[PATCH] D138872: [InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 1

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D138872#3960754 <https://reviews.llvm.org/D138872#3960754>, @lebedev.ri wrote:

> In D138872#3960604 <https://reviews.llvm.org/D138872#3960604>, @spatel wrote:
>
>> Patch updated:
>>
>> 1. Added documentation comment for the general transform.
>> 2. Added TODO for one-use restriction.
>
> Actually, i guess i was thinking about canonicalizing all insertelements @ constant idx into shuffles
> (+the shuffle to convert the scalar into an appropriately-sized vector),
> but i guess that is almost borderline bad?
> So LG i suppose.

Ah, I see. Yes, we could turn all insert-to-constant-index into shuffles after bitcasting a scalar to a <1 x N> type. The insertelt is the direct representation though, so that's a tough sell. TBH even with a trunc, it's a borderline canonicalization; it could go either way. I think the backend is fine either way too, but I'll watch for fallout.


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