[Openmp-commits] [openmp] r274369 - Improve performance of #pragma omp single

[Jonathan Peyton via Openmp-commits]

Author: jlpeyton
Date: Fri Jul  1 12:37:49 2016
New Revision: 274369

URL: http://llvm.org/viewvc/llvm-project?rev=274369&view=rev
Log:
Improve performance of #pragma omp single

EPCC Performance of single is considerably worse than plain barrier.
Adding a read-only check to the code before the atomic compare-and-store
helps considerably.

Patch by Terry Wilmarth.

Differential Revision: http://reviews.llvm.org/D21893

Modified:
    openmp/trunk/runtime/src/kmp_runtime.c

Modified: openmp/trunk/runtime/src/kmp_runtime.c
URL: http://llvm.org/viewvc/llvm-project/openmp/trunk/runtime/src/kmp_runtime.c?rev=274369&r1=274368&r2=274369&view=diff
==============================================================================
--- openmp/trunk/runtime/src/kmp_runtime.c (original)
+++ openmp/trunk/runtime/src/kmp_runtime.c Fri Jul  1 12:37:49 2016
@@ -777,8 +777,10 @@ __kmp_enter_single( int gtid, ident_t *i
            single block
         */
         /* TODO: Should this be acquire or release? */
-        status = KMP_COMPARE_AND_STORE_ACQ32(&team->t.t_construct, old_this,
-                                             th->th.th_local.this_construct);
+        if (team->t.t_construct == old_this) {
+            status = KMP_COMPARE_AND_STORE_ACQ32(&team->t.t_construct, old_this,
+                                                 th->th.th_local.this_construct);
+        }
 #if USE_ITT_BUILD
         if ( __itt_metadata_add_ptr && __kmp_forkjoin_frames_mode == 3 && KMP_MASTER_GTID(gtid) &&
 #if OMP_40_ENABLED