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From: Thomas Rodgers <rodgertq@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-8622] libstdc++: Minor codegen improvement for atomic wait spinloop Date: Tue, 26 Jul 2022 22:33:28 +0000 (GMT) [thread overview] Message-ID: <20220726223328.147F93858412@sourceware.org> (raw) https://gcc.gnu.org/g:f281d9dd1ed63825efa234df63d3f2ae6cc9022a commit r12-8622-gf281d9dd1ed63825efa234df63d3f2ae6cc9022a Author: Thomas Rodgers <trodgers@redhat.com> Date: Tue Jul 5 17:42:42 2022 -0700 libstdc++: Minor codegen improvement for atomic wait spinloop This patch merges the spin loops in the atomic wait implementation which is a minor codegen improvement. libstdc++-v3/ChangeLog: * include/bits/atomic_wait.h (__atomic_spin): Merge spin loops. (cherry picked from commit e75da2ace6b6f634237259ef62cfb2d3d34adb10) Diff: --- libstdc++-v3/include/bits/atomic_wait.h | 17 +++++++---------- 1 file changed, 7 insertions(+), 10 deletions(-) diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h index 125b1cad886..a6d55d3af8a 100644 --- a/libstdc++-v3/include/bits/atomic_wait.h +++ b/libstdc++-v3/include/bits/atomic_wait.h @@ -142,8 +142,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION #endif } - constexpr auto __atomic_spin_count_1 = 12; - constexpr auto __atomic_spin_count_2 = 4; + constexpr auto __atomic_spin_count_relax = 12; + constexpr auto __atomic_spin_count = 16; struct __default_spin_policy { @@ -157,18 +157,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION bool __atomic_spin(_Pred& __pred, _Spin __spin = _Spin{ }) noexcept { - for (auto __i = 0; __i < __atomic_spin_count_1; ++__i) + for (auto __i = 0; __i < __atomic_spin_count; ++__i) { if (__pred()) return true; - __detail::__thread_relax(); - } - for (auto __i = 0; __i < __atomic_spin_count_2; ++__i) - { - if (__pred()) - return true; - __detail::__thread_yield(); + if (__i < __atomic_spin_count_relax) + __detail::__thread_relax(); + else + __detail::__thread_yield(); } while (__spin())
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