[PATCH v8 04/19] gas: implement parsing of object attributes v2

[Jan Beulich]

On 15.08.2025 11:51, Matthieu Longo wrote:
> On 2025-08-15 10:41, Jan Beulich wrote:
>> On 15.08.2025 11:40, Matthieu Longo wrote:
>>> On 2025-08-15 09:18, Jan Beulich wrote:
>>>> On 14.08.2025 18:41, Matthieu Longo wrote:
>>>>> On 2025-08-14 16:40, Jan Beulich wrote:
>>>>>> On 14.08.2025 17:29, Matthieu Longo wrote:
>>>>>>> On 2025-08-06 16:20, Richard Earnshaw wrote:
>>>>>>>> On 06/08/2025 16:05, Jan Beulich wrote:
>>>>>>>>> On 06.08.2025 16:02, Matthieu Longo wrote:
>>>>>>>>>> On 2025-07-31 15:40, Jan Beulich wrote:
>>>>>>>>>>> On 15.07.2025 13:39, Matthieu Longo wrote:
>>>>>>>>>>>> The usage of those directives will error if the following requirements
>>>>>>>>>>>> are breached:
>>>>>>>>>>>> - If the subsection X has been previously declared, the
>>>>>>>>>>>> comprehension and
>>>>>>>>>>>>        encoding parameters of the current .aeabi_subsection that
>>>>>>>>>>>> redeclares X
>>>>>>>>>>>>        have to match with the previous declaration.
>>>>>>>>>>>
>>>>>>>>>>> If I'm reading things right, such a re-declaration still needs to
>>>>>>>>>>> provide
>>>>>>>>>>> not only the name, but also "comprehension" and encoding. Just for
>>>>>>>>>>> it to
>>>>>>>>>>> be checked against the earlier decl. As with ordinary sections, I think
>>>>>>>>>>> it would be nice if an "incomplete" re-decl would be accepted.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Unlike code where there is a genuine usecase of switching to and from
>>>>>>>>>> sections, all the build attributes are in a file scope so there is not
>>>>>>>>>> really much need to split them up. For instance, GCC on AArch64 targets
>>>>>>>>>> will dump them all in one chunk.
>>>>>>>>>> No strong opinion against the idea, so I don't mind adding it if you
>>>>>>>>>> think that it would be valuable for users.
>>>>>>>>>>
>>>>>>>>>> NB: This would require amending the spec, and discussing it with LLVM.
>>>>>>>>>
>>>>>>>>> Well, I don't know how things will typically look in assembly files not
>>>>>>>>> produced by a compiler or alike. If "re-declarations" aren't expected to
>>>>>>>>> be common, perhaps leave as is. Question then is whether to permit
>>>>>>>>> "re-declarations" at all; disallowing them would simplify a few things.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I wouldn't expect a compiler to need to do redeclarations, but I could
>>>>>>>> envisage a use case for this when using include files to build up a
>>>>>>>> file.  In that case, I'd expect the attributes to be stand-alone and add
>>>>>>>> the attributes specific to each hunk.  If you wanted to have it work
>>>>>>>> where only the first specification of a section defined the flags, you'd
>>>>>>>> end up either with a pre-declaration of the section, or having some
>>>>>>>> weird constraint on include order.
>>>>>>>>
>>>>>>>> So I certainly think we need checking of the headers, but perhaps we'd
>>>>>>>> also want checking of the added attributes as well (unless gas were to
>>>>>>>> be able to merge re-declarations of a single attribute): for example,
>>>>>>>> changing an attribute value from 3 to 2 might be completely incorrect,
>>>>>>>> but keeping 3 might not be correct either - it depends very much on the
>>>>>>>> context.
>>>>>>>
>>>>>>> With the current implementation, gas raises an error if an attribute is
>>>>>>> redeclared with a different value.
>>>>>>> Merging would be the ideal, but since there is no use case for it now,
>>>>>>> could we delay the merge implementation to when a real use case will
>>>>>>> emerge ?
>>>>>>
>>>>>> I guess that's going to be okay. I'd like to ask though that you mention
>>>>>> the aspect in the description then.
>>>>>
>>>>> It is already here in the description.
>>>>>
>>>>> The usage of those directives will error if the following requirements
>>>>>     are breached:
>>>>> - If the subsection X has been previously declared, the comprehension
>>>>> and encoding parameters of the current .aeabi_subsection that redeclares
>>>>> X have to match with the previous declaration.
>>>>> - The type of the value set via .aeabi_attribute has to align with the
>>>>> current subsection.
>>>>> - If the tag N has already been declared for the current subsection, a
>>>>> later assignment to tag N is tolerated only if the newly set value is
>>>>> equal to the former one.
>>>>>
>>>>> I guess that the issue is the density and length of the description.
>>>>
>>>> No, the issue is that what I'm asking for isn't there: I'm after you
>>>> mentioning that these strict requirements could in principle be relaxed,
>>>> or in other words, that the checking right now is deliberately more
>>>> strict than absolutely necessary.
>>>
>>> Thanks for the clarification.
>>>
>>> Is this description better ?
>>>
>>> If the tag N has already been declared for the current subsection, a
>>> later assignment to tag N is tolerated only if the newly set value is
>>> equal to the former one. This check is stricter than needed. Ideally,
>>> the tag N's values should be merged together, and an error should be
>>> raised only if an incompatibility is detected. Because the attributes
>>> are set in one chunk by GCC, there is no real use case for such a merge.
>>
>> That reads okay to me, yes. Something similar then for subsections?
> 
> For subsections, no. The parameters have to match as soon as the name 
> matches, so the check need to be strict and no merge is possible.

Yet my earlier comment were primarily on subsections, comparing the
situation with the re-declaration of (ordinary) sections. Why is it that,
once a subsection was declared, it can't be re-entered by just providing
its name, if the other attributes have to match anyway?

Jan