在 2007-03-27二的 22:17 -0700，Reid Spencer写道：
> 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.
I looked into the context, the *else* case is possible, but, as the
NewBits are :
APInt NewBits(APInt::getHighBitsSet(BitWidth, BitWidth - SrcBitWidth));
So ~NewBits should be a Mask of SrcBitWidth, and as KnownZero, KnownOne
are just zexted from SrcBitWidth to BitWidth, so, the &= operation will
be useless.
>
> 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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