https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99277
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 will
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
