[RFC] Add gnu::diagnose_as attribute

[Matthias Kretz]

Hi,

before requesting comments on the implementation of this patch, I'd like to 
know if there's interest in making use of it at all.

In a nutshell, this solves the problem of bad signal-to-noise ratio with TS 
implementations and potentially implementation-internal types.

I've been using the diagnose_as attribute on my parallelism_v2 simd 
implementation for two months now, and it significantly improved my 
productivity. Like this:

namespace std _GLIBCXX_VISIBILITY(default)
{
  _GLIBCXX_BEGIN_NAMESPACE_VERSION
  namespace experimental 
  {
    inline namespace parallelism_v2 [[__gnu__::__diagnose_as__("stdₓ")]] 
    {
      namespace simd_abi [[__gnu__::__diagnose_as__("simd_abi")]]
      {
        struct [[__gnu__::__diagnose_as__("scalar")]] _Scalar;
        using scalar = _Scalar;

        template <int _Np>
          struct [[__gnu__::__diagnose_as__("fixed_size")]] _Fixed;
        template <int _Np>
          using fixed_size = _Fixed<_Np>;
[...]

Effects:
1. __PRETTY_FUNCTION__ in my test output:

   PASS: void invoke_test(int) [with V = stdₓ::simd<double, 
simd_abi::_VecBuiltin<16> >; <template-parameter-1-2> = stdₓ::simd<double, 
simd_abi::_VecBuiltin<16> >]

   instead of 
  
   PASS: void invoke_test(int) [with V = 
std::experimental::parallelism_v2::simd<double, 
std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16> >; <template-
parameter-1-2> = std::experimental::parallelism_v2::simd<double, 
std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16> >]

2. Error messages:

   required from 'static constexpr stdₓ::_SimdWrapper<_Tp, _Np> 
stdₓ::_SimdImplBuiltin<_Abi, <template-parameter-1-2> 
>::_S_fma(stdₓ::_SimdWrapper<_Tp, _Np>, stdₓ::_SimdWrapper<_Tp, _Np>, 
stdₓ::_SimdWrapper<_Tp, _Np>) [with _Tp = double; long unsigned int _Np = 2; 
_Abi = simd_abi::_VecBuiltin<16>; <template-parameter-1-2> = 
stdₓ::__detail::_MachineFlagsTemplate<31, 9>]'

   instead of

   required from 'static constexpr 
std::experimental::parallelism_v2::_SimdWrapper<_Tp, _Np> 
std::experimental::parallelism_v2::_SimdImplBuiltin<_Abi, <template-
parameter-1-2> >::_S_fma(std::experimental::parallelism_v2::_SimdWrapper<_Tp, 
_Np>, std::experimental::parallelism_v2::_SimdWrapper<_Tp, _Np>, 
std::experimental::parallelism_v2::_SimdWrapper<_Tp, _Np>) [with _Tp = double; 
long unsigned int _Np = 2; _Abi = 
std::experimental::parallelism_v2::simd_abi::_VecBuiltin<16>; <template-
parameter-1-2> = 
std::experimental::parallelism_v2::__detail::_MachineFlagsTemplate<31, 9>]'


Since the main user of the attribute would be the standard library (I 
believe), this patch only makes sense to pursue if there's interest in using 
it in libstdc++. I'd like to use it for stdₓ::simd. Would you accept such a 
patch? I guess using unicode in the alias name would not be acceptable, 
though?



From: Matthias Kretz <kretz@kde.org>

This attribute overrides the diagnostics output string for the entity it
appertains to. The motivation is to improve QoI for library TS
implementations, where diagnostics have a very bad signal-to-noise ratio
due to the long namespaces involved.

gcc/ChangeLog:
	* doc/invoke.texi: Document -fno-diagnostics-use-aliases.

gcc/c-family/ChangeLog:
	* c.opt (fdiagnostics-use-aliases): New diagnostics flag.

gcc/cp/ChangeLog:
	* error.c (dump_scope): When printing the name of a namespace,
	look for the diagnose_as attribute. If found, print the
	associated string instead of calling dump_decl.
	(dump_aggr_type): If the type has a diagnose_as attribute, print
	the associated string instead of printing the original type
	name.
	* name-lookup.c (handle_namespace_attrs): Handle the diagnose_as
	attribute. Ensure exactly one string argument. Ensure previous
	diagnose_as attributes used the same name.
	* tree.c (cxx_attribute_table): Add diagnose_as attribute to the
	table.
	(check_diagnose_as_redeclaration): New function; copied and
	adjusted from check_abi_tag_redeclaration.
	(handle_diagnose_as_attribute): New function; copied and
	adjusted from handle_abi_tag_attribute.
---
 gcc/c-family/c.opt   |   4 ++
 gcc/cp/error.c       |  19 +++++++-
 gcc/cp/name-lookup.c |  27 ++++++++++
 gcc/cp/tree.c        | 114 +++++++++++++++++++++++++++++++++++++++++++
 gcc/doc/invoke.texi  |   9 +++-
 5 files changed, 170 insertions(+), 3 deletions(-)


--
──────────────────────────────────────────────────────────────────────────
 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
──────────────────────────────────────────────────────────────────────────
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