[Bug middle-end/77964] [7 Regression] Linux kernel firmware loader miscompiled

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77964

--- Comment #17 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Jiri Slaby from comment #16)
> (In reply to Jakub Jelinek from comment #15)
> > lots of them that rely on pointer arithmetics being defined only within the
> > same object.
> 
> Sure, but the two pointers (taken implicitly of the arrays) are within the
> same object. So I do not see, why it wouldn't work? I.e. where exactly this
> breaks the C specs?

No.  In C
extern struct builtin_fw __start_builtin_fw[];
extern struct builtin_fw __end_builtin_fw[];
declares two external arrays, thus they are two independent objects.  It is
like if you have:
int a[10];
int b[10];
in your program, although they might be allocated adjacent, such that
int *p = &a[10]; int *q = &b[0]; memcmp (&p, &q, sizeof (p)) == 0;
&b[0] - &a[0] is still UB.
What you do with __start_*/__end_* symbols is nothing you can define in C, you
need linker support or asm for that, and to use it without UB you also need to
use an optimization barrier that has been suggested.