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InstCombine: Don't fold (X <<s log(INT_MIN)) /s INT_MIN to X

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Consider the case where X is 2.  (2 <<s 31)/s-2147483648 is zero but we
would fold to X.  Note that this is valid when we are in the unsigned
domain because we require NUW: 2 <<u 31 results in poison.

This fixes PR21245.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219568 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2014-10-11 10:20:04 +00:00
parent 9043f74acb
commit 171825a8ce
2 changed files with 19 additions and 1 deletions
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3
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
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@@ -762,7 +762,8 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
}
}
if ((IsSigned && match(LHS, m_NSWShl(m_Value(X), m_APInt(C1)))) ||
if ((IsSigned && match(LHS, m_NSWShl(m_Value(X), m_APInt(C1))) &&
*C1 != C1->getBitWidth() - 1) ||
(!IsSigned && match(LHS, m_NUWShl(m_Value(X), m_APInt(C1))))) {
APInt Quotient(C1->getBitWidth(), /*Val=*/0ULL, IsSigned);
APInt C1Shifted = APInt::getOneBitSet(