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

[Jan Beulich]

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?

Jan