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From: "thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99277] New: C++2a synchronisation is inefficient in
 GCC 11 and can't be fixed later
Date: Thu, 25 Feb 2021 23:27:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99277

            Bug ID: 99277
           Summary: C++2a synchronisation is inefficient in GCC 11 and
                    can't be fixed later
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Requesting that this be treated as a blocker for GCC 11.

Detailed post:
https://gcc.gnu.org/pipermail/libstdc++/2021-February/052043.html

Summary:
Since everything in this implementation is inline and, once released, it wi=
ll
tie our hands until the next ABI break (libstdc++.so.7). And no, inline
namespaces and ABI tags are no different than ABI breaks, they only make the
ABI break more or less silent in the process.

Here's a summary of the findings:

 1) everything is inline
 2) futex code is still behind a lot of code calling into std::_Hash_bytes
 3) other int-sized (incl. enums) atomics don't use futex
 4) std::latch and std::counting_semaphore defaults preclude from using
    futex on Linux
 5) std::barrier implementation also uses a type that futex(2) can't handle=