[PATCH] D86468: [WIP][GlobalISel] CSE copies

Wed Aug 26 05:54:40 PDT 2020

arsenm added a comment.

In D86468#2238317 <https://reviews.llvm.org/D86468#2238317>, @foad wrote:

> In D86468#2234049 <https://reviews.llvm.org/D86468#2234049>, @arsenm wrote:
>
>> I always thought all the stuff SelectionDAG::getNode tried to do was insane, and a large part of why the DAG is so slow.
>
> If you're ever going to do these simplifications, then surely it makes sense to do them up front so you don't have to create the MIR, rather than doing them in a later combine/simplification pass?
>
>> How often do these trivially simplifiable situations actually get produced during legalization and combines? I would expect it's pretty rare
>
> I tried a quick hack to simplify integer arithmetic in MIRBuilder, and you can see the effect on AMDGPU legalization tests here: https://github.com/jayfoad/llvm-project/commit/00d98a70c4dc8e8db2f8465104a5e65ca12409f5

These look like some of the cases I want to have an optimizing combiner during the legalizer, but I'm not sure it means they should be done in the MIRBuilder. Some of these cases I think are avoidable. For example, I don't think we take the best path to legalize merge/unmerge, which in turn ends up producing more of these trivial situations

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