[PATCH] D125070: [InstCombine] Simplify chain of GEP with constant indices

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

In D125070#3505105 <https://reviews.llvm.org/D125070#3505105>, @huangjd wrote:

> @nikic
>
>> 2. Combine constant GEPs. We already do this, but sometimes fail based on element type mismatch. This can be fixed either by canonicalizing all constant GEPs to `i8` (probably better) or by doing so only when it allows combining GEPs that otherwise couldn't be combined.
>
> Where are the tests located for combining constant GEP?  I think currently it only work with one direction (merging the second one into the first), so I will need to make it work both ways.

There are some tests starting here:; https://github.com/llvm/llvm-project/blob/34b6f206cbab8471abf29739dab981bd8b868a65/llvm/test/Transforms/InstCombine/opaque-ptr.ll#L172 The code for this is in https://github.com/llvm/llvm-project/blob/34b6f206cbab8471abf29739dab981bd8b868a65/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp#L2025.

Though as already mentioned, the most general fix for this is to canonicalize all constant GEPs to use `i8` element type, which ensures that two constant GEPs can always be combined.


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