[PATCH] D122233: [AggressiveInstCombine] Add `shufflevector` instr support to `TruncInstCombine`

Tue Mar 22 09:32:34 PDT 2022

lebedev.ri added a comment.

In D122233#3399767 <https://reviews.llvm.org/D122233#3399767>, @anton-afanasyev wrote:

> In D122233#3399664 <https://reviews.llvm.org/D122233#3399664>, @lebedev.ri wrote:
>
>> Do we expect that unused inputs of the shuffle has been already replaced with undef?
>
> I don't see this could be an issue. For instance, `@unary_shuffle()` test-case contains shuffle with `undef` (line 25). Do you mean this case?

I mean, what if we have a two-input shuffle, and one of the operands is unused as per the shuffle mask.
Then, said operand can be replaced with `undef`, which doesn't affect the narrowing, while the original operand might?
I guess it's a theoretical question, mainly.

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  https://reviews.llvm.org/D122233/new/

https://reviews.llvm.org/D122233