Re: [PATCH, libstdc++] Improve slightly __cxa_guard_acquire

[Benjamin De Kosnik <bkoz at redhat dot com>]

Here's the patch as applied to trunk in rev. 191042. I'll apply it to
4.7 this weekend as long as nobody yelps.

-benjamin

2012-09-06  Thiago Macieira  <thiago.macieira@intel.com>

	PR libstdc++/54172
        * libsupc++/guard.cc (__cxa_guard_acquire): Exit the loop earlier if
        we detect that another thread has had success. Don't compare_exchange
        from a finished state back to a waiting state. Comment.

diff --git a/libstdc++-v3/libsupc++/guard.cc b/libstdc++-v3/libsupc++/guard.cc
index adc9608..60165cd 100644
--- a/libstdc++-v3/libsupc++/guard.cc
+++ b/libstdc++-v3/libsupc++/guard.cc
@@ -244,13 +244,13 @@ namespace __cxxabiv1
     if (__gthread_active_p ())
       {
 	int *gi = (int *) (void *) g;
-	int expected(0);
 	const int guard_bit = _GLIBCXX_GUARD_BIT;
 	const int pending_bit = _GLIBCXX_GUARD_PENDING_BIT;
 	const int waiting_bit = _GLIBCXX_GUARD_WAITING_BIT;
 
 	while (1)
 	  {
+	    int expected(0);
 	    if (__atomic_compare_exchange_n(gi, &expected, pending_bit, false,
 					    __ATOMIC_ACQ_REL,
 					    __ATOMIC_RELAXED))
@@ -264,13 +264,26 @@ namespace __cxxabiv1
 		// Already initialized.
 		return 0;	
 	      }
+
 	     if (expected == pending_bit)
 	       {
+		 // Use acquire here.
 		 int newv = expected | waiting_bit;
 		 if (!__atomic_compare_exchange_n(gi, &expected, newv, false,
 						  __ATOMIC_ACQ_REL, 
-						  __ATOMIC_RELAXED))
-		   continue;
+						  __ATOMIC_ACQUIRE))
+		   {
+		     if (expected == guard_bit)
+		       {
+			 // Make a thread that failed to set the
+			 // waiting bit exit the function earlier,
+			 // if it detects that another thread has
+			 // successfully finished initialising.
+			 return 0;
+		       }
+		     if (expected == 0)
+		       continue;
+		   }
 		 
 		 expected = newv;
 	       }