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

[Eric Gallager <egall@gwmail.gwu.edu>]

On Thu, Jun 16, 2022 at 3:37 PM Vit Kabele <vit.kabele@sysgo.com> 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.

Hi, thanks for taking the time to improve -Wpadded; I have been
wishing that GCC's implementation of -Wpadded would print this
information for a while now and thought there was a bug open for it,
but can't seem to find it now...

>
> This removes the need of using pahole or similar tools, or manually
> determining the padding size.
>
> Tested on x86_64-pc-linux-gnu.
>
> gcc/ChangeLog:
>
>         * stor-layout.cc (finalize_record_size): Improve warning message
>
> 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);
> +  }

Style nit: indentation seems off; check your tabs vs. spaces etc.

>
>    if (warn_packed && TREE_CODE (rli->t) == RECORD_TYPE
>        && TYPE_PACKED (rli->t) && ! rli->packed_maybe_necessary
> --
> 2.30.2