Use "rep; nop" inside a spin-lock loop on x86/x86-64

This helps on a hyperthreaded CPU by yielding to the other thread in a
spinlock loop.

1 files changed, 2 insertions, 0 deletions

diff --git a/includes/rts/SpinLock.h b/includes/rts/SpinLock.h
index 9bfb35bbac..3d0b56cfcb 100644
--- a/includes/rts/SpinLock.h
+++ b/includes/rts/SpinLock.h
@@ -49,6 +49,7 @@ spin:
     r = cas((StgVolatilePtr)&(p->lock), 1, 0);
     if (r == 0) {
         p->spin++;
+        busy_wait_nop();
         goto spin;
     }
 }
@@ -76,6 +77,7 @@ INLINE_HEADER void ACQUIRE_SPIN_LOCK(SpinLock * p)
     StgWord32 r = 0;
     do {
         r = cas((StgVolatilePtr)p, 1, 0);
+        busy_wait_nop();
     } while(r == 0);
 }