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

[Qing Zhao <qing.zhao@oracle.com>]

> On Jan 10, 2023, at 3:06 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Mon, 9 Jan 2023, Qing Zhao wrote:
> 
>> 
>> 
>>> On Jan 9, 2023, at 2:11 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Thu, 22 Dec 2022, Qing Zhao wrote:
>>> 
>>>> 
>>>> 
>>>>> On Dec 22, 2022, at 2:09 AM, Richard Biener <rguenther@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 <rguenther@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.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>."
>>>>>> 
>>>>>> 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?
>>> 
>>> I wonder if that function should check DECL_NOT_FLEXARRAY?
>> 
>> The function ?strict_flex_array_level_of? is intended to query the LEVEL of strict_flex_array, only check DECL_NOT_FLEXARRAY is not enough. 
>> 
>> So, I think the major question here is: 
>> 
>> Do we need  the LEVEL of strict_flex_array information in the Middle end?
>> 
>> The current major use of LEVEL of strict_flex_array in the middle end is two places:
>> 
>> 	1. In the routine ?component_ref_size?: to determine the size of the trailing array based on the level of the strict_flex_array.
>>        2. In the routine ?array_bounds_checker::check_array_ref?: to issue different information for -Wstrict-flex-array based on different level.
>> 
>> 
>> Just double checked the above 1, and 2. Without LEVEL of strict_flex_array info, 1 should be fine
>> 2, as you mentioned previously, the major impact will be that the LEVEL information is lost in the issued message, but that might be not a big
>> issue.
>> 
>> So, I will try to eliminate the reference to ?flag_strict_flex_array? in the middle end, replace it with ?DECL_NOT_FLEXARRAY?, and come up with
>> an updated patch for this change.
>> 
>> How do you think?
> 
> Yes, that sounds good.
Will do that.
> 
>>> 
>>>>> 
>>>>> 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?
>>> 
>>> I think the main information is preserved in diagnosing the flex vs.
>>> non-flex array assumption.
>> Yes. Agreed.
>> 
>>> 
>>>>> 
>>>>>>> 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? 
>>> 
>>> I think in similar situations we try to DWIM, but in some cases it will
>>> result in "garbage" behavior.  I don't think there's anything we can
>>> do here besides trying to diagnose such mismatches with -flto at the WPA
>>> stage.
>> 
>> Shall we issue warning for such mismatches? Where is the place I can add such warnings?
> 
> I'm not sure - we'd have to restrict it to "used" types and in principle
> only when actual objects pass from one TU to another with different
> flex-array semantics.  Otherwise we'll risk tons of diagnostics when
> people "forget" -fstrict-flex-arrays on some TUs but pull in common
> headers.
> 
> The C++ ODR diagnostics reside in ipa-devirt.cc, I'm not sure diagnostics
> on flex arrays would fit there.
> 
> I just wanted to bring this up, I do not have a good idea how or where
> to implement it.
Thanks for the information.  Will study this a little bit later.

Qing
> 
> Richard.
> 
>> thanks.
>> 
>> Qing
>>> 
>>> Richard.
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
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