[InstCombine] lshr (sext iM X to iN), N-M --> zext (ashr X, min(N-M, M-1)) to iN
This is a follow-up to https://reviews.llvm.org/D33879 / https://reviews.llvm.org/rL304939 ,
and was discussed in https://reviews.llvm.org/D33338.
We prefer this form because a narrower shift may be cheaper, and we can more easily fold a
zext than a sext.
http://rise4fun.com/Alive/slVe
Name: shz
%s = sext i8 %x to i12
%r = lshr i12 %s, 4
=>
%a = ashr i8 %x, 4
%r = zext i8 %a to i12
llvm-svn: 305190
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 3f2ddca..8cec865 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -682,11 +682,11 @@
return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, Mask));
}
- if (match(Op0, m_SExt(m_Value(X)))) {
+ if (match(Op0, m_SExt(m_Value(X))) &&
+ (!Ty->isIntegerTy() || shouldChangeType(Ty, X->getType()))) {
// Are we moving the sign bit to the low bit and widening with high zeros?
unsigned SrcTyBitWidth = X->getType()->getScalarSizeInBits();
- if (ShAmt == BitWidth - 1 &&
- (!Ty->isIntegerTy() || shouldChangeType(Ty, X->getType()))) {
+ if (ShAmt == BitWidth - 1) {
// lshr (sext i1 X to iN), N-1 --> zext X to iN
if (SrcTyBitWidth == 1)
return new ZExtInst(X, Ty);
@@ -698,7 +698,13 @@
}
}
- // TODO: Convert to ashr+zext if the shift equals the extension amount.
+ // lshr (sext iM X to iN), N-M --> zext (ashr X, min(N-M, M-1)) to iN
+ if (ShAmt == BitWidth - SrcTyBitWidth && Op0->hasOneUse()) {
+ // The new shift amount can't be more than the narrow source type.
+ unsigned NewShAmt = std::min(ShAmt, SrcTyBitWidth - 1);
+ Value *AShr = Builder->CreateAShr(X, NewShAmt);
+ return new ZExtInst(AShr, Ty);
+ }
}
if (match(Op0, m_LShr(m_Value(X), m_APInt(ShOp1)))) {