[PATCH] D134142: [InstCombine] Handle integer extension in `select` patterns using the condition as value

Tue Sep 20 09:53:52 PDT 2022

spatel added a comment.

In D134142#3802960 <https://reviews.llvm.org/D134142#3802960>, @nikic wrote:

> In D134142#3802765 <https://reviews.llvm.org/D134142#3802765>, @spatel wrote:
>
>> In D134142#3802599 <https://reviews.llvm.org/D134142#3802599>, @nikic wrote:
>>
>>> This kinda looks like foldSelectValueEquivalence() with a trivial comparison (i.e. interpret `c` as `c == 1`).
>>
>> Agree - but we'd have to account for the missing icmp that code is expecting? Not sure that's worth the effort and reduced readability, so this patch LGTM.
>
> It looks like we don't even need foldSelectValueEquivalence here -- we already have simplifyWithOpReplaced() folds for these. They are currently behind a guard for i1 selects, but if we move them outside we get most of the mileage: https://gist.github.com/nikic/86420628e72dd137aa56d2317f3825b0
>
> The main caveat is that this does not handle vectors. Not that I actually care about vectors, but for the sake of propriety we should probably update simplifyWithOpReplaced() to allow vectors and only reject the problematic cross-lane operations inside.

That also misses the basic patterns with a 'not' before the extend. I like the smaller patch, but given that this one covers more cases, I'd still go with this patch as-is.

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https://reviews.llvm.org/D134142