Re: [PATCH] Add gnu::diagnose_as attribute

[Matthias Kretz <m.kretz@gsi.de>]

On Thursday, 27 May 2021 19:39:48 CEST Jason Merrill wrote:
> On 5/4/21 7:13 AM, Matthias Kretz wrote:
> > 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.
> > 
> > On Tuesday, 27 April 2021 11:46:48 CEST Jonathan Wakely wrote:
> >> I think it's a great idea and would like to use it for all the TS
> >> implementations where there is some inline namespace that the user
> >> doesn't care about. std::experimental::fundamentals_v1:: would be much
> >> better as just std::experimental::, or something like std::[LFTS]::.
> 
> Hmm, how much of the benefit could we get from a flag (probably on by
> default) to skip inline namespaces in diagnostics?

I'd say about 20% for the TS's. Even std::experimental::simd (i.e. without the 
'::parallelism_v2' part) is still rather noisy. I want stdₓ::simd, std-x::simd 
or std::[PTS2]::simd or whatever shortest shorthand Jonathan will allow. ;)

For PR89370, the benefit would be ~2%:

'template<class _Tp> std::__cxx11::basic_string<_CharT, _Traits, 
_Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> 
std::__cxx11::basic_string<_CharT, _Traits, 
_Alloc>::insert(std::__cxx11::basic_string<_CharT, _Traits, 
_Alloc>::size_type, const _Tp&, std::__cxx11::basic_string<_CharT, _Traits, 
_Alloc>::size_type, std::__cxx11::basic_string<_CharT, _Traits, 
_Alloc>::size_type) [with _Tp = _Tp; _CharT = char; _Traits = 
std::char_traits<char>; _Alloc = std::allocator<char>]'

would then only turn into:

'template<class _Tp> std::basic_string<_CharT, _Traits, 
_Alloc>::_If_sv<_Tp, std::basic_string<_CharT, _Traits, _Alloc>&> 
std::basic_string<_CharT, _Traits, 
_Alloc>::insert(std::basic_string<_CharT, _Traits, 
_Alloc>::size_type, const _Tp&, std::basic_string<_CharT, _Traits, 
_Alloc>::size_type, std::basic_string<_CharT, _Traits, 
_Alloc>::size_type) [with _Tp = _Tp; _CharT = char; _Traits = 
std::char_traits<char>; _Alloc = std::allocator<char>]'

instead of:

'template<class _Tp> std::string::_If_sv<_Tp, std::string&> 
std::string::insert<_Tp>(std::string::size_type, const _Tp&, 
std::string::size_type, std::string::size_type)'


Also hiding all inline namespace by default might make some error messages 
harder to understand:

namespace Vir {
  inline namespace foo {
    struct A {};
  }
  struct A {};
}
using Vir::A;


<source>:7:12: error: reference to 'A' is ambiguous
<source>:3:12: note: candidates are: 'struct Vir::A'
<source>:5:10: note:                 'struct Vir::A'

> > With the attribute, it is possible to solve PR89370 and make
> > std::__cxx11::basic_string<_CharT, _Traits, _Alloc> appear as
> > std::string in diagnostic output without extra hacks to recognize the
> > type.
> 
> That sounds wrong to me; std::string is the <char> instantiation, not
> the template.  Your patch doesn't make it possible to apply this
> attribute to class template instantiations, does it?

Yes, it does.

Initially, when I tried to improve the TS experience, it didn't. When Jonathan 
showed PR89370 to me I tried to make [[gnu::diagnose_as]] more generic & 
useful. Since there's no obvious syntax to apply an attribute to a template 
instantiation, I had to be creative. This is from my pending std::string 
patch:

--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -299,7 +299,8 @@ namespace std
 #if _GLIBCXX_USE_CXX11_ABI
 namespace std
 {
-  inline namespace __cxx11 __attribute__((__abi_tag__ ("cxx11"))) { }
+  inline namespace __cxx11
+    __attribute__((__abi_tag__ ("cxx11"), __diagnose_as__("std"))) { }
 }
 namespace __gnu_cxx
 {
--- a/libstdc++-v3/include/bits/stringfwd.h
+++ b/libstdc++-v3/include/bits/stringfwd.h
@@ -76,24 +76,24 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 _GLIBCXX_END_NAMESPACE_CXX11
 
   /// A string of @c char
-  typedef basic_string<char>    string;   
+  typedef basic_string<char> string [[__gnu__::__diagnose_as__("string")]];
 
 #ifdef _GLIBCXX_USE_WCHAR_T
   /// A string of @c wchar_t
-  typedef basic_string<wchar_t> wstring;   
+  typedef basic_string<wchar_t> wstring
     [[__gnu__::__diagnose_as__("wstring")]];
 #endif
[...]

The part of my frontend patch that makes this work is in 
handle_diagnose_as_attribute:

+  if (TREE_CODE (*node) == TYPE_DECL)
+    {
+      // Apply the attribute to the type alias itself.
+      decl = *node;
+      tree type = TREE_TYPE (*node);
+      if (CLASS_TYPE_P (type) && CLASSTYPE_TEMPLATE_INSTANTIATION (type))
+	{
+	  if (COMPLETE_OR_OPEN_TYPE_P (type))
+	    warning (OPT_Wattributes,
+		     "ignoring %qE attribute applied to template %qT after "
+		     "instantiation", name, type);
+	  else
+	    {
+	      type = strip_typedefs(type, nullptr, 0);
+	      // And apply the attribute to the specialization on the RHS.
+	      tree attributes = tree_cons (name, args, NULL_TREE);
+	      decl_attributes (&type, attributes,
+			       ATTR_FLAG_TYPE_IN_PLACE, NULL_TREE);
+	    }
+	}
+    }

-- 
──────────────────────────────────────────────────────────────────────────
 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
──────────────────────────────────────────────────────────────────────────