Note to self: Don’t use a compiler from the Mesozoic era

Updating my compiler is a suprisingly effective debugging strategy

18 February 2025

Earlier this year I was writing code to run on one of my university’s supercomputers and to make things a bit more convenient for myself I was developing locally on my laptop and zapping my source changes to the cluster by working in a private git repository hosted on GitHub. Again for convenience I compiled my code locally on my laptop and did small test runs so I can analyse performance before putting them on the supercomputer however I noticed that a specific loop I wrote wasn’t being vectorised even though I knew the supercomputer used AMD EPYC 7702 CPUs which I triple checked supports AVX2 instructions so I was expecting to see instructions using the 256 bit YMM registers as the compiler emitted on my own computer.

I was very lucky I was feeling lazy and decided to try a more recent compiler version because that just worked. Fast forwarding GCC 3 years from 2018 to 2021 fixed my issue and the compiler vectorised my code as I expected.

A bit more recently I had something very similar: I was having a go at compiling Lean4 for WebAssembly and, since I don’t like waiting for my code to compile, I had the terrific idea of compiling everything using my university’s supercomputer. I saw a weird error message complaining about the use of va_arg which I didn’t have the patience to read. I updated the compiler version and lo-and-behold things started working again. More precisely I ran into another issue but I fixed my original issue and I was happy.

So yea, update your compiler.