[Bug c++/107462] New: Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()

[Bug c++/107462] New: Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()

[marko.makela at mariadb dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107462

            Bug ID: 107462
           Summary: Missed optimization of std::atomic::fetch_xxx "null
                    operations"  to std::atomic::load()
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marko.makela at mariadb dot com
  Target Milestone: ---

In g++-12 -c -O2 targeting x86-64, the fetch_xxx() variants are translated to
LOCK XADD or loops around LOCK CMPXCHG, instead of the simple MOV. I did not
test other targets.

#include <atomic>
uint32_t load(const std::atomic<uint32_t> &a)
{ return a.load(std::memory_order_relaxed); }
uint32_t load_add(std::atomic<uint32_t> &a)
{ return a.fetch_add(0, std::memory_order_relaxed); }
uint32_t load_sub(std::atomic<uint32_t> &a)
{ return a.fetch_sub(0, std::memory_order_relaxed); }
uint32_t load_or(std::atomic<uint32_t> &a)
{ return a.fetch_or(0, std::memory_order_relaxed); }
uint32_t load_xor(std::atomic<uint32_t> &a)
{ return a.fetch_xor(0, std::memory_order_relaxed); }
uint32_t load_and(std::atomic<uint32_t> &a)
{ return a.fetch_and(~uint32_t(0), std::memory_order_relaxed); }

clang++-15 would translate each function to the same x86-64 code:
   0:   8b 07                   mov    (%rdi),%eax
   2:   c3                      ret
When using the default std::memory_order_seq_cst, an MFENCE instruction would
be emitted before the MOV except in load().

[Bug tree-optimization/107462] Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107462

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |tree-optimization
           Keywords|                            |missed-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There might be a dup of this bug already but gcc does not try to optimize fetch
and add to just fetch if the operation was going to be a nop.

[Bug tree-optimization/107462] Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107462

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-05-19

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.