[InstCombine] sink sext after ashr
Narrow ops are better for bit-tracking, and in the case of vectors,
may enable better codegen.
As the trunc test shows, this can allow follow-on simplifications.
There's a block of code in visitTrunc that deals with shifted ops
with FIXME comments. It may be possible to remove some of that now,
but I want to make sure there are no problems with this step first.
http://rise4fun.com/Alive/Y3a
Name: hoist_ashr_ahead_of_sext_1
%s = sext i8 %x to i32
%r = ashr i32 %s, 3 ; shift value is < than source bit width
=>
%a = ashr i8 %x, 3
%r = sext i8 %a to i32
Name: hoist_ashr_ahead_of_sext_2
%s = sext i8 %x to i32
%r = ashr i32 %s, 8 ; shift value is >= than source bit width
=>
%a = ashr i8 %x, 7 ; so clamp this shift value
%r = sext i8 %a to i32
Name: junc_the_trunc
%a = sext i16 %v to i32
%s = ashr i32 %a, 18
%t = trunc i32 %s to i16
=>
%t = ashr i16 %v, 15
llvm-svn: 310942
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 098079a..a19cdf7 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -803,6 +803,15 @@
return BinaryOperator::CreateAShr(X, ConstantInt::get(Ty, AmtSum));
}
+ if (match(Op0, m_OneUse(m_SExt(m_Value(X)))) &&
+ (Ty->isVectorTy() || shouldChangeType(Ty, X->getType()))) {
+ // ashr (sext X), C --> sext (ashr X, C')
+ Type *SrcTy = X->getType();
+ ShAmt = std::min(ShAmt, SrcTy->getScalarSizeInBits() - 1);
+ Value *NewSh = Builder.CreateAShr(X, ConstantInt::get(SrcTy, ShAmt));
+ return new SExtInst(NewSh, Ty);
+ }
+
// If the shifted-out value is known-zero, then this is an exact shift.
if (!I.isExact() &&
MaskedValueIsZero(Op0, APInt::getLowBitsSet(BitWidth, ShAmt), 0, &I)) {