[PATCH] D150922: [KnownBits] Return `0` for poison {s,u}div inputs

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D150922#4357610 <https://reviews.llvm.org/D150922#4357610>, @goldstein.w.n wrote:

> In D150922#4357344 <https://reviews.llvm.org/D150922#4357344>, @nikic wrote:
>
>> IMHO we shouldn't go out of the way to handle such cases -- if we need to handle this for other reasons (e.g. for shift amounts, or for conflicts) that's one thing, but explicitly checking for poison result for known constant cases is pretty pointless -- those will get folded to actual poison values by code that can do so.
>
> The INT_MIN / -1 case isn't necessary, but the LHS.isZero() || RHS.isZero() cases reduce the number of
> checks we need elsewhere (specifically now we know MinTZ < BitWidth)
>
> Edit: But I have no particularly strong feelings either way. the Zero, Zero case just
> seemed to be the simplest way to handle it. The INT_MIN, -1 case was just to
> be consistent. If you feel either/both should be dropped I have no objections.

Is this referring to the code in D150923 <https://reviews.llvm.org/D150923>? That is we do the early isZero() check here and then divComputeLowBit() from that patch has an implicit precondition that the denominator is not zero? Adding an explicit `< BitWidth` check there seems more obvious to me, unless it's more than that?


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https://reviews.llvm.org/D150922