Re: [committed] libstdc++: Disable over-zealous warnings about std::string copies [PR103332]

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Fri, 10 Dec 2021 at 17:17, Jakub Jelinek wrote:
>
> On Fri, Dec 10, 2021 at 10:11:04AM -0700, Martin Sebor via Gcc-patches wrote:
> > On 12/10/21 9:41 AM, Jakub Jelinek wrote:
> > > On Fri, Dec 10, 2021 at 09:35:50AM -0700, Martin Sebor via Gcc-patches wrote:
> > > > The above was just a quick proof of concept experiment.  You're
> > > > of course right that the final solution can't be so crude(*).
> > > > But if the required functions are always_inline (I think member
> > > > functions defined in the body of the class implicitly are
> > >
> > > They are not, and can't be, nothing says that such member functions
> > > can't use constructs that make it uninlinable (with always_inline
> > > that would be an error), or are way too large that inlining is not
> > > desirable, etc.  They are just implicitly inline.
> >
> > The functions we're talking about are the trivial max_size()
> > members of std::string and allocator traits.  They just return
> > a constant.
> >
> > But I see I was wrong, even member functions have to be explicitly
> > declared always_inline to be guaranteed to be inlined even at -O0.
> > I don't think that should be an issue for the trivial max_size()
> > (at least not for the std::string specialization).
>
> Note, if those functions are declared constexpr, without -fno-inline
> (default at -O0), then cp_fold will try to evaluate such calls to constant
> expressions already, effectively "inlining" them.

Every member function of std::string is constexpr in C++20, but not
before. But we could add constexpr to internal _M_xxx functions if
that benefits optimization.

For std::basic_string::max_size() it has to call another function
provided by the allocator, but for the std::string (i.e.
std::basic_string<char, char_traits<char>, allocator<char>>)
specialization we know what the allocator is going to tell us.

--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -1071,7 +1071,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
      _GLIBCXX20_CONSTEXPR
      size_type
      max_size() const _GLIBCXX_NOEXCEPT
-      { return (_Alloc_traits::max_size(_M_get_allocator()) - 1) / 2; }
+      {
+       if _GLIBCXX17_CONSTEXPR (__are_same<allocator_type, allocator<char>>)
+         return size_t(-1) / 2;
+       else
+         return (_Alloc_traits::max_size(_M_get_allocator()) - 1) / 2;
+      }

      /**
       *  @brief  Resizes the %string to the specified number of characters.