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Teach GetMinSignBits about SCEVAddExprs.

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llvm-svn: 74045
This commit is contained in:
Dan Gohman 2009-06-24 01:05:09 +00:00
parent 0e5051b251
commit 6f0aa30f94
1 changed files with 32 additions and 0 deletions
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32
lib/Analysis/ScalarEvolution.cpp
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@ -2402,6 +2402,38 @@ ScalarEvolution::GetMinSignBits(const SCEV* S) {
getTypeSizeInBits(C->getOperand()->getType()));
}
if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
unsigned BitWidth = getTypeSizeInBits(A->getType());
// Special case decrementing a value (ADD X, -1):
if (const SCEVConstant *CRHS = dyn_cast<SCEVConstant>(A->getOperand(0)))
if (CRHS->isAllOnesValue()) {
SmallVector<const SCEV *, 4> OtherOps(A->op_begin() + 1, A->op_end());
const SCEV *OtherOpsAdd = getAddExpr(OtherOps);
unsigned LZ = GetMinLeadingZeros(OtherOpsAdd);
// If the input is known to be 0 or 1, the output is 0/-1, which is all
// sign bits set.
if (LZ == BitWidth - 1)
return BitWidth;
// If we are subtracting one from a positive number, there is no carry
// out of the result.
if (LZ > 0)
return GetMinSignBits(OtherOpsAdd);
}
// Add can have at most one carry bit. Thus we know that the output
// is, at worst, one more bit than the inputs.
unsigned Min = BitWidth;
for (unsigned i = 0, e = A->getNumOperands(); i != e; ++i) {
unsigned N = GetMinSignBits(A->getOperand(i));
Min = std::min(Min, N) - 1;
if (Min == 0) return 1;
}
return 1;
}
if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
// For a SCEVUnknown, ask ValueTracking.
return ComputeNumSignBits(U->getValue(), TD);