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Before replacing a phi node with a different value, it 

needs to be checked that this won't break LCSSA form.
Change the existing checking method to a more direct one:
rather than seeing if all predecessors belong to the loop,
check that the replacing value is either not in any loop or
is in a loop that contains the phi node.

llvm-svn: 119556
This commit is contained in:
Duncan Sands 2010-11-17 20:49:12 +00:00
parent 6c37ceb182
commit 39d77131a1
1 changed files with 15 additions and 11 deletions
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26
llvm/lib/Analysis/ScalarEvolution.cpp
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@ -2879,18 +2879,22 @@ const SCEV *ScalarEvolution::createNodeForPHI(PHINode *PN) {
// risks breaking LCSSA form. Instcombine would normally zap these, but
// it doesn't have DominatorTree information, so it may miss cases.
if (Value *V = SimplifyInstruction(PN, TD, DT)) {
// TODO: The following check is suboptimal. For example, it is pointless
// if V is a constant. Since the problematic case is if V is defined inside
// a deeper loop, it would be better to check for that directly.
bool AllSameLoop = true;
Loop *PNLoop = LI->getLoopFor(PN->getParent());
for (size_t i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (LI->getLoopFor(PN->getIncomingBlock(i)) != PNLoop) {
AllSameLoop = false;
break;
}
if (AllSameLoop)
Instruction *I = dyn_cast<Instruction>(V);
// Only instructions are problematic for preserving LCSSA form.
if (!I)
return getSCEV(V);
// If the instruction is not defined in a loop, then it can be used freely.
Loop *ILoop = LI->getLoopFor(I->getParent());
if (!ILoop)
return getSCEV(I);
// If the instruction is defined in the same loop as the phi node, or in a
// loop that contains the phi node loop as an inner loop, then using it as
// a replacement for the phi node will not break LCSSA form.
Loop *PNLoop = LI->getLoopFor(PN->getParent());
if (ILoop->contains(PNLoop))
return getSCEV(I);
}
// If it's not a loop phi, we can't handle it yet.