Re: [PATCH] c: Extend the -Wpadded message with actual padding size

[Marek Polacek <polacek@redhat.com>]

On Thu, Jun 16, 2022 at 09:37:32PM +0200, Vit Kabele wrote:
> When the compiler warns about padding struct to alignment boundary, it
> now also informs the user about the size of the alignment that needs to
> be added to get rid of the warning.
> 
> This removes the need of using pahole or similar tools, or manually
> determining the padding size.

Thanks for the patch, it looks reasonable, with the formatting fixed.
It would be nice to have a testcase, at least something like

struct S {
  __UINT64_TYPE__ i;
  char c;
};

The problem is what value to check for, on 32-bit arches the padding is
probably 3 bytes large and on 64-bit arches probably 7 bytes.  So I think
you could use __attribute__((aligned (8))) and then it's always 7.

> Tested on x86_64-pc-linux-gnu.
> 
> gcc/ChangeLog:
> 
> 	* stor-layout.cc (finalize_record_size): Improve warning message

Missing '.' at the end.

> 
> Signed-off-by: Vit Kabele <vit.kabele@sysgo.com>
> ---
>  gcc/stor-layout.cc | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc
> index 765f22f68b9..57ddb001780 100644
> --- a/gcc/stor-layout.cc
> +++ b/gcc/stor-layout.cc
> @@ -1781,7 +1781,14 @@ finalize_record_size (record_layout_info rli)
>        && simple_cst_equal (unpadded_size, TYPE_SIZE (rli->t)) == 0
>        && input_location != BUILTINS_LOCATION
>        && !TYPE_ARTIFICIAL (rli->t))
> -    warning (OPT_Wpadded, "padding struct size to alignment boundary");
> +  {
> +      tree padding_size
> +		= size_binop (MINUS_EXPR,
> +			TYPE_SIZE_UNIT (rli->t), unpadded_size_unit);
> +      warning (OPT_Wpadded,
> +		   "padding struct size to alignment boundary with %E bytes",
> +		   padding_size);
> +  }
>  
>    if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
>        && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
> -- 
> 2.30.2
> 

Marek