> On Dec 22, 2022, at 2:09 AM, Richard Biener <rguent...@suse.de> wrote:
>
> On Wed, 21 Dec 2022, Qing Zhao wrote:
>
>> Hi, Richard,
>>
>> Thanks a lot for your comments.
>>
>>> On Dec 21, 2022, at 2:12 AM, Richard Biener <rguent...@suse.de> wrote:
>>>
>>> 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.z...@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>."
>>
>> Okay.
>>>
>>>> </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
>> Okay.
>>>
>>>> + 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.
>>
>> Okay.
>>
>>>
>>> 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 need to know the level value of the strict_flex_arrays on the struct
>> field to issue proper warnings at different levels. DECL_NOT_FLEXARRAY
>> does not include such info. So, what should I do? Streaming the
>> flag_strict_flex_arrays with LTO?
>
> But you do
>
> if (compref)
> {
> /* Try to determine special array member type for this
> COMPONENT_REF. */
> sam = component_ref_sam_type (arg);
> /* Get the level of strict_flex_array for this array field. */
> tree afield_decl = TREE_OPERAND (arg, 1);
> strict_flex_array_level = strict_flex_array_level_of (afield_decl);
>
> I see that function doesn't look at DECL_NOT_FLEXARRAY but just
> checks attributes (those are streamed in LTO).
Yes, checked both flag_strict_flex_arrays and attributes.
There are two places in middle end calling “strict_flex_array_level_of”
function,
one inside “array_bounds_checker::check_array_ref”, another one inside
“component_ref_size”.
Shall we check DECL_NOT_FLEXARRAY field instead of calling
“strict_flex_array_level_of” in both places?
>
> OK, so I suppose the diagnostic itself would become just less precise
> as in "trailing array %qT should not be used as a flexible array member"
> without the "for level N and above" part of the diagnostic?
Yes, that might be the major impact.
If only check DECL_NOT_FLEXARRAY, we will lose such information. Does that
matter?
>
>>> 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).
>
> This mixing also means streaming -fstrict-flex-arrays won't be of much
> help in general.
Then under such situation, i.e, different -fstrict-flex-arrays levels for the
same structure from different TUs, how should we handle it?
>
>> Yes, good point, I will check on this part.
>>
>> BTW, a stupid question: what does ODR mean?
>
> It's the One-Definition-Rule (of C++). Basically we'd diagnose
> same struct declarations with different -fstrict-flex-arrays setting.
> I see we miss comparing DECL_NOT_FLEXARRAY for tree merging, I'm
> testing a patch to fix that now.
Okay, thanks a lot for the help.
Qing
>
> Richard.
>
>> thanks.
>>
>> Qing
>>>
>>> Thanks,
>>> Richard.
>>
>>
>
> --
> Richard Biener <rguent...@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
> HRB 36809 (AG Nuernberg)
