[PATCH] D106222: [X86] Don't use known bits to simplify shuffles.

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added a comment.

In D106222#2886166 <https://reviews.llvm.org/D106222#2886166>, @efriedma wrote:

> In D106222#2885959 <https://reviews.llvm.org/D106222#2885959>, @RKSimon wrote:
>
>> I think we're going to have to add something to the SelectionDAG::computeKnownBits/ComputeNumSignBits doxygen comments explaining that the value could have the determined known/signbits BUT it could be a poison value (or if a vector any demanded element could be poison value).
>
> Sure, probably all these functions could use a comment like that: computeKnownBits, ComputeNumSignBits, haveNoCommonBitsSet, isKnownToBeAPowerOfTwo, computeOverflowKind, MaskedValueIsAllOnes, MaskedValueIsZero, SignBitIsZero, isKnownNeverNaN, isSplatValue, etc.  And probably the ValueTracking equivalents, too.  (Some of the ValueTracking functions already have comments which subtly indicate this, e.g. "exactly one bit set when defined.")

+1, this clarification looks great to me.


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https://reviews.llvm.org/D106222