[libvirt] [PATCH] Improve on virAtomic implementation

[Stefan Berger]

This patch improves on the previously added virAtomicInt operations
by testing for the compiler and if GCC >= 4.1 (not found in docs prior to
that) is used on Linux and has the appropriate processor (that I have
access to) then use the implementation based on the gcc-builtins.

I also did not look at other systems (cygwin, win32) that do not need
access to virAtomic right now.

---
  src/util/viratomic.h |   88 
+++++++++++++++++++++++++++++++++++++++++----------
  1 file changed, 72 insertions(+), 16 deletions(-)

Index: libvirt-acl/src/util/viratomic.h
===================================================================
--- libvirt-acl.orig/src/util/viratomic.h
+++ libvirt-acl/src/util/viratomic.h
@@ -30,6 +30,22 @@
  typedef struct _virAtomicInt virAtomicInt;
  typedef virAtomicInt *virAtomicIntPtr;

+# define __VIR_ATOMIC_USES_LOCK
+
+# if defined(__GNUC__)
+#  if ((__GNUC__ == 4) && (__GNUC_MINOR__ >= 1)) || (__GNUC__ > 4)
+#   if defined(__linux__)
+#    if defined(__i386__) || defined(__x86_64__) || \
+        defined(__powerpc64__) || defined(__powerpc__)
+#     undef __VIR_ATOMIC_USES_LOCK
+#    endif
+#   endif
+#  endif
+# endif
+
+
+# ifdef __VIR_ATOMIC_USES_LOCK
+
  struct _virAtomicInt {
      virMutex lock;
      int value;
@@ -42,22 +58,6 @@ virAtomicIntInit(virAtomicIntPtr vaip)
      return virMutexInit(&vaip->lock);
  }

-static inline void
-virAtomicIntSet(virAtomicIntPtr vaip, int value)
-{
-     virMutexLock(&vaip->lock);
-
-     vaip->value = value;
-
-     virMutexUnlock(&vaip->lock);
-}
-
-static inline int
-virAtomicIntRead(virAtomicIntPtr vaip)
-{
-     return vaip->value;
-}
-
  static inline int
  virAtomicIntAdd(virAtomicIntPtr vaip, int add)
  {
@@ -88,4 +88,60 @@ virAtomicIntSub(virAtomicIntPtr vaip, in
      return ret;
  }

+# else /* __VIR_ATOMIC_USES_LOCK */
+
+struct _virAtomicInt {
+    int value;
+};
+
+static inline int
+virAtomicIntInit(virAtomicIntPtr vaip)
+{
+    vaip->value = 0;
+    return 0;
+}
+
+static inline int
+virAtomicIntAdd(virAtomicIntPtr vaip, int add)
+{
+    return __sync_add_and_fetch(&vaip->value, add);
+}
+
+static inline int
+virAtomicIntSub(virAtomicIntPtr vaip, int sub)
+{
+    return __sync_sub_and_fetch(&vaip->value, sub);
+}
+
+# endif /* __VIR_ATOMIC_USES_LOCK */
+
+
+
+/* common operations that need no locking or build on others */
+
+
+static inline void
+virAtomicIntSet(virAtomicIntPtr vaip, int value)
+{
+     vaip->value = value;
+}
+
+static inline int
+virAtomicIntRead(virAtomicIntPtr vaip)
+{
+     return *(volatile int *)&vaip->value;
+}
+
+static inline int
+virAtomicIntInc(virAtomicIntPtr vaip)
+{
+    return virAtomicIntAdd(vaip, 1);
+}
+
+static inline int
+virAtomicIntDec(virAtomicIntPtr vaip)
+{
+    return virAtomicIntSub(vaip, 1);
+}
+
  #endif /* __VIR_ATOMIC_H */