is true, and the block that will
227 /// be entered if the condition is false.
230 static Value *GetIfCondition(BasicBlock *BB,
231 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
232 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
233 "Function can only handle blocks with 2 predecessors!");
234 BasicBlock *Pred1 = *pred_begin(BB);
235 BasicBlock *Pred2 = *++pred_begin(BB);
237 // We can only handle branches. Other control flow will be lowered to
238 // branches if possible anyway.
239 if (!isa<BranchInst>(Pred1->getTerminator()) ||
240 !isa<BranchInst>(Pred2->getTerminator()))
242 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
243 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
245 // Eliminate code duplication by ensuring that Pred1Br is conditional if
247 if (Pred2Br->isConditional()) {
248 // If both branches are conditional, we don't have an "if statement". In
249 // reality, we could transform this case, but since the condition will be
250 // required anyway, we stand no chance of eliminating it, so the xform is
251 // probably not profitable.
252 if (Pred1Br->isConditional())
255 std::swap(Pred1, Pred2);
256 std::swap(Pred1Br, Pred2Br);
259 if (Pred1Br->isConditional()) {
260 // If we found a conditional branch predecessor, make sure that it branches
261 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
262 if (Pred1Br->getSuccessor(0) == BB &&
263 Pred1Br->getSuccessor(1) == Pred2) {
266 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
267 Pred1Br->getSuccessor(1) == BB) {
271 // We know that one arm of the conditional goes to BB, so the other must
272 // go somewhere unrelated, and this must not be an "if statement".
276 // The only thing we have to watch out for here is to make sure that Pred2
277 // doesn't have incoming edges from other blocks. If it does, the condition
278 // doesn't dominate BB.
279 if (++pred_begin(Pred2) != pred_end(Pred2))
282 return Pred1Br->getCondition();
285 // Ok, if we got here, both predecessors end with an unconditional branch to
286 // BB. Don't panic! If both blocks only have a single (identical)
287 // predecessor, and THAT is a conditional branch, then we're all ok!
288 if (pred_begin(Pred1) == pred_end(Pred1) ||
289 ++pred_begin(Pred1) != pred_end(Pred1) ||
290 pred_begin(Pred2) == pred_end(Pred2) ||
291 ++pred_begin(Pred2) != pred_end(Pred2) ||
292 *pred_begin(Pred1) != *pred_begin(Pred2))
295 // Otherwise, if this is a conditional branch, then we can use it!
296 BasicBlock *CommonPred = *pred_begin(Pred1);
297 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
298 assert(BI->isConditional() && "Two successors but not conditional?");
299 if (BI->getSuccessor(0) == Pred1) {
306 return BI->getCondition();
312 // If we have a merge point of an "if condition" as accepted above, return true
313 // if the specified value dominates the block. We don't handle the true
314 // generality of domination here, just a special case which works well enough
317 // If AggressiveInsts is non-null, and if V does not dominate BB, we check to
318 // see if V (which must be an instruction) is cheap to compute and is
319 // non-trapping. If both are true, the instruction is inserted into the set and
321 static bool DominatesMergePoint(Value *V, BasicBlock *BB,
322 std::set<Instruction*> *AggressiveInsts) {
323 Instruction *I = dyn_cast<Instruction>(V);
325 // Non-instructions all dominate instructions, but not all constantexprs
326 // can be executed unconditionally.
327 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V))
332 BasicBlock *PBB = I->getParent();
334 // We don't want to allow weird loops that might have the "if condition" in
335 // the bottom of this block.
336 if (PBB == BB) return false;
338 // If this instruction is defined in a block that contains an unconditional
339 // branch to BB, then it must be in the 'conditional' part of the "if
341 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
342 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
343 if (!AggressiveInsts) return false;
344 // Okay, it looks like the instruction IS in the "condition". Check to
345 // see if its a cheap instruction to unconditionally compute, and if it
346 // only uses stuff defined outside of the condition. If so, hoist it out.
347 switch (I->getOpcode()) {
348 default: return false; // Cannot hoist this out safely.
349 case Instruction::Load:
350 // We can hoist loads that are non-volatile and obviously cannot trap.
351 if (cast<LoadInst>(I)->isVolatile())
353 if (!isa<AllocaInst>(I->getOperand(0)) &&
354 !isa<Constant>(I->getOperand(0)))
357 // Finally, we have to check to make sure there are no instructions
358 // before the load in its basic block, as we are going to hoist the loop
359 // out to its predecessor.
360 if (PBB->begin() != BasicBlock::iterator(I))
363 case Instruction::Add:
364 case Instruction::Sub:
365 case Instruction::And:
366 case Instruction::Or:
367 case Instruction::Xor:
368 case Instruction::Shl:
369 case Instruction::LShr:
370 case Instruction::AShr:
371 case Instruction::ICmp:
372 case Instruction::FCmp:
373 break; // These are all cheap and non-trapping instructions.
376 // Okay, we can only really hoist these out if their operands are not
377 // defined in the conditional region.
378 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
379 if (!DominatesMergePoint(I->getOperand(i), BB, 0))
381 // Okay, it's safe to do this! Remember this instruction.
382 AggressiveInsts->insert(I);
388 // GatherConstantSetEQs - Given a potentially 'or'd together collection of
389 // icmp_eq instructions that compare a value against a constant, return the
390 // value being compared, and stick the constant into the Values vector.
391 static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
392 if (Instruction *Inst = dyn_cast<Instruction>(V))
393 if (Inst->getOpcode() == Instruction::ICmp &&
394 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_EQ) {
395 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
397 return Inst->getOperand(0);
398 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
400 return Inst->getOperand(1);
402 } else if (Inst->getOpcode() == Instruction::Or) {
403 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
404 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
411 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
412 // setne instructions that compare a value against a constant, return the value
413 // being compared, and stick the constant into the Values vector.
414 static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
415 if (Instruction *Inst = dyn_cast<Instruction>(V))
416 if (Inst->getOpcode() == Instruction::ICmp &&
417 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_NE) {
418 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
420 return Inst->getOperand(0);
421 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
423 return Inst->getOperand(1);
425 } else if (Inst->getOpcode() == Instruction::And) {
426 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
427 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
436 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
437 /// bunch of comparisons of one value against constants, return the value and
438 /// the constants being compared.
439 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
440 std::vector<ConstantInt*> &Values) {
441 if (Cond->getOpcode() == Instruction::Or) {
442 CompVal = GatherConstantSetEQs(Cond, Values);
444 // Return true to indicate that the condition is true if the CompVal is
445 // equal to one of the constants.
447 } else if (Cond->getOpcode() == Instruction::And) {
448 CompVal = GatherConstantSetNEs(Cond, Values);
450 // Return false to indicate that the condition is false if the CompVal is
451 // equal to one of the constants.
457 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
458 /// has no side effects, nuke it. If it uses any instructions that become dead
459 /// because the instruction is now gone, nuke them too.
460 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
461 if (!isInstructionTriviallyDead(I)) return;
463 std::vector<Instruction*> InstrsToInspect;
464 InstrsToInspect.push_back(I);
466 while (!InstrsToInspect.empty()) {
467 I = InstrsToInspect.back();
468 InstrsToInspect.pop_back();
470 if (!isInstructionTriviallyDead(I)) continue;
472 // If I is in the work list multiple times, remove previous instances.
473 for (unsigned i = 0, e = InstrsToInspect.size(); i != e; ++i)
474 if (InstrsToInspect[i] == I) {
475 InstrsToInspect.erase(InstrsToInspect.begin()+i);
479 // Add operands of dead instruction to worklist.
480 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
481 if (Instruction *OpI = dyn_cast<Instruction>(I->getOperand(i)))
482 InstrsToInspect.push_back(OpI);
484 // Remove dead instruction.
485 I->eraseFromParent();
489 // isValueEqualityComparison - Return true if the specified terminator checks to
490 // see if a value is equal to constant integer value.
491 static Value *isValueEqualityComparison(TerminatorInst *TI) {
492 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
493 // Do not permit merging of large switch instructions into their
494 // predecessors unless there is only one predecessor.
495 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
496 pred_end(SI->getParent())) > 128)
499 return SI->getCondition();
501 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
502 if (BI->isConditional() && BI->getCondition()->hasOneUse())
503 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition()))
504 if ((ICI->getPredicate() == ICmpInst::ICMP_EQ ||
505 ICI->getPredicate() == ICmpInst::ICMP_NE) &&
506 isa<ConstantInt>(ICI->getOperand(1)))
507 return ICI->getOperand(0);
511 // Given a value comparison instruction, decode all of the 'cases' that it
512 // represents and return the 'default' block.
514 GetValueEqualityComparisonCases(TerminatorInst *TI,
515 std::vector<std::pair<ConstantInt*,
516 BasicBlock*> > &Cases) {
517 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
518 Cases.reserve(SI->getNumCases());
519 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
520 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i)));
521 return SI->getDefaultDest();
524 BranchInst *BI = cast<BranchInst>(TI);
525 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition());
526 Cases.push_back(std::make_pair(cast<ConstantInt>(ICI->getOperand(1)),
527 BI->getSuccessor(ICI->getPredicate() ==
528 ICmpInst::ICMP_NE)));
529 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ);
533 // EliminateBlockCases - Given an vector of bb/value pairs, remove any entries
534 // in the list that match the specified block.
535 static void EliminateBlockCases(BasicBlock *BB,
536 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) {
537 for (unsigned i = 0, e = Cases.size(); i != e; ++i)
538 if (Cases[i].second == BB) {
539 Cases.erase(Cases.begin()+i);
544 // ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as
547 ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1,
548 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) {
549 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2;
551 // Make V1 be smaller than V2.
552 if (V1->size() > V2->size())
555 if (V1->size() == 0) return false;
556 if (V1->size() == 1) {
558 ConstantInt *TheVal = (*V1)[0].first;
559 for (unsigned i = 0, e = V2->size(); i != e; ++i)
560 if (TheVal == (*V2)[i].first)
564 // Otherwise, just sort both lists and compare element by element.
565 std::sort(V1->begin(), V1->end());
566 std::sort(V2->begin(), V2->end());
567 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size();
568 while (i1 != e1 && i2 != e2) {
569 if ((*V1)[i1].first == (*V2)[i2].first)
571 if ((*V1)[i1].first < (*V2)[i2].first)
579 // SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a
580 // terminator instruction and its block is known to only have a single
581 // predecessor block, check to see if that predecessor is also a value
582 // comparison with the same value, and if that comparison determines the outcome
583 // of this comparison. If so, simplify TI. This does a very limited form of
585 static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI,
587 Value *PredVal = isValueEqualityComparison(Pred->getTerminator());
588 if (!PredVal) return false; // Not a value comparison in predecessor.
590 Value *ThisVal = isValueEqualityComparison(TI);
591 assert(ThisVal && "This isn't a value comparison!!");
592 if (ThisVal != PredVal) return false; // Different predicates.
594 // Find out information about when control will move from Pred to TI's block.
595 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
596 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(),
598 EliminateBlockCases(PredDef, PredCases); // Remove default from cases.
600 // Find information about how control leaves this block.
601 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases;
602 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases);
603 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases.
605 // If TI's block is the default block from Pred's comparison, potentially
606 // simplify TI based on this knowledge.
607 if (PredDef == TI->getParent()) {
608 // If we are here, we know that the value is none of those cases listed in
609 // PredCases. If there are any cases in ThisCases that are in PredCases, we
611 if (ValuesOverlap(PredCases, ThisCases)) {
612 if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) {
613 // Okay, one of the successors of this condbr is dead. Convert it to a
615 assert(ThisCases.size() == 1 && "Branch can only have one case!");
616 Value *Cond = BTI->getCondition();
617 // Insert the new branch.
618 Instruction *NI = new BranchInst(ThisDef, TI);
620 // Remove PHI node entries for the dead edge.
621 ThisCases[0].second->removePredecessor(TI->getParent());
623 DOUT << "Threading pred instr: " << *Pred->getTerminator()
624 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n";
626 TI->eraseFromParent(); // Nuke the old one.
627 // If condition is now dead, nuke it.
628 if (Instruction *CondI = dyn_cast<Instruction>(Cond))
629 ErasePossiblyDeadInstructionTree(CondI);
633 SwitchInst *SI = cast<SwitchInst>(TI);
634 // Okay, TI has cases that are statically dead, prune them away.
635 std::set<Constant*> DeadCases;
636 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
637 DeadCases.insert(PredCases[i].first);
639 DOUT << "Threading pred instr: " << *Pred->getTerminator()
640 << "Through successor TI: " << *TI;
642 for (unsigned i = SI->getNumCases()-1; i != 0; --i)
643 if (DeadCases.count(SI->getCaseValue(i))) {
644 SI->getSuccessor(i)->removePredecessor(TI->getParent());
648 DOUT << "Leaving: " << *TI << "\n";
654 // Otherwise, TI's block must correspond to some matched value. Find out
655 // which value (or set of values) this is.
656 ConstantInt *TIV = 0;
657 BasicBlock *TIBB = TI->getParent();
658 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
659 if (PredCases[i].second == TIBB)
661 TIV = PredCases[i].first;
663 return false; // Cannot handle multiple values coming to this block.
664 assert(TIV && "No edge from pred to succ?");
666 // Okay, we found the one constant that our value can be if we get into TI's
667 // BB. Find out which successor will unconditionally be branched to.
668 BasicBlock *TheRealDest = 0;
669 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i)
670 if (ThisCases[i].first == TIV) {
671 TheRealDest = ThisCases[i].second;
675 // If not handled by any explicit cases, it is handled by the default case.
676 if (TheRealDest == 0) TheRealDest = ThisDef;
678 // Remove PHI node entries for dead edges.
679 BasicBlock *CheckEdge = TheRealDest;
680 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI)
681 if (*SI != CheckEdge)
682 (*SI)->removePredecessor(TIBB);
686 // Insert the new branch.
687 Instruction *NI = new BranchInst(TheRealDest, TI);
689 DOUT << "Threading pred instr: " << *Pred->getTerminator()
690 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n";
691 Instruction *Cond = 0;
692 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
693 Cond = dyn_cast<Instruction>(BI->getCondition());
694 TI->eraseFromParent(); // Nuke the old one.
696 if (Cond) ErasePossiblyDeadInstructionTree(Cond);
702 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
703 // equality comparison instruction (either a switch or a branch on "X == c").
704 // See if any of the predecessors of the terminator block are value comparisons
705 // on the same value. If so, and if safe to do so, fold them together.
706 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
707 BasicBlock *BB = TI->getParent();
708 Value *CV = isValueEqualityComparison(TI); // CondVal
709 assert(CV && "Not a comparison?");
710 bool Changed = false;
712 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
713 while (!Preds.empty()) {
714 BasicBlock *Pred = Preds.back();
717 // See if the predecessor is a comparison with the same value.
718 TerminatorInst *PTI = Pred->getTerminator();
719 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
721 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
722 // Figure out which 'cases' to copy from SI to PSI.
723 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
724 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
726 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
727 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
729 // Based on whether the default edge from PTI goes to BB or not, fill in
730 // PredCases and PredDefault with the new switch cases we would like to
732 std::vector<BasicBlock*> NewSuccessors;
734 if (PredDefault == BB) {
735 // If this is the default destination from PTI, only the edges in TI
736 // that don't occur in PTI, or that branch to BB will be activated.
737 std::set<ConstantInt*> PTIHandled;
738 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
739 if (PredCases[i].second != BB)
740 PTIHandled.insert(PredCases[i].first);
742 // The default destination is BB, we don't need explicit targets.
743 std::swap(PredCases[i], PredCases.back());
744 PredCases.pop_back();
748 // Reconstruct the new switch statement we will be building.
749 if (PredDefault != BBDefault) {
750 PredDefault->removePredecessor(Pred);
751 PredDefault = BBDefault;
752 NewSuccessors.push_back(BBDefault);
754 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
755 if (!PTIHandled.count(BBCases[i].first) &&
756 BBCases[i].second != BBDefault) {
757 PredCases.push_back(BBCases[i]);
758 NewSuccessors.push_back(BBCases[i].second);
762 // If this is not the default destination from PSI, only the edges
763 // in SI that occur in PSI with a destination of BB will be
765 std::set<ConstantInt*> PTIHandled;
766 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
767 if (PredCases[i].second == BB) {
768 PTIHandled.insert(PredCases[i].first);
769 std::swap(PredCases[i], PredCases.back());
770 PredCases.pop_back();
774 // Okay, now we know which constants were sent to BB from the
775 // predecessor. Figure out where they will all go now.
776 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
777 if (PTIHandled.count(BBCases[i].first)) {
778 // If this is one we are capable of getting...
779 PredCases.push_back(BBCases[i]);
780 NewSuccessors.push_back(BBCases[i].second);
781 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
784 // If there are any constants vectored to BB that TI doesn't handle,
785 // they must go to the default destination of TI.
786 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
787 E = PTIHandled.end(); I != E; ++I) {
788 PredCases.push_back(std::make_pair(*I, BBDefault));
789 NewSuccessors.push_back(BBDefault);
793 // Okay, at this point, we know which new successor Pred will get. Make
794 // sure we update the number of entries in the PHI nodes for these
796 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
797 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
799 // Now that the successors are updated, create the new Switch instruction.
800 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI);
801 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
802 NewSI->addCase(PredCases[i].first, PredCases[i].second);
804 Instruction *DeadCond = 0;
805 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
806 // If PTI is a branch, remember the condition.
807 DeadCond = dyn_cast<Instruction>(BI->getCondition());
808 Pred->getInstList().erase(PTI);
810 // If the condition is dead now, remove the instruction tree.
811 if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond);
813 // Okay, last check. If BB is still a successor of PSI, then we must
814 // have an infinite loop case. If so, add an infinitely looping block
815 // to handle the case to preserve the behavior of the code.
816 BasicBlock *InfLoopBlock = 0;
817 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
818 if (NewSI->getSuccessor(i) == BB) {
819 if (InfLoopBlock == 0) {
820 // Insert it at the end of the loop, because it's either code,
821 // or it won't matter if it's hot. :)
822 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
823 new BranchInst(InfLoopBlock, InfLoopBlock);
825 NewSI->setSuccessor(i, InfLoopBlock);
834 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
835 /// BB2, hoist any common code in the two blocks up into the branch block. The
836 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
837 static bool HoistThenElseCodeToIf(BranchInst *BI) {
838 // This does very trivial matching, with limited scanning, to find identical
839 // instructions in the two blocks. In particular, we don't want to get into
840 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As
841 // such, we currently just scan for obviously identical instructions in an
843 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination.
844 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination
846 Instruction *I1 = BB1->begin(), *I2 = BB2->begin();
847 if (I1->getOpcode() != I2->getOpcode() || isa<PHINode>(I1) ||
848 isa<InvokeInst>(I1) || !I1->isIdenticalTo(I2))
851 // If we get here, we can hoist at least one instruction.
852 BasicBlock *BIParent = BI->getParent();
855 // If we are hoisting the terminator instruction, don't move one (making a
856 // broken BB), instead clone it, and remove BI.
857 if (isa<TerminatorInst>(I1))
858 goto HoistTerminator;
860 // For a normal instruction, we just move one to right before the branch,
861 // then replace all uses of the other with the first. Finally, we remove
862 // the now redundant second instruction.
863 BIParent->getInstList().splice(BI, BB1->getInstList(), I1);
864 if (!I2->use_empty())
865 I2->replaceAllUsesWith(I1);
866 BB2->getInstList().erase(I2);
870 } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2));
875 // Okay, it is safe to hoist the terminator.
876 Instruction *NT = I1->clone();
877 BIParent->getInstList().insert(BI, NT);
878 if (NT->getType() != Type::VoidTy) {
879 I1->replaceAllUsesWith(NT);
880 I2->replaceAllUsesWith(NT);
884 // Hoisting one of the terminators from our successor is a great thing.
885 // Unfortunately, the successors of the if/else blocks may have PHI nodes in
886 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI
887 // nodes, so we insert select instruction to compute the final result.
888 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects;
889 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) {
891 for (BasicBlock::iterator BBI = SI->begin();
892 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
893 Value *BB1V = PN->getIncomingValueForBlock(BB1);
894 Value *BB2V = PN->getIncomingValueForBlock(BB2);
896 // These values do not agree. Insert a select instruction before NT
897 // that determines the right value.
898 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)];
900 SI = new SelectInst(BI->getCondition(), BB1V, BB2V,
901 BB1V->getName()+"."+BB2V->getName(), NT);
902 // Make the PHI node use the select for all incoming values for BB1/BB2
903 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
904 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2)
905 PN->setIncomingValue(i, SI);
910 // Update any PHI nodes in our new successors.
911 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI)
912 AddPredecessorToBlock(*SI, BIParent, BB1);
914 BI->eraseFromParent();
918 /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
919 /// across this block.
920 static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
921 BranchInst *BI = cast<BranchInst>(BB->getTerminator());
924 // If this basic block contains anything other than a PHI (which controls the
925 // branch) and branch itself, bail out. FIXME: improve this in the future.
926 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI, ++Size) {
927 if (Size > 10) return false; // Don't clone large BB's.
929 // We can only support instructions that are do not define values that are
930 // live outside of the current basic block.
931 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
933 Instruction *U = cast<Instruction>(*UI);
934 if (U->getParent() != BB || isa<PHINode>(U)) return false;
937 // Looks ok, continue checking.
943 /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
944 /// that is defined in the same block as the branch and if any PHI entries are
945 /// constants, thread edges corresponding to that entry to be branches to their
946 /// ultimate destination.
947 static bool FoldCondBranchOnPHI(BranchInst *BI) {
948 BasicBlock *BB = BI->getParent();
949 PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
950 // NOTE: we currently cannot transform this case if the PHI node is used
951 // outside of the block.
952 if (!PN || PN->getParent() != BB || !PN->hasOneUse())
955 // Degenerate case of a single entry PHI.
956 if (PN->getNumIncomingValues() == 1) {
957 if (PN->getIncomingValue(0) != PN)
958 PN->replaceAllUsesWith(PN->getIncomingValue(0));
960 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
961 PN->eraseFromParent();
965 // Now we know that this block has multiple preds and two succs.
966 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
968 // Okay, this is a simple enough basic block. See if any phi values are
970 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
972 if ((CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i))) &&
973 CB->getType() == Type::Int1Ty) {
974 // Okay, we now know that all edges from PredBB should be revectored to
975 // branch to RealDest.
976 BasicBlock *PredBB = PN->getIncomingBlock(i);
977 BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue());
979 if (RealDest == BB) continue; // Skip self loops.
981 // The dest block might have PHI nodes, other predecessors and other
982 // difficult cases. Instead of being smart about this, just insert a new
983 // block that jumps to the destination block, effectively splitting
984 // the edge we are about to create.
985 BasicBlock *EdgeBB = new BasicBlock(RealDest->getName()+".critedge",
986 RealDest->getParent(), RealDest);
987 new BranchInst(RealDest, EdgeBB);
989 for (BasicBlock::iterator BBI = RealDest->begin();
990 (PN = dyn_cast<PHINode>(BBI)); ++BBI) {
991 Value *V = PN->getIncomingValueForBlock(BB);
992 PN->addIncoming(V, EdgeBB);
995 // BB may have instructions that are being threaded over. Clone these
996 // instructions into EdgeBB. We know that there will be no uses of the
997 // cloned instructions outside of EdgeBB.
998 BasicBlock::iterator InsertPt = EdgeBB->begin();
999 std::map<Value*, Value*> TranslateMap; // Track translated values.
1000 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
1001 if (PHINode *PN = dyn_cast<PHINode>(BBI)) {
1002 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB);
1004 // Clone the instruction.
1005 Instruction *N = BBI->clone();
1006 if (BBI->hasName()) N->setName(BBI->getName()+".c");
1008 // Update operands due to translation.
1009 for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i) {
1010 std::map<Value*, Value*>::iterator PI =
1011 TranslateMap.find(N->getOperand(i));
1012 if (PI != TranslateMap.end())
1013 N->setOperand(i, PI->second);
1016 // Check for trivial simplification.
1017 if (Constant *C = ConstantFoldInstruction(N)) {
1018 TranslateMap[BBI] = C;
1019 delete N; // Constant folded away, don't need actual inst
1021 // Insert the new instruction into its new home.
1022 EdgeBB->getInstList().insert(InsertPt, N);
1023 if (!BBI->use_empty())
1024 TranslateMap[BBI] = N;
1029 // Loop over all of the edges from PredBB to BB, changing them to branch
1030 // to EdgeBB instead.
1031 TerminatorInst *PredBBTI = PredBB->getTerminator();
1032 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i)
1033 if (PredBBTI->getSuccessor(i) == BB) {
1034 BB->removePredecessor(PredBB);
1035 PredBBTI->setSuccessor(i, EdgeBB);
1038 // Recurse, simplifying any other constants.
1039 return FoldCondBranchOnPHI(BI) | true;
1046 /// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry
1047 /// PHI node, see if we can eliminate it.
1048 static bool FoldTwoEntryPHINode(PHINode *PN) {
1049 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
1050 // statement", which has a very simple dominance structure. Basically, we
1051 // are trying to find the condition that is being branched on, which
1052 // subsequently causes this merge to happen. We really want control
1053 // dependence information for this check, but simplifycfg can't keep it up
1054 // to date, and this catches most of the cases we care about anyway.
1056 BasicBlock *BB = PN->getParent();
1057 BasicBlock *IfTrue, *IfFalse;
1058 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
1059 if (!IfCond) return false;
1061 // Okay, we found that we can merge this two-entry phi node into a select.
1062 // Doing so would require us to fold *all* two entry phi nodes in this block.
1063 // At some point this becomes non-profitable (particularly if the target
1064 // doesn't support cmov's). Only do this transformation if there are two or
1065 // fewer PHI nodes in this block.
1066 unsigned NumPhis = 0;
1067 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I)
1071 DOUT << "FOUND IF CONDITION! " << *IfCond << " T: "
1072 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n";
1074 // Loop over the PHI's seeing if we can promote them all to select
1075 // instructions. While we are at it, keep track of the instructions
1076 // that need to be moved to the dominating block.
1077 std::set<Instruction*> AggressiveInsts;
1079 BasicBlock::iterator AfterPHIIt = BB->begin();
1080 while (isa<PHINode>(AfterPHIIt)) {
1081 PHINode *PN = cast<PHINode>(AfterPHIIt++);
1082 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) {
1083 if (PN->getIncomingValue(0) != PN)
1084 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1086 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
1087 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
1088 &AggressiveInsts) ||
1089 !DominatesMergePoint(PN->getIncomingValue(1), BB,
1090 &AggressiveInsts)) {
1095 // If we all PHI nodes are promotable, check to make sure that all
1096 // instructions in the predecessor blocks can be promoted as well. If
1097 // not, we won't be able to get rid of the control flow, so it's not
1098 // worth promoting to select instructions.
1099 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0;
1100 PN = cast<PHINode>(BB->begin());
1101 BasicBlock *Pred = PN->getIncomingBlock(0);
1102 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1104 DomBlock = *pred_begin(Pred);
1105 for (BasicBlock::iterator I = Pred->begin();
1106 !isa<TerminatorInst>(I); ++I)
1107 if (!AggressiveInsts.count(I)) {
1108 // This is not an aggressive instruction that we can promote.
1109 // Because of this, we won't be able to get rid of the control
1110 // flow, so the xform is not worth it.
1115 Pred = PN->getIncomingBlock(1);
1116 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
1118 DomBlock = *pred_begin(Pred);
1119 for (BasicBlock::iterator I = Pred->begin();
1120 !isa<TerminatorInst>(I); ++I)
1121 if (!AggressiveInsts.count(I)) {
1122 // This is not an aggressive instruction that we can promote.
1123 // Because of this, we won't be able to get rid of the control
1124 // flow, so the xform is not worth it.
1129 // If we can still promote the PHI nodes after this gauntlet of tests,
1130 // do all of the PHI's now.
1132 // Move all 'aggressive' instructions, which are defined in the
1133 // conditional parts of the if's up to the dominating block.
1135 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1136 IfBlock1->getInstList(),
1138 IfBlock1->getTerminator());
1141 DomBlock->getInstList().splice(DomBlock->getTerminator(),
1142 IfBlock2->getInstList(),
1144 IfBlock2->getTerminator());
1147 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
1148 // Change the PHI node into a select instruction.
1150 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
1152 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
1154 Value *NV = new SelectInst(IfCond, TrueVal, FalseVal, "", AfterPHIIt);
1155 PN->replaceAllUsesWith(NV);
1158 BB->getInstList().erase(PN);
1164 /// ConstantIntOrdering - This class implements a stable ordering of constant
1165 /// integers that does not depend on their address. This is important for
1166 /// applications that sort ConstantInt's to ensure uniqueness.
1167 struct ConstantIntOrdering {
1168 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
1169 return LHS->getZExtValue() < RHS->getZExtValue();
1174 // SimplifyCFG - This function is used to do simplification of a CFG. For
1175 // example, it adjusts branches to branches to eliminate the extra hop, it
1176 // eliminates unreachable basic blocks, and does other "peephole" optimization
1177 // of the CFG. It returns true if a modification was made.
1179 // WARNING: The entry node of a function may not be simplified.
1181 bool llvm::SimplifyCFG(BasicBlock *BB) {
1182 bool Changed = false;
1183 Function *M = BB->getParent();
1185 assert(BB && BB->getParent() && "Block not embedded in function!");
1186 assert(BB->getTerminator() && "Degenerate basic block encountered!");
1187 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
1189 // Remove basic blocks that have no predecessors... which are unreachable.
1190 if (pred_begin(BB) == pred_end(BB) ||
1191 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
1192 DOUT << "Removing BB: \n" << *BB;
1194 // Loop through all of our successors and make sure they know that one
1195 // of their predecessors is going away.
1196 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
1197 SI->removePredecessor(BB);
1199 while (!BB->empty()) {
1200 Instruction &I = BB->back();
1201 // If this instruction is used, replace uses with an arbitrary
1202 // value. Because control flow can't get here, we don't care
1203 // what we replace the value with. Note that since this block is
1204 // unreachable, and all values contained within it must dominate their
1205 // uses, that all uses will eventually be removed.
1207 // Make all users of this instruction use undef instead
1208 I.replaceAllUsesWith(UndefValue::get(I.getType()));
1210 // Remove the instruction from the basic block
1211 BB->getInstList().pop_back();
1213 M->getBasicBlockList().erase(BB);
1217 // Check to see if we can constant propagate this terminator instruction
1219 Changed |= ConstantFoldTerminator(BB);
1221 // If this is a returning block with only PHI nodes in it, fold the return
1222 // instruction into any unconditional branch predecessors.
1224 // If any predecessor is a conditional branch that just selects among
1225 // different return values, fold the replace the branch/return with a select
1227 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1228 BasicBlock::iterator BBI = BB->getTerminator();
1229 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
1230 // Find predecessors that end with branches.
1231 std::vector<BasicBlock*> UncondBranchPreds;
1232 std::vector<BranchInst*> CondBranchPreds;
1233 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1234 TerminatorInst *PTI = (*PI)->getTerminator();
1235 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
1236 if (BI->isUnconditional())
1237 UncondBranchPreds.push_back(*PI);
1239 CondBranchPreds.push_back(BI);
1242 // If we found some, do the transformation!
1243 if (!UncondBranchPreds.empty()) {
1244 while (!UncondBranchPreds.empty()) {
1245 BasicBlock *Pred = UncondBranchPreds.back();
1246 DOUT << "FOLDING: " << *BB
1247 << "INTO UNCOND BRANCH PRED: " << *Pred;
1248 UncondBranchPreds.pop_back();
1249 Instruction *UncondBranch = Pred->getTerminator();
1250 // Clone the return and add it to the end of the predecessor.
1251 Instruction *NewRet = RI->clone();
1252 Pred->getInstList().push_back(NewRet);
1254 // If the return instruction returns a value, and if the value was a
1255 // PHI node in "BB", propagate the right value into the return.
1256 if (NewRet->getNumOperands() == 1)
1257 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
1258 if (PN->getParent() == BB)
1259 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
1260 // Update any PHI nodes in the returning block to realize that we no
1261 // longer branch to them.
1262 BB->removePredecessor(Pred);
1263 Pred->getInstList().erase(UncondBranch);
1266 // If we eliminated all predecessors of the block, delete the block now.
1267 if (pred_begin(BB) == pred_end(BB))
1268 // We know there are no successors, so just nuke the block.
1269 M->getBasicBlockList().erase(BB);
1274 // Check out all of the conditional branches going to this return
1275 // instruction. If any of them just select between returns, change the
1276 // branch itself into a select/return pair.
1277 while (!CondBranchPreds.empty()) {
1278 BranchInst *BI = CondBranchPreds.back();
1279 CondBranchPreds.pop_back();
1280 BasicBlock *TrueSucc = BI->getSuccessor(0);
1281 BasicBlock *FalseSucc = BI->getSuccessor(1);
1282 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
1284 // Check to see if the non-BB successor is also a return block.
1285 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
1286 // Check to see if there are only PHI instructions in this block.
1287 BasicBlock::iterator OSI = OtherSucc->getTerminator();
1288 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
1289 // Okay, we found a branch that is going to two return nodes. If
1290 // there is no return value for this function, just change the
1291 // branch into a return.
1292 if (RI->getNumOperands() == 0) {
1293 TrueSucc->removePredecessor(BI->getParent());
1294 FalseSucc->removePredecessor(BI->getParent());
1295 new ReturnInst(0, BI);
1296 BI->getParent()->getInstList().erase(BI);
1300 // Otherwise, figure out what the true and false return values are
1301 // so we can insert a new select instruction.
1302 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
1303 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
1305 // Unwrap any PHI nodes in the return blocks.
1306 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
1307 if (TVPN->getParent() == TrueSucc)
1308 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1309 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
1310 if (FVPN->getParent() == FalseSucc)
1311 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1313 // In order for this transformation to be safe, we must be able to
1314 // unconditionally execute both operands to the return. This is
1315 // normally the case, but we could have a potentially-trapping
1316 // constant expression that prevents this transformation from being
1318 if ((!isa<ConstantExpr>(TrueValue) ||
1319 !cast<ConstantExpr>(TrueValue)->canTrap()) &&
1320 (!isa<ConstantExpr>(TrueValue) ||
1321 !cast<ConstantExpr>(TrueValue)->canTrap())) {
1322 TrueSucc->removePredecessor(BI->getParent());
1323 FalseSucc->removePredecessor(BI->getParent());
1325 // Insert a new select instruction.
1327 Value *BrCond = BI->getCondition();
1328 if (TrueValue != FalseValue)
1329 NewRetVal = new SelectInst(BrCond, TrueValue,
1330 FalseValue, "retval", BI);
1332 NewRetVal = TrueValue;
1334 DOUT << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
1335 << "\n " << *BI << "Select = " << *NewRetVal
1336 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc;
1338 new ReturnInst(NewRetVal, BI);
1339 BI->eraseFromParent();
1340 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond))
1341 if (isInstructionTriviallyDead(BrCondI))
1342 BrCondI->eraseFromParent();
1349 } else if (isa<UnwindInst>(BB->begin())) {
1350 // Check to see if the first instruction in this block is just an unwind.
1351 // If so, replace any invoke instructions which use this as an exception
1352 // destination with call instructions, and any unconditional branch
1353 // predecessor with an unwind.
1355 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1356 while (!Preds.empty()) {
1357 BasicBlock *Pred = Preds.back();
1358 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) {
1359 if (BI->isUnconditional()) {
1360 Pred->getInstList().pop_back(); // nuke uncond branch
1361 new UnwindInst(Pred); // Use unwind.
1364 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
1365 if (II->getUnwindDest() == BB) {
1366 // Insert a new branch instruction before the invoke, because this
1367 // is now a fall through...
1368 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1369 Pred->getInstList().remove(II); // Take out of symbol table
1371 // Insert the call now...
1372 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1373 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1375 CI->setCallingConv(II->getCallingConv());
1376 // If the invoke produced a value, the Call now does instead
1377 II->replaceAllUsesWith(CI);
1385 // If this block is now dead, remove it.
1386 if (pred_begin(BB) == pred_end(BB)) {
1387 // We know there are no successors, so just nuke the block.
1388 M->getBasicBlockList().erase(BB);
1392 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1393 if (isValueEqualityComparison(SI)) {
1394 // If we only have one predecessor, and if it is a branch on this value,
1395 // see if that predecessor totally determines the outcome of this switch.
1396 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1397 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
1398 return SimplifyCFG(BB) || 1;
1400 // If the block only contains the switch, see if we can fold the block
1401 // away into any preds.
1402 if (SI == &BB->front())
1403 if (FoldValueComparisonIntoPredecessors(SI))
1404 return SimplifyCFG(BB) || 1;
1406 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
1407 if (BI->isUnconditional()) {
1408 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
1409 while (isa<PHINode>(*BBI)) ++BBI;
1411 BasicBlock *Succ = BI->getSuccessor(0);
1412 if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
1413 Succ != BB) // Don't hurt infinite loops!
1414 if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ))
1417 } else { // Conditional branch
1418 if (isValueEqualityComparison(BI)) {
1419 // If we only have one predecessor, and if it is a branch on this value,
1420 // see if that predecessor totally determines the outcome of this
1422 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1423 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
1424 return SimplifyCFG(BB) || 1;
1426 // This block must be empty, except for the setcond inst, if it exists.
1427 BasicBlock::iterator I = BB->begin();
1429 (&*I == cast<Instruction>(BI->getCondition()) &&
1431 if (FoldValueComparisonIntoPredecessors(BI))
1432 return SimplifyCFG(BB) | true;
1435 // If this is a branch on a phi node in the current block, thread control
1436 // through this block if any PHI node entries are constants.
1437 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
1438 if (PN->getParent() == BI->getParent())
1439 if (FoldCondBranchOnPHI(BI))
1440 return SimplifyCFG(BB) | true;
1442 // If this basic block is ONLY a setcc and a branch, and if a predecessor
1443 // branches to us and one of our successors, fold the setcc into the
1444 // predecessor and use logical operations to pick the right destination.
1445 BasicBlock *TrueDest = BI->getSuccessor(0);
1446 BasicBlock *FalseDest = BI->getSuccessor(1);
1447 if (Instruction *Cond = dyn_cast<Instruction>(BI->getCondition()))
1448 if ((isa<CmpInst>(Cond) || isa<BinaryOperator>(Cond)) &&
1449 Cond->getParent() == BB && &BB->front() == Cond &&
1450 Cond->getNext() == BI && Cond->hasOneUse() &&
1451 TrueDest != BB && FalseDest != BB)
1452 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
1453 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1454 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
1455 BasicBlock *PredBlock = *PI;
1456 if (PBI->getSuccessor(0) == FalseDest ||
1457 PBI->getSuccessor(1) == TrueDest) {
1458 // Invert the predecessors condition test (xor it with true),
1459 // which allows us to write this code once.
1461 BinaryOperator::createNot(PBI->getCondition(),
1462 PBI->getCondition()->getName()+".not", PBI);
1463 PBI->setCondition(NewCond);
1464 BasicBlock *OldTrue = PBI->getSuccessor(0);
1465 BasicBlock *OldFalse = PBI->getSuccessor(1);
1466 PBI->setSuccessor(0, OldFalse);
1467 PBI->setSuccessor(1, OldTrue);
1470 if ((PBI->getSuccessor(0) == TrueDest && FalseDest != BB) ||
1471 (PBI->getSuccessor(1) == FalseDest && TrueDest != BB)) {
1472 // Clone Cond into the predecessor basic block, and or/and the
1473 // two conditions together.
1474 Instruction *New = Cond->clone();
1475 PredBlock->getInstList().insert(PBI, New);
1476 New->takeName(Cond);
1477 Cond->setName(New->getName()+".old");
1478 Instruction::BinaryOps Opcode =
1479 PBI->getSuccessor(0) == TrueDest ?
1480 Instruction::Or : Instruction::And;
1482 BinaryOperator::create(Opcode, PBI->getCondition(),
1483 New, "bothcond", PBI);
1484 PBI->setCondition(NewCond);
1485 if (PBI->getSuccessor(0) == BB) {
1486 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1487 PBI->setSuccessor(0, TrueDest);
1489 if (PBI->getSuccessor(1) == BB) {
1490 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1491 PBI->setSuccessor(1, FalseDest);
1493 return SimplifyCFG(BB) | 1;
1497 // Scan predessor blocks for conditional branchs.
1498 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1499 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1500 if (PBI != BI && PBI->isConditional()) {
1502 // If this block ends with a branch instruction, and if there is a
1503 // predecessor that ends on a branch of the same condition, make
1504 // this conditional branch redundant.
1505 if (PBI->getCondition() == BI->getCondition() &&
1506 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1507 // Okay, the outcome of this conditional branch is statically
1508 // knowable. If this block had a single pred, handle specially.
1509 if (BB->getSinglePredecessor()) {
1510 // Turn this into a branch on constant.
1511 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1512 BI->setCondition(ConstantInt::get(Type::Int1Ty, CondIsTrue));
1513 return SimplifyCFG(BB); // Nuke the branch on constant.
1516 // Otherwise, if there are multiple predecessors, insert a PHI
1517 // that merges in the constant and simplify the block result.
1518 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1519 PHINode *NewPN = new PHINode(Type::Int1Ty,
1520 BI->getCondition()->getName()+".pr",
1522 for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1523 if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
1524 PBI != BI && PBI->isConditional() &&
1525 PBI->getCondition() == BI->getCondition() &&
1526 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1527 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1528 NewPN->addIncoming(ConstantInt::get(Type::Int1Ty,
1531 NewPN->addIncoming(BI->getCondition(), *PI);
1534 BI->setCondition(NewPN);
1535 // This will thread the branch.
1536 return SimplifyCFG(BB) | true;
1540 // If this is a conditional branch in an empty block, and if any
1541 // predecessors is a conditional branch to one of our destinations,
1542 // fold the conditions into logical ops and one cond br.
1543 if (&BB->front() == BI) {
1545 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
1547 } else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
1548 PBIOp = 0; BIOp = 1;
1549 } else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
1550 PBIOp = 1; BIOp = 0;
1551 } else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
1557 // Check to make sure that the other destination of this branch
1558 // isn't BB itself. If so, this is an infinite loop that will
1559 // keep getting unwound.
1560 if (PBIOp != -1 && PBI->getSuccessor(PBIOp) == BB)
1563 // Do not perform this transformation if it would require
1564 // insertion of a large number of select instructions. For targets
1565 // without predication/cmovs, this is a big pessimization.
1567 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1569 unsigned NumPhis = 0;
1570 for (BasicBlock::iterator II = CommonDest->begin();
1571 isa<PHINode>(II); ++II, ++NumPhis) {
1573 // Disable this xform.
1580 // Finally, if everything is ok, fold the branches to logical ops.
1582 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1583 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
1585 // If OtherDest *is* BB, then this is a basic block with just
1586 // a conditional branch in it, where one edge (OtherDesg) goes
1587 // back to the block. We know that the program doesn't get
1588 // stuck in the infinite loop, so the condition must be such
1589 // that OtherDest isn't branched through. Forward to CommonDest,
1590 // and avoid an infinite loop at optimizer time.
1591 if (OtherDest == BB)
1592 OtherDest = CommonDest;
1594 DOUT << "FOLDING BRs:" << *PBI->getParent()
1595 << "AND: " << *BI->getParent();
1597 // BI may have other predecessors. Because of this, we leave
1598 // it alone, but modify PBI.
1600 // Make sure we get to CommonDest on True&True directions.
1601 Value *PBICond = PBI->getCondition();
1603 PBICond = BinaryOperator::createNot(PBICond,
1604 PBICond->getName()+".not",
1606 Value *BICond = BI->getCondition();
1608 BICond = BinaryOperator::createNot(BICond,
1609 BICond->getName()+".not",
1611 // Merge the conditions.
1613 BinaryOperator::createOr(PBICond, BICond, "brmerge", PBI);
1615 // Modify PBI to branch on the new condition to the new dests.
1616 PBI->setCondition(Cond);
1617 PBI->setSuccessor(0, CommonDest);
1618 PBI->setSuccessor(1, OtherDest);
1620 // OtherDest may have phi nodes. If so, add an entry from PBI's
1621 // block that are identical to the entries for BI's block.
1623 for (BasicBlock::iterator II = OtherDest->begin();
1624 (PN = dyn_cast<PHINode>(II)); ++II) {
1625 Value *V = PN->getIncomingValueForBlock(BB);
1626 PN->addIncoming(V, PBI->getParent());
1629 // We know that the CommonDest already had an edge from PBI to
1630 // it. If it has PHIs though, the PHIs may have different
1631 // entries for BB and PBI's BB. If so, insert a select to make
1633 for (BasicBlock::iterator II = CommonDest->begin();
1634 (PN = dyn_cast<PHINode>(II)); ++II) {
1635 Value * BIV = PN->getIncomingValueForBlock(BB);
1636 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
1637 Value *PBIV = PN->getIncomingValue(PBBIdx);
1639 // Insert a select in PBI to pick the right value.
1640 Value *NV = new SelectInst(PBICond, PBIV, BIV,
1641 PBIV->getName()+".mux", PBI);
1642 PN->setIncomingValue(PBBIdx, NV);
1646 DOUT << "INTO: " << *PBI->getParent();
1648 // This basic block is probably dead. We know it has at least
1649 // one fewer predecessor.
1650 return SimplifyCFG(BB) | true;
1655 } else if (isa<UnreachableInst>(BB->getTerminator())) {
1656 // If there are any instructions immediately before the unreachable that can
1657 // be removed, do so.
1658 Instruction *Unreachable = BB->getTerminator();
1659 while (Unreachable != BB->begin()) {
1660 BasicBlock::iterator BBI = Unreachable;
1662 if (isa<CallInst>(BBI)) break;
1663 // Delete this instruction
1664 BB->getInstList().erase(BBI);
1668 // If the unreachable instruction is the first in the block, take a gander
1669 // at all of the predecessors of this instruction, and simplify them.
1670 if (&BB->front() == Unreachable) {
1671 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
1672 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
1673 TerminatorInst *TI = Preds[i]->getTerminator();
1675 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1676 if (BI->isUnconditional()) {
1677 if (BI->getSuccessor(0) == BB) {
1678 new UnreachableInst(TI);
1679 TI->eraseFromParent();
1683 if (BI->getSuccessor(0) == BB) {
1684 new BranchInst(BI->getSuccessor(1), BI);
1685 BI->eraseFromParent();
1686 } else if (BI->getSuccessor(1) == BB) {
1687 new BranchInst(BI->getSuccessor(0), BI);
1688 BI->eraseFromParent();
1692 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
1693 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1694 if (SI->getSuccessor(i) == BB) {
1695 BB->removePredecessor(SI->getParent());
1700 // If the default value is unreachable, figure out the most popular
1701 // destination and make it the default.
1702 if (SI->getSuccessor(0) == BB) {
1703 std::map<BasicBlock*, unsigned> Popularity;
1704 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1705 Popularity[SI->getSuccessor(i)]++;
1707 // Find the most popular block.
1708 unsigned MaxPop = 0;
1709 BasicBlock *MaxBlock = 0;
1710 for (std::map<BasicBlock*, unsigned>::iterator
1711 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
1712 if (I->second > MaxPop) {
1714 MaxBlock = I->first;
1718 // Make this the new default, allowing us to delete any explicit
1720 SI->setSuccessor(0, MaxBlock);
1723 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
1725 if (isa<PHINode>(MaxBlock->begin()))
1726 for (unsigned i = 0; i != MaxPop-1; ++i)
1727 MaxBlock->removePredecessor(SI->getParent());
1729 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1730 if (SI->getSuccessor(i) == MaxBlock) {
1736 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
1737 if (II->getUnwindDest() == BB) {
1738 // Convert the invoke to a call instruction. This would be a good
1739 // place to note that the call does not throw though.
1740 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1741 II->removeFromParent(); // Take out of symbol table
1743 // Insert the call now...
1744 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
1745 CallInst *CI = new CallInst(II->getCalledValue(), Args,
1747 CI->setCallingConv(II->getCallingConv());
1748 // If the invoke produced a value, the Call does now instead.
1749 II->replaceAllUsesWith(CI);
1756 // If this block is now dead, remove it.
1757 if (pred_begin(BB) == pred_end(BB)) {
1758 // We know there are no successors, so just nuke the block.
1759 M->getBasicBlockList().erase(BB);
1765 // Merge basic blocks into their predecessor if there is only one distinct
1766 // pred, and if there is only one distinct successor of the predecessor, and
1767 // if there are no PHI nodes.
1769 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
1770 BasicBlock *OnlyPred = *PI++;
1771 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
1772 if (*PI != OnlyPred) {
1773 OnlyPred = 0; // There are multiple different predecessors...
1777 BasicBlock *OnlySucc = 0;
1778 if (OnlyPred && OnlyPred != BB && // Don't break self loops
1779 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
1780 // Check to see if there is only one distinct successor...
1781 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
1783 for (; SI != SE; ++SI)
1784 if (*SI != OnlySucc) {
1785 OnlySucc = 0; // There are multiple distinct successors!
1791 DOUT << "Merging: " << *BB << "into: " << *OnlyPred;
1793 // Resolve any PHI nodes at the start of the block. They are all
1794 // guaranteed to have exactly one entry if they exist, unless there are
1795 // multiple duplicate (but guaranteed to be equal) entries for the
1796 // incoming edges. This occurs when there are multiple edges from
1797 // OnlyPred to OnlySucc.
1799 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
1800 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1801 BB->getInstList().pop_front(); // Delete the phi node.
1804 // Delete the unconditional branch from the predecessor.
1805 OnlyPred->getInstList().pop_back();
1807 // Move all definitions in the successor to the predecessor.
1808 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
1810 // Make all PHI nodes that referred to BB now refer to Pred as their
1812 BB->replaceAllUsesWith(OnlyPred);
1814 // Inherit predecessors name if it exists.
1815 if (!OnlyPred->hasName())
1816 OnlyPred->takeName(BB);
1818 // Erase basic block from the function.
1819 M->getBasicBlockList().erase(BB);
1824 // Otherwise, if this block only has a single predecessor, and if that block
1825 // is a conditional branch, see if we can hoist any code from this block up
1826 // into our predecessor.
1828 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
1829 if (BI->isConditional()) {
1830 // Get the other block.
1831 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
1832 PI = pred_begin(OtherBB);
1834 if (PI == pred_end(OtherBB)) {
1835 // We have a conditional branch to two blocks that are only reachable
1836 // from the condbr. We know that the condbr dominates the two blocks,
1837 // so see if there is any identical code in the "then" and "else"
1838 // blocks. If so, we can hoist it up to the branching block.
1839 Changed |= HoistThenElseCodeToIf(BI);
1843 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1844 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1845 // Change br (X == 0 | X == 1), T, F into a switch instruction.
1846 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
1847 Instruction *Cond = cast<Instruction>(BI->getCondition());
1848 // If this is a bunch of seteq's or'd together, or if it's a bunch of
1849 // 'setne's and'ed together, collect them.
1851 std::vector<ConstantInt*> Values;
1852 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
1853 if (CompVal && CompVal->getType()->isInteger()) {
1854 // There might be duplicate constants in the list, which the switch
1855 // instruction can't handle, remove them now.
1856 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
1857 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
1859 // Figure out which block is which destination.
1860 BasicBlock *DefaultBB = BI->getSuccessor(1);
1861 BasicBlock *EdgeBB = BI->getSuccessor(0);
1862 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
1864 // Create the new switch instruction now.
1865 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI);
1867 // Add all of the 'cases' to the switch instruction.
1868 for (unsigned i = 0, e = Values.size(); i != e; ++i)
1869 New->addCase(Values[i], EdgeBB);
1871 // We added edges from PI to the EdgeBB. As such, if there were any
1872 // PHI nodes in EdgeBB, they need entries to be added corresponding to
1873 // the number of edges added.
1874 for (BasicBlock::iterator BBI = EdgeBB->begin();
1875 isa<PHINode>(BBI); ++BBI) {
1876 PHINode *PN = cast<PHINode>(BBI);
1877 Value *InVal = PN->getIncomingValueForBlock(*PI);
1878 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
1879 PN->addIncoming(InVal, *PI);
1882 // Erase the old branch instruction.
1883 (*PI)->getInstList().erase(BI);
1885 // Erase the potentially condition tree that was used to computed the
1886 // branch condition.
1887 ErasePossiblyDeadInstructionTree(Cond);
1892 // If there is a trivial two-entry PHI node in this basic block, and we can
1893 // eliminate it, do so now.
1894 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1895 if (PN->getNumIncomingValues() == 2)
1896 Changed |= FoldTwoEntryPHINode(PN);