1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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"
24 #include "llvm/ADT/SmallVector.h"
25 #include "llvm/ADT/SmallPtrSet.h"
32 /// SafeToMergeTerminators - Return true if it is safe to merge these two
33 /// terminator instructions together.
35 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
36 if (SI1 == SI2) return false; // Can't merge with self!
38 // It is not safe to merge these two switch instructions if they have a common
39 // successor, and if that successor has a PHI node, and if *that* PHI node has
40 // conflicting incoming values from the two switch blocks.
41 BasicBlock *SI1BB = SI1->getParent();
42 BasicBlock *SI2BB = SI2->getParent();
43 SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
45 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
46 if (SI1Succs.count(*I))
47 for (BasicBlock::iterator BBI = (*I)->begin();
48 isa<PHINode>(BBI); ++BBI) {
49 PHINode *PN = cast<PHINode>(BBI);
50 if (PN->getIncomingValueForBlock(SI1BB) !=
51 PN->getIncomingValueForBlock(SI2BB))
58 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
59 /// now be entries in it from the 'NewPred' block. The values that will be
60 /// flowing into the PHI nodes will be the same as those coming in from
61 /// ExistPred, an existing predecessor of Succ.
62 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
63 BasicBlock *ExistPred) {
64 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
65 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
66 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
68 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
69 PHINode *PN = cast<PHINode>(I);
70 Value *V = PN->getIncomingValueForBlock(ExistPred);
71 PN->addIncoming(V, NewPred);
75 // CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
76 // almost-empty BB ending in an unconditional branch to Succ, into succ.
78 // Assumption: Succ is the single successor for BB.
80 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
81 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
83 // Check to see if one of the predecessors of BB is already a predecessor of
84 // Succ. If so, we cannot do the transformation if there are any PHI nodes
85 // with incompatible values coming in from the two edges!
87 if (isa<PHINode>(Succ->front())) {
88 SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
89 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
91 if (BBPreds.count(*PI)) {
92 // Loop over all of the PHI nodes checking to see if there are
93 // incompatible values coming in.
94 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
95 PHINode *PN = cast<PHINode>(I);
96 // Loop up the entries in the PHI node for BB and for *PI if the
97 // values coming in are non-equal, we cannot merge these two blocks
98 // (instead we should insert a conditional move or something, then
100 if (PN->getIncomingValueForBlock(BB) !=
101 PN->getIncomingValueForBlock(*PI))
102 return false; // Values are not equal...
107 // Finally, if BB has PHI nodes that are used by things other than the PHIs in
108 // Succ and Succ has predecessors that are not Succ and not Pred, we cannot
109 // fold these blocks, as we don't know whether BB dominates Succ or not to
110 // update the PHI nodes correctly.
111 if (!isa<PHINode>(BB->begin()) || Succ->getSinglePredecessor()) return true;
113 // If the predecessors of Succ are only BB, handle it.
115 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
120 if (IsSafe) return true;
122 // If the PHI nodes in BB are only used by instructions in Succ, we are ok if
123 // BB and Succ have no common predecessors.
124 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) {
125 PHINode *PN = cast<PHINode>(I);
126 for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end(); UI != E;
128 if (cast<Instruction>(*UI)->getParent() != Succ)
132 // Scan the predecessor sets of BB and Succ, making sure there are no common
133 // predecessors. Common predecessors would cause us to build a phi node with
134 // differing incoming values, which is not legal.
135 SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
136 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI)
137 if (BBPreds.count(*PI))
143 /// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional
144 /// branch to Succ, and contains no instructions other than PHI nodes and the
145 /// branch. If possible, eliminate BB.
146 static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
148 // If our successor has PHI nodes, then we need to update them to include
149 // entries for BB's predecessors, not for BB itself. Be careful though,
150 // if this transformation fails (returns true) then we cannot do this
153 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
155 DOUT << "Killing Trivial BB: \n" << *BB;
157 if (isa<PHINode>(Succ->begin())) {
158 // If there is more than one pred of succ, and there are PHI nodes in
159 // the successor, then we need to add incoming edges for the PHI nodes
161 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
163 // Loop over all of the PHI nodes in the successor of BB.
164 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
165 PHINode *PN = cast<PHINode>(I);
166 Value *OldVal = PN->removeIncomingValue(BB, false);
167 assert(OldVal && "No entry in PHI for Pred BB!");
169 // If this incoming value is one of the PHI nodes in BB, the new entries
170 // in the PHI node are the entries from the old PHI.
171 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
172 PHINode *OldValPN = cast<PHINode>(OldVal);
173 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
174 PN->addIncoming(OldValPN->getIncomingValue(i),
175 OldValPN->getIncomingBlock(i));
177 // Add an incoming value for each of the new incoming values.
178 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i)
179 PN->addIncoming(OldVal, BBPreds[i]);
184 if (isa<PHINode>(&BB->front())) {
185 SmallVector<BasicBlock*, 16>
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 if (I->getOperand(0)->getType()->isFPOrFPVector())
374 return false; // FP arithmetic might trap.
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 SmallVector<Instruction*, 16> 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 a 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 SmallPtrSet<Constant*, 16> 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 SmallVector<BasicBlock*, 16> 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 SmallVector<BasicBlock*, 8> 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);
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 Value *NV = new SelectInst(IfCond, TrueVal, FalseVal, "", AfterPHIIt);
1157 PN->replaceAllUsesWith(NV);
1160 BB->getInstList().erase(PN);
1166 /// ConstantIntOrdering - This class implements a stable ordering of constant
1167 /// integers that does not depend on their address. This is important for
1168 /// applications that sort ConstantInt's to ensure uniqueness.
1169 struct ConstantIntOrdering {
1170 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
1171 return LHS->getValue().ult(RHS->getValue());
1176 // SimplifyCFG - This function is used to do simplification of a CFG. For
1177 // example, it adjusts branches to branches to eliminate the extra hop, it
1178 // eliminates unreachable basic blocks, and does other "peephole" optimization
1179 // of the CFG. It returns true if a modification was made.
1181 // WARNING: The entry node of a function may not be simplified.
1183 bool llvm::SimplifyCFG(BasicBlock *BB) {
1184 bool Changed = false;
1185 Function *M = BB->getParent();
1187 assert(BB && BB->getParent() && "Block not embedded in function!");
1188 assert(BB->getTerminator() && "Degenerate basic block encountered!");
1189 assert(&BB->getParent()->getEntryBlock() != BB &&
1190 "Can't Simplify entry block!");
1192 // Remove basic blocks that have no predecessors... which are unreachable.
1193 if (pred_begin(BB) == pred_end(BB) ||
1194 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
1195 DOUT << "Removing BB: \n" << *BB;
1197 // Loop through all of our successors and make sure they know that one
1198 // of their predecessors is going away.
1199 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
1200 SI->removePredecessor(BB);
1202 while (!BB->empty()) {
1203 Instruction &I = BB->back();
1204 // If this instruction is used, replace uses with an arbitrary
1205 // value. Because control flow can't get here, we don't care
1206 // what we replace the value with. Note that since this block is
1207 // unreachable, and all values contained within it must dominate their
1208 // uses, that all uses will eventually be removed.
1210 // Make all users of this instruction use undef instead
1211 I.replaceAllUsesWith(UndefValue::get(I.getType()));
1213 // Remove the instruction from the basic block
1214 BB->getInstList().pop_back();
1216 M->getBasicBlockList().erase(BB);
1220 // Check to see if we can constant propagate this terminator instruction
1222 Changed |= ConstantFoldTerminator(BB);
1224 // If this is a returning block with only PHI nodes in it, fold the return
1225 // instruction into any unconditional branch predecessors.
1227 // If any predecessor is a conditional branch that just selects among
1228 // different return values, fold the replace the branch/return with a select
1230 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1231 BasicBlock::iterator BBI = BB->getTerminator();
1232 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
1233 // Find predecessors that end with branches.
1234 SmallVector<BasicBlock*, 8> UncondBranchPreds;
1235 SmallVector<BranchInst*, 8> CondBranchPreds;
1236 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
1237 TerminatorInst *PTI = (*PI)->getTerminator();
1238 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
1239 if (BI->isUnconditional())
1240 UncondBranchPreds.push_back(*PI);
1242 CondBranchPreds.push_back(BI);
1245 // If we found some, do the transformation!
1246 if (!UncondBranchPreds.empty()) {
1247 while (!UncondBranchPreds.empty()) {
1248 BasicBlock *Pred = UncondBranchPreds.back();
1249 DOUT << "FOLDING: " << *BB
1250 << "INTO UNCOND BRANCH PRED: " << *Pred;
1251 UncondBranchPreds.pop_back();
1252 Instruction *UncondBranch = Pred->getTerminator();
1253 // Clone the return and add it to the end of the predecessor.
1254 Instruction *NewRet = RI->clone();
1255 Pred->getInstList().push_back(NewRet);
1257 // If the return instruction returns a value, and if the value was a
1258 // PHI node in "BB", propagate the right value into the return.
1259 if (NewRet->getNumOperands() == 1)
1260 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
1261 if (PN->getParent() == BB)
1262 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
1263 // Update any PHI nodes in the returning block to realize that we no
1264 // longer branch to them.
1265 BB->removePredecessor(Pred);
1266 Pred->getInstList().erase(UncondBranch);
1269 // If we eliminated all predecessors of the block, delete the block now.
1270 if (pred_begin(BB) == pred_end(BB))
1271 // We know there are no successors, so just nuke the block.
1272 M->getBasicBlockList().erase(BB);
1277 // Check out all of the conditional branches going to this return
1278 // instruction. If any of them just select between returns, change the
1279 // branch itself into a select/return pair.
1280 while (!CondBranchPreds.empty()) {
1281 BranchInst *BI = CondBranchPreds.back();
1282 CondBranchPreds.pop_back();
1283 BasicBlock *TrueSucc = BI->getSuccessor(0);
1284 BasicBlock *FalseSucc = BI->getSuccessor(1);
1285 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
1287 // Check to see if the non-BB successor is also a return block.
1288 if (isa<ReturnInst>(OtherSucc->getTerminator())) {
1289 // Check to see if there are only PHI instructions in this block.
1290 BasicBlock::iterator OSI = OtherSucc->getTerminator();
1291 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
1292 // Okay, we found a branch that is going to two return nodes. If
1293 // there is no return value for this function, just change the
1294 // branch into a return.
1295 if (RI->getNumOperands() == 0) {
1296 TrueSucc->removePredecessor(BI->getParent());
1297 FalseSucc->removePredecessor(BI->getParent());
1298 new ReturnInst(0, BI);
1299 BI->getParent()->getInstList().erase(BI);
1303 // Otherwise, figure out what the true and false return values are
1304 // so we can insert a new select instruction.
1305 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
1306 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
1308 // Unwrap any PHI nodes in the return blocks.
1309 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
1310 if (TVPN->getParent() == TrueSucc)
1311 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
1312 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
1313 if (FVPN->getParent() == FalseSucc)
1314 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
1316 // In order for this transformation to be safe, we must be able to
1317 // unconditionally execute both operands to the return. This is
1318 // normally the case, but we could have a potentially-trapping
1319 // constant expression that prevents this transformation from being
1321 if ((!isa<ConstantExpr>(TrueValue) ||
1322 !cast<ConstantExpr>(TrueValue)->canTrap()) &&
1323 (!isa<ConstantExpr>(TrueValue) ||
1324 !cast<ConstantExpr>(TrueValue)->canTrap())) {
1325 TrueSucc->removePredecessor(BI->getParent());
1326 FalseSucc->removePredecessor(BI->getParent());
1328 // Insert a new select instruction.
1330 Value *BrCond = BI->getCondition();
1331 if (TrueValue != FalseValue)
1332 NewRetVal = new SelectInst(BrCond, TrueValue,
1333 FalseValue, "retval", BI);
1335 NewRetVal = TrueValue;
1337 DOUT << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:"
1338 << "\n " << *BI << "Select = " << *NewRetVal
1339 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc;
1341 new ReturnInst(NewRetVal, BI);
1342 BI->eraseFromParent();
1343 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond))
1344 if (isInstructionTriviallyDead(BrCondI))
1345 BrCondI->eraseFromParent();
1352 } else if (isa<UnwindInst>(BB->begin())) {
1353 // Check to see if the first instruction in this block is just an unwind.
1354 // If so, replace any invoke instructions which use this as an exception
1355 // destination with call instructions, and any unconditional branch
1356 // predecessor with an unwind.
1358 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
1359 while (!Preds.empty()) {
1360 BasicBlock *Pred = Preds.back();
1361 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) {
1362 if (BI->isUnconditional()) {
1363 Pred->getInstList().pop_back(); // nuke uncond branch
1364 new UnwindInst(Pred); // Use unwind.
1367 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
1368 if (II->getUnwindDest() == BB) {
1369 // Insert a new branch instruction before the invoke, because this
1370 // is now a fall through...
1371 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1372 Pred->getInstList().remove(II); // Take out of symbol table
1374 // Insert the call now...
1375 SmallVector<Value*,8> Args(II->op_begin()+3, II->op_end());
1376 CallInst *CI = new CallInst(II->getCalledValue(),
1377 Args.begin(), Args.end(), II->getName(), BI);
1378 CI->setCallingConv(II->getCallingConv());
1379 CI->setParamAttrs(II->getParamAttrs());
1380 // If the invoke produced a value, the Call now does instead
1381 II->replaceAllUsesWith(CI);
1389 // If this block is now dead, remove it.
1390 if (pred_begin(BB) == pred_end(BB)) {
1391 // We know there are no successors, so just nuke the block.
1392 M->getBasicBlockList().erase(BB);
1396 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1397 if (isValueEqualityComparison(SI)) {
1398 // If we only have one predecessor, and if it is a branch on this value,
1399 // see if that predecessor totally determines the outcome of this switch.
1400 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1401 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred))
1402 return SimplifyCFG(BB) || 1;
1404 // If the block only contains the switch, see if we can fold the block
1405 // away into any preds.
1406 if (SI == &BB->front())
1407 if (FoldValueComparisonIntoPredecessors(SI))
1408 return SimplifyCFG(BB) || 1;
1410 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
1411 if (BI->isUnconditional()) {
1412 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
1413 while (isa<PHINode>(*BBI)) ++BBI;
1415 BasicBlock *Succ = BI->getSuccessor(0);
1416 if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
1417 Succ != BB) // Don't hurt infinite loops!
1418 if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ))
1421 } else { // Conditional branch
1422 if (isValueEqualityComparison(BI)) {
1423 // If we only have one predecessor, and if it is a branch on this value,
1424 // see if that predecessor totally determines the outcome of this
1426 if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
1427 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred))
1428 return SimplifyCFG(BB) || 1;
1430 // This block must be empty, except for the setcond inst, if it exists.
1431 BasicBlock::iterator I = BB->begin();
1433 (&*I == cast<Instruction>(BI->getCondition()) &&
1435 if (FoldValueComparisonIntoPredecessors(BI))
1436 return SimplifyCFG(BB) | true;
1439 // If this is a branch on a phi node in the current block, thread control
1440 // through this block if any PHI node entries are constants.
1441 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition()))
1442 if (PN->getParent() == BI->getParent())
1443 if (FoldCondBranchOnPHI(BI))
1444 return SimplifyCFG(BB) | true;
1446 // If this basic block is ONLY a setcc and a branch, and if a predecessor
1447 // branches to us and one of our successors, fold the setcc into the
1448 // predecessor and use logical operations to pick the right destination.
1449 BasicBlock *TrueDest = BI->getSuccessor(0);
1450 BasicBlock *FalseDest = BI->getSuccessor(1);
1451 if (Instruction *Cond = dyn_cast<Instruction>(BI->getCondition())) {
1452 BasicBlock::iterator CondIt = Cond;
1453 if ((isa<CmpInst>(Cond) || isa<BinaryOperator>(Cond)) &&
1454 Cond->getParent() == BB && &BB->front() == Cond &&
1455 &*++CondIt == BI && Cond->hasOneUse() &&
1456 TrueDest != BB && FalseDest != BB)
1457 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
1458 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1459 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
1460 BasicBlock *PredBlock = *PI;
1461 if (PBI->getSuccessor(0) == FalseDest ||
1462 PBI->getSuccessor(1) == TrueDest) {
1463 // Invert the predecessors condition test (xor it with true),
1464 // which allows us to write this code once.
1466 BinaryOperator::createNot(PBI->getCondition(),
1467 PBI->getCondition()->getName()+".not", PBI);
1468 PBI->setCondition(NewCond);
1469 BasicBlock *OldTrue = PBI->getSuccessor(0);
1470 BasicBlock *OldFalse = PBI->getSuccessor(1);
1471 PBI->setSuccessor(0, OldFalse);
1472 PBI->setSuccessor(1, OldTrue);
1475 if ((PBI->getSuccessor(0) == TrueDest && FalseDest != BB) ||
1476 (PBI->getSuccessor(1) == FalseDest && TrueDest != BB)) {
1477 // Clone Cond into the predecessor basic block, and or/and the
1478 // two conditions together.
1479 Instruction *New = Cond->clone();
1480 PredBlock->getInstList().insert(PBI, New);
1481 New->takeName(Cond);
1482 Cond->setName(New->getName()+".old");
1483 Instruction::BinaryOps Opcode =
1484 PBI->getSuccessor(0) == TrueDest ?
1485 Instruction::Or : Instruction::And;
1487 BinaryOperator::create(Opcode, PBI->getCondition(),
1488 New, "bothcond", PBI);
1489 PBI->setCondition(NewCond);
1490 if (PBI->getSuccessor(0) == BB) {
1491 AddPredecessorToBlock(TrueDest, PredBlock, BB);
1492 PBI->setSuccessor(0, TrueDest);
1494 if (PBI->getSuccessor(1) == BB) {
1495 AddPredecessorToBlock(FalseDest, PredBlock, BB);
1496 PBI->setSuccessor(1, FalseDest);
1498 return SimplifyCFG(BB) | 1;
1503 // Scan predessor blocks for conditional branches.
1504 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1505 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1506 if (PBI != BI && PBI->isConditional()) {
1508 // If this block ends with a branch instruction, and if there is a
1509 // predecessor that ends on a branch of the same condition, make
1510 // this conditional branch redundant.
1511 if (PBI->getCondition() == BI->getCondition() &&
1512 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1513 // Okay, the outcome of this conditional branch is statically
1514 // knowable. If this block had a single pred, handle specially.
1515 if (BB->getSinglePredecessor()) {
1516 // Turn this into a branch on constant.
1517 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1518 BI->setCondition(ConstantInt::get(Type::Int1Ty, CondIsTrue));
1519 return SimplifyCFG(BB); // Nuke the branch on constant.
1522 // Otherwise, if there are multiple predecessors, insert a PHI
1523 // that merges in the constant and simplify the block result.
1524 if (BlockIsSimpleEnoughToThreadThrough(BB)) {
1525 PHINode *NewPN = new PHINode(Type::Int1Ty,
1526 BI->getCondition()->getName()+".pr",
1528 for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1529 if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
1530 PBI != BI && PBI->isConditional() &&
1531 PBI->getCondition() == BI->getCondition() &&
1532 PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
1533 bool CondIsTrue = PBI->getSuccessor(0) == BB;
1534 NewPN->addIncoming(ConstantInt::get(Type::Int1Ty,
1537 NewPN->addIncoming(BI->getCondition(), *PI);
1540 BI->setCondition(NewPN);
1541 // This will thread the branch.
1542 return SimplifyCFG(BB) | true;
1546 // If this is a conditional branch in an empty block, and if any
1547 // predecessors is a conditional branch to one of our destinations,
1548 // fold the conditions into logical ops and one cond br.
1549 if (&BB->front() == BI) {
1551 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
1553 } else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
1554 PBIOp = 0; BIOp = 1;
1555 } else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
1556 PBIOp = 1; BIOp = 0;
1557 } else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
1563 // Check to make sure that the other destination of this branch
1564 // isn't BB itself. If so, this is an infinite loop that will
1565 // keep getting unwound.
1566 if (PBIOp != -1 && PBI->getSuccessor(PBIOp) == BB)
1569 // Do not perform this transformation if it would require
1570 // insertion of a large number of select instructions. For targets
1571 // without predication/cmovs, this is a big pessimization.
1573 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1575 unsigned NumPhis = 0;
1576 for (BasicBlock::iterator II = CommonDest->begin();
1577 isa<PHINode>(II); ++II, ++NumPhis) {
1579 // Disable this xform.
1586 // Finally, if everything is ok, fold the branches to logical ops.
1588 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
1589 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
1591 // If OtherDest *is* BB, then this is a basic block with just
1592 // a conditional branch in it, where one edge (OtherDesg) goes
1593 // back to the block. We know that the program doesn't get
1594 // stuck in the infinite loop, so the condition must be such
1595 // that OtherDest isn't branched through. Forward to CommonDest,
1596 // and avoid an infinite loop at optimizer time.
1597 if (OtherDest == BB)
1598 OtherDest = CommonDest;
1600 DOUT << "FOLDING BRs:" << *PBI->getParent()
1601 << "AND: " << *BI->getParent();
1603 // BI may have other predecessors. Because of this, we leave
1604 // it alone, but modify PBI.
1606 // Make sure we get to CommonDest on True&True directions.
1607 Value *PBICond = PBI->getCondition();
1609 PBICond = BinaryOperator::createNot(PBICond,
1610 PBICond->getName()+".not",
1612 Value *BICond = BI->getCondition();
1614 BICond = BinaryOperator::createNot(BICond,
1615 BICond->getName()+".not",
1617 // Merge the conditions.
1619 BinaryOperator::createOr(PBICond, BICond, "brmerge", PBI);
1621 // Modify PBI to branch on the new condition to the new dests.
1622 PBI->setCondition(Cond);
1623 PBI->setSuccessor(0, CommonDest);
1624 PBI->setSuccessor(1, OtherDest);
1626 // OtherDest may have phi nodes. If so, add an entry from PBI's
1627 // block that are identical to the entries for BI's block.
1629 for (BasicBlock::iterator II = OtherDest->begin();
1630 (PN = dyn_cast<PHINode>(II)); ++II) {
1631 Value *V = PN->getIncomingValueForBlock(BB);
1632 PN->addIncoming(V, PBI->getParent());
1635 // We know that the CommonDest already had an edge from PBI to
1636 // it. If it has PHIs though, the PHIs may have different
1637 // entries for BB and PBI's BB. If so, insert a select to make
1639 for (BasicBlock::iterator II = CommonDest->begin();
1640 (PN = dyn_cast<PHINode>(II)); ++II) {
1641 Value * BIV = PN->getIncomingValueForBlock(BB);
1642 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
1643 Value *PBIV = PN->getIncomingValue(PBBIdx);
1645 // Insert a select in PBI to pick the right value.
1646 Value *NV = new SelectInst(PBICond, PBIV, BIV,
1647 PBIV->getName()+".mux", PBI);
1648 PN->setIncomingValue(PBBIdx, NV);
1652 DOUT << "INTO: " << *PBI->getParent();
1654 // This basic block is probably dead. We know it has at least
1655 // one fewer predecessor.
1656 return SimplifyCFG(BB) | true;
1661 } else if (isa<UnreachableInst>(BB->getTerminator())) {
1662 // If there are any instructions immediately before the unreachable that can
1663 // be removed, do so.
1664 Instruction *Unreachable = BB->getTerminator();
1665 while (Unreachable != BB->begin()) {
1666 BasicBlock::iterator BBI = Unreachable;
1668 if (isa<CallInst>(BBI)) break;
1669 // Delete this instruction
1670 BB->getInstList().erase(BBI);
1674 // If the unreachable instruction is the first in the block, take a gander
1675 // at all of the predecessors of this instruction, and simplify them.
1676 if (&BB->front() == Unreachable) {
1677 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB));
1678 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
1679 TerminatorInst *TI = Preds[i]->getTerminator();
1681 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1682 if (BI->isUnconditional()) {
1683 if (BI->getSuccessor(0) == BB) {
1684 new UnreachableInst(TI);
1685 TI->eraseFromParent();
1689 if (BI->getSuccessor(0) == BB) {
1690 new BranchInst(BI->getSuccessor(1), BI);
1691 BI->eraseFromParent();
1692 } else if (BI->getSuccessor(1) == BB) {
1693 new BranchInst(BI->getSuccessor(0), BI);
1694 BI->eraseFromParent();
1698 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
1699 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1700 if (SI->getSuccessor(i) == BB) {
1701 BB->removePredecessor(SI->getParent());
1706 // If the default value is unreachable, figure out the most popular
1707 // destination and make it the default.
1708 if (SI->getSuccessor(0) == BB) {
1709 std::map<BasicBlock*, unsigned> Popularity;
1710 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1711 Popularity[SI->getSuccessor(i)]++;
1713 // Find the most popular block.
1714 unsigned MaxPop = 0;
1715 BasicBlock *MaxBlock = 0;
1716 for (std::map<BasicBlock*, unsigned>::iterator
1717 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) {
1718 if (I->second > MaxPop) {
1720 MaxBlock = I->first;
1724 // Make this the new default, allowing us to delete any explicit
1726 SI->setSuccessor(0, MaxBlock);
1729 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from
1731 if (isa<PHINode>(MaxBlock->begin()))
1732 for (unsigned i = 0; i != MaxPop-1; ++i)
1733 MaxBlock->removePredecessor(SI->getParent());
1735 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
1736 if (SI->getSuccessor(i) == MaxBlock) {
1742 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
1743 if (II->getUnwindDest() == BB) {
1744 // Convert the invoke to a call instruction. This would be a good
1745 // place to note that the call does not throw though.
1746 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
1747 II->removeFromParent(); // Take out of symbol table
1749 // Insert the call now...
1750 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
1751 CallInst *CI = new CallInst(II->getCalledValue(),
1752 Args.begin(), Args.end(),
1754 CI->setCallingConv(II->getCallingConv());
1755 CI->setParamAttrs(II->getParamAttrs());
1756 // If the invoke produced a value, the Call does now instead.
1757 II->replaceAllUsesWith(CI);
1764 // If this block is now dead, remove it.
1765 if (pred_begin(BB) == pred_end(BB)) {
1766 // We know there are no successors, so just nuke the block.
1767 M->getBasicBlockList().erase(BB);
1773 // Merge basic blocks into their predecessor if there is only one distinct
1774 // pred, and if there is only one distinct successor of the predecessor, and
1775 // if there are no PHI nodes.
1777 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
1778 BasicBlock *OnlyPred = *PI++;
1779 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
1780 if (*PI != OnlyPred) {
1781 OnlyPred = 0; // There are multiple different predecessors...
1785 BasicBlock *OnlySucc = 0;
1786 if (OnlyPred && OnlyPred != BB && // Don't break self loops
1787 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
1788 // Check to see if there is only one distinct successor...
1789 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
1791 for (; SI != SE; ++SI)
1792 if (*SI != OnlySucc) {
1793 OnlySucc = 0; // There are multiple distinct successors!
1799 DOUT << "Merging: " << *BB << "into: " << *OnlyPred;
1801 // Resolve any PHI nodes at the start of the block. They are all
1802 // guaranteed to have exactly one entry if they exist, unless there are
1803 // multiple duplicate (but guaranteed to be equal) entries for the
1804 // incoming edges. This occurs when there are multiple edges from
1805 // OnlyPred to OnlySucc.
1807 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
1808 PN->replaceAllUsesWith(PN->getIncomingValue(0));
1809 BB->getInstList().pop_front(); // Delete the phi node.
1812 // Delete the unconditional branch from the predecessor.
1813 OnlyPred->getInstList().pop_back();
1815 // Move all definitions in the successor to the predecessor.
1816 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
1818 // Make all PHI nodes that referred to BB now refer to Pred as their
1820 BB->replaceAllUsesWith(OnlyPred);
1822 // Inherit predecessors name if it exists.
1823 if (!OnlyPred->hasName())
1824 OnlyPred->takeName(BB);
1826 // Erase basic block from the function.
1827 M->getBasicBlockList().erase(BB);
1832 // Otherwise, if this block only has a single predecessor, and if that block
1833 // is a conditional branch, see if we can hoist any code from this block up
1834 // into our predecessor.
1836 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
1837 if (BI->isConditional()) {
1838 // Get the other block.
1839 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB);
1840 PI = pred_begin(OtherBB);
1842 if (PI == pred_end(OtherBB)) {
1843 // We have a conditional branch to two blocks that are only reachable
1844 // from the condbr. We know that the condbr dominates the two blocks,
1845 // so see if there is any identical code in the "then" and "else"
1846 // blocks. If so, we can hoist it up to the branching block.
1847 Changed |= HoistThenElseCodeToIf(BI);
1851 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
1852 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
1853 // Change br (X == 0 | X == 1), T, F into a switch instruction.
1854 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
1855 Instruction *Cond = cast<Instruction>(BI->getCondition());
1856 // If this is a bunch of seteq's or'd together, or if it's a bunch of
1857 // 'setne's and'ed together, collect them.
1859 std::vector<ConstantInt*> Values;
1860 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
1861 if (CompVal && CompVal->getType()->isInteger()) {
1862 // There might be duplicate constants in the list, which the switch
1863 // instruction can't handle, remove them now.
1864 std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
1865 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
1867 // Figure out which block is which destination.
1868 BasicBlock *DefaultBB = BI->getSuccessor(1);
1869 BasicBlock *EdgeBB = BI->getSuccessor(0);
1870 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
1872 // Create the new switch instruction now.
1873 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI);
1875 // Add all of the 'cases' to the switch instruction.
1876 for (unsigned i = 0, e = Values.size(); i != e; ++i)
1877 New->addCase(Values[i], EdgeBB);
1879 // We added edges from PI to the EdgeBB. As such, if there were any
1880 // PHI nodes in EdgeBB, they need entries to be added corresponding to
1881 // the number of edges added.
1882 for (BasicBlock::iterator BBI = EdgeBB->begin();
1883 isa<PHINode>(BBI); ++BBI) {
1884 PHINode *PN = cast<PHINode>(BBI);
1885 Value *InVal = PN->getIncomingValueForBlock(*PI);
1886 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
1887 PN->addIncoming(InVal, *PI);
1890 // Erase the old branch instruction.
1891 (*PI)->getInstList().erase(BI);
1893 // Erase the potentially condition tree that was used to computed the
1894 // branch condition.
1895 ErasePossiblyDeadInstructionTree(Cond);
1900 // If there is a trivial two-entry PHI node in this basic block, and we can
1901 // eliminate it, do so now.
1902 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
1903 if (PN->getNumIncomingValues() == 2)
1904 Changed |= FoldTwoEntryPHINode(PN);