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Teach GetMinSignBits about SCEVAddExprs.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74045 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -2402,6 +2402,38 @@ ScalarEvolution::GetMinSignBits(const SCEV* S) {
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getTypeSizeInBits(C->getOperand()->getType()));
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}
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if (const SCEVAddExpr *A = dyn_cast<SCEVAddExpr>(S)) {
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unsigned BitWidth = getTypeSizeInBits(A->getType());
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// Special case decrementing a value (ADD X, -1):
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if (const SCEVConstant *CRHS = dyn_cast<SCEVConstant>(A->getOperand(0)))
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if (CRHS->isAllOnesValue()) {
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SmallVector<const SCEV *, 4> OtherOps(A->op_begin() + 1, A->op_end());
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const SCEV *OtherOpsAdd = getAddExpr(OtherOps);
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unsigned LZ = GetMinLeadingZeros(OtherOpsAdd);
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// If the input is known to be 0 or 1, the output is 0/-1, which is all
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// sign bits set.
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if (LZ == BitWidth - 1)
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return BitWidth;
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// If we are subtracting one from a positive number, there is no carry
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// out of the result.
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if (LZ > 0)
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return GetMinSignBits(OtherOpsAdd);
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}
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// Add can have at most one carry bit. Thus we know that the output
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// is, at worst, one more bit than the inputs.
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unsigned Min = BitWidth;
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for (unsigned i = 0, e = A->getNumOperands(); i != e; ++i) {
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unsigned N = GetMinSignBits(A->getOperand(i));
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Min = std::min(Min, N) - 1;
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if (Min == 0) return 1;
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}
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return 1;
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}
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if (const SCEVUnknown *U = dyn_cast<SCEVUnknown>(S)) {
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// For a SCEVUnknown, ask ValueTracking.
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return ComputeNumSignBits(U->getValue(), TD);
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