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InstCombine: Fix miscompile in X % -Y -> X % Y transform

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We assumed that negation operations of the form (0 - %Z) resulted in a
negative number.  This isn't true if %Z was originally negative.
Substituting the negative number into the remainder operation may result
in undefined behavior because the dividend might be INT_MIN.

This fixes PR21256.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219639 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-10-13 22:37:51 +00:00
parent 3a143ce2e7
commit af6be11a60
3 changed files with 9 additions and 14 deletions
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12
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -1324,15 +1324,15 @@ Instruction *InstCombiner::visitSRem(BinaryOperator &I) {
if (Instruction *Common = commonIRemTransforms(I)) if (Instruction *Common = commonIRemTransforms(I))
return Common; return Common;
if (Value *RHSNeg = dyn_castNegVal(Op1)) {
if (!isa<Constant>(RHSNeg) || const APInt *Y;
(isa<ConstantInt>(RHSNeg) && // X % -Y -> X % Y
cast<ConstantInt>(RHSNeg)->getValue().isStrictlyPositive())) { if (match(Op1, m_APInt(Y)) && Y->isNegative() && !Y->isMinSignedValue()) {
// X % -Y -> X % Y
Worklist.AddValue(I.getOperand(1)); Worklist.AddValue(I.getOperand(1));
I.setOperand(1, RHSNeg); I.setOperand(1, ConstantInt::get(I.getType(), -*Y));
return &I; return &I;
} }
}
// If the sign bits of both operands are zero (i.e. we can prove they are // If the sign bits of both operands are zero (i.e. we can prove they are
// unsigned inputs), turn this into a urem. // unsigned inputs), turn this into a urem.

6
test/Transforms/InstCombine/apint-sub.ll
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@ -95,12 +95,6 @@ define i1024 @test14(i1024 %A) {
ret i1024 %D ret i1024 %D
} }
define i14 @test15(i14 %A, i14 %B) {
%C = sub i14 0, %A ; <i14> [#uses=1]
%D = srem i14 %B, %C ; <i14> [#uses=1]
ret i14 %D
}
define i51 @test16(i51 %A) { define i51 @test16(i51 %A) {
%X = sdiv i51 %A, 1123 ; <i51> [#uses=1] %X = sdiv i51 %A, 1123 ; <i51> [#uses=1]
%Y = sub i51 0, %X ; <i51> [#uses=1] %Y = sub i51 0, %X ; <i51> [#uses=1]

5
test/Transforms/InstCombine/sub.ll
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@ -142,8 +142,9 @@ define i32 @test15(i32 %A, i32 %B) {
%D = srem i32 %B, %C %D = srem i32 %B, %C
ret i32 %D ret i32 %D
; CHECK-LABEL: @test15( ; CHECK-LABEL: @test15(
; CHECK: %D = srem i32 %B, %A ; CHECK: %[[sub:.*]] = sub i32 0, %A
; CHECK: ret i32 %D ; CHECK-NEXT: %[[rem:.*]] = srem i32 %B, %[[sub]]
; CHECK: ret i32 %[[rem]]
} }
define i32 @test16(i32 %A) { define i32 @test16(i32 %A) {