Sheng,
A correction to my last email ..
On Tue, 2007-03-27 at 22:13 -0700, Reid Spencer wrote:
> > // If the sign bit of the input is known set or clear, then we know the
> > // top bits of the result.
> > - APInt InSignBit(APInt::getSignBit(SrcTy->getBitWidth()));
> > - InSignBit.zext(BitWidth);
> > - if ((KnownZero & InSignBit) != 0) { // Input sign bit known
> > zero
> > + APInt NewBits(APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth));
> > + if (KnownZero[SrcBitWidth-1]) // Input sign bit known zero
>
> This doesn't look correct nor efficient to me. It should be testing for
> only the sign bit. YOu need an & not an |. You're trying to determine
> if the sign bit of the SrcTy is set in KnownZero, right? So, why not:
>
> if (KnownZero.get(SrcBitWidth-1))
>
> ?
My eyes are tired tonight. I didn't see the [...] and thought the [ was
| so please disregard. This looks okay.
>
> > KnownZero |= NewBits;
> > - KnownOne &= ~NewBits;
> > - } else if ((KnownOne & InSignBit) != 0) { // Input sign bit known
> > set
> > + else if (KnownOne[SrcBitWidth-1]) // Input sign bit known set
>
> Same issue as above.
>
> > KnownOne |= NewBits;
> > - KnownZero &= ~NewBits;
> > - } else { // Input sign bit unknown
> > - KnownZero &= ~NewBits;
> > - KnownOne &= ~NewBits;
> > - }
>
> Why did you delete this? What if the sign bit is unknown? (neither known
> one nor known zero). Please revert.
This however is still a problem. It is quite possible for KnownZero[X]
and KnownOne[X] to both be false.
Reid.
>
> > return;
> > }
> > case Instruction::Shl:
> > @@ -760,7 +759,6 @@
> > if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
> > // Compute the new bits that are at the top now.
> > uint64_t ShiftAmt = SA->getZExtValue();
> > - APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
> >
> > // Unsigned shift right.
> > APInt Mask2(Mask.shl(ShiftAmt));
> > @@ -768,16 +766,16 @@
> > assert((KnownZero & KnownOne) == 0&&"Bits known to be one AND
> > zero?");
> > KnownZero = APIntOps::lshr(KnownZero, ShiftAmt);
> > KnownOne = APIntOps::lshr(KnownOne, ShiftAmt);
> > - KnownZero |= HighBits; // high bits known zero.
> > + // high bits known zero.
> > + KnownZero |= APInt::getHighBitsSet(BitWidth, ShiftAmt);
> > return;
> > }
> > break;
> > case Instruction::AShr:
> > - // (ushr X, C1) & C2 == 0 iff (-1 >> C1) & C2 == 0
> > + // (ashr X, C1) & C2 == 0 iff (-1 >> C1) & C2 == 0
> > if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {
> > // Compute the new bits that are at the top now.
> > uint64_t ShiftAmt = SA->getZExtValue();
> > - APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
> >
> > // Signed shift right.
> > APInt Mask2(Mask.shl(ShiftAmt));
> > @@ -789,11 +787,11 @@
> > // Handle the sign bits and adjust to where it is now in the mask.
> > APInt SignBit(APInt::getSignBit(BitWidth).lshr(ShiftAmt));
> >
> > - if ((KnownZero & SignBit) != 0) { // New bits are known zero.
> > + APInt HighBits(APInt::getHighBitsSet(BitWidth, ShiftAmt));
> > + if (KnownZero[BitWidth-ShiftAmt-1]) // New bits are known zero.
> > KnownZero |= HighBits;
> > - } else if ((KnownOne & SignBit) != 0) { // New bits are known one.
> > + else if (KnownOne[BitWidth-ShiftAmt-1]) // New bits are known one.
> > KnownOne |= HighBits;
> > - }
> > return;
> > }
> > break;
> >
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits@cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
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