From 65038515ee4465d20103b2c32b337a76a2c63b53 Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Sat, 5 Jan 2019 09:48:16 +0000 Subject: [InstCombine] Relax cttz/ctlz with select on zero The cttz/ctlz intrinsics have a parameter specifying whether the result is undefined for zero. cttz(x, false) can be relaxed to cttz(x, true) if x is known non-zero, and in fact such an optimization is already performed. However, this currently doesn't work if x is non-zero as a result of a select rather than an explicit branch. This patch adds handling for this case, thus allowing x != 0 ? cttz(x, false) : y to simplify to x != 0 ? cttz(x, true) : y. Differential Revision: https://reviews.llvm.org/D55786 llvm-svn: 350463 --- .../Transforms/InstCombine/InstCombineSelect.cpp | 23 ++++++++++++++-------- 1 file changed, 15 insertions(+), 8 deletions(-) (limited to 'llvm/lib/Transforms') diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp index b1e0ffa6fa7..ebbe2afe3ec 100644 --- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp +++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp @@ -709,17 +709,18 @@ static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal, match(Count, m_Trunc(m_Value(V)))) Count = V; + // 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(m_Specific(CmpLHS))) && + !match(Count, m_Intrinsic(m_Specific(CmpLHS)))) + return nullptr; + + IntrinsicInst *II = cast(Count); + // 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(m_Specific(CmpLHS))) || - match(Count, m_Intrinsic(m_Specific(CmpLHS)))) { - IntrinsicInst *II = cast(Count); + if (match(ValueOnZero, m_SpecificInt(SizeOfInBits))) { // Explicitly clear the 'undef_on_zero' flag. IntrinsicInst *NewI = cast(II->clone()); NewI->setArgOperand(1, ConstantInt::getFalse(NewI->getContext())); @@ -727,6 +728,12 @@ static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal, return Builder.CreateZExtOrTrunc(NewI, ValueOnZero->getType()); } + // If the ValueOnZero is not the bitwidth, we can at least make use of the + // fact that the cttz/ctlz result will not be used if the input is zero, so + // it's okay to relax it to undef for that case. + if (II->hasOneUse() && !match(II->getArgOperand(1), m_One())) + II->setArgOperand(1, ConstantInt::getTrue(II->getContext())); + return nullptr; } -- cgit v1.2.3