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[InstCombine] Teach how to fold a select into a cttz/ctlz with the 'is_zero_undef' flag.

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This patch teaches the Instruction Combiner how to fold a cttz/ctlz followed by
a icmp plus select into a single cttz/ctlz with flag 'is_zero_undef' cleared.

Added test InstCombine/select-cmp-cttz-ctlz.ll.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227197 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrea Di Biagio
2015-01-27 15:58:14 +00:00
parent 0b2455c374
commit 944d86558e
3 changed files with 367 additions and 2 deletions
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63
lib/Transforms/InstCombine/InstCombineSelect.cpp
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@@ -437,6 +437,66 @@ static Value *foldSelectICmpAndOr(const SelectInst &SI, Value *TrueVal,
return Builder->CreateOr(V, Y);
}
/// Attempt to fold a cttz/ctlz followed by a icmp plus select into a single
/// call to cttz/ctlz with flag 'is_zero_undef' cleared.
///
/// For example, we can fold the following code sequence:
/// \code
/// %0 = tail call i32 @llvm.cttz.i32(i32 %x, i1 true)
/// %1 = icmp ne i32 %x, 0
/// %2 = select i1 %1, i32 %0, i32 32
/// \code
///
/// into:
/// %0 = tail call i32 @llvm.cttz.i32(i32 %x, i1 false)
static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal,
InstCombiner::BuilderTy *Builder) {
ICmpInst::Predicate Pred = ICI->getPredicate();
Value *CmpLHS = ICI->getOperand(0);
Value *CmpRHS = ICI->getOperand(1);
// Check if the condition value compares a value for equality against zero.
if (!ICI->isEquality() || !match(CmpRHS, m_Zero()))
return nullptr;
Value *Count = FalseVal;
Value *ValueOnZero = TrueVal;
if (Pred == ICmpInst::ICMP_NE)
std::swap(Count, ValueOnZero);
// Skip zero extend/truncate.
Value *V = nullptr;
if (match(Count, m_ZExt(m_Value(V))) ||
match(Count, m_Trunc(m_Value(V))))
Count = V;
// Check if the value propagated on zero is a constant number equal to the
// sizeof in bits of 'Count'.
unsigned SizeOfInBits = Count->getType()->getScalarSizeInBits();
if (!match(ValueOnZero, m_SpecificInt(SizeOfInBits)))
return nullptr;
// Check that 'Count' is a call to intrinsic cttz/ctlz. Also check that the
// input to the cttz/ctlz is used as LHS for the compare instruction.
if (match(Count, m_Intrinsic<Intrinsic::cttz>(m_Specific(CmpLHS))) ||
match(Count, m_Intrinsic<Intrinsic::ctlz>(m_Specific(CmpLHS)))) {
IntrinsicInst *II = cast<IntrinsicInst>(Count);
IRBuilder<> Builder(II);
if (cast<ConstantInt>(II->getArgOperand(1))->isOne()) {
// Explicitly clear the 'undef_on_zero' flag.
IntrinsicInst *NewI = cast<IntrinsicInst>(II->clone());
Type *Ty = NewI->getArgOperand(1)->getType();
NewI->setArgOperand(1, Constant::getNullValue(Ty));
Builder.Insert(NewI);
Count = NewI;
}
return Builder.CreateZExtOrTrunc(Count, ValueOnZero->getType());
}
return nullptr;
}
/// visitSelectInstWithICmp - Visit a SelectInst that has an
/// ICmpInst as its first operand.
///
@@ -665,6 +725,9 @@ Instruction *InstCombiner::visitSelectInstWithICmp(SelectInst &SI,
if (Value *V = foldSelectICmpAndOr(SI, TrueVal, FalseVal, Builder))
return ReplaceInstUsesWith(SI, V);
if (Value *V = foldSelectCttzCtlz(ICI, TrueVal, FalseVal, Builder))
return ReplaceInstUsesWith(SI, V);
return Changed ? &SI : nullptr;
}