1 //===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // Peephole optimize the CFG.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Transforms/Utils/Local.h"
15 #include "llvm/Constants.h"
16 #include "llvm/Instructions.h"
17 #include "llvm/Type.h"
18 #include "llvm/Support/CFG.h"
24 // PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
25 // predecessors from "BB". This is a little tricky because "Succ" has PHI
26 // nodes, which need to have extra slots added to them to hold the merge edges
27 // from BB's predecessors, and BB itself might have had PHI nodes in it. This
28 // function returns true (failure) if the Succ BB already has a predecessor that
29 // is a predecessor of BB and incoming PHI arguments would not be discernible.
31 // Assumption: Succ is the single successor for BB.
33 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
34 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
36 if (!isa<PHINode>(Succ->front()))
37 return false; // We can make the transformation, no problem.
39 // If there is more than one predecessor, and there are PHI nodes in
40 // the successor, then we need to add incoming edges for the PHI nodes
42 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
44 // Check to see if one of the predecessors of BB is already a predecessor of
45 // Succ. If so, we cannot do the transformation if there are any PHI nodes
46 // with incompatible values coming in from the two edges!
48 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
49 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
50 // Loop over all of the PHI nodes checking to see if there are
51 // incompatible values coming in.
52 for (BasicBlock::iterator I = Succ->begin();
53 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
54 // Loop up the entries in the PHI node for BB and for *PI if the values
55 // coming in are non-equal, we cannot merge these two blocks (instead we
56 // should insert a conditional move or something, then merge the
58 int Idx1 = PN->getBasicBlockIndex(BB);
59 int Idx2 = PN->getBasicBlockIndex(*PI);
60 assert(Idx1 != -1 && Idx2 != -1 &&
61 "Didn't have entries for my predecessors??");
62 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
63 return true; // Values are not equal...
67 // Loop over all of the PHI nodes in the successor BB
68 for (BasicBlock::iterator I = Succ->begin();
69 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
70 Value *OldVal = PN->removeIncomingValue(BB, false);
71 assert(OldVal && "No entry in PHI for Pred BB!");
73 // If this incoming value is one of the PHI nodes in BB...
74 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
75 PHINode *OldValPN = cast<PHINode>(OldVal);
76 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
77 End = BBPreds.end(); PredI != End; ++PredI) {
78 PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
81 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
82 End = BBPreds.end(); PredI != End; ++PredI) {
83 // Add an incoming value for each of the new incoming values...
84 PN->addIncoming(OldVal, *PredI);
91 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
92 /// presumably PHI nodes in it), check to see if the merge at this block is due
93 /// to an "if condition". If so, return the boolean condition that determines
94 /// which entry into BB will be taken. Also, return by references the block
95 /// that will be entered from if the condition is true, and the block that will
96 /// be entered if the condition is false.
99 static Value *GetIfCondition(BasicBlock *BB,
100 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
101 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
102 "Function can only handle blocks with 2 predecessors!");
103 BasicBlock *Pred1 = *pred_begin(BB);
104 BasicBlock *Pred2 = *++pred_begin(BB);
106 // We can only handle branches. Other control flow will be lowered to
107 // branches if possible anyway.
108 if (!isa<BranchInst>(Pred1->getTerminator()) ||
109 !isa<BranchInst>(Pred2->getTerminator()))
111 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
112 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
114 // Eliminate code duplication by ensuring that Pred1Br is conditional if
116 if (Pred2Br->isConditional()) {
117 // If both branches are conditional, we don't have an "if statement". In
118 // reality, we could transform this case, but since the condition will be
119 // required anyway, we stand no chance of eliminating it, so the xform is
120 // probably not profitable.
121 if (Pred1Br->isConditional())
124 std::swap(Pred1, Pred2);
125 std::swap(Pred1Br, Pred2Br);
128 if (Pred1Br->isConditional()) {
129 // If we found a conditional branch predecessor, make sure that it branches
130 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
131 if (Pred1Br->getSuccessor(0) == BB &&
132 Pred1Br->getSuccessor(1) == Pred2) {
135 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
136 Pred1Br->getSuccessor(1) == BB) {
140 // We know that one arm of the conditional goes to BB, so the other must
141 // go somewhere unrelated, and this must not be an "if statement".
145 // The only thing we have to watch out for here is to make sure that Pred2
146 // doesn't have incoming edges from other blocks. If it does, the condition
147 // doesn't dominate BB.
148 if (++pred_begin(Pred2) != pred_end(Pred2))
151 return Pred1Br->getCondition();
154 // Ok, if we got here, both predecessors end with an unconditional branch to
155 // BB. Don't panic! If both blocks only have a single (identical)
156 // predecessor, and THAT is a conditional branch, then we're all ok!
157 if (pred_begin(Pred1) == pred_end(Pred1) ||
158 ++pred_begin(Pred1) != pred_end(Pred1) ||
159 pred_begin(Pred2) == pred_end(Pred2) ||
160 ++pred_begin(Pred2) != pred_end(Pred2) ||
161 *pred_begin(Pred1) != *pred_begin(Pred2))
164 // Otherwise, if this is a conditional branch, then we can use it!
165 BasicBlock *CommonPred = *pred_begin(Pred1);
166 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
167 assert(BI->isConditional() && "Two successors but not conditional?");
168 if (BI->getSuccessor(0) == Pred1) {
175 return BI->getCondition();
181 // If we have a merge point of an "if condition" as accepted above, return true
182 // if the specified value dominates the block. We don't handle the true
183 // generality of domination here, just a special case which works well enough
185 static bool DominatesMergePoint(Value *V, BasicBlock *BB) {
186 if (Instruction *I = dyn_cast<Instruction>(V)) {
187 BasicBlock *PBB = I->getParent();
188 // If this instruction is defined in a block that contains an unconditional
189 // branch to BB, then it must be in the 'conditional' part of the "if
191 if (isa<BranchInst>(PBB->getTerminator()) &&
192 cast<BranchInst>(PBB->getTerminator())->isUnconditional() &&
193 cast<BranchInst>(PBB->getTerminator())->getSuccessor(0) == BB)
196 // We also don't want to allow wierd loops that might have the "if
197 // condition" in the bottom of this block.
198 if (PBB == BB) return false;
201 // Non-instructions all dominate instructions.
205 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
206 // instructions that compare a value against a constant, return the value being
207 // compared, and stick the constant into the Values vector.
208 static Value *GatherConstantSetEQs(Value *V, std::vector<Constant*> &Values) {
209 if (Instruction *Inst = dyn_cast<Instruction>(V))
210 if (Inst->getOpcode() == Instruction::SetEQ) {
211 if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
213 return Inst->getOperand(0);
214 } else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
216 return Inst->getOperand(1);
218 } else if (Inst->getOpcode() == Instruction::Or) {
219 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
220 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
227 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
228 // setne instructions that compare a value against a constant, return the value
229 // being compared, and stick the constant into the Values vector.
230 static Value *GatherConstantSetNEs(Value *V, std::vector<Constant*> &Values) {
231 if (Instruction *Inst = dyn_cast<Instruction>(V))
232 if (Inst->getOpcode() == Instruction::SetNE) {
233 if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
235 return Inst->getOperand(0);
236 } else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
238 return Inst->getOperand(1);
240 } else if (Inst->getOpcode() == Instruction::Cast) {
241 // Cast of X to bool is really a comparison against zero.
242 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
243 Values.push_back(Constant::getNullValue(Inst->getOperand(0)->getType()));
244 return Inst->getOperand(0);
245 } else if (Inst->getOpcode() == Instruction::And) {
246 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
247 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
256 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
257 /// bunch of comparisons of one value against constants, return the value and
258 /// the constants being compared.
259 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
260 std::vector<Constant*> &Values) {
261 if (Cond->getOpcode() == Instruction::Or) {
262 CompVal = GatherConstantSetEQs(Cond, Values);
264 // Return true to indicate that the condition is true if the CompVal is
265 // equal to one of the constants.
267 } else if (Cond->getOpcode() == Instruction::And) {
268 CompVal = GatherConstantSetNEs(Cond, Values);
270 // Return false to indicate that the condition is false if the CompVal is
271 // equal to one of the constants.
277 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
278 /// has no side effects, nuke it. If it uses any instructions that become dead
279 /// because the instruction is now gone, nuke them too.
280 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
281 if (isInstructionTriviallyDead(I)) {
282 std::vector<Value*> Operands(I->op_begin(), I->op_end());
283 I->getParent()->getInstList().erase(I);
284 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
285 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
286 ErasePossiblyDeadInstructionTree(OpI);
290 /// SafeToMergeTerminators - Return true if it is safe to merge these two
291 /// terminator instructions together.
293 static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
294 if (SI1 == SI2) return false; // Can't merge with self!
296 // It is not safe to merge these two switch instructions if they have a common
297 // successor, and if that successor has a PHI node, and if that PHI node has
298 // conflicting incoming values from the two switch blocks.
299 BasicBlock *SI1BB = SI1->getParent();
300 BasicBlock *SI2BB = SI2->getParent();
301 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
303 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
304 if (SI1Succs.count(*I))
305 for (BasicBlock::iterator BBI = (*I)->begin();
306 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
307 if (PN->getIncomingValueForBlock(SI1BB) !=
308 PN->getIncomingValueForBlock(SI2BB))
314 /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
315 /// now be entries in it from the 'NewPred' block. The values that will be
316 /// flowing into the PHI nodes will be the same as those coming in from
317 /// ExistPred, and existing predecessor of Succ.
318 static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
319 BasicBlock *ExistPred) {
320 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
321 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
322 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
324 for (BasicBlock::iterator I = Succ->begin();
325 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
326 Value *V = PN->getIncomingValueForBlock(ExistPred);
327 PN->addIncoming(V, NewPred);
331 // isValueEqualityComparison - Return true if the specified terminator checks to
332 // see if a value is equal to constant integer value.
333 static Value *isValueEqualityComparison(TerminatorInst *TI) {
334 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI))
335 return SI->getCondition();
336 if (BranchInst *BI = dyn_cast<BranchInst>(TI))
337 if (BI->isConditional() && BI->getCondition()->hasOneUse())
338 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
339 if ((SCI->getOpcode() == Instruction::SetEQ ||
340 SCI->getOpcode() == Instruction::SetNE) &&
341 isa<ConstantInt>(SCI->getOperand(1)))
342 return SCI->getOperand(0);
346 // Given a value comparison instruction, decode all of the 'cases' that it
347 // represents and return the 'default' block.
349 GetValueEqualityComparisonCases(TerminatorInst *TI,
350 std::vector<std::pair<ConstantInt*,
351 BasicBlock*> > &Cases) {
352 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
353 Cases.reserve(SI->getNumCases());
354 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
355 Cases.push_back(std::make_pair(cast<ConstantInt>(SI->getCaseValue(i)),
356 SI->getSuccessor(i)));
357 return SI->getDefaultDest();
360 BranchInst *BI = cast<BranchInst>(TI);
361 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
362 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
363 BI->getSuccessor(SCI->getOpcode() ==
364 Instruction::SetNE)));
365 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
369 // FoldValueComparisonIntoPredecessors - The specified terminator is a value
370 // equality comparison instruction (either a switch or a branch on "X == c").
371 // See if any of the predecessors of the terminator block are value comparisons
372 // on the same value. If so, and if safe to do so, fold them together.
373 static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
374 BasicBlock *BB = TI->getParent();
375 Value *CV = isValueEqualityComparison(TI); // CondVal
376 assert(CV && "Not a comparison?");
377 bool Changed = false;
379 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
380 while (!Preds.empty()) {
381 BasicBlock *Pred = Preds.back();
384 // See if the predecessor is a comparison with the same value.
385 TerminatorInst *PTI = Pred->getTerminator();
386 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
388 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
389 // Figure out which 'cases' to copy from SI to PSI.
390 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
391 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
393 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
394 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
396 // Based on whether the default edge from PTI goes to BB or not, fill in
397 // PredCases and PredDefault with the new switch cases we would like to
399 std::vector<BasicBlock*> NewSuccessors;
401 if (PredDefault == BB) {
402 // If this is the default destination from PTI, only the edges in TI
403 // that don't occur in PTI, or that branch to BB will be activated.
404 std::set<ConstantInt*> PTIHandled;
405 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
406 if (PredCases[i].second != BB)
407 PTIHandled.insert(PredCases[i].first);
409 // The default destination is BB, we don't need explicit targets.
410 std::swap(PredCases[i], PredCases.back());
411 PredCases.pop_back();
415 // Reconstruct the new switch statement we will be building.
416 if (PredDefault != BBDefault) {
417 PredDefault->removePredecessor(Pred);
418 PredDefault = BBDefault;
419 NewSuccessors.push_back(BBDefault);
421 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
422 if (!PTIHandled.count(BBCases[i].first) &&
423 BBCases[i].second != BBDefault) {
424 PredCases.push_back(BBCases[i]);
425 NewSuccessors.push_back(BBCases[i].second);
429 // If this is not the default destination from PSI, only the edges
430 // in SI that occur in PSI with a destination of BB will be
432 std::set<ConstantInt*> PTIHandled;
433 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
434 if (PredCases[i].second == BB) {
435 PTIHandled.insert(PredCases[i].first);
436 std::swap(PredCases[i], PredCases.back());
437 PredCases.pop_back();
441 // Okay, now we know which constants were sent to BB from the
442 // predecessor. Figure out where they will all go now.
443 for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
444 if (PTIHandled.count(BBCases[i].first)) {
445 // If this is one we are capable of getting...
446 PredCases.push_back(BBCases[i]);
447 NewSuccessors.push_back(BBCases[i].second);
448 PTIHandled.erase(BBCases[i].first);// This constant is taken care of
451 // If there are any constants vectored to BB that TI doesn't handle,
452 // they must go to the default destination of TI.
453 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
454 E = PTIHandled.end(); I != E; ++I) {
455 PredCases.push_back(std::make_pair(*I, BBDefault));
456 NewSuccessors.push_back(BBDefault);
460 // Okay, at this point, we know which new successor Pred will get. Make
461 // sure we update the number of entries in the PHI nodes for these
463 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
464 AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
466 // Now that the successors are updated, create the new Switch instruction.
467 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PTI);
468 for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
469 NewSI->addCase(PredCases[i].first, PredCases[i].second);
470 Pred->getInstList().erase(PTI);
472 // Okay, last check. If BB is still a successor of PSI, then we must
473 // have an infinite loop case. If so, add an infinitely looping block
474 // to handle the case to preserve the behavior of the code.
475 BasicBlock *InfLoopBlock = 0;
476 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
477 if (NewSI->getSuccessor(i) == BB) {
478 if (InfLoopBlock == 0) {
479 // Insert it at the end of the loop, because it's either code,
480 // or it won't matter if it's hot. :)
481 InfLoopBlock = new BasicBlock("infloop", BB->getParent());
482 new BranchInst(InfLoopBlock, InfLoopBlock);
484 NewSI->setSuccessor(i, InfLoopBlock);
494 // SimplifyCFG - This function is used to do simplification of a CFG. For
495 // example, it adjusts branches to branches to eliminate the extra hop, it
496 // eliminates unreachable basic blocks, and does other "peephole" optimization
497 // of the CFG. It returns true if a modification was made.
499 // WARNING: The entry node of a function may not be simplified.
501 bool llvm::SimplifyCFG(BasicBlock *BB) {
502 bool Changed = false;
503 Function *M = BB->getParent();
505 assert(BB && BB->getParent() && "Block not embedded in function!");
506 assert(BB->getTerminator() && "Degenerate basic block encountered!");
507 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
509 // Remove basic blocks that have no predecessors... which are unreachable.
510 if (pred_begin(BB) == pred_end(BB) ||
511 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
512 //cerr << "Removing BB: \n" << BB;
514 // Loop through all of our successors and make sure they know that one
515 // of their predecessors is going away.
516 for_each(succ_begin(BB), succ_end(BB),
517 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
519 while (!BB->empty()) {
520 Instruction &I = BB->back();
521 // If this instruction is used, replace uses with an arbitrary
522 // constant value. Because control flow can't get here, we don't care
523 // what we replace the value with. Note that since this block is
524 // unreachable, and all values contained within it must dominate their
525 // uses, that all uses will eventually be removed.
527 // Make all users of this instruction reference the constant instead
528 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
530 // Remove the instruction from the basic block
531 BB->getInstList().pop_back();
533 M->getBasicBlockList().erase(BB);
537 // Check to see if we can constant propagate this terminator instruction
539 Changed |= ConstantFoldTerminator(BB);
541 // Check to see if this block has no non-phi instructions and only a single
542 // successor. If so, replace references to this basic block with references
544 succ_iterator SI(succ_begin(BB));
545 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
547 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
548 while (isa<PHINode>(*BBI)) ++BBI;
550 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
551 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
553 if (Succ != BB) { // Arg, don't hurt infinite loops!
554 // If our successor has PHI nodes, then we need to update them to
555 // include entries for BB's predecessors, not for BB itself.
556 // Be careful though, if this transformation fails (returns true) then
557 // we cannot do this transformation!
559 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
560 //cerr << "Killing Trivial BB: \n" << BB;
561 std::string OldName = BB->getName();
563 std::vector<BasicBlock*>
564 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
566 // Move all PHI nodes in BB to Succ if they are alive, otherwise
568 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
570 BB->getInstList().erase(BB->begin()); // Nuke instruction...
572 // The instruction is alive, so this means that Succ must have
573 // *ONLY* had BB as a predecessor, and the PHI node is still valid
574 // now. Simply move it into Succ, because we know that BB
575 // strictly dominated Succ.
576 BB->getInstList().remove(BB->begin());
577 Succ->getInstList().push_front(PN);
579 // We need to add new entries for the PHI node to account for
580 // predecessors of Succ that the PHI node does not take into
581 // account. At this point, since we know that BB dominated succ,
582 // this means that we should any newly added incoming edges should
583 // use the PHI node as the value for these edges, because they are
586 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
587 if (OldSuccPreds[i] != BB)
588 PN->addIncoming(PN, OldSuccPreds[i]);
591 // Everything that jumped to BB now goes to Succ...
592 BB->replaceAllUsesWith(Succ);
594 // Delete the old basic block...
595 M->getBasicBlockList().erase(BB);
597 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
598 Succ->setName(OldName);
600 //cerr << "Function after removal: \n" << M;
607 // If this is a returning block with only PHI nodes in it, fold the return
608 // instruction into any unconditional branch predecessors.
609 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
610 BasicBlock::iterator BBI = BB->getTerminator();
611 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
612 // Find predecessors that end with unconditional branches.
613 std::vector<BasicBlock*> UncondBranchPreds;
614 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
615 TerminatorInst *PTI = (*PI)->getTerminator();
616 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
617 if (BI->isUnconditional())
618 UncondBranchPreds.push_back(*PI);
621 // If we found some, do the transformation!
622 if (!UncondBranchPreds.empty()) {
623 while (!UncondBranchPreds.empty()) {
624 BasicBlock *Pred = UncondBranchPreds.back();
625 UncondBranchPreds.pop_back();
626 Instruction *UncondBranch = Pred->getTerminator();
627 // Clone the return and add it to the end of the predecessor.
628 Instruction *NewRet = RI->clone();
629 Pred->getInstList().push_back(NewRet);
631 // If the return instruction returns a value, and if the value was a
632 // PHI node in "BB", propagate the right value into the return.
633 if (NewRet->getNumOperands() == 1)
634 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
635 if (PN->getParent() == BB)
636 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
637 // Update any PHI nodes in the returning block to realize that we no
638 // longer branch to them.
639 BB->removePredecessor(Pred);
640 Pred->getInstList().erase(UncondBranch);
643 // If we eliminated all predecessors of the block, delete the block now.
644 if (pred_begin(BB) == pred_end(BB))
645 // We know there are no successors, so just nuke the block.
646 M->getBasicBlockList().erase(BB);
651 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
652 // Check to see if the first instruction in this block is just an unwind.
653 // If so, replace any invoke instructions which use this as an exception
654 // destination with call instructions.
656 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
657 while (!Preds.empty()) {
658 BasicBlock *Pred = Preds.back();
659 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
660 if (II->getUnwindDest() == BB) {
661 // Insert a new branch instruction before the invoke, because this
662 // is now a fall through...
663 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
664 Pred->getInstList().remove(II); // Take out of symbol table
666 // Insert the call now...
667 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
668 CallInst *CI = new CallInst(II->getCalledValue(), Args,
670 // If the invoke produced a value, the Call now does instead
671 II->replaceAllUsesWith(CI);
679 // If this block is now dead, remove it.
680 if (pred_begin(BB) == pred_end(BB)) {
681 // We know there are no successors, so just nuke the block.
682 M->getBasicBlockList().erase(BB);
686 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
687 if (FoldValueComparisonIntoPredecessors(SI))
688 return SimplifyCFG(BB) || 1;
689 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
690 if (Value *CompVal = isValueEqualityComparison(BB->getTerminator())) {
691 // This block must be empty, except for the setcond inst, if it exists.
692 BasicBlock::iterator I = BB->begin();
694 (&*I == cast<Instruction>(BI->getCondition()) &&
696 if (FoldValueComparisonIntoPredecessors(BI))
697 return SimplifyCFG(BB) || 1;
701 // Merge basic blocks into their predecessor if there is only one distinct
702 // pred, and if there is only one distinct successor of the predecessor, and
703 // if there are no PHI nodes.
705 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
706 BasicBlock *OnlyPred = *PI++;
707 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
708 if (*PI != OnlyPred) {
709 OnlyPred = 0; // There are multiple different predecessors...
713 BasicBlock *OnlySucc = 0;
714 if (OnlyPred && OnlyPred != BB && // Don't break self loops
715 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
716 // Check to see if there is only one distinct successor...
717 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
719 for (; SI != SE; ++SI)
720 if (*SI != OnlySucc) {
721 OnlySucc = 0; // There are multiple distinct successors!
727 //cerr << "Merging: " << BB << "into: " << OnlyPred;
728 TerminatorInst *Term = OnlyPred->getTerminator();
730 // Resolve any PHI nodes at the start of the block. They are all
731 // guaranteed to have exactly one entry if they exist, unless there are
732 // multiple duplicate (but guaranteed to be equal) entries for the
733 // incoming edges. This occurs when there are multiple edges from
734 // OnlyPred to OnlySucc.
736 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
737 PN->replaceAllUsesWith(PN->getIncomingValue(0));
738 BB->getInstList().pop_front(); // Delete the phi node...
741 // Delete the unconditional branch from the predecessor...
742 OnlyPred->getInstList().pop_back();
744 // Move all definitions in the successor to the predecessor...
745 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
747 // Make all PHI nodes that referred to BB now refer to Pred as their
749 BB->replaceAllUsesWith(OnlyPred);
751 std::string OldName = BB->getName();
753 // Erase basic block from the function...
754 M->getBasicBlockList().erase(BB);
756 // Inherit predecessors name if it exists...
757 if (!OldName.empty() && !OnlyPred->hasName())
758 OnlyPred->setName(OldName);
763 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
764 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
765 // Change br (X == 0 | X == 1), T, F into a switch instruction.
766 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
767 Instruction *Cond = cast<Instruction>(BI->getCondition());
768 // If this is a bunch of seteq's or'd together, or if it's a bunch of
769 // 'setne's and'ed together, collect them.
771 std::vector<Constant*> Values;
772 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
773 if (CompVal && CompVal->getType()->isInteger()) {
774 // There might be duplicate constants in the list, which the switch
775 // instruction can't handle, remove them now.
776 std::sort(Values.begin(), Values.end());
777 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
779 // Figure out which block is which destination.
780 BasicBlock *DefaultBB = BI->getSuccessor(1);
781 BasicBlock *EdgeBB = BI->getSuccessor(0);
782 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
784 // Create the new switch instruction now.
785 SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
787 // Add all of the 'cases' to the switch instruction.
788 for (unsigned i = 0, e = Values.size(); i != e; ++i)
789 New->addCase(Values[i], EdgeBB);
791 // We added edges from PI to the EdgeBB. As such, if there were any
792 // PHI nodes in EdgeBB, they need entries to be added corresponding to
793 // the number of edges added.
794 for (BasicBlock::iterator BBI = EdgeBB->begin();
795 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
796 Value *InVal = PN->getIncomingValueForBlock(*PI);
797 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
798 PN->addIncoming(InVal, *PI);
801 // Erase the old branch instruction.
802 (*PI)->getInstList().erase(BI);
804 // Erase the potentially condition tree that was used to computed the
806 ErasePossiblyDeadInstructionTree(Cond);
811 // If there is a trivial two-entry PHI node in this basic block, and we can
812 // eliminate it, do so now.
813 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
814 if (PN->getNumIncomingValues() == 2) {
815 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
816 // statement", which has a very simple dominance structure. Basically, we
817 // are trying to find the condition that is being branched on, which
818 // subsequently causes this merge to happen. We really want control
819 // dependence information for this check, but simplifycfg can't keep it up
820 // to date, and this catches most of the cases we care about anyway.
822 BasicBlock *IfTrue, *IfFalse;
823 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
824 //std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
825 // << IfTrue->getName() << " F: " << IfFalse->getName() << "\n";
827 // Figure out where to insert instructions as necessary.
828 BasicBlock::iterator AfterPHIIt = BB->begin();
829 while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
831 BasicBlock::iterator I = BB->begin();
832 while (PHINode *PN = dyn_cast<PHINode>(I)) {
835 // If we can eliminate this PHI by directly computing it based on the
836 // condition, do so now. We can't eliminate PHI nodes where the
837 // incoming values are defined in the conditional parts of the branch,
838 // so check for this.
840 if (DominatesMergePoint(PN->getIncomingValue(0), BB) &&
841 DominatesMergePoint(PN->getIncomingValue(1), BB)) {
843 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
845 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
847 // FIXME: when we have a 'select' statement, we can be completely
848 // generic and clean here and let the instcombine pass clean up
849 // after us, by folding the select instructions away when possible.
851 if (TrueVal == FalseVal) {
852 // Degenerate case...
853 PN->replaceAllUsesWith(TrueVal);
854 BB->getInstList().erase(PN);
856 } else if (isa<ConstantBool>(TrueVal) &&
857 isa<ConstantBool>(FalseVal)) {
858 if (TrueVal == ConstantBool::True) {
859 // The PHI node produces the same thing as the condition.
860 PN->replaceAllUsesWith(IfCond);
862 // The PHI node produces the inverse of the condition. Insert a
863 // "NOT" instruction, which is really a XOR.
865 BinaryOperator::createNot(IfCond, IfCond->getName()+".inv",
867 PN->replaceAllUsesWith(InverseCond);
869 BB->getInstList().erase(PN);
871 } else if (isa<ConstantInt>(TrueVal) && isa<ConstantInt>(FalseVal)){
872 // If this is a PHI of two constant integers, we insert a cast of
873 // the boolean to the integer type in question, giving us 0 or 1.
874 // Then we multiply this by the difference of the two constants,
875 // giving us 0 if false, and the difference if true. We add this
876 // result to the base constant, giving us our final value. We
877 // rely on the instruction combiner to eliminate many special
878 // cases, like turning multiplies into shifts when possible.
879 std::string Name = PN->getName(); PN->setName("");
880 Value *TheCast = new CastInst(IfCond, TrueVal->getType(),
882 Constant *TheDiff = ConstantExpr::get(Instruction::Sub,
883 cast<Constant>(TrueVal),
884 cast<Constant>(FalseVal));
886 if (TheDiff != ConstantInt::get(TrueVal->getType(), 1))
887 V = BinaryOperator::create(Instruction::Mul, TheCast,
888 TheDiff, TheCast->getName()+".scale",
890 if (!cast<Constant>(FalseVal)->isNullValue())
891 V = BinaryOperator::create(Instruction::Add, V, FalseVal,
892 V->getName()+".offs", AfterPHIIt);
893 PN->replaceAllUsesWith(V);
894 BB->getInstList().erase(PN);
896 } else if (isa<ConstantInt>(FalseVal) &&
897 cast<Constant>(FalseVal)->isNullValue()) {
898 // If the false condition is an integral zero value, we can
899 // compute the PHI by multiplying the condition by the other
901 std::string Name = PN->getName(); PN->setName("");
902 Value *TheCast = new CastInst(IfCond, TrueVal->getType(),
903 Name+".c", AfterPHIIt);
904 Value *V = BinaryOperator::create(Instruction::Mul, TrueVal,
905 TheCast, Name, AfterPHIIt);
906 PN->replaceAllUsesWith(V);
907 BB->getInstList().erase(PN);
909 } else if (isa<ConstantInt>(TrueVal) &&
910 cast<Constant>(TrueVal)->isNullValue()) {
911 // If the true condition is an integral zero value, we can compute
912 // the PHI by multiplying the inverse condition by the other
914 std::string Name = PN->getName(); PN->setName("");
915 Value *NotCond = BinaryOperator::createNot(IfCond, Name+".inv",
917 Value *TheCast = new CastInst(NotCond, TrueVal->getType(),
918 Name+".inv", AfterPHIIt);
919 Value *V = BinaryOperator::create(Instruction::Mul, FalseVal,
920 TheCast, Name, AfterPHIIt);
921 PN->replaceAllUsesWith(V);
922 BB->getInstList().erase(PN);