Re: gcc-13/changes.html: Mention -fstrict-flex-arrays and its impact

[Richard Biener <rguenther@suse.de>]

On Tue, 20 Dec 2022, Qing Zhao wrote:

> Hi,
> 
> This is the patch for mentioning -fstrict-flex-arrays and -Warray-bounds=2 changes in gcc-13/changes.html.
> 
> Let me know if you have any comment or suggestions.

Some copy editing below

> Thanks.
> 
> Qing.
> 
> =======================================
> From c022076169b4f1990b91f7daf4cc52c6c5535228 Mon Sep 17 00:00:00 2001
> From: Qing Zhao <qing.zhao@oracle.com>
> Date: Tue, 20 Dec 2022 16:13:04 +0000
> Subject: [PATCH] gcc-13/changes: Mention -fstrict-flex-arrays and its impact.
> 
> ---
>  htdocs/gcc-13/changes.html | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/htdocs/gcc-13/changes.html b/htdocs/gcc-13/changes.html
> index 689178f9..47b3d40f 100644
> --- a/htdocs/gcc-13/changes.html
> +++ b/htdocs/gcc-13/changes.html
> @@ -39,6 +39,10 @@ a work-in-progress.</p>
>      <li>Legacy debug info compression option <code>-gz=zlib-gnu</code> was removed
>        and the option is ignored right now.</li>
>      <li>New debug info compression option value <code>-gz=zstd</code> has been added.</li>
> +    <li><code>-Warray-bounds=2</code> will no longer issue warnings for out of bounds
> +      accesses to trailing struct members of one-element array type anymore. Please
> +      add <code>-fstrict-flex-arrays=level</code> to control how the compiler treat
> +      trailing arrays of structures as flexible array members. </li>

"Instead it diagnoses accesses to trailing arrays according to 
<code>-fstrict-flex-arrays</code>."

>  </ul>
>  
>  
> @@ -409,6 +413,17 @@ a work-in-progress.</p>
>  <h2>Other significant improvements</h2>
>  
>  <!-- <h3 id="uninitialized">Eliminating uninitialized variables</h3> -->
> +<h3 id="flexible array">Treating trailing arrays as flexible array members</h3>
> +
> +<ul>
> + <li>GCC can now control when to treat the trailing array of a structure as a 
> +     flexible array member for the purpose of accessing the elements of such
> +     an array. By default, all trailing arrays of structures are treated as

all trailing arrays in aggregates are treated
 
> +     flexible array members. Use the new command-line option
> +     <code>-fstrict-flex-array=level</code> to control how GCC treats the trailing
> +     array of a structure as a flexible array member at different levels.

<code>-fstrict-flex-arrays</code> to control which trailing array
members are streated as flexible arrays.

I've also just now noticed that there's now a flag_strict_flex_arrays
check in the middle-end (in array bound diagnostics) but this option
isn't streamed or handled with LTO.  I think you want to replace that
with the appropriate DECL_NOT_FLEXARRAY check.  We might also want
to see how inlining accesses from TUs with different -fstrict-flex-arrays
setting behaves when accessing the same structure (and whether we might
want to issue an ODR style diagnostic there).

Thanks,
Richard.