[PATCH] D123394: [CodeGen] Late cleanup of redundant address/immediate definitions.

[Jay Foad via Phabricator via llvm-commits]

foad added a comment.

I can understand the temptation to commit this patch. I also agree with Chris that it's useful as a diagnostic tool to find deficiencies that should probably be fixed elsewhere. That's what I did with D133702 <https://reviews.llvm.org/D133702>.

> Do you have a sense of what (in practice) is causing the dead operations to get emitted?

Speaking only for AMDGPU, I think there are quite a few different reasons. So far I've seen:

- Missed optimizations in globalisel (D133702 <https://reviews.llvm.org/D133702>).
- PrologEpilogInserter calls TargetRegisterInfo::eliminateFrameIndex which materializes a frame offset into a register, too late for the resulting register-immediate moves to be commoned up.
- RegisterCoalescer(?) not doing a good job with register tuples? The problem is that a 64-bit constant like 0x1 is materialized in a register pair like s[0:1], and then 32-bit 0x0 is materialized in s1 -- but s1 was already 0 because it was the high part of the 64-bit constant.
- Some problems related to CFG structurization: I suspect that the highly-AMDGPU-specific parts of this like the SIOptimizeExecMasking pass are not doing a good enough job of cleaning up redundant code.


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https://reviews.llvm.org/D123394