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Allow compatible extension attributes for tail calls

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If the tail-callee and caller give the same bits via the same signext/zeroext
attribute then a tail-call should be allowed, since the extension has already
been done by the callee.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188159 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Tim Northover
2013-08-12 09:45:46 +00:00
parent 674c91bb6e
commit 6a4e44f0de
3 changed files with 95 additions and 17 deletions
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50
lib/CodeGen/Analysis.cpp
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@@ -320,6 +320,7 @@ static const Value *getNoopInput(const Value *V,
static bool slotOnlyDiscardsData(const Value *RetVal, const Value *CallVal,
SmallVectorImpl<unsigned> &RetIndices,
SmallVectorImpl<unsigned> &CallIndices,
bool AllowDifferingSizes,
const TargetLoweringBase &TLI) {
// Trace the sub-value needed by the return value as far back up the graph as
@@ -350,7 +351,8 @@ static bool slotOnlyDiscardsData(const Value *RetVal, const Value *CallVal,
// all the bits that are needed by the "ret" have been provided by the "tail
// call". FIXME: with sufficiently cunning bit-tracking, we could look through
// extensions too.
if (BitsProvided < BitsRequired)
if (BitsProvided < BitsRequired ||
(!AllowDifferingSizes && BitsProvided != BitsRequired))
return false;
return true;
@@ -516,19 +518,38 @@ bool llvm::isInTailCallPosition(ImmutableCallSite CS,
// return type is.
if (isa<UndefValue>(Ret->getOperand(0))) return true;
// Conservatively require the attributes of the call to match those of
// the return. Ignore noalias because it doesn't affect the call sequence.
const Function *F = ExitBB->getParent();
AttributeSet CallerAttrs = F->getAttributes();
if (AttrBuilder(CallerAttrs, AttributeSet::ReturnIndex).
removeAttribute(Attribute::NoAlias) !=
AttrBuilder(CallerAttrs, AttributeSet::ReturnIndex).
removeAttribute(Attribute::NoAlias))
return false;
// Make sure the attributes attached to each return are compatible.
AttrBuilder CallerAttrs(ExitBB->getParent()->getAttributes(),
AttributeSet::ReturnIndex);
AttrBuilder CalleeAttrs(cast<CallInst>(I)->getAttributes(),
AttributeSet::ReturnIndex);
// It's not safe to eliminate the sign / zero extension of the return value.
if (CallerAttrs.hasAttribute(AttributeSet::ReturnIndex, Attribute::ZExt) ||
CallerAttrs.hasAttribute(AttributeSet::ReturnIndex, Attribute::SExt))
// Noalias is completely benign as far as calling convention goes, it
// shouldn't affect whether the call is a tail call.
CallerAttrs = CallerAttrs.removeAttribute(Attribute::NoAlias);
CalleeAttrs = CalleeAttrs.removeAttribute(Attribute::NoAlias);
bool AllowDifferingSizes = true;
if (CallerAttrs.contains(Attribute::ZExt)) {
if (!CalleeAttrs.contains(Attribute::ZExt))
return false;
AllowDifferingSizes = false;
CallerAttrs.removeAttribute(Attribute::ZExt);
CalleeAttrs.removeAttribute(Attribute::ZExt);
} else if (CallerAttrs.contains(Attribute::SExt)) {
if (!CalleeAttrs.contains(Attribute::SExt))
return false;
AllowDifferingSizes = false;
CallerAttrs.removeAttribute(Attribute::SExt);
CalleeAttrs.removeAttribute(Attribute::SExt);
}
// If they're still different, there's some facet we don't understand
// (currently only "inreg", but in future who knows). It may be OK but the
// only safe option is to reject the tail call.
if (CallerAttrs != CalleeAttrs)
return false;
const Value *RetVal = Ret->getOperand(0), *CallVal = I;
@@ -570,7 +591,8 @@ bool llvm::isInTailCallPosition(ImmutableCallSite CS,
// Finally, we can check whether the value produced by the tail call at this
// index is compatible with the value we return.
if (!slotOnlyDiscardsData(RetVal, CallVal, TmpRetPath, TmpCallPath, TLI))
if (!slotOnlyDiscardsData(RetVal, CallVal, TmpRetPath, TmpCallPath,
AllowDifferingSizes, TLI))
return false;
CallEmpty = !nextRealType(CallSubTypes, CallPath);