/*
* XCHG - the atomic exchange instruction. Used for locking closures
- * during updates (see LOCK_CLOSURE below) and the MVar primops.
+ * during updates (see lockClosure() below) and the MVar primops.
+ *
+ * NB: the xchg instruction is implicitly locked, so we do not need
+ * a lock prefix here.
*/
INLINE_HEADER StgWord
xchg(StgPtr p, StgWord w)
StgWord result;
result = w;
__asm__ __volatile__ (
- "xchgl %1,%0"
+ "xchg %1,%0"
:"+r" (result), "+m" (*p)
: /* no input-only operands */
);
return result;
}
+/*
+ * CMPXCHG - the single-word atomic compare-and-exchange instruction. Used
+ * in the STM implementation.
+ */
+INLINE_HEADER StgWord
+cas(StgVolatilePtr p, StgWord o, StgWord n)
+{
+ __asm__ __volatile__ (
+ "lock cmpxchg %3,%1"
+ :"=a"(o), "=m" (*(volatile unsigned int *)p)
+ :"0" (o), "r" (n));
+ return o;
+}
+
+/*
+ * Write barrier - ensure that all preceding writes have happened
+ * before all following writes.
+ *
+ * We need to tell both the compiler AND the CPU about the barrier.
+ * This is a brute force solution; better results might be obtained by
+ * using volatile type declarations to get fine-grained ordering
+ * control in C, and optionally a memory barrier instruction on CPUs
+ * that require it (not x86 or x86_64).
+ */
+INLINE_HEADER void
+wb(void) {
+#if i386_HOST_ARCH || x86_64_HOST_ARCH
+ __asm__ __volatile__ ("" : : : "memory");
+#else
+#error memory barriers unimplemented on this architecture
+#endif
+}
+
+/*
+ * Locking/unlocking closures
+ *
+ * This is used primarily in the implementation of MVars.
+ */
+#define SPIN_COUNT 4000
+
INLINE_HEADER StgInfoTable *
lockClosure(StgClosure *p)
{
+#if i386_HOST_ARCH || x86_64_HOST_ARCH
StgWord info;
-#if 0
do {
- info = xchg((P_)&p->header.info, (W_)&stg_WHITEHOLE_info);
- if (info != (W_)&stg_WHITEHOLE_info) return (StgInfoTable *)info;
+ nat i = 0;
+ do {
+ info = xchg((P_)&p->header.info, (W_)&stg_WHITEHOLE_info);
+ if (info != (W_)&stg_WHITEHOLE_info) return (StgInfoTable *)info;
+ } while (++i < SPIN_COUNT);
yieldThread();
} while (1);
#else
- info = p->header.info;
+ ACQUIRE_SM_LOCK
+#endif
+}
+
+INLINE_HEADER void
+unlockClosure(StgClosure *p, StgInfoTable *info)
+{
+#if i386_HOST_ARCH || x86_64_HOST_ARCH
+ // This is a strictly ordered write, so we need a wb():
+ wb();
+ p->header.info = info;
+#else
+ RELEASE_SM_LOCK;
#endif
}
-#endif /* SMP */
+#else /* !SMP */
+
+#define wb() /* nothing */
+
+INLINE_HEADER StgWord
+xchg(StgPtr p, StgWord w)
+{
+ StgWord old = *p;
+ *p = w;
+ return old;
+}
+
+#endif /* !SMP */
#endif /* SMP_H */