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* [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member
@ 2023-03-31 14:01 barry.revzin at gmail dot com
2023-03-31 14:38 ` [Bug c++/109362] " barry.revzin at gmail dot com
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: barry.revzin at gmail dot com @ 2023-03-31 14:01 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
Bug ID: 109362
Summary: codegen adds unnecessary extra add when reading atomic
member
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
This program:
#include <atomic>
struct S {
long size;
std::atomic<char*> read_ptr;
auto peek() const -> const char* {
return read_ptr.load(std::memory_order_acquire);
}
};
auto with_atomic(S const& v) {
while (true) {
if (v.peek()) {
return true;
}
}
}
emits (on gcc 12.2 -O3):
with_atomic(S const&):
add rdi, 8
.L2:
mov rax, QWORD PTR [rdi]
test rax, rax
je .L2
mov eax, 1
ret
But that add is completely necessary, the mov could just be:
mov rax, QWORD PTR [rdi + 8]
which is what clang (16.0 -O3) generates:
with_atomic(S const&): # @with_atomic(S const&)
.LBB0_1: # =>This Inner Loop Header: Depth=1
mov rax, qword ptr [rdi + 8]
test rax, rax
je .LBB0_1
mov al, 1
ret
It's not just an extra add, it's consuming an extra register - which has more
downstream optimization effects.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109362] codegen adds unnecessary extra add when reading atomic member
2023-03-31 14:01 [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member barry.revzin at gmail dot com
@ 2023-03-31 14:38 ` barry.revzin at gmail dot com
2023-03-31 15:26 ` jakub at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: barry.revzin at gmail dot com @ 2023-03-31 14:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
--- Comment #1 from Barry Revzin <barry.revzin at gmail dot com> ---
Sorry, in this reduced example, it doesn't actually consume an extra register -
only rdi is used.
In this slightly less reduced example:
#include <atomic>
struct S {
std::atomic<long> size;
std::atomic<char*> read_ptr;
auto peek() const -> const char* {
auto const s = size.load(std::memory_order_acquire);
return read_ptr.load(std::memory_order_acquire);
}
};
auto with_atomic(S const& v) {
while (true) {
if (v.peek()) {
return true;
}
}
}
the codegen is:
with_atomic(WithAtomic const&):
lea rdx, [rdi+8]
.L2:
mov rax, QWORD PTR [rdi]
mov rax, QWORD PTR [rdx]
test rax, rax
je .L2
mov eax, 1
ret
which now does consume rdx unnecessarily.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109362] codegen adds unnecessary extra add when reading atomic member
2023-03-31 14:01 [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member barry.revzin at gmail dot com
2023-03-31 14:38 ` [Bug c++/109362] " barry.revzin at gmail dot com
@ 2023-03-31 15:26 ` jakub at gcc dot gnu.org
2023-03-31 15:27 ` jakub at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2023-03-31 15:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |FIXED
CC| |jakub at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This got fixed with r13-137-gee1cb43bc76de800efa0ade6 on the trunk.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109362] codegen adds unnecessary extra add when reading atomic member
2023-03-31 14:01 [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member barry.revzin at gmail dot com
2023-03-31 14:38 ` [Bug c++/109362] " barry.revzin at gmail dot com
2023-03-31 15:26 ` jakub at gcc dot gnu.org
@ 2023-03-31 15:27 ` jakub at gcc dot gnu.org
2023-03-31 15:29 ` barry.revzin at gmail dot com
2023-04-01 7:01 ` [Bug tree-optimization/109362] " cvs-commit at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2023-03-31 15:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Reduced C testcase would be
struct S { long a, b; };
int
foo (struct S *v)
{
while (1)
{
__atomic_load_n (&v->a, __ATOMIC_ACQUIRE);
if (__atomic_load_n (&v->b, __ATOMIC_ACQUIRE))
return 1;
}
}
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug c++/109362] codegen adds unnecessary extra add when reading atomic member
2023-03-31 14:01 [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member barry.revzin at gmail dot com
` (2 preceding siblings ...)
2023-03-31 15:27 ` jakub at gcc dot gnu.org
@ 2023-03-31 15:29 ` barry.revzin at gmail dot com
2023-04-01 7:01 ` [Bug tree-optimization/109362] " cvs-commit at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: barry.revzin at gmail dot com @ 2023-03-31 15:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
--- Comment #4 from Barry Revzin <barry.revzin at gmail dot com> ---
Awesome!
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/109362] codegen adds unnecessary extra add when reading atomic member
2023-03-31 14:01 [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member barry.revzin at gmail dot com
` (3 preceding siblings ...)
2023-03-31 15:29 ` barry.revzin at gmail dot com
@ 2023-04-01 7:01 ` cvs-commit at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2023-04-01 7:01 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109362
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:87d3bc53b177037699f7f8dda3a3d17e647c459d
commit r13-6966-g87d3bc53b177037699f7f8dda3a3d17e647c459d
Author: Jakub Jelinek <jakub@redhat.com>
Date: Sat Apr 1 09:00:22 2023 +0200
testsuite: Add testcase for already fixed PR [PR109362]
This PR got fixed with r13-137.
Add a testcase to make sure it doesn't reappear.
2023-04-01 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/109362
* gcc.target/i386/pr109362.c: New test.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-04-01 7:01 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-03-31 14:01 [Bug c++/109362] New: codegen adds unnecessary extra add when reading atomic member barry.revzin at gmail dot com
2023-03-31 14:38 ` [Bug c++/109362] " barry.revzin at gmail dot com
2023-03-31 15:26 ` jakub at gcc dot gnu.org
2023-03-31 15:27 ` jakub at gcc dot gnu.org
2023-03-31 15:29 ` barry.revzin at gmail dot com
2023-04-01 7:01 ` [Bug tree-optimization/109362] " cvs-commit at gcc dot gnu.org
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