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CoverageMappingReader behaves O(N^2) slowly with hundreds thousand of Expansions #113173

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chapuni opened this issue Oct 21, 2024 · 0 comments · May be fixed by #122589
Open

CoverageMappingReader behaves O(N^2) slowly with hundreds thousand of Expansions #113173

chapuni opened this issue Oct 21, 2024 · 0 comments · May be fixed by #122589
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coverage tools:llvm-cov

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@chapuni
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chapuni commented Oct 21, 2024

For example RISCVInstructionSelector.cpp (possibly since #110782):

  • NumRegions 892,077
  • NumFiles 387,470 (Expansions)

It takes bunch of minutes for me.

llvm-cov is not required for reproduction. Just add -Xclang -dump-coverage-mapping onto -profile-instr-generate -fcoverage-mapping. The dump calls CoverageMappingReader for encoded covmap.
chapuni added a commit that referenced this issue Jan 11, 2025
@chapuni
[Coverage] Improve performance of propagating Counter of Expansions
e5fbf84 
This improves `-dump-coverage-mapping` (939,498 lines) for
`RISCVInstructionSelector.cpp` from 30m to 1m18s and also improves
`llvm-cov` for `check-llvm` from 33m to 24s.

The current implementation behaved O(N^2) order with hundreds
thousands of Expansions.

This assumes:
  - Records are partitioned by FileID.
    - ExpandedFileID doesn't point FileID==0, since it is the root.
  - The Count in Expansion is propagated from 1st Record in
    ExpandedFileID.

Therefore another fact below can be assumed.
  - Propagation chain consists of Expansions at each 1st Record.

This scans the Record at most a few times. O(N) is expected.

Fixes #113173
@chapuni chapuni linked a pull request Jan 11, 2025 that will close this issue
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[Coverage] Improve performance of propagating Counter of Expansions llvm/llvm-project
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@chapuni @EugeneZelenko