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[DAG] computeKnownBits - Move ISD::SRA handling into KnownBits::ashr

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As discussed on D90527, we should be trying to move shift handling functionality into KnownBits to avoid code duplication in SelectionDAG/GlobalISel/ValueTracking.
This commit is contained in:
Simon Pilgrim 2020-11-03 18:09:15 +00:00
parent b3e56d6425
commit 977dc8c300
3 changed files with 25 additions and 7 deletions
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4
include/llvm/Support/KnownBits.h
View File

@ -278,6 +278,10 @@ public:
/// NOTE: RHS (shift amount) bitwidth doesn't need to be the same as LHS.
static KnownBits lshr(const KnownBits &LHS, const KnownBits &RHS);
/// Compute known bits for ashr(LHS, RHS).
/// NOTE: RHS (shift amount) bitwidth doesn't need to be the same as LHS.
static KnownBits ashr(const KnownBits &LHS, const KnownBits &RHS);
/// Insert the bits from a smaller known bits starting at bitPosition.
void insertBits(const KnownBits &SubBits, unsigned BitPosition) {
Zero.insertBits(SubBits.Zero, BitPosition);

11
lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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@ -2979,13 +2979,10 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
Known.Zero.setHighBits(ShMinAmt->getZExtValue());
break;
case ISD::SRA:
if (const APInt *ShAmt = getValidShiftAmountConstant(Op, DemandedElts)) {
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
unsigned Shift = ShAmt->getZExtValue();
// Sign extend known zero/one bit (else is unknown).
Known.Zero.ashrInPlace(Shift);
Known.One.ashrInPlace(Shift);
}
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
Known = KnownBits::ashr(Known, Known2);
// TODO: Add minimum shift high known sign bits.
break;
case ISD::FSHL:
case ISD::FSHR:

17
lib/Support/KnownBits.cpp
View File

@ -192,6 +192,23 @@ KnownBits KnownBits::lshr(const KnownBits &LHS, const KnownBits &RHS) {
return Known;
}
KnownBits KnownBits::ashr(const KnownBits &LHS, const KnownBits &RHS) {
unsigned BitWidth = LHS.getBitWidth();
KnownBits Known(BitWidth);
if (RHS.isConstant() && RHS.getConstant().ult(BitWidth)) {
unsigned Shift = RHS.getConstant().getZExtValue();
Known = LHS;
Known.Zero.ashrInPlace(Shift);
Known.One.ashrInPlace(Shift);
return Known;
}
// TODO: Minimum shift amount high bits are known sign bits.
// TODO: No matter the shift amount, the leading sign bits will stay.
return Known;
}
KnownBits KnownBits::abs() const {
// If the source's MSB is zero then we know the rest of the bits already.
if (isNonNegative())