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* [Bug c++/107462] New: Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()
@ 2022-10-30 9:29 marko.makela at mariadb dot com
2022-10-30 11:02 ` [Bug tree-optimization/107462] " pinskia at gcc dot gnu.org
2023-05-19 3:08 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: marko.makela at mariadb dot com @ 2022-10-30 9:29 UTC (permalink / raw)
To: gcc-bugs
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().
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/107462] Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()
2022-10-30 9:29 [Bug c++/107462] New: Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load() marko.makela at mariadb dot com
@ 2022-10-30 11:02 ` pinskia at gcc dot gnu.org
2023-05-19 3:08 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-10-30 11:02 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/107462] Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load()
2022-10-30 9:29 [Bug c++/107462] New: Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load() marko.makela at mariadb dot com
2022-10-30 11:02 ` [Bug tree-optimization/107462] " pinskia at gcc dot gnu.org
@ 2023-05-19 3:08 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-05-19 3:08 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-05-19 3:08 UTC | newest]
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2022-10-30 9:29 [Bug c++/107462] New: Missed optimization of std::atomic::fetch_xxx "null operations" to std::atomic::load() marko.makela at mariadb dot com
2022-10-30 11:02 ` [Bug tree-optimization/107462] " pinskia at gcc dot gnu.org
2023-05-19 3:08 ` pinskia at gcc dot gnu.org
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