[Pkg-rust-maintainers] Bug#976374: Bug#973414: libmozjs-78-0: invalid opcodes when launching GDM on AMD Geode

[Martin-Éric Racine]

to 10. jouluk. 2020 klo 21.43 Martin-Éric Racine
(martin-eric.racine at iki.fi) kirjoitti:
>
> to 10. jouluk. 2020 klo 21.21 Simon McVittie (smcv at debian.org) kirjoitti:
> > Debian's rustc has a patch to reduce the i386 baseline from upstream's
> > pentium4 to pentiumpro
> > https://sources.debian.org/src/rustc/1.48.0+dfsg1-1/debian/patches/d-i686-baseline.patch/
> > but apparently that's not sufficient for a Geode LX. i686 is in a weird
> > situation where the Pentium Pro was the *first* i686 CPU, but is not a
> > *baseline* i686 CPU.
> >
> > Presumably the root cause of #976374 in librsvg, which also contains
> > Rust code, is the same.
>
> I cannot help but wonder what platform defaults GCC uses on i386.
> Given how the baseline kernel for i386 (linux-image-686 i.e. Linux for
> older PCs) is configured for Geode LX, I would presume the GCC
> defaults are aligned to match. AFAIK that kernel is compiled for Geode
> and for Generic 686, which I presume means only using the flags that
> intersect both CPU variants. It might be a good starting point for the
> Rust compiler too.

I'm no expert on x86 instruction sets, but looking at GCC options
suggests that "i686" would be the same as -march=pentiumpro and
-mtune=generic, which seems to provide support for a broader range of
i686-family chips. It's just a guess (I know nothing of Rust's
building environment), but I wouldn't be surprised if the above patch
also affected -mtune, which is perhaps too tight of a CPU spec.

Again, checking what compiler and linker options are used to build
linux-image-686 might provide a clearer view of what instruction set
is a safe baseline for the i386 port.

Martin-Éric