Re: [PATCH, ARM] Fox target/69180] #pragma GCC target should not warn about redefined macros

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

Hi Christian,

On 07/01/16 15:40, Christian Bruel wrote:
> as discussed with Kyrill (https://gcc.gnu.org/ml/gcc-patches/2016-01/msg00307.html), this patch avoids confusing (for the testsuite) macro redefinition warning or pedantic errors when the user changes FP versions implicitly with a #pragma 
> GCC target. The warning is kept when the macro is redefined explicitly by the user.
>
> tested on arm-linux-gnueabi for {,-mfpu=neon-fp-armv8,-mfpu=neon}

> Index: config/arm/arm-c.c
> ===================================================================
> --- config/arm/arm-c.c	(revision 232101)
> +++ config/arm/arm-c.c	(working copy)
> @@ -23,6 +23,7 @@
>   #include "c-family/c-common.h"
>   #include "tm_p.h"
>   #include "c-family/c-pragma.h"
> +#include "stringpool.h"
>   
>   /* Output C specific EABI object attributes.  These can not be done in
>      arm.c because they require information from the C frontend.  */
> @@ -245,8 +246,18 @@ arm_pragma_target_parse (tree args, tree
>   
>         /* Update macros.  */
>         gcc_assert (cur_opt->x_target_flags == target_flags);
> -      /* This one can be redefined by the pragma without warning.  */
> -      cpp_undef (parse_in, "__ARM_FP");
> +
> +      /* Don't warn for macros that have context sensitive values depending on
> +	 other attributes.
> +	 See warn_of_redefinition, Reset after cpp_create_definition.  */
> +      tree acond_macro = get_identifier ("__ARM_NEON_FP");
> +      C_CPP_HASHNODE (acond_macro)->flags |= NODE_CONDITIONAL ;
> +
> +      acond_macro = get_identifier ("__ARM_FP");
> +      C_CPP_HASHNODE (acond_macro)->flags |= NODE_CONDITIONAL;
> +
> +      acond_macro = get_identifier ("__ARM_FEATURE_LDREX");
> +      C_CPP_HASHNODE (acond_macro)->flags |= NODE_CONDITIONAL;

I see this mechanism also being used by rs6000, s390 and spu but I'm not very familiar with it.
Could you please provide a short explanatino of what NODE_CONDITIONAL means?
I suspec this is ok, but I'd like to get a better understanding of what's going on here.

Thanks,
Kyrill