The GMP developers felt it was a compiler bug, so I think I will leave it at that. But thanks for the tips. > On 26 Oct 2019, at 00:32, David Blaikie <dblaikie at gmail.com> wrote: > > UBSan doesn't catch everything - you could also try ASan and/or valgrind, etc. (MSan if you want, but that's a bit fussier/more work to use)