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(1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2

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rdar://problem/6501862


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132316 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Stuart Hastings
2011-05-30 20:00:33 +00:00
parent 67dc11352d
commit acbf107d9b
2 changed files with 41 additions and 0 deletions
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17
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
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@ -135,6 +135,23 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
return BinaryOperator::CreateAdd(Add, Builder->CreateMul(C1, CI));
}
}
// (1 - X) * (-2) -> (x - 1) * 2, for all positive nonzero powers of 2
// The "* 2" thus becomes a potential shifting opportunity.
{
const APInt & Val = CI->getValue();
const APInt &PosVal = Val.abs();
if (Val.isNegative() && PosVal.isPowerOf2()) {
Value *X = 0;
if (match(Op0, m_Sub(m_One(), m_Value(X)))) {
// ConstantInt::get(Op0->getType(), 2);
Value *Sub = Builder->CreateSub(X, ConstantInt::get(X->getType(), 1),
"dec1");
return BinaryOperator::CreateMul(Sub, ConstantInt::get(X->getType(),
PosVal));
}
}
}
}
// Simplify mul instructions with a constant RHS.