[PATCH] D59227: [GlobalISel][Utils] Teach getConstantVRegVal how to look through trunc and z|sext

[Quentin Colombet via Phabricator via llvm-commits]

qcolombet added a comment.

Hi Matt,

>   Why wouldn’t the combiner fold casts of constants?

The problem is, let say you have G_CONSTANT being legal only for {s16, s32}
Now consider:

  %0 = G_CONSTANT i14

The legalizer will then transform that into:

  %tmp = G_CONSTANT i16
  %0 = G_TRUNC i16 %tmp to i14

If you were to do the constant folding within the MachineIRBuilder, this would then be transformed back into:

  %0 = G_CONSTANT i14

And now you have your loop.

We could do that in the combiner and then we don't need to look through this, However, in the middle of legalization, we will still have the truncates and extensions that may not be cleaned up at all given point in the legalization and that's when we would miss to get the constant value.

> Do any of these testcases hit the legalization loop problem you mentioned?

No, they just happen to exercise the code I added.

The problem I have arises during legalization with an out-of-tree target.
Essentially, we promote some illegal operation, then end-up with extensions in the way of the constants and some custom legalization fails (without this patch) because we don't support non-constant patterns.


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  rL LLVM

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https://reviews.llvm.org/D59227