From: Thomas Rodgers <trodgers@redhat.com>
Date: Wed, 6 Jul 2022 00:42:42 +0000 (-0700)
Subject: libstdc++: Minor codegen improvement for atomic wait spinloop
X-Git-Tag: basepoints/gcc-14~5358
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e75da2ace6b6f634237259ef62cfb2d3d34adb10;p=thirdparty%2Fgcc.git

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.
---

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())