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InstCombine: Fix a potential bug in 0 - (X sdiv C) -> (X sdiv -C)

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While *most* (X sdiv 1) operations will get caught by InstSimplify, it
is still possible for a sdiv to appear in the worklist which hasn't been
simplified yet.

This means that it is possible for 0 - (X sdiv 1) to get transformed
into (X sdiv -1); dividing by -1 can make the transform produce undef
values instead of the proper result.

Sorry for the lack of testcase, it's a bit problematic because it relies
on the exact order of operations in the worklist.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215818 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2014-08-16 09:23:42 +00:00
parent cb698b26a1
commit 3bbb4b15ae
3 changed files with 26 additions and 1 deletions
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2
lib/Transforms/InstCombine/InstCombineAddSub.cpp
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@ -1583,7 +1583,7 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
// 0 - (X sdiv C) -> (X sdiv -C) provided the negation doesn't overflow.
if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) && match(Op0, m_Zero()) &&
!C->isMinSignedValue())
!C->isMinSignedValue() && !C->isOneValue())
return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));
// 0 - (X << Y) -> (-X << Y) when X is freely negatable.