[thibautg]

I own a MacBook Air M1. Today, I noticed that my Homebrew Python (3.9.1 on Intel) installation had reverted to the default one (3.8 on ARM). I forced a reinstall with homebrew and I got an ‘illegal hardware instruction’ error when running python3. It was corrected by building from source (with the -s switch: brew reinstall -s python3).

I found out about this issue on Github and I learned that the M1 chip does not support AVX (which is indeed documented) and that Homebrew developers assumed that all CPUs supported by Big Sur were compatible with AVX instruction set (which is true except for the M1 under Rosetta 2).

I found it interesting to see how they rapidly corrected the situation by merging a partial revert. I also found their CI infrastructure interesting to watch while it was live-testing the commit.

[klodolph]

This is fascinating. I can understand why the Homebrew devs might make the decision, but it seems the right way to do this is just to, e.g.,

    cc -dM -E -xc /dev/null | less

Anything with a preprocessor macro is supported on all OS configurations that you are targeting. You can see, for example, that __SSE4_1__ is defined, so SSE 4.1 is ok to use without checking it at runtime with e.g. sysctlbyname("hw.optional.sse4_1", &enabled, ...)

I have always thought that it is worth a bit of paranoia to try and do the check for optional features the “correct” way, whatever method the OS advertises (like sysctl for macOS) rather than doing something like cpuid. After all, every once in a while it’s possible to run into a configuration where the CPU does support some particular feature but the OS does not, and if you rely on cpuid you could be up shit creek without a paddle.

[bodyfour]

It was a mistaken assumption from the Homebrew developers who didn't realize AVX would cause problems with Rosetta2.

Within a few days problems became evident and Homebrew turned the compiler flag back off. Now it's just a question of the best way of remediating the already-compiled packages in the simplest way possible.

Really the fact that this is now a front-page-of-HN story is going to cause more confusion to people. It was just a short-lived bug.