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Transform (x&C)>V into (x&C)!=0 where possible

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When the least bit of C is greater than V, (x&C) must be greater than V
if it is not zero, so the comparison can be simplified.

Although this was suggested in Target/X86/README.txt, it benefits any
architecture with a directly testable form of AND.

Patch by Kevin Schoedel


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170576 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Paul Redmond
2012-12-19 19:47:13 +00:00
parent 433cb080ba
commit 6da2e22dff
3 changed files with 27 additions and 37 deletions
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10
lib/Transforms/InstCombine/InstCombineCompares.cpp
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@ -1226,6 +1226,16 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
ICI.setOperand(0, NewAnd);
return &ICI;
}
// Replace ((X & AndCST) > RHSV) with ((X & AndCST) != 0), if any
// bit set in (X & AndCST) will produce a result greater than RHSV.
if (ICI.getPredicate() == ICmpInst::ICMP_UGT) {
unsigned NTZ = AndCST->getValue().countTrailingZeros();
if ((NTZ < AndCST->getBitWidth()) &&
APInt::getOneBitSet(AndCST->getBitWidth(), NTZ).ugt(RHSV))
return new ICmpInst(ICmpInst::ICMP_NE, LHSI,
Constant::getNullValue(RHS->getType()));
}
}
// Try to optimize things like "A[i]&42 == 0" to index computations.