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[InstCombine] Teach canEvaluateTruncated to handle arithmetic shift (including those with vector splat shift amount) 

Differential Revision: https://reviews.llvm.org/D36784

llvm-svn: 311050
This commit is contained in:
Amjad Aboud 2017-08-16 22:42:38 +00:00
parent cbcffb173c
commit 86111c6696
2 changed files with 43 additions and 0 deletions
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17
llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -373,6 +373,23 @@ static bool canEvaluateTruncated(Value *V, Type *Ty, InstCombiner &IC,
}
break;
}
case Instruction::AShr: {
// If this is a truncate of an arithmetic shr, we can truncate it to a
// smaller ashr iff we know that all the bits from the sign bit of the
// original type and the sign bit of the truncate type are similar.
// TODO: It is enough to check that the bits we would be shifting in are
// similar to sign bit of the truncate type.
const APInt *Amt;
if (match(I->getOperand(1), m_APInt(Amt))) {
uint32_t OrigBitWidth = OrigTy->getScalarSizeInBits();
uint32_t BitWidth = Ty->getScalarSizeInBits();
if (Amt->getLimitedValue(BitWidth) < BitWidth &&
OrigBitWidth - BitWidth <
IC.ComputeNumSignBits(I->getOperand(0), 0, CxtI))
return canEvaluateTruncated(I->getOperand(0), Ty, IC, CxtI);
}
break;
}
case Instruction::Trunc:
// trunc(trunc(x)) -> trunc(x)
return true;

26
llvm/test/Transforms/InstCombine/trunc.ll
View File

@ -89,6 +89,32 @@ define i32 @test6(i64 %A) {
ret i32 %D
}
define i32 @trunc_ashr(i32 %X) {
; CHECK-LABEL: @trunc_ashr(
; CHECK-NEXT: [[B:%.*]] = or i32 [[X:%.*]], -2147483648
; CHECK-NEXT: [[C:%.*]] = ashr i32 [[B]], 8
; CHECK-NEXT: ret i32 [[C]]
;
%A = zext i32 %X to i36
%B = or i36 %A, -2147483648 ; 0xF80000000
%C = ashr i36 %B, 8
%T = trunc i36 %C to i32
ret i32 %T
}
define <2 x i32> @trunc_ashr_vec(<2 x i32> %X) {
; CHECK-LABEL: @trunc_ashr_vec(
; CHECK-NEXT: [[B:%.*]] = or <2 x i32> [[X:%.*]], <i32 -2147483648, i32 -2147483648>
; CHECK-NEXT: [[C:%.*]] = ashr <2 x i32> [[B]], <i32 8, i32 8>
; CHECK-NEXT: ret <2 x i32> [[C]]
;
%A = zext <2 x i32> %X to <2 x i36>
%B = or <2 x i36> %A, <i36 -2147483648, i36 -2147483648> ; 0xF80000000
%C = ashr <2 x i36> %B, <i36 8, i36 8>
%T = trunc <2 x i36> %C to <2 x i32>
ret <2 x i32> %T
}
define i92 @test7(i64 %A) {
; CHECK-LABEL: @test7(
; CHECK-NEXT: [[TMP1:%.*]] = lshr i64 %A, 32