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Boost the power of phi node constant folding slightly: if all

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operands are the phi node itself or undef, then return undef.
This logic already existed at a higher level so in practice it
shouldn't make the slightest difference.  Note that this code
could be replaced by a call to PN->hasConstantValue().  However
since we bail out the moment we see a non-constant operand, it
is more efficient to have a specialized version of that logic.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119041 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan Sands 2010-11-14 12:53:18 +00:00
parent b08ca275bd
commit c0362d5c6e
1 changed files with 17 additions and 11 deletions
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28
lib/Analysis/ConstantFolding.cpp
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@ -695,20 +695,26 @@ static Constant *SymbolicallyEvaluateGEP(Constant *const *Ops, unsigned NumOps,
/// instructions like loads and stores, which have no constant expression form.
///
Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
// Handle PHI nodes specially here...
if (PHINode *PN = dyn_cast<PHINode>(I)) {
if (PN->getNumIncomingValues() == 0)
return UndefValue::get(PN->getType());
Constant *CommonValue = 0;
Constant *Result = dyn_cast<Constant>(PN->getIncomingValue(0));
if (Result == 0) return 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *Incoming = PN->getIncomingValue(i);
// If the incoming value is equal to the phi node itself or is undef then
// skip it.
if (Incoming == PN || isa<UndefValue>(Incoming))
continue;
// If the incoming value is not a constant, or is a different constant to
// the one we saw previously, then give up.
Constant *C = dyn_cast<Constant>(Incoming);
if (!C || (CommonValue && C != CommonValue))
return 0;
CommonValue = C;
}
// Handle PHI nodes specially here...
for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) != Result && PN->getIncomingValue(i) != PN)
return 0; // Not all the same incoming constants...
// If we reach here, all incoming values are the same constant.
return Result;
// If we reach here, all incoming values are the same constant or undef.
return CommonValue ? CommonValue : UndefValue::get(PN->getType());
}
// Scan the operand list, checking to see if they are all constants, if so,