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 <[email protected]>
> >
> > 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);
+ }
+ }
+ }
--
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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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