comments

includes/stg/SMP.h

@@ -312,12 +312,22 @@ atomic_dec(StgVolatilePtr p)
 #endif
 }
 
+/*
+ * Some architectures have a way to tell the CPU that we're in a
+ * busy-wait loop, and the processor should look for something else to
+ * do (such as run another hardware thread).
+ */
 EXTERN_INLINE void
 busy_wait_nop(void)
 {
 #if defined(i386_HOST_ARCH) || defined(x86_64_HOST_ARCH)
+    // On Intel, the busy-wait-nop instruction is called "pause",
+    // which is actually represented as a nop with the rep prefix.
+    // On processors before the P4 this behaves as a nop; on P4 and
+    // later it might do something clever like yield to another
+    // hyperthread.  In any case, Intel recommends putting one
+    // of these in a spin lock loop.
     __asm__ __volatile__ ("rep; nop");
-    //
 #else
     // nothing
 #endif