Force serial reduction when team size == 1

[jlpeyton]

Summary

This change enforces the use of the serial reduction method when the team size is 1 even if KMP_FORCE_REDUCTION is set. Using the serial reduction method avoids unnecessary synchronization and should always be used when the team size is 1.

[hfinkel]

LGTM

[AndreyChurbanov]

Committed revision 245209.

runtime/src/kmp_runtime.c

#endif
         #else
             #error "Unknown or unsupported architecture"
         #endif
 
     }
 
     // KMP_FORCE_REDUCTION
 
-    if( __kmp_force_reduction_method != reduction_method_not_defined ) {
+    // If the team is serialized (team_size == 1), ignore the forced reduction
+    // method and stay with the unsynchronized method (empty_reduce_block)
+    if( __kmp_force_reduction_method != reduction_method_not_defined && team_size != 1) {
 
         PACKED_REDUCTION_METHOD_T forced_retval;
 
         int atomic_available, tree_available;
 
         switch( ( forced_retval = __kmp_force_reduction_method ) )
         {
             case critical_reduce_block:
                 KMP_ASSERT( lck );              // lck should be != 0
-                if( team_size <= 1 ) {
-                    forced_retval = empty_reduce_block;
-                }
                 break;
 
             case atomic_reduce_block:
                 atomic_available = FAST_REDUCTION_ATOMIC_METHOD_GENERATED;
                 KMP_ASSERT( atomic_available ); // atomic_available should be != 0
                 break;
 
             case tree_reduce_block: