unix: add atomic-ops.h

Add cmpxchgi(), cmpxchgl() and cpu_relax() functions that we can use
as simple primitives to build spinlocks out of.

PR-URL: https://github.com/libuv/libuv/pull/197
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

[NB: This is a back-port of commit a3c3b37 from the v1.x branch.]

config-unix.mk

@@ -186,7 +186,7 @@ src/.buildstamp src/unix/.buildstamp test/.buildstamp:
 	mkdir -p $(@D)
 	touch $@
 
-src/unix/%.o src/unix/%.pic.o: src/unix/%.c include/uv.h include/uv-private/uv-unix.h src/unix/internal.h src/unix/.buildstamp $(DTRACE_HEADER)
+src/unix/%.o src/unix/%.pic.o: src/unix/%.c include/uv.h include/uv-private/uv-unix.h src/unix/atomic-ops.h src/unix/internal.h src/unix/.buildstamp $(DTRACE_HEADER)
 	$(CC) $(CSTDFLAG) $(CPPFLAGS) $(CFLAGS) -c $< -o $@
 
 src/%.o src/%.pic.o: src/%.c include/uv.h include/uv-private/uv-unix.h src/.buildstamp

src/unix/atomic-ops.h

@@ -0,0 +1,60 @@
+/* Copyright (c) 2013, Ben Noordhuis <info@bnoordhuis.nl>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef UV_ATOMIC_OPS_H_
+#define UV_ATOMIC_OPS_H_
+
+#include "internal.h"  /* UV_UNUSED */
+
+UV_UNUSED(static int cmpxchgi(int* ptr, int oldval, int newval));
+UV_UNUSED(static long cmpxchgl(long* ptr, long oldval, long newval));
+UV_UNUSED(static void cpu_relax(void));
+
+/* Prefer hand-rolled assembly over the gcc builtins because the latter also
+ * issue full memory barriers.
+ */
+UV_UNUSED(static int cmpxchgi(int* ptr, int oldval, int newval)) {
+#if defined(__i386__) || defined(__x86_64__)
+  int out;
+  __asm__ __volatile__ ("lock; cmpxchg %2, %1;"
+                        : "=a" (out), "+m" (*(volatile int*) ptr)
+                        : "r" (newval), "0" (oldval)
+                        : "memory");
+  return out;
+#else
+  return __sync_val_compare_and_swap(ptr, oldval, newval);
+#endif
+}
+
+UV_UNUSED(static long cmpxchgl(long* ptr, long oldval, long newval)) {
+#if defined(__i386__) || defined(__x86_64__)
+  long out;
+  __asm__ __volatile__ ("lock; cmpxchg %2, %1;"
+                        : "=a" (out), "+m" (*(volatile long*) ptr)
+                        : "r" (newval), "0" (oldval)
+                        : "memory");
+  return out;
+#else
+  return __sync_val_compare_and_swap(ptr, oldval, newval);
+#endif
+}
+
+UV_UNUSED(static void cpu_relax(void)) {
+#if defined(__i386__) || defined(__x86_64__)
+  __asm__ __volatile__ ("rep; nop");  /* a.k.a. PAUSE */
+#endif
+}
+
+#endif  /* UV_ATOMIC_OPS_H_ */

uv.gyp

@@ -133,6 +133,7 @@
             'include/uv-private/uv-darwin.h',
             'include/uv-private/uv-bsd.h',
             'src/unix/async.c',
+            'src/unix/atomic-ops.h',
             'src/unix/core.c',
             'src/unix/dl.c',
             'src/unix/error.c',