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When folding away a (shl (shr)) pair, we need to check that the bits that will BECOME the low

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bits are zero, not that the current low bits are zero.  Fixes <rdar://problem/8606771>.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117953 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Owen Anderson 2010-11-01 21:08:20 +00:00
parent 99f535242c
commit 648b20d5db
2 changed files with 21 additions and 1 deletions
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2
lib/Transforms/InstCombine/InstCombineShifts.cpp
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@ -157,7 +157,7 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
if (CI->getZExtValue() > NumBits) {
unsigned LowBits = CI->getZExtValue() - NumBits;
if (MaskedValueIsZero(I->getOperand(0),
APInt::getLowBitsSet(TypeWidth, LowBits)))
APInt::getLowBitsSet(TypeWidth, LowBits) << NumBits))
return true;
}

20
test/Transforms/InstCombine/2010-11-01-lshr-mask.ll Normal file
View File

@ -0,0 +1,20 @@
; RUN: opt -instcombine -S < %s | FileCheck %s
; <rdar://problem/8606771>
define i32 @main(i32 %argc) nounwind ssp {
entry:
%tmp3151 = trunc i32 %argc to i8
%tmp3161 = or i8 %tmp3151, -17
%tmp3162 = and i8 %tmp3151, 122
%tmp3163 = xor i8 %tmp3162, -17
%tmp4114 = shl i8 %tmp3163, 6
%tmp4115 = xor i8 %tmp4114, %tmp3163
%tmp4120 = xor i8 %tmp3161, %tmp4115
; CHECK: lshr i8 %tmp4115, 1
; CHECK-NOT: shl i8 %tmp4126, 6
%tmp4126 = lshr i8 %tmp4120, 7
%tmp4127 = mul i8 %tmp4126, 64
%tmp4086 = zext i8 %tmp4127 to i32
; CHECK: ret i32
ret i32 %tmp4086
}