[Bug target/102566] New: [i386] GCC should emit LOCK BTS for simple bit-test-and-set operations with std::atomic

[thiago at kde dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102566

            Bug ID: 102566
           Summary: [i386] GCC should emit LOCK BTS for simple
                    bit-test-and-set operations with std::atomic
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Simple test:

$ cat test.cpp
#include <atomic>
bool tbit(std::atomic<int> &i)
{
    return i.fetch_or(1, std::memory_order_relaxed) & 1;
}

The sequence x.fetch_or(singlebit_constant) & singlebit_constant can be
implemented by a LOCK BTS sequence. The above should emit:

    lock bts $1, (%rdi)
    setb %al
    ret

But instead it emits a cmpxchg loop - see https://gcc.godbolt.org/z/99enKaffa.

This was found reviewing MariaDB lightweight-mutex code, which uses the sign
bit to indicate a contended mutex. See this commit[1] by one of their
maintainers for the removal of fetch_or because it emits an extra loop.

Bonus: LOCK BTR can be used in the sequence x.fetch_and(~single_bit_constant) &
single_bit_constant

[1]
https://github.com/dr-m/atomic_sync/commit/d5e22b2d42cdbac7a15d242bf1446377555c4041