Re: [PATCH 1/2] Don't warn function alignment if warn_if_not_aligned_p is true

[Martin Sebor <msebor at gmail dot com>]

On 08/21/2017 06:21 AM, H.J. Lu wrote:
> When warn_if_not_aligned_p is true, a warning will be issued on function
> declaration later.  There is no need to warn function alignment when
> warn_if_not_aligned_p is true.
>
> OK for trunk?
>
> H.J.
> --
> 	* c-attribs.c (common_handle_aligned_attribute): Don't warn
> 	function alignment if warn_if_not_aligned_p is true.
> ---
>  gcc/c-family/c-attribs.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
> index 5f79468407f..78969532543 100644
> --- a/gcc/c-family/c-attribs.c
> +++ b/gcc/c-family/c-attribs.c
> @@ -1754,9 +1754,12 @@ common_handle_aligned_attribute (tree *node, tree args, int flags,
>        This formally comes from the c++11 specification but we are
>        doing it for the GNU attribute syntax as well.  */
>      *no_add_attrs = true;
> -  else if (TREE_CODE (decl) == FUNCTION_DECL
> +  else if (!warn_if_not_aligned_p
> +	   && TREE_CODE (decl) == FUNCTION_DECL
>  	   && DECL_ALIGN (decl) > (1U << i) * BITS_PER_UNIT)
>      {
> +      /* Don't warn function alignment here if warn_if_not_aligned_p is
> +	 true.  It will be warned later.  */
>        if (DECL_USER_ALIGN (decl))
>  	error ("alignment for %q+D was previously specified as %d "
>  	       "and may not be decreased", decl,

Your comment refers to warning but the code here uses error().
That raises two questions for me: a) will the later diagnostic
really be a warning or an error, and if a warning, under what
option will it be issued? and b) why is an error appropriate
here when a warning is appropriate elsewhere (most other
attribute conflicts are at present diagnosed with -Wattributes).

My main motivation for these questions is to understand the
rationale for warning for vs rejecting conflicts so that
a consistent general solution can be implemented for all
attributes (i.e., along the lines of my patch here:
https://gcc.gnu.org/ml/gcc-patches/2017-08/msg01457.html)

Martin