[Bug c/126552] const-qualified local register variable used as an asm operand is silently discarded

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D126552

--- Comment #8 from Bernie Innocenti <bernie at codewiz dot org> ---
To clarify the constraints we're working with, the repro code comes from the
expansion of type-generic macros like these:

 #define LP7NR(offs, name, t1, v1, r1, t2, v2, r2, t3, v3, r3, t4, v4, r4, =
t5,
v5, r5, t6, v6, r6, t7, v7, r7, bt, bn) \
 ({                                              \
   t1 _##name##_v1 =3D (v1);                       \
   t2 _##name##_v2 =3D (v2);                       \
   t3 _##name##_v3 =3D (v3);                       \
   t4 _##name##_v4 =3D (v4);                       \
   t5 _##name##_v5 =3D (v5);                       \
   t6 _##name##_v6 =3D (v6);                       \
   t7 _##name##_v7 =3D (v7);                       \
   {                                             \
      register int _d0 __asm("d0");              \
      register int _d1 __asm("d1");              \
      register int _a0 __asm("a0");              \
      register int _a1 __asm("a1");              \
      register t3 _n3 __asm(#r3) =3D _##name##_v3; \
      register t4 _n4 __asm(#r4) =3D _##name##_v4; \
      register t5 _n5 __asm(#r5) =3D _##name##_v5; \
      register t6 _n6 __asm(#r6) =3D _##name##_v6; \
      register t7 _n7 __asm(#r7) =3D _##name##_v7; \
      register t1 _n1 __asm(#r1) =3D _##name##_v1; \
      register t2 _n2 __asm(#r2) =3D _##name##_v2; \
      register void *const _##name##_bn __asm("a6") =3D (bn); \
      __asm volatile ("jsr a6@(-"#offs":W)"      \
      : "=3Dr" (_d0), "=3Dr" (_d1), "=3Dr" (_a0), "=3Dr" (_a1) \
      : "r" (_##name##_bn), "rf"(_n1), "rf"(_n2), "rf"(_n3), "rf"(_n4),
"rf"(_n5), "rf"(_n6), "rf"(_n7) \
      : "fp0", "fp1", "cc", "memory");          \
   }                                            \
 })


The full set is here:

  https://github.com/adtools/fd2pragma/blob/master/Include/inline/macros.h

Following Drea's and Jakub's recommendation, I experimented with using hard
register constraints on __GNUC__ >=3D 16:

=20
https://github.com/codewiz/sfdc/blob/inline-macros-amigaports/include/inlin=
e/macros.h

This works only for GCC 16 (*), and only with -mlra (and there might be good
reasons if it's not already the default on m68k).

Plus, we still need a solution for GCC 13 and GCC 15, which are still activ=
ely
used by Amiga projects.=