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"
23 // PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
24 // predecessors from "BB". This is a little tricky because "Succ" has PHI
25 // nodes, which need to have extra slots added to them to hold the merge edges
26 // from BB's predecessors, and BB itself might have had PHI nodes in it. This
27 // function returns true (failure) if the Succ BB already has a predecessor that
28 // is a predecessor of BB and incoming PHI arguments would not be discernible.
30 // Assumption: Succ is the single successor for BB.
32 static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
33 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
35 if (!isa<PHINode>(Succ->front()))
36 return false; // We can make the transformation, no problem.
38 // If there is more than one predecessor, and there are PHI nodes in
39 // the successor, then we need to add incoming edges for the PHI nodes
41 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
43 // Check to see if one of the predecessors of BB is already a predecessor of
44 // Succ. If so, we cannot do the transformation if there are any PHI nodes
45 // with incompatible values coming in from the two edges!
47 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
48 if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
49 // Loop over all of the PHI nodes checking to see if there are
50 // incompatible values coming in.
51 for (BasicBlock::iterator I = Succ->begin();
52 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
53 // Loop up the entries in the PHI node for BB and for *PI if the values
54 // coming in are non-equal, we cannot merge these two blocks (instead we
55 // should insert a conditional move or something, then merge the
57 int Idx1 = PN->getBasicBlockIndex(BB);
58 int Idx2 = PN->getBasicBlockIndex(*PI);
59 assert(Idx1 != -1 && Idx2 != -1 &&
60 "Didn't have entries for my predecessors??");
61 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
62 return true; // Values are not equal...
66 // Loop over all of the PHI nodes in the successor BB
67 for (BasicBlock::iterator I = Succ->begin();
68 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
69 Value *OldVal = PN->removeIncomingValue(BB, false);
70 assert(OldVal && "No entry in PHI for Pred BB!");
72 // If this incoming value is one of the PHI nodes in BB...
73 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
74 PHINode *OldValPN = cast<PHINode>(OldVal);
75 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
76 End = BBPreds.end(); PredI != End; ++PredI) {
77 PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
80 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
81 End = BBPreds.end(); PredI != End; ++PredI) {
82 // Add an incoming value for each of the new incoming values...
83 PN->addIncoming(OldVal, *PredI);
90 /// GetIfCondition - Given a basic block (BB) with two predecessors (and
91 /// presumably PHI nodes in it), check to see if the merge at this block is due
92 /// to an "if condition". If so, return the boolean condition that determines
93 /// which entry into BB will be taken. Also, return by references the block
94 /// that will be entered from if the condition is true, and the block that will
95 /// be entered if the condition is false.
98 static Value *GetIfCondition(BasicBlock *BB,
99 BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
100 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
101 "Function can only handle blocks with 2 predecessors!");
102 BasicBlock *Pred1 = *pred_begin(BB);
103 BasicBlock *Pred2 = *++pred_begin(BB);
105 // We can only handle branches. Other control flow will be lowered to
106 // branches if possible anyway.
107 if (!isa<BranchInst>(Pred1->getTerminator()) ||
108 !isa<BranchInst>(Pred2->getTerminator()))
110 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
111 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
113 // Eliminate code duplication by ensuring that Pred1Br is conditional if
115 if (Pred2Br->isConditional()) {
116 // If both branches are conditional, we don't have an "if statement". In
117 // reality, we could transform this case, but since the condition will be
118 // required anyway, we stand no chance of eliminating it, so the xform is
119 // probably not profitable.
120 if (Pred1Br->isConditional())
123 std::swap(Pred1, Pred2);
124 std::swap(Pred1Br, Pred2Br);
127 if (Pred1Br->isConditional()) {
128 // If we found a conditional branch predecessor, make sure that it branches
129 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
130 if (Pred1Br->getSuccessor(0) == BB &&
131 Pred1Br->getSuccessor(1) == Pred2) {
134 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
135 Pred1Br->getSuccessor(1) == BB) {
139 // We know that one arm of the conditional goes to BB, so the other must
140 // go somewhere unrelated, and this must not be an "if statement".
144 // The only thing we have to watch out for here is to make sure that Pred2
145 // doesn't have incoming edges from other blocks. If it does, the condition
146 // doesn't dominate BB.
147 if (++pred_begin(Pred2) != pred_end(Pred2))
150 return Pred1Br->getCondition();
153 // Ok, if we got here, both predecessors end with an unconditional branch to
154 // BB. Don't panic! If both blocks only have a single (identical)
155 // predecessor, and THAT is a conditional branch, then we're all ok!
156 if (pred_begin(Pred1) == pred_end(Pred1) ||
157 ++pred_begin(Pred1) != pred_end(Pred1) ||
158 pred_begin(Pred2) == pred_end(Pred2) ||
159 ++pred_begin(Pred2) != pred_end(Pred2) ||
160 *pred_begin(Pred1) != *pred_begin(Pred2))
163 // Otherwise, if this is a conditional branch, then we can use it!
164 BasicBlock *CommonPred = *pred_begin(Pred1);
165 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
166 assert(BI->isConditional() && "Two successors but not conditional?");
167 if (BI->getSuccessor(0) == Pred1) {
174 return BI->getCondition();
180 // If we have a merge point of an "if condition" as accepted above, return true
181 // if the specified value dominates the block. We don't handle the true
182 // generality of domination here, just a special case which works well enough
184 static bool DominatesMergePoint(Value *V, BasicBlock *BB) {
185 if (Instruction *I = dyn_cast<Instruction>(V)) {
186 BasicBlock *PBB = I->getParent();
187 // If this instruction is defined in a block that contains an unconditional
188 // branch to BB, then it must be in the 'conditional' part of the "if
190 if (isa<BranchInst>(PBB->getTerminator()) &&
191 cast<BranchInst>(PBB->getTerminator())->isUnconditional() &&
192 cast<BranchInst>(PBB->getTerminator())->getSuccessor(0) == BB)
195 // We also don't want to allow wierd loops that might have the "if
196 // condition" in the bottom of this block.
197 if (PBB == BB) return false;
200 // Non-instructions all dominate instructions.
204 // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
205 // instructions that compare a value against a constant, return the value being
206 // compared, and stick the constant into the Values vector.
207 static Value *GatherConstantSetEQs(Value *V, std::vector<Constant*> &Values) {
208 if (Instruction *Inst = dyn_cast<Instruction>(V))
209 if (Inst->getOpcode() == Instruction::SetEQ) {
210 if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
212 return Inst->getOperand(0);
213 } else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
215 return Inst->getOperand(1);
217 } else if (Inst->getOpcode() == Instruction::Or) {
218 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
219 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
226 // GatherConstantSetNEs - Given a potentially 'and'd together collection of
227 // setne instructions that compare a value against a constant, return the value
228 // being compared, and stick the constant into the Values vector.
229 static Value *GatherConstantSetNEs(Value *V, std::vector<Constant*> &Values) {
230 if (Instruction *Inst = dyn_cast<Instruction>(V))
231 if (Inst->getOpcode() == Instruction::SetNE) {
232 if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
234 return Inst->getOperand(0);
235 } else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
237 return Inst->getOperand(1);
239 } else if (Inst->getOpcode() == Instruction::Cast) {
240 // Cast of X to bool is really a comparison against zero.
241 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
242 Values.push_back(Constant::getNullValue(Inst->getOperand(0)->getType()));
243 return Inst->getOperand(0);
244 } else if (Inst->getOpcode() == Instruction::And) {
245 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
246 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
255 /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
256 /// bunch of comparisons of one value against constants, return the value and
257 /// the constants being compared.
258 static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
259 std::vector<Constant*> &Values) {
260 if (Cond->getOpcode() == Instruction::Or) {
261 CompVal = GatherConstantSetEQs(Cond, Values);
263 // Return true to indicate that the condition is true if the CompVal is
264 // equal to one of the constants.
266 } else if (Cond->getOpcode() == Instruction::And) {
267 CompVal = GatherConstantSetNEs(Cond, Values);
269 // Return false to indicate that the condition is false if the CompVal is
270 // equal to one of the constants.
276 /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
277 /// has no side effects, nuke it. If it uses any instructions that become dead
278 /// because the instruction is now gone, nuke them too.
279 static void ErasePossiblyDeadInstructionTree(Instruction *I) {
280 if (isInstructionTriviallyDead(I)) {
281 std::vector<Value*> Operands(I->op_begin(), I->op_end());
282 I->getParent()->getInstList().erase(I);
283 for (unsigned i = 0, e = Operands.size(); i != e; ++i)
284 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
285 ErasePossiblyDeadInstructionTree(OpI);
289 // SimplifyCFG - This function is used to do simplification of a CFG. For
290 // example, it adjusts branches to branches to eliminate the extra hop, it
291 // eliminates unreachable basic blocks, and does other "peephole" optimization
292 // of the CFG. It returns true if a modification was made.
294 // WARNING: The entry node of a function may not be simplified.
296 bool llvm::SimplifyCFG(BasicBlock *BB) {
297 bool Changed = false;
298 Function *M = BB->getParent();
300 assert(BB && BB->getParent() && "Block not embedded in function!");
301 assert(BB->getTerminator() && "Degenerate basic block encountered!");
302 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
304 // Check to see if the first instruction in this block is just an unwind. If
305 // so, replace any invoke instructions which use this as an exception
306 // destination with call instructions.
308 if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->getTerminator()))
309 if (BB->begin() == BasicBlock::iterator(UI)) { // Empty block?
310 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
311 while (!Preds.empty()) {
312 BasicBlock *Pred = Preds.back();
313 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
314 if (II->getUnwindDest() == BB) {
315 // Insert a new branch instruction before the invoke, because this
316 // is now a fall through...
317 BranchInst *BI = new BranchInst(II->getNormalDest(), II);
318 Pred->getInstList().remove(II); // Take out of symbol table
320 // Insert the call now...
321 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
322 CallInst *CI = new CallInst(II->getCalledValue(), Args,
324 // If the invoke produced a value, the Call now does instead
325 II->replaceAllUsesWith(CI);
334 // Remove basic blocks that have no predecessors... which are unreachable.
335 if (pred_begin(BB) == pred_end(BB)) {
336 //cerr << "Removing BB: \n" << BB;
338 // Loop through all of our successors and make sure they know that one
339 // of their predecessors is going away.
340 for_each(succ_begin(BB), succ_end(BB),
341 std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
343 while (!BB->empty()) {
344 Instruction &I = BB->back();
345 // If this instruction is used, replace uses with an arbitrary
346 // constant value. Because control flow can't get here, we don't care
347 // what we replace the value with. Note that since this block is
348 // unreachable, and all values contained within it must dominate their
349 // uses, that all uses will eventually be removed.
351 // Make all users of this instruction reference the constant instead
352 I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
354 // Remove the instruction from the basic block
355 BB->getInstList().pop_back();
357 M->getBasicBlockList().erase(BB);
361 // Check to see if we can constant propagate this terminator instruction
363 Changed |= ConstantFoldTerminator(BB);
365 // Check to see if this block has no non-phi instructions and only a single
366 // successor. If so, replace references to this basic block with references
368 succ_iterator SI(succ_begin(BB));
369 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
371 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
372 while (isa<PHINode>(*BBI)) ++BBI;
374 if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
375 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
377 if (Succ != BB) { // Arg, don't hurt infinite loops!
378 // If our successor has PHI nodes, then we need to update them to
379 // include entries for BB's predecessors, not for BB itself.
380 // Be careful though, if this transformation fails (returns true) then
381 // we cannot do this transformation!
383 if (!PropagatePredecessorsForPHIs(BB, Succ)) {
384 //cerr << "Killing Trivial BB: \n" << BB;
385 std::string OldName = BB->getName();
387 std::vector<BasicBlock*>
388 OldSuccPreds(pred_begin(Succ), pred_end(Succ));
390 // Move all PHI nodes in BB to Succ if they are alive, otherwise
392 while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
394 BB->getInstList().erase(BB->begin()); // Nuke instruction...
396 // The instruction is alive, so this means that Succ must have
397 // *ONLY* had BB as a predecessor, and the PHI node is still valid
398 // now. Simply move it into Succ, because we know that BB
399 // strictly dominated Succ.
400 BB->getInstList().remove(BB->begin());
401 Succ->getInstList().push_front(PN);
403 // We need to add new entries for the PHI node to account for
404 // predecessors of Succ that the PHI node does not take into
405 // account. At this point, since we know that BB dominated succ,
406 // this means that we should any newly added incoming edges should
407 // use the PHI node as the value for these edges, because they are
410 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
411 if (OldSuccPreds[i] != BB)
412 PN->addIncoming(PN, OldSuccPreds[i]);
415 // Everything that jumped to BB now goes to Succ...
416 BB->replaceAllUsesWith(Succ);
418 // Delete the old basic block...
419 M->getBasicBlockList().erase(BB);
421 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
422 Succ->setName(OldName);
424 //cerr << "Function after removal: \n" << M;
431 // If this is a returning block with only PHI nodes in it, fold the return
432 // instruction into any unconditional branch predecessors.
433 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
434 BasicBlock::iterator BBI = BB->getTerminator();
435 if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
436 // Find predecessors that end with unconditional branches.
437 std::vector<BasicBlock*> UncondBranchPreds;
438 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
439 TerminatorInst *PTI = (*PI)->getTerminator();
440 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
441 if (BI->isUnconditional())
442 UncondBranchPreds.push_back(*PI);
445 // If we found some, do the transformation!
446 if (!UncondBranchPreds.empty()) {
447 while (!UncondBranchPreds.empty()) {
448 BasicBlock *Pred = UncondBranchPreds.back();
449 UncondBranchPreds.pop_back();
450 Instruction *UncondBranch = Pred->getTerminator();
451 // Clone the return and add it to the end of the predecessor.
452 Instruction *NewRet = RI->clone();
453 Pred->getInstList().push_back(NewRet);
455 // If the return instruction returns a value, and if the value was a
456 // PHI node in "BB", propagate the right value into the return.
457 if (NewRet->getNumOperands() == 1)
458 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
459 if (PN->getParent() == BB)
460 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
461 // Update any PHI nodes in the returning block to realize that we no
462 // longer branch to them.
463 BB->removePredecessor(Pred);
464 Pred->getInstList().erase(UncondBranch);
467 // If we eliminated all predecessors of the block, delete the block now.
468 if (pred_begin(BB) == pred_end(BB))
469 // We know there are no successors, so just nuke the block.
470 M->getBasicBlockList().erase(BB);
478 // Merge basic blocks into their predecessor if there is only one distinct
479 // pred, and if there is only one distinct successor of the predecessor, and
480 // if there are no PHI nodes.
482 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
483 BasicBlock *OnlyPred = *PI++;
484 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
485 if (*PI != OnlyPred) {
486 OnlyPred = 0; // There are multiple different predecessors...
490 BasicBlock *OnlySucc = 0;
491 if (OnlyPred && OnlyPred != BB && // Don't break self loops
492 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
493 // Check to see if there is only one distinct successor...
494 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
496 for (; SI != SE; ++SI)
497 if (*SI != OnlySucc) {
498 OnlySucc = 0; // There are multiple distinct successors!
504 //cerr << "Merging: " << BB << "into: " << OnlyPred;
505 TerminatorInst *Term = OnlyPred->getTerminator();
507 // Resolve any PHI nodes at the start of the block. They are all
508 // guaranteed to have exactly one entry if they exist, unless there are
509 // multiple duplicate (but guaranteed to be equal) entries for the
510 // incoming edges. This occurs when there are multiple edges from
511 // OnlyPred to OnlySucc.
513 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
514 PN->replaceAllUsesWith(PN->getIncomingValue(0));
515 BB->getInstList().pop_front(); // Delete the phi node...
518 // Delete the unconditional branch from the predecessor...
519 OnlyPred->getInstList().pop_back();
521 // Move all definitions in the successor to the predecessor...
522 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
524 // Make all PHI nodes that referred to BB now refer to Pred as their
526 BB->replaceAllUsesWith(OnlyPred);
528 std::string OldName = BB->getName();
530 // Erase basic block from the function...
531 M->getBasicBlockList().erase(BB);
533 // Inherit predecessors name if it exists...
534 if (!OldName.empty() && !OnlyPred->hasName())
535 OnlyPred->setName(OldName);
540 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
541 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
542 // Change br (X == 0 | X == 1), T, F into a switch instruction.
543 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
544 Instruction *Cond = cast<Instruction>(BI->getCondition());
545 // If this is a bunch of seteq's or'd together, or if it's a bunch of
546 // 'setne's and'ed together, collect them.
548 std::vector<Constant*> Values;
549 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
550 if (CompVal && CompVal->getType()->isInteger()) {
551 // There might be duplicate constants in the list, which the switch
552 // instruction can't handle, remove them now.
553 std::sort(Values.begin(), Values.end());
554 Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
556 // Figure out which block is which destination.
557 BasicBlock *DefaultBB = BI->getSuccessor(1);
558 BasicBlock *EdgeBB = BI->getSuccessor(0);
559 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
561 // Create the new switch instruction now.
562 SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
564 // Add all of the 'cases' to the switch instruction.
565 for (unsigned i = 0, e = Values.size(); i != e; ++i)
566 New->addCase(Values[i], EdgeBB);
568 // We added edges from PI to the EdgeBB. As such, if there were any
569 // PHI nodes in EdgeBB, they need entries to be added corresponding to
570 // the number of edges added.
571 for (BasicBlock::iterator BBI = EdgeBB->begin();
572 PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
573 Value *InVal = PN->getIncomingValueForBlock(*PI);
574 for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
575 PN->addIncoming(InVal, *PI);
578 // Erase the old branch instruction.
579 (*PI)->getInstList().erase(BI);
581 // Erase the potentially condition tree that was used to computed the
583 ErasePossiblyDeadInstructionTree(Cond);
588 // If there is a trivial two-entry PHI node in this basic block, and we can
589 // eliminate it, do so now.
590 if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
591 if (PN->getNumIncomingValues() == 2) {
592 // Ok, this is a two entry PHI node. Check to see if this is a simple "if
593 // statement", which has a very simple dominance structure. Basically, we
594 // are trying to find the condition that is being branched on, which
595 // subsequently causes this merge to happen. We really want control
596 // dependence information for this check, but simplifycfg can't keep it up
597 // to date, and this catches most of the cases we care about anyway.
599 BasicBlock *IfTrue, *IfFalse;
600 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
601 //std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
602 // << IfTrue->getName() << " F: " << IfFalse->getName() << "\n";
604 // Figure out where to insert instructions as necessary.
605 BasicBlock::iterator AfterPHIIt = BB->begin();
606 while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
608 BasicBlock::iterator I = BB->begin();
609 while (PHINode *PN = dyn_cast<PHINode>(I)) {
612 // If we can eliminate this PHI by directly computing it based on the
613 // condition, do so now. We can't eliminate PHI nodes where the
614 // incoming values are defined in the conditional parts of the branch,
615 // so check for this.
617 if (DominatesMergePoint(PN->getIncomingValue(0), BB) &&
618 DominatesMergePoint(PN->getIncomingValue(1), BB)) {
620 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
622 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
624 // FIXME: when we have a 'select' statement, we can be completely
625 // generic and clean here and let the instcombine pass clean up
626 // after us, by folding the select instructions away when possible.
628 if (TrueVal == FalseVal) {
629 // Degenerate case...
630 PN->replaceAllUsesWith(TrueVal);
631 BB->getInstList().erase(PN);
633 } else if (isa<ConstantBool>(TrueVal) &&
634 isa<ConstantBool>(FalseVal)) {
635 if (TrueVal == ConstantBool::True) {
636 // The PHI node produces the same thing as the condition.
637 PN->replaceAllUsesWith(IfCond);
639 // The PHI node produces the inverse of the condition. Insert a
640 // "NOT" instruction, which is really a XOR.
642 BinaryOperator::createNot(IfCond, IfCond->getName()+".inv",
644 PN->replaceAllUsesWith(InverseCond);
646 BB->getInstList().erase(PN);
648 } else if (isa<ConstantInt>(TrueVal) && isa<ConstantInt>(FalseVal)){
649 // If this is a PHI of two constant integers, we insert a cast of
650 // the boolean to the integer type in question, giving us 0 or 1.
651 // Then we multiply this by the difference of the two constants,
652 // giving us 0 if false, and the difference if true. We add this
653 // result to the base constant, giving us our final value. We
654 // rely on the instruction combiner to eliminate many special
655 // cases, like turning multiplies into shifts when possible.
656 std::string Name = PN->getName(); PN->setName("");
657 Value *TheCast = new CastInst(IfCond, TrueVal->getType(),
659 Constant *TheDiff = ConstantExpr::get(Instruction::Sub,
660 cast<Constant>(TrueVal),
661 cast<Constant>(FalseVal));
663 if (TheDiff != ConstantInt::get(TrueVal->getType(), 1))
664 V = BinaryOperator::create(Instruction::Mul, TheCast,
665 TheDiff, TheCast->getName()+".scale",
667 if (!cast<Constant>(FalseVal)->isNullValue())
668 V = BinaryOperator::create(Instruction::Add, V, FalseVal,
669 V->getName()+".offs", AfterPHIIt);
670 PN->replaceAllUsesWith(V);
671 BB->getInstList().erase(PN);
673 } else if (isa<ConstantInt>(FalseVal) &&
674 cast<Constant>(FalseVal)->isNullValue()) {
675 // If the false condition is an integral zero value, we can
676 // compute the PHI by multiplying the condition by the other
678 std::string Name = PN->getName(); PN->setName("");
679 Value *TheCast = new CastInst(IfCond, TrueVal->getType(),
680 Name+".c", AfterPHIIt);
681 Value *V = BinaryOperator::create(Instruction::Mul, TrueVal,
682 TheCast, Name, AfterPHIIt);
683 PN->replaceAllUsesWith(V);
684 BB->getInstList().erase(PN);
686 } else if (isa<ConstantInt>(TrueVal) &&
687 cast<Constant>(TrueVal)->isNullValue()) {
688 // If the true condition is an integral zero value, we can compute
689 // the PHI by multiplying the inverse condition by the other
691 std::string Name = PN->getName(); PN->setName("");
692 Value *NotCond = BinaryOperator::createNot(IfCond, Name+".inv",
694 Value *TheCast = new CastInst(NotCond, TrueVal->getType(),
695 Name+".inv", AfterPHIIt);
696 Value *V = BinaryOperator::create(Instruction::Mul, FalseVal,
697 TheCast, Name, AfterPHIIt);
698 PN->replaceAllUsesWith(V);
699 BB->getInstList().erase(PN);