Re: [PATCH] Add gnu::diagnose_as attribute

[Jason Merrill <jason@redhat.com>]

On 6/15/21 4:56 PM, Matthias Kretz wrote:
> On Tuesday, 15 June 2021 17:51:20 CEST Jason Merrill wrote:
>> On 6/11/21 6:01 AM, Matthias Kretz wrote:
>>> For reference I'll attach my stdx::simd diagnose_as patch.
>>>
>>> We could also talk about extending the feature to provide more information
>>> about the diagnose_as substition. E.g. print a list of all diagnose_as
>>> substitutions, which were used, at the end of the output stream. Or
>>> simpler, print "note: some identifiers were simplified, use
>>> -fno-diagnostics-use- aliases to see their real names".
>>
>> Or perhaps before the first use of a name that doesn't correspond to a
>> source-level name.
> 
> Right. I guess that would be even easier to implement than printing it at the
> end.
> 
>>> -struct _Scalar;
>>> +  struct [[__gnu__::__diagnose_as__("scalar")]] _Scalar;
>>>
>>>   template <int _Np>
>>>
>>> -  struct _Fixed;
>>> +  struct [[__gnu__::__diagnose_as__("fixed_size")]] _Fixed;
>>
>> Thes two could be the variant of the attribute without an explicit
>> string, attached to the alias-declaration.
> 
> Agreed. (since you don't have implementation concerns...)
> 
>>> +using __sse [[__gnu__::__diagnose_as__("[SSE]")]] = _VecBuiltin<16>;
>>> +using __avx [[__gnu__::__diagnose_as__("[AVX]")]] = _VecBuiltin<32>;
>>> +using __avx512 [[__gnu__::__diagnose_as__("[AVX512]")]] =
>>> _VecBltnBtmsk<64>; +  using __odr_helper [[__gnu__::__diagnose_as__("[ODR
>>> helper]")]]
>> These [] names seem like minimal improvements over the __ names that you
>> would get from the attribute without an explicit string.
> 
> Right. It would, however, give the user an identifier that I don't want them
> to use in their code. We could argue "it has a double-underscore and it's not
> a documented implementation-defined type, so you're shooting yourself in the
> foot". Or we could just avoid the issue altogether. I agree this is not a huge
> issue.
> 
>>> +	inline namespace parallelism_v2
>>> [[__gnu__::__diagnose_as__("std\u2093")]] {
>> This could go on std::experimental itself, along with my proposed change
>> to hide inline namespaces by default (with a note similar to the one above).
> 
> Yes, with the following consequences:
> 
> * If only the std::experimental::parallelism_v2::simd headers set the
> diagnose_as attribute on std::experimental, the #inclusion of <experimental/
> simd> changes the diagnostics of all other TS implementations.
> 
> * If all TS implementations set the diagnose_as attribute, then it's basically
> impossible to go back to the long and scary name. Which is what we really
> should do as soon as there's both a std::simd and a stdₓ::simd. Attaching the
> diagnose_as attribute to the inline namespace allows for better granularity,
> even if it's maybe not good enough for some TSs.
> 
> * If `namespace std { namespace experimental [[gnu::diagnose_as("foo")]] {`
> turns the scope into 'foo::' and not 'std::foo::' (not sure what you intended)
> then I could still attach the attribute to the inline namespace.
> 
> 
> So, yes, I could improve stdx::simd with what you propose. IMHO it wouldn't be
> as good as what I can do with the patch at hand, though.
> 
> IIUC, your main concern is that my proposed diagnose_as *can* be used to make
> diagnostics worse, by replacing names with strings that are not valid
> identifiers. Of course, whoever uses the attribute to that effect should have
> a good reason to do so. Is your other concern that using the attribute in a
> "good" way is repetitive? Would you be happier if I make the string argument
> to the attribute optional for type aliases?

Yes, and namespace aliases.

Jason