Re: Help needed for debugging OCaml failure on m68k

[Stéphane Glondu <glondu@debian.org>]

Le 19/06/2024 à 12:45, John Paul Adrian Glaubitz a écrit :
> > > > To reproduce the problem quickly:
> > > > - unpack ocaml 5.2.0 source package
> > > > - ./configure --enable-imprecise-c99-float-ops
> > > > - make coldstart
> > > >
> > > > Is there some subtlety with thread local variables on m68k?
> > >
> > > Can you please try reproduce the issue on the porterbox mitchy.debian.net first to
> > > make sure it's not related to the QEMU build environment on the buildds?
> >
> > I can reproduce the issue on mitchy.debian.net.
>
> OK, then it's actually a bug.
>
> One important thing to know is that the natural alignment on m68k is actually 16 bits
> and not 32 bits which causes quite some issues with various upstream projects.
>
> We're currently planning on switching the alignment on m68k to 32 bits and chances are
> that this could this issue as well.
>
> Can you maybe try passing "-malign-int" to CFLAGS/CXXFLAGS when building OCaml on m68k
> to verify this hypothesis? Please note that this also breaks the SysV ABI, so it's not
> possible to easily do this on a per-package basis.

I observe the same behaviour with "-malign-int": the address of 
caml_state (a thread local variable) changes unexpectedly (goes from 
0x402e5fac to 0x402e7454) after the following goto:


https://salsa.debian.org/ocaml-team/ocaml/-/blob/debian/experimental/runtime/interp.c?ref_type=heads#L295

which leads to:


https://salsa.debian.org/ocaml-team/ocaml/-/blob/debian/experimental/runtime/interp.c?ref_type=heads#L819

...confirmed by adding:

  fprintf(stderr, "&caml_state = %p\n", &caml_state);

before the goto and after the "Instruct(BRANCH):".


Cheers,

--
Stéphane