Re: [PATCH] libgcc: Fix uninitialized RA signing on AArch64 [PR107678]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jan 12, 2023 at 03:22:56PM +0000, Richard Sandiford wrote:
> If we have a new enum, I think we should handle it explicitly.  The fact
> that the information isn't propagated between frames is a key part of
> the semantics.
> 
> >> Another option is to just define the arch dependent value for how field
> >> in the arch code, right now it is unsigned char type, so using say
> >> (unsigned char) ~0 or (unsigned char) ~0 and (unsigned char) ~1 as arch
> >> specific values might be ok too.
> >
> > Yet another option would be to define 1-2 extra REG_ values in the generic
> > unwind-dw2.h header, but name them
> >   REG_ARCH_SPECIFIC_1,
> >   REG_ARCH_SPECIFIC_2,
> > or so, and then the machine specific code can
> > #define REG_AARCH64_TOGGLE_ON REG_ARCH_SPECIFIC_1
> > Of course, all this depends on whether the arch specific codes can be
> > handled in uw_update_context_1 by doing break; there and nothing else.
> 
> Yeah, personally I'd prefer for target-independent code to provide
> the toggle representation, even if it isn't widely used.

I can live even with that, I just hope it won't make code generation worse
on other targets.
Anyway, I understood aarch64 needs 2 states for the signing, so one would
be REG_TOGGLE_ON and the other anything else?
Users can always create (invalid?) unwind info where they save the magic
register, make it undefined etc.

And
void bar (void), baz (void), boo (void), qux (void), corge (void);
enum {
  REG_UNSAVED,
  REG_SAVED_OFFSET,
  REG_SAVED_REG,
  REG_SAVED_EXP,
  REG_SAVED_VAL_OFFSET,
  REG_SAVED_VAL_EXP,
  REG_UNDEFINED
#ifdef ANOTHER
  , REG_TOGGLE_ON
#endif
};

void
foo (unsigned char c)
{
  switch (c)
    {
      case REG_UNSAVED:
      case REG_UNDEFINED:
#ifdef ANOTHER
      case REG_TOGGLE_ON:
#endif
        break;
          
      case REG_SAVED_OFFSET:
        bar ();
        break;
     
      case REG_SAVED_REG:
        baz ();
        break;
  
      case REG_SAVED_EXP:
        boo ();
        break;
  
      case REG_SAVED_VAL_OFFSET:
        qux ();
        break;

      case REG_SAVED_VAL_EXP:
        corge ();
        break;
    }
}
suggests that it doesn't, already cfg pass turns the implicit default:
into something that covers even the REG_UNSAVED, REG_UNDEFINED and maybe
REG_TOGGLE_ON values into default:

	Jakub