[llvm-dev] [GlobalISel] Narrowing uneven/non-pow-2 types

[Dominik Montada via llvm-dev]

Hi all,

recently when working with GlobalISel we have often encountered cases in 
the legalizer where instructions could not be narrowed because the 
narrowing code relies on G_UNMERGE_VALUES and therefore requires the 
source type to be a multiple of the narrow type. Often times these 
instructions can be widened without any problem to a fitting type.

This has us writing legalization rules like `.widenScalarToNextPow2(0, 
/*MinSize*/ 32).maxScalar(0, s32)` instead of the much simpler 
`.clampScalar(0, s32 ,s32)`.

Although this works and has the desired effect, we feel like that such a 
rule requires internal knowledge of the legalizer, which can change at 
any point in the future. Ideally we would only want to say `clampScalar` 
and let the legalizer handle the rest, even if it requires extending the 
type before it can be narrowed.

We were now wondering if such a behavior in the legalizer is even 
desired? I.e. that an instruction is extended to an easier-to-handle 
type even though narrowing was requested (as long as the narrowing is 
done in the end)?

Would be great to get some feedback so we know whether we can start 
adding support for some instructions or if we just need to rewrite some 
of our legalization rules using the internal knowledge.

Regards,

Dominik

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