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Instcombine's ShrinkDemandedConstant may strip bits out of constants,

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obscuring what would otherwise be a low-bits mask.  Use ComputeMaskedBits
to compute what ShrinkDemandedConstant knew about to reconstruct a
low-bits mask value.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73540 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman
2009-06-16 19:52:01 +00:00
parent 876eac9da5
commit 61ffa8e373
2 changed files with 32 additions and 3 deletions
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21
lib/Analysis/ScalarEvolution.cpp
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@@ -68,6 +68,7 @@
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CommandLine.h"
@@ -2394,14 +2395,28 @@ SCEVHandle ScalarEvolution::createSCEV(Value *V) {
if (CI->isAllOnesValue())
return getSCEV(U->getOperand(0));
const APInt &A = CI->getValue();
unsigned Ones = A.countTrailingOnes();
if (APIntOps::isMask(Ones, A))
// Instcombine's ShrinkDemandedConstant may strip bits out of
// constants, obscuring what would otherwise be a low-bits mask.
// Use ComputeMaskedBits to compute what ShrinkDemandedConstant
// knew about to reconstruct a low-bits mask value.
unsigned LZ = A.countLeadingZeros();
unsigned BitWidth = A.getBitWidth();
APInt AllOnes = APInt::getAllOnesValue(BitWidth);
APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
ComputeMaskedBits(U->getOperand(0), AllOnes, KnownZero, KnownOne, TD);
APInt EffectiveMask = APInt::getLowBitsSet(BitWidth, BitWidth - LZ);
if (LZ != 0 && !((~A & ~KnownZero) & EffectiveMask)) {
return
getZeroExtendExpr(getTruncateExpr(getSCEV(U->getOperand(0)),
IntegerType::get(Ones)),
IntegerType::get(BitWidth - LZ)),
U->getType());
}
}
break;
case Instruction::Or:
// If the RHS of the Or is a constant, we may have something like:
// X*4+1 which got turned into X*4|1. Handle this as an Add so loop