https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89714
Bug ID: 89714
Summary: allow for overriding the thread-detection mechanism
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgcc
Assignee: unassigned at gcc dot gnu.org
Reporter: tneumann at users dot sourceforge.net
Target Milestone: ---
Currently the libgcc code uses a weak symbol to recognize if the program is
linked again pthreads (in __gthread_active_p()), and switches between
implementations at runtime based the presence of the weak symbol.
But if we know statically that we will always run multi-threaded, this dynamic
dispatch is quite wasteful.
This is particular visible for std C++ classes that indirectly inline this
code, for example std::shared_ptr. When copying a std::shared_ptr this check is
performed twice, each time with two code paths.
If we force __gthread_active_p to always return true the assembler code is
quite a bit shorter (and has less branches and executes less instructions, of
course).
This can be seen with the quick hack here
https://godbolt.org/z/946lzO
Changing the first line to #if 1 reduces the generated code significantly.
It would be good to offer a similar mechanism without having to resort to
horrible hacks.
