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InstCombine: fold (A << C) == (B << C) --> ((A^B) & (~0U >> C)) == 0

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Anding and comparing with zero can be done in a single instruction on
most archs so this is a bit cheaper.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233291 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer
2015-03-26 17:12:06 +00:00
parent 27d12f3e6e
commit 8f820e9495
3 changed files with 45 additions and 38 deletions
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15
lib/Transforms/InstCombine/InstCombineCompares.cpp
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@@ -3553,6 +3553,21 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
}
}
// (A << C) == (B << C) --> ((A^B) & (~0U >> C)) == 0
if (match(Op0, m_OneUse(m_Shl(m_Value(A), m_ConstantInt(Cst1)))) &&
match(Op1, m_OneUse(m_Shl(m_Value(B), m_Specific(Cst1))))) {
unsigned TypeBits = Cst1->getBitWidth();
unsigned ShAmt = (unsigned)Cst1->getLimitedValue(TypeBits);
if (ShAmt < TypeBits && ShAmt != 0) {
Value *Xor = Builder->CreateXor(A, B, I.getName() + ".unshifted");
APInt AndVal = APInt::getLowBitsSet(TypeBits, TypeBits - ShAmt);
Value *And = Builder->CreateAnd(Xor, Builder->getInt(AndVal),
I.getName() + ".mask");
return new ICmpInst(I.getPredicate(), And,
Constant::getNullValue(Cst1->getType()));
}
}
// Transform "icmp eq (trunc (lshr(X, cst1)), cst" to
// "icmp (and X, mask), cst"
uint64_t ShAmt = 0;