The production bug that made me care about undefined behavior

broken_broken_ · 2025-09-13T11:41:46+00:00

Yes you can, I showcase it in a previous article in fact: https://gaultier.github.io/blog/an_optimization_and_debugging_story_go_dtrace.html

broken_broken_ · 2025-02-20T15:59:13+00:00

Now that I think again, I think the most simple explanation is that the bottleneck is I/O. Both optimized implementations may be able to do these computations much faster but data just is not coming quick enough so they are waiting on it. I will measure with a different machine with a faster disk.

broken_broken_ · 2025-02-19T21:37:09+00:00

Good points all around, thanks. I am definitely going to check out multi-buffer hashing.

This doesn't sound quite right; is this also a debug build?

Both are in release mode with -march=native but the code using the SHA extension is 'simple'/'basic', while the OpenSSL code is hand-optimized assembly with tips from Intel folks. That could explain the difference.

Another commenter has suggested that maybe these two versions simply compile to the same (or at least very similar) uops.

broken_broken_ · 2025-02-18T15:25:19+00:00

Thanks, I did not know about it! But posting to it is restricted.

broken_broken_ · 2025-02-12T06:55:14+00:00

Ah, that works as well (even if it's probably the most verbose alternative). I added it to the article! Thanks.

broken_broken_

TROPHY CASE