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MergeFunction can't merge functions because of jump tables #92201

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yamt opened this issue May 15, 2024 · 3 comments · May be fixed by #92498
Open

MergeFunction can't merge functions because of jump tables #92201

yamt opened this issue May 15, 2024 · 3 comments · May be fixed by #92498
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llvm:optimizations missed-optimization

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@yamt
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yamt commented May 15, 2024

i observed MergeFunc missing merging opportunities due to switch jump tables.

workarounds i can think of:

  • -fno-jump-tables
  • move mergefunc before switch jump tables transformation
  • move mergefunc after constmerge, which would merge switch jump tables
@nikic
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nikic commented May 15, 2024

move mergefunc after constmerge, which would merge switch jump tables

This sounds reasonable to me. I think it is a lot more likely that MergeFuncs can benefit from ConstMerge than the other way around.

@PiJoules
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@yamt do you have a small example of IR where there's missed merging?

@yamt
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yamt commented May 15, 2024

@yamt do you have a small example of IR where there's missed merging?

see:
https://github.com/yamt/garbage/tree/ebf39eafc3e61df66f109cc1115b3fde0fa79327/llvm/mergefunc/1

mergefunc can't merge functions because of tables:
https://github.com/yamt/garbage/blob/ebf39eafc3e61df66f109cc1115b3fde0fa79327/llvm/mergefunc/1/log#L5855-L5914

in the final output, functions are identical because of later constmerge:
https://github.com/yamt/garbage/blob/ebf39eafc3e61df66f109cc1115b3fde0fa79327/llvm/mergefunc/1/a.ll

@yamt yamt linked a pull request May 17, 2024 that will close this issue
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yamt added a commit to yamt/llvm-project that referenced this issue May 17, 2024
@yamt
[Pipelines] Perform mergefunc after constmerge
91eadcf 
Constmerge can fold switch jump tables, possibly making functions
identical again. It can help mergefunc.
On the otherhand, the opposite seems unlikely.

Fixes llvm#92201
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[Pipelines] Perform mergefunc after constmerge yamt/llvm-project
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@nikic @yamt @PiJoules @EugeneZelenko