void DemandedBits::determineLiveOperandBits(
const Instruction *UserI, const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB, KnownBits &Known, KnownBits &Known2) {
unsigned BitWidth = AB.getBitWidth();
// We're called once per operand, but for some instructions, we need to
// compute known bits of both operands in order to determine the live bits of
// either (when both operands are instructions themselves). We don't,
// however, want to do this twice, so we cache the result in APInts that live
// in the caller. For the two-relevant-operands case, both operand values are
// provided here.
auto ComputeKnownBits =
[&](unsigned BitWidth, const Value *V1, const Value *V2) {
const DataLayout &DL = I->getModule()->getDataLayout();
Known = KnownBits(BitWidth);
computeKnownBits(V1, Known, DL, 0, &AC, UserI, &DT);
if (V2) {
Known2 = KnownBits(BitWidth);
computeKnownBits(V2, Known2, DL, 0, &AC, UserI, &DT);
}
};
switch (UserI->getOpcode()) {
default: break;
case Instruction::Call:
case Instruction::Invoke:
if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(UserI))
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::bswap:
// The alive bits of the input are the swapped alive bits of
// the output.
AB = AOut.byteSwap();
break;
case Intrinsic::bitreverse:
// The alive bits of the input are the reversed alive bits of
// the output.
AB = AOut.reverseBits();
break;
case Intrinsic::ctlz:
if (OperandNo == 0) {
// We need some output bits, so we need all bits of the
// input to the left of, and including, the leftmost bit
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getHighBitsSet(BitWidth,
std::min(BitWidth, Known.countMaxLeadingZeros()+1));
}
break;
case Intrinsic::cttz:
if (OperandNo == 0) {
// We need some output bits, so we need all bits of the
// input to the right of, and including, the rightmost bit
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getLowBitsSet(BitWidth,
std::min(BitWidth, Known.countMaxTrailingZeros()+1));
}
break;
}
break;
case Instruction::Add:
case Instruction::Sub:
case Instruction::Mul:
// Find the highest live output bit. We don't need any more input
// bits than that (adds, and thus subtracts, ripple only to the
// left).
AB = APInt::getLowBitsSet(BitWidth, AOut.getActiveBits());
break;
case Instruction::Shl:
if (OperandNo == 0)
if (auto *ShiftAmtC = dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = ShiftAmtC->getLimitedValue(BitWidth - 1);
AB = AOut.lshr(ShiftAmt);
// If the shift is nuw/nsw, then the high bits are not dead
// (because we've promised that they *must* be zero).
const ShlOperator *S = cast<ShlOperator>(UserI);
if (S->hasNoSignedWrap())
AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt+1);
else if (S->hasNoUnsignedWrap())
AB |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::LShr:
if (OperandNo == 0)
if (auto *ShiftAmtC = dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = ShiftAmtC->getLimitedValue(BitWidth - 1);
AB = AOut.shl(ShiftAmt);
// If the shift is exact, then the low bits are not dead
// (they must be zero).
if (cast<LShrOperator>(UserI)->isExact())
AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::AShr:
if (OperandNo == 0)
if (auto *ShiftAmtC = dyn_cast<ConstantInt>(UserI->getOperand(1))) {
uint64_t ShiftAmt = ShiftAmtC->getLimitedValue(BitWidth - 1);
AB = AOut.shl(ShiftAmt);
// Because the high input bit is replicated into the
// high-order bits of the result, if we need any of those
// bits, then we must keep the highest input bit.
if ((AOut & APInt::getHighBitsSet(BitWidth, ShiftAmt))
.getBoolValue())
AB.setSignBit();
// If the shift is exact, then the low bits are not dead
// (they must be zero).
if (cast<AShrOperator>(UserI)->isExact())
AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
}
break;
case Instruction::And:
AB = AOut;
// For bits that are known zero, the corresponding bits in the
// other operand are dead (unless they're both zero, in which
// case they can't both be dead, so just mark the LHS bits as
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~Known2.Zero;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(Known.Zero & ~Known2.Zero);
}
break;
case Instruction::Or:
AB = AOut;
// For bits that are known one, the corresponding bits in the
// other operand are dead (unless they're both one, in which
// case they can't both be dead, so just mark the LHS bits as
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~Known2.One;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(Known.One & ~Known2.One);
}
break;
case Instruction::Xor:
case Instruction::PHI:
AB = AOut;
break;
case Instruction::Trunc:
AB = AOut.zext(BitWidth);
break;
case Instruction::ZExt:
AB = AOut.trunc(BitWidth);
break;
case Instruction::SExt:
AB = AOut.trunc(BitWidth);
// Because the high input bit is replicated into the
// high-order bits of the result, if we need any of those
// bits, then we must keep the highest input bit.
if ((AOut & APInt::getHighBitsSet(AOut.getBitWidth(),
AOut.getBitWidth() - BitWidth))
.getBoolValue())
AB.setSignBit();
break;
case Instruction::Select:
if (OperandNo != 0)
AB = AOut;
break;
}
}
