Add an option to bind initial thread at the start of application

[AndreyChurbanov]

Summary

Introduced environment variable KMP_INITIAL_THREAD_BIND that allows to bind initial thread of OpenMP application to be bound at the very beginning. To do so call middle library initialization from compiler generated function __kmpc_begin(). The middle initialization includes machine topology discovery and binding of the thread according to requested method if any.

This change is done by request from customer who wanted serial start of application be executed on the same core as the master thread of parallel region.

[jlpeyton]

LGTM

openmp/trunk/runtime/src/kmp_csupport.cpp

  *
  * Initialize the runtime library. This call is optional; if it is not made then
  * it will be implicitly called by attempts to use other library functions.
  *
  */
 void
 __kmpc_begin(ident_t *loc, kmp_int32 flags)
 {
+    // By default __kmpc_begin() is no-op.
+    char *env;
+    if ((env = getenv( "KMP_INITIAL_THREAD_BIND" )) != NULL &&
+        __kmp_str_match_true( env )) {
+        __kmp_middle_initialize();
+        KC_TRACE(10, ("__kmpc_begin: middle initialization called\n" ));
+    } else if (__kmp_ignore_mppbeg() == FALSE) {
         // By default __kmp_ignore_mppbeg() returns TRUE.
-    if (__kmp_ignore_mppbeg() == FALSE) {
         __kmp_internal_begin();
-
         KC_TRACE( 10, ("__kmpc_begin: called\n" ) );
     }
 }
 
 /*!
  * @ingroup STARTUP_SHUTDOWN
  * @param loc source location information
  *