Re: C++2a synchronisation inefficient in GCC 11

[Thiago Macieira <thiago.macieira@intel.com>]

On Wednesday, 3 March 2021 06:30:20 PST Jonathan Wakely wrote:
> Yes, exactly. They could still do that if some additional option/macro
> was needed to get to those new facilities though.
> 
> My concern is that if we add a new "I know what I'm doing" option,
> everybody defines it (because that's what blog posts and youtube
> videos and conference talks tell them to do, because that gives them
> the new shiny things) and then it just becomes a default.
> 
> Maybe Qt wouldn't define it, but plenty would.

Well, if you define the "void warranty" option and you break things, you get 
to keep both pieces.

Another option is to fiddle with the include path. 

$ gcc -v -E -xc++ /dev/null
Using built-in specs.
[...]
#include <...> search starts here:
 /usr/lib64/gcc/x86_64-generic-linux/10/../../../../include/c++/10
 /usr/lib64/gcc/x86_64-generic-linux/10/../../../../include/c++/10/x86_64-
generic-linux
 /usr/lib64/gcc/x86_64-generic-linux/10/../../../../include/c++/10/backward
 /usr/lib64/gcc/x86_64-generic-linux/10/include
 /usr/local/include
 /usr/lib64/gcc/x86_64-generic-linux/10/include-fixed
 /usr/include

That "backward" has always got me thinking. What if we had a command-line 
option like -fexperimental-c++ option that added the $CXXROOT/experimental 
(complete with its bits/ and $ARCH subdirs)? That way, the regular 
atomic_base.h could do:

#if __cplusplus > 201703L && __has_include(<bits/atomic_wait.h>)

And the second conditional would fail unless the option is provided.

The advantage of a compiler option is that you need each DSO to consciously 
add something to their build systems and that can't be done by header-only 
libraries. That's like MSVC's /std:c++latest, which is documented by MS as 
"here there be dragons".

GCC can also generate some extra bit of assembly to mark the final executable 
as using experimental API, so distributors can easily tell that unfinished API 
was used. I've successfully done this for Qt using ELF versions.

$ rpm -qR kwin5 | grep PRIVATE
libQt5Core.so.5(Qt_5.15.2_PRIVATE_API)(64bit)
libQt5Gui.so.5(Qt_5.15.2_PRIVATE_API)(64bit)

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel DPG Cloud Engineering