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* [Bug libstdc++/101406] New: shared_ptr in _S_atomic mode still uses __atomic_add_dispatch()
@ 2021-07-10 20:51 marc.mutz at kdab dot com
2021-07-11 8:52 ` [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation marc.mutz at kdab dot com
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: marc.mutz at kdab dot com @ 2021-07-10 20:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101406
Bug ID: 101406
Summary: shared_ptr in _S_atomic mode still uses
__atomic_add_dispatch()
Product: gcc
Version: 11.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: marc.mutz at kdab dot com
Target Milestone: ---
Consider
// https://godbolt.org/z/efTW6MoEh
void test_copy(const std::shared_ptr<int> &sp)
{ auto copy = sp; }
vs.
// https://godbolt.org/z/3aoGq1f9P
void test_copy(const boost::shared_ptr<int> &sp)
{ auto copy = sp; }
In the first cast, over 70 lines of assembler are emitted, in the second,
around 30. This seems to be in large part because in
_Sp_counted_base::_M_add_ref_copy(), you're using __atomic_add_dispatch() even
if _Lp is _S_atomic. It seems to me that a specialisation of this function
template for _S_atomic calling just __atomic_add() is missing:
https://godbolt.org/z/crPz9hGe7
Probably same for the deref case, too.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation
2021-07-10 20:51 [Bug libstdc++/101406] New: shared_ptr in _S_atomic mode still uses __atomic_add_dispatch() marc.mutz at kdab dot com
@ 2021-07-11 8:52 ` marc.mutz at kdab dot com
2021-07-12 10:44 ` redi at gcc dot gnu.org
2021-07-12 14:21 ` marc.mutz at kdab dot com
2 siblings, 0 replies; 4+ messages in thread
From: marc.mutz at kdab dot com @ 2021-07-11 8:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101406
--- Comment #1 from Marc Mutz <marc.mutz at kdab dot com> ---
Comparison to the other two major standard library implementations:
https://godbolt.org/z/crPo44rxo
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation
2021-07-10 20:51 [Bug libstdc++/101406] New: shared_ptr in _S_atomic mode still uses __atomic_add_dispatch() marc.mutz at kdab dot com
2021-07-11 8:52 ` [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation marc.mutz at kdab dot com
@ 2021-07-12 10:44 ` redi at gcc dot gnu.org
2021-07-12 14:21 ` marc.mutz at kdab dot com
2 siblings, 0 replies; 4+ messages in thread
From: redi at gcc dot gnu.org @ 2021-07-12 10:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101406
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |WONTFIX
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Marc Mutz from comment #0)
> Consider
>
> // https://godbolt.org/z/efTW6MoEh
N.B. -DNDEBUG has no effect on anything in libstdc++, it's not allowed to use
<assert.h> anywhere.
> void test_copy(const std::shared_ptr<int> &sp)
> { auto copy = sp; }
>
> vs.
>
> // https://godbolt.org/z/3aoGq1f9P
> void test_copy(const boost::shared_ptr<int> &sp)
> { auto copy = sp; }
>
> In the first cast, over 70 lines of assembler are emitted, in the second,
> around 30. This seems to be in large part because in
> _Sp_counted_base::_M_add_ref_copy(), you're using __atomic_add_dispatch()
> even if _Lp is _S_atomic.
That's by design:
https://gcc.gnu.org/onlinedocs/libstdc++/manual/memory.html#shared_ptr.policy
"For all three policies, reference count increments and decrements are done via
the functions in ext/atomicity.h, which detect if the program is
multi-threaded. If only one thread of execution exists in the program then less
expensive non-atomic operations are used."
> It seems to me that a specialisation of this
> function template for _S_atomic calling just __atomic_add() is missing:
No, the _S_atomic policy doesn't mean "use atomics unconditionally". We still
go through __atomic_add_dispatch so that we avoid the atomics in
single-threaded programs.
With a new glibc the check for multiple threads is cheaper and should optimize
better.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation
2021-07-10 20:51 [Bug libstdc++/101406] New: shared_ptr in _S_atomic mode still uses __atomic_add_dispatch() marc.mutz at kdab dot com
2021-07-11 8:52 ` [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation marc.mutz at kdab dot com
2021-07-12 10:44 ` redi at gcc dot gnu.org
@ 2021-07-12 14:21 ` marc.mutz at kdab dot com
2 siblings, 0 replies; 4+ messages in thread
From: marc.mutz at kdab dot com @ 2021-07-12 14:21 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101406
Marc Mutz <marc.mutz at kdab dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|WONTFIX |FIXED
--- Comment #3 from Marc Mutz <marc.mutz at kdab dot com> ---
Ok, it doesn't seem to affect execution speed much, but oh my, the code size :/
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-07-12 14:21 UTC | newest]
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2021-07-10 20:51 [Bug libstdc++/101406] New: shared_ptr in _S_atomic mode still uses __atomic_add_dispatch() marc.mutz at kdab dot com
2021-07-11 8:52 ` [Bug libstdc++/101406] poor codegen for shared_ptr copy & destory, compared to boost::shared_ptr and libc++'s implementation marc.mutz at kdab dot com
2021-07-12 10:44 ` redi at gcc dot gnu.org
2021-07-12 14:21 ` marc.mutz at kdab dot com
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