A big performance regression in benchmarks

[ljedrz]

rustc performance data shows a pretty big regression between 74bd074e and 6050e52. I don't know if this is expected, but I'm putting this out there in case others see the dent in perf charts and wonder what had happened.

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EDIT(@eddyb): we how have a single-PR perf comparison showing the regression for #69718.

#70156, the other PR in the range, is much more of a mixed bag (but only for a small subset of runs).

[jonas-schievink]

cc @arlosi @eddyb

[Mark-Simulacrum]

9e55101 is in the queue: https://perf.rust-lang.org/status.html.

[arlosi]

It looks like this was caused by #69718. I'll start investigating why.

I timed several compilations of ripgrep locally in debug with and without #69718.

master (sec): 41 40 39 40 40
revert (sec): 34 34 34 36 34

[eddyb]

For the record, this is the single-PR perf comparison showing the regression from #69718.
I'll update the issue description to minimize potential confusion.

@arlosi Two things I'm seeing:

1. there are way worse regressions than ripgrep, you can focus on the worse ones to get better contrast in measurements (e.g. tokio-webpush-simple-debug even has "patched incremental" runs slowed down a lot, unlike many others, clap-rs-debug being another example I guess)
2. the detailed profile (if you click the percentage for a run) shows a lot of LLVM differences, e.g. for tokio-webpush-simple-debug "clean", and most of the regressions are -debug, with effectively no -check ones, so I think we might be able to get away without a revert, just adding a flag controlling whether LLVM sees these hashes (assuming that's the source of the slowdown)

[eddyb]

We have perf data for the fix (#70803), which at a glance looks right.

Before the regression to after the fix looks like noise, and so does after the regression to before the fix, so I believe this confirms the fix is complete.

For the cost of MD5 hashing compared to SipHash, based on the changes to -check runs in the original regression, it looks insignificant.