* [committed] libstdc++: Micro-optimization for std::optional [PR112480]
@ 2023-11-13 23:22 Jonathan Wakely
0 siblings, 0 replies; only message in thread
From: Jonathan Wakely @ 2023-11-13 23:22 UTC (permalink / raw)
To: libstdc++, gcc-patches
Tested x86_64-linux. Pushed to trunk.
-- >8 --
This small change removes a branch when clearing a std::optional<T> for
types with no-op destructors. For types where the destructor can be
optimized away (e.g. because it's trivial, or empty and can be inlined)
the _M_destroy() function does nothing but set _M_engaged to false.
Setting _M_engaged=false unconditionally is cheaper than only doing it
when initially true, because it allows the compiler to remove a branch.
The compiler thinks it would be incorrect to unconditionally introduce a
store there, because it could conflict with reads in other threads, so
it won't do that optimization itself. We know it's safe to do because
we're in a non-const member function, so the standard forbids any
potentially concurrent calls to other member functions of the same
object. Making the store unconditional can't create a data race that
isn't already present in the program.
libstdc++-v3/ChangeLog:
PR libstdc++/112480
* include/std/optional (_Optional_payload_base::_M_reset): Set
_M_engaged to false unconditionally.
---
libstdc++-v3/include/std/optional | 2 ++
1 file changed, 2 insertions(+)
diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index 53450c760d9..a8c97717b72 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -311,6 +311,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
if (this->_M_engaged)
_M_destroy();
+ else // This seems redundant but improves codegen, see PR 112480.
+ this->_M_engaged = false;
}
};
--
2.41.0
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2023-11-13 23:23 UTC | newest]
Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-11-13 23:22 [committed] libstdc++: Micro-optimization for std::optional [PR112480] Jonathan Wakely
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).