Call sched_yield() in Unsafe implementation

[Andrew Haley]

In the fallback code in natUnsafe.cc -- which is used when gcc doesn't
support atomic builtins -- we don't yield when a lock is contended.
I've done a few experiments and on the machines I have it is always
much faster to yield the instant we realize a lock is held by another
thread.  This may not be true for some high-end SMP boxes, but they
won't be calling this code.

Andrew.


2007-11-16  Andrew Haley  <aph@redhat.com>

        * sun/misc/natUnsafe.cc (spinlock contructor): Call sched_yield().

Index: sun/misc/natUnsafe.cc
===================================================================
--- sun/misc/natUnsafe.cc       (revision 130234)
+++ sun/misc/natUnsafe.cc       (working copy)
@@ -1,6 +1,6 @@
 // natUnsafe.cc - Implementation of sun.misc.Unsafe native methods.
 
-/* Copyright (C) 2006
+/* Copyright (C) 2006, 2007
    Free Software Foundation
 
    This file is part of libgcj.
@@ -32,7 +32,7 @@
 spinlock ()
   {
     while (! compare_and_swap (&lock, 0, 1))
-      ;
+      _Jv_ThreadYield ();
   }
   ~spinlock ()
   {

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