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When converting phi nodes into select instructions, we shouldn't promote PHI 

nodes unless we KNOW that we are able to promote all of them.

This fixes: test/Regression/Transforms/SimplifyCFG/PhiNoEliminate.ll

llvm-svn: 16973
This commit is contained in:
Chris Lattner 2004-10-14 05:13:36 +00:00
parent 6299141c4e
commit 45c35b1d1f
1 changed files with 92 additions and 40 deletions
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132
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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@ -185,7 +185,13 @@ static Value *GetIfCondition(BasicBlock *BB,
// if the specified value dominates the block. We don't handle the true
// generality of domination here, just a special case which works well enough
// for us.
static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
//
// If AggressiveInsts is non-null, and if V does not dominate BB, we check to
// see if V (which must be an instruction) is cheap to compute and is
// non-trapping. If both are true, the instruction is inserted into the set and
// true is returned.
static bool DominatesMergePoint(Value *V, BasicBlock *BB,
std::set<Instruction*> *AggressiveInsts) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I) return true; // Non-instructions all dominate instructions.
BasicBlock *PBB = I->getParent();
@ -199,7 +205,7 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
// statement".
if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
if (!AllowAggressive) return false;
if (!AggressiveInsts) return false;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if its a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
@ -232,9 +238,10 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
// Okay, we can only really hoist these out if their operands are not
// defined in the conditional region.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (!DominatesMergePoint(I->getOperand(i), BB, false))
if (!DominatesMergePoint(I->getOperand(i), BB, 0))
return false;
// Okay, it's safe to do this!
// Okay, it's safe to do this! Remember this instruction.
AggressiveInsts->insert(I);
}
return true;
@ -1038,54 +1045,99 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
<< IfTrue->getName() << " F: " << IfFalse->getName() << "\n");
// Figure out where to insert instructions as necessary.
// Loop over the PHI's seeing if we can promote them all to select
// instructions. While we are at it, keep track of the instructions
// that need to be moved to the dominating block.
std::set<Instruction*> AggressiveInsts;
bool CanPromote = true;
BasicBlock::iterator AfterPHIIt = BB->begin();
while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
while (isa<PHINode>(AfterPHIIt)) {
PHINode *PN = cast<PHINode>(AfterPHIIt++);
if (PN->getIncomingValue(0) == PN->getIncomingValue(1))
PN->replaceAllUsesWith(PN->getIncomingValue(0));
else if (!DominatesMergePoint(PN->getIncomingValue(0), BB,
&AggressiveInsts) ||
!DominatesMergePoint(PN->getIncomingValue(1), BB,
&AggressiveInsts)) {
CanPromote = false;
break;
}
}
BasicBlock::iterator I = BB->begin();
while (PHINode *PN = dyn_cast<PHINode>(I)) {
++I;
// Did we eliminate all PHI's?
CanPromote |= AfterPHIIt == BB->begin();
// If we can eliminate this PHI by directly computing it based on the
// condition, do so now. We can't eliminate PHI nodes where the
// incoming values are defined in the conditional parts of the branch,
// so check for this.
//
if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
// If we all PHI nodes are promotable, check to make sure that all
// instructions in the predecessor blocks can be promoted as well. If
// not, we won't be able to get rid of the control flow, so it's not
// worth promoting to select instructions.
BasicBlock *DomBlock, *IfBlock1 = 0, *IfBlock2 = 0;
if (CanPromote) {
PN = cast<PHINode>(BB->begin());
BasicBlock *Pred = PN->getIncomingBlock(0);
if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
IfBlock1 = Pred;
DomBlock = *pred_begin(Pred);
for (BasicBlock::iterator I = Pred->begin();
!isa<TerminatorInst>(I); ++I)
if (!AggressiveInsts.count(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control
// flow, so the xform is not worth it.
CanPromote = false;
break;
}
}
Pred = PN->getIncomingBlock(1);
if (CanPromote &&
cast<BranchInst>(Pred->getTerminator())->isUnconditional()) {
IfBlock2 = Pred;
DomBlock = *pred_begin(Pred);
for (BasicBlock::iterator I = Pred->begin();
!isa<TerminatorInst>(I); ++I)
if (!AggressiveInsts.count(I)) {
// This is not an aggressive instruction that we can promote.
// Because of this, we won't be able to get rid of the control
// flow, so the xform is not worth it.
CanPromote = false;
break;
}
}
}
// If we can still promote the PHI nodes after this gauntlet of tests,
// do all of the PHI's now.
if (CanPromote) {
// Move all 'aggressive' instructions, which are defined in the
// conditional parts of the if's up to the dominating block.
if (IfBlock1) {
DomBlock->getInstList().splice(DomBlock->getTerminator(),
IfBlock1->getInstList(),
IfBlock1->begin(),
IfBlock1->getTerminator());
}
if (IfBlock2) {
DomBlock->getInstList().splice(DomBlock->getTerminator(),
IfBlock2->getInstList(),
IfBlock2->begin(),
IfBlock2->getTerminator());
}
while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
// Change the PHI node into a select instruction.
Value *TrueVal =
PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
Value *FalseVal =
PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
// If one of the incoming values is defined in the conditional
// region, move it into it's predecessor block, which we know is
// safe.
if (!DominatesMergePoint(TrueVal, BB, false)) {
Instruction *TrueI = cast<Instruction>(TrueVal);
BasicBlock *OldBB = TrueI->getParent();
OldBB->getInstList().remove(TrueI);
BasicBlock *NewBB = *pred_begin(OldBB);
NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
}
if (!DominatesMergePoint(FalseVal, BB, false)) {
Instruction *FalseI = cast<Instruction>(FalseVal);
BasicBlock *OldBB = FalseI->getParent();
OldBB->getInstList().remove(FalseI);
BasicBlock *NewBB = *pred_begin(OldBB);
NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
}
// Change the PHI node into a select instruction.
BasicBlock::iterator InsertPos = PN;
while (isa<PHINode>(InsertPos)) ++InsertPos;
std::string Name = PN->getName(); PN->setName("");
PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
Name, InsertPos));
Name, AfterPHIIt));
BB->getInstList().erase(PN);
Changed = true;
}
Changed = true;
}
}
}