[PATCH] D84963: [ValueTracking] Add basic computeKnownBits support for llvm.abs intrinsic

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D84963#2186141 <https://reviews.llvm.org/D84963#2186141>, @spatel wrote:

> The SelectionDAG version does the following, so it would make sense to adapt these here:
>
>   // If the source's MSB is zero then we know the rest of the bits already.
>   if (Known2.isNonNegative()) {
>     Known.Zero = Known2.Zero;
>     Known.One = Known2.One;
>     break;
>   }
>   
>   // We only know that the absolute values's MSB will be zero iff there is
>   // a set bit that isn't the sign bit (otherwise it could be INT_MIN).
>   Known2.One.clearSignBit();
>   if (Known2.One.getBoolValue()) {
>     Known.Zero = APInt::getSignMask(BitWidth);
>     break;
>   }

I did think about doing the first change, but I wondered if we would likely end up just removing the ABS in instcombine in that case.


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