Re: [PATCH roland/arm-atomic-warn] Fiddle ARM atomic.h to avoid -Wvolatile-register-var warnings.

[Richard Henderson <rth at twiddle dot net>]

On 08/08/2012 01:44 PM, Roland McGrath wrote:
> +   We use the union trick rather than simply using __typeof (...) in the
> +   declarations of A_OLDVAL et al because when NEWVAL or OLDVAL is of the
> +   form *PTR and PTR has a 'volatile ... *' type, then __typeof (*PTR) has
> +   a 'volatile ...' type and this triggers -Wvolatile-register-var to
> +   complain about 'register volatile ... asm ("reg")'.  */
>  #elif defined __thumb2__
>  /* Thumb-2 has ldrex/strex.  However it does not have barrier instructions,
>     so we still need to use the kernel helper.  */
>  #define __arch_compare_and_exchange_val_32_acq(mem, newval, oldval) \
> -  ({ register __typeof (oldval) a_oldval asm ("r0");			      \
> -     register __typeof (oldval) a_newval asm ("r1") = (newval);		      \
> +  ({ union { __typeof (oldval) a; uint32_t v; } oldval_arg = { .a = (oldval) };\
> +     union { __typeof (newval) a; uint32_t v; } newval_arg = { .a = (newval) };\
> +     register uint32_t a_oldval asm ("r0");				      \
> +     register uint32_t a_newval asm ("r1") = newval_arg.v;		      \
>       register __typeof (mem) a_ptr asm ("r2") = (mem);			      \
> -     register __typeof (oldval) a_tmp asm ("r3");			      \
> -     register __typeof (oldval) a_oldval2 asm ("r4") = (oldval);	      \
> +     register uint32_t a_tmp asm ("r3");				      \
> +     register uint32_t a_oldval2 asm ("r4") = oldval_arg.v;		      \
>       __asm__ __volatile__						      \
>  	     ("0:\tldr\t%[tmp],[%[ptr]]\n\t"				      \

Is it really worth working so hard to avoid the gcc builtins?
Yes, gcc before 4.7 doesn't have __atomic to avoid the extra
barriers, but at least 4.7 and later will.

May I ask what gcc version you're targeting, anyway?


r~