ntdll: make NtDelayExecution a bit more efficient

[Graham]

Hi folks,

This is my first attempt at submitting a patch to Wine. If I've done
anything incorrectly, please let me know.

This patch is vaguely related to bug 24558. It eliminates a few
syscalls in NtDelayExecution:

1. If the caller requests a zero-wait yield, then do just that, and
nothing more.

2. If you're about to block on select(), then I don't see any point in
preceding that with a call to sched_yield(). I've also shuffled the
calls to NtGetSystemTime() so that it is only called once prior to the
first invocation of select().

I tested this with StarCraft II, and it appears to work as intended. I
don't expect a measurable performance gain, but the difference is
visible via "strace -c". Please let me know what you think. Thanks.

-- graham
-------------- next part --------------
diff --git a/dlls/ntdll/sync.c b/dlls/ntdll/sync.c
index 58dde65..d99d91d 100644
--- a/dlls/ntdll/sync.c
+++ b/dlls/ntdll/sync.c
@@ -1213,30 +1213,31 @@ NTSTATUS WINAPI NtDelayExecution( BOOLEAN alertable, const LARGE_INTEGER *timeou
     {
         for (;;) select( 0, NULL, NULL, NULL, NULL );
     }
+    else if (!timeout->QuadPart)
+    {
+        NtYieldExecution();
+    }
     else
     {
         LARGE_INTEGER now;
         timeout_t when, diff;
 
+        NtQuerySystemTime( &now );
+
         if ((when = timeout->QuadPart) < 0)
         {
-            NtQuerySystemTime( &now );
             when = now.QuadPart - when;
         }
 
-        /* Note that we yield after establishing the desired timeout */
-        NtYieldExecution();
-        if (!when) return STATUS_SUCCESS;
-
         for (;;)
         {
             struct timeval tv;
-            NtQuerySystemTime( &now );
             diff = (when - now.QuadPart + 9) / 10;
             if (diff <= 0) break;
             tv.tv_sec  = diff / 1000000;
             tv.tv_usec = diff % 1000000;
             if (select( 0, NULL, NULL, NULL, &tv ) != -1) break;
+            NtQuerySystemTime( &now );
         }
     }
     return STATUS_SUCCESS;