contrib: Skip llvm-cov rendering for deterministic fuzz inputs #35608

1. 
   HowHsu commented at 11:55 AM on June 26, 2026: contributor

   Summary

   deterministic-fuzz-coverage rendered a full llvm-cov show report for every
   fuzz input on both runs and diffed the two reports, even though almost all
   inputs are deterministic. llvm-cov show is by far the most expensive step, so
   the vast majority of that work produced two identical reports that were
   immediately discarded.
   Compare the merged .profdata first: identical counters are a sufficient
   condition for deterministic coverage, so when the two profdata files match the
   llvm-cov show rendering is skipped entirely. Only when they differ does it
   fall back to rendering and diffing the reports to confirm and locate the
   difference.
   The pass/fail result is unchanged: identical profdata implies identical
   counters, which implies identical llvm-cov show output (the report is a
   deterministic function of the profdata and the binary). Any profdata difference
   falls back to the exact same llvm-cov diff as before.

   Benchmark

   standalone coverage build (-DBUILD_FOR_FUZZING=ON +
   -fprofile-instr-generate -fcoverage-mapping, no sanitizer), par=4,
   qa-assets corpora:

   target	inputs	before	after	speedup
   addition_overflow	112	32s	10s	3.2x
   process_message	2386	719s	247s	2.9x

   On process_message every input is deterministic, so all 2386 take the
   profdata fast path and zero llvm-cov show reports are rendered, down from
   2*2386 + 2 = 4774.

2. DrahtBot added the label Scripts and tools on Jun 26, 2026
3. 
   DrahtBot commented at 11:55 AM on June 26, 2026: contributor

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   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35608.

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   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #35809 (contrib: add deterministic fuzz coverage mode by HowHsu)

   If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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4. contrib: Skip llvm-cov rendering for deterministic fuzz inputs

   deterministic-fuzz-coverage rendered a full `llvm-cov show` report for
   every input on both runs and diffed the two reports. `llvm-cov show`
   is by far the most expensive step, so almost all of that work produced
   two identical reports that were immediately thrown away.
   
   Compare the merged profdata first instead: identical counters are a
   sufficient condition for deterministic coverage, so when the two
   profdata files match we skip rendering entirely. Only when they differ
   do we fall back to rendering and diffing the `llvm-cov show` reports to
   render and locate the difference.

   87ecdf2cf0
5. HowHsu force-pushed on Jun 26, 2026
6. DrahtBot added the label CI failed on Jun 26, 2026
7. DrahtBot removed the label CI failed on Jun 26, 2026
8. sedited requested review from maflcko on Jul 24, 2026

Review Requested
maflcko