Further atomic ops portability improvements and bug fixes.

* Don't play tricks for a more efficient pg_atomic_clear_flag() in the
  generic gcc implementation. The old version was broken on gcc < 4.7
  on !x86 platforms. Per buildfarm member chipmunk.
* Make usage of __atomic() fences depend on HAVE_GCC__ATOMIC_INT32_CAS
  instead of HAVE_GCC__ATOMIC_INT64_CAS - there's platforms with 32bit
  support that don't support 64bit atomics.
* Blindly fix two superflous #endif in generic-xlc.h
* Check for --disable-atomics in platforms but x86.

src/include/port/atomics/arch-x86.h

@@ -78,6 +78,7 @@ typedef struct pg_atomic_uint64
 #endif
 
 #endif /* defined(HAVE_ATOMICS) */
+
 #endif /* defined(__GNUC__) && !defined(__INTEL_COMPILER) */
 
 #if defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS)
@@ -160,6 +161,18 @@ pg_atomic_test_set_flag_impl(volatile pg_atomic_flag *ptr)
 	return _res == 0;
 }
 
+#define PG_HAVE_ATOMIC_CLEAR_FLAG
+static inline void
+pg_atomic_clear_flag_impl(volatile pg_atomic_flag *ptr)
+{
+	/*
+	 * On a TSO architecture like x86 it's sufficient to use a compiler
+	 * barrier to achieve release semantics.
+	 */
+	__asm__ __volatile__("" ::: "memory");
+	ptr->value = 0;
+}
+
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
 static inline bool
 pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,

src/include/port/atomics/generic-acc.h

@@ -25,6 +25,10 @@
 
 #include <machine/sys/inline.h>
 
+#define pg_compiler_barrier_impl()	_Asm_sched_fence()
+
+#if defined(HAVE_ATOMICS)
+
 /* IA64 always has 32/64 bit atomics */
 
 #define PG_HAVE_ATOMIC_U32_SUPPORT
@@ -39,10 +43,13 @@ typedef struct pg_atomic_uint64
 	volatile uint64 value;
 } pg_atomic_uint64;
 
-#define pg_compiler_barrier_impl()	_Asm_sched_fence()
+#endif /* defined(HAVE_ATOMICS) */
+
 
 #if defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS)
 
+#if defined(HAVE_ATOMICS)
+
 #define MINOR_FENCE (_Asm_fence) (_UP_CALL_FENCE | _UP_SYS_FENCE | \
 								 _DOWN_CALL_FENCE | _DOWN_SYS_FENCE )
 
@@ -96,4 +103,6 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 
 #undef MINOR_FENCE
 
+#endif /* defined(HAVE_ATOMICS) */
+
 #endif /* defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS) */

src/include/port/atomics/generic-gcc.h

@@ -40,19 +40,19 @@
  * definitions where possible, and use this only as a fallback.
  */
 #if !defined(pg_memory_barrier_impl)
-#	if defined(HAVE_GCC__ATOMIC_INT64_CAS)
+#	if defined(HAVE_GCC__ATOMIC_INT32_CAS)
 #		define pg_memory_barrier_impl()		__atomic_thread_fence(__ATOMIC_SEQ_CST)
 #	elif (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1))
 #		define pg_memory_barrier_impl()		__sync_synchronize()
 #	endif
 #endif /* !defined(pg_memory_barrier_impl) */
 
-#if !defined(pg_read_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT64_CAS)
+#if !defined(pg_read_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT32_CAS)
 /* acquire semantics include read barrier semantics */
 #		define pg_read_barrier_impl()		__atomic_thread_fence(__ATOMIC_ACQUIRE)
 #endif
 
-#if !defined(pg_write_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT64_CAS)
+#if !defined(pg_write_barrier_impl) && defined(HAVE_GCC__ATOMIC_INT32_CAS)
 /* release semantics include write barrier semantics */
 #		define pg_write_barrier_impl()		__atomic_thread_fence(__ATOMIC_RELEASE)
 #endif
@@ -139,13 +139,7 @@ pg_atomic_unlocked_test_flag_impl(volatile pg_atomic_flag *ptr)
 static inline void
 pg_atomic_clear_flag_impl(volatile pg_atomic_flag *ptr)
 {
-	/*
-	 * XXX: It would be nicer to use __sync_lock_release here, but gcc insists
-	 * on making that an atomic op which is far to expensive and a stronger
-	 * guarantee than what we actually need.
-	 */
-	pg_write_barrier_impl();
-	ptr->value = 0;
+	__sync_lock_release(&ptr->value);
 }
 #endif
 

src/include/port/atomics/generic-msvc.h

@@ -32,6 +32,8 @@
 #define pg_memory_barrier_impl()	MemoryBarrier()
 #endif
 
+#if defined(HAVE_ATOMICS)
+
 #define PG_HAVE_ATOMIC_U32_SUPPORT
 typedef struct pg_atomic_uint32
 {
@@ -44,9 +46,13 @@ typedef struct pg_atomic_uint64
 	volatile uint64 value;
 } pg_atomic_uint64;
 
+#endif /* defined(HAVE_ATOMICS) */
+
 
 #if defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS)
 
+#if defined(HAVE_ATOMICS)
+
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
 static inline bool
 pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
@@ -100,4 +106,6 @@ pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
 }
 #endif /* _WIN64 */
 
+#endif /* HAVE_ATOMICS */
+
 #endif /* defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS) */

src/include/port/atomics/generic-sunpro.h

@@ -17,6 +17,8 @@
  * -------------------------------------------------------------------------
  */
 
+#if defined(HAVE_ATOMICS)
+
 /* Older versions of the compiler don't have atomic.h... */
 #ifdef HAVE_ATOMIC_H
 
@@ -36,9 +38,13 @@ typedef struct pg_atomic_uint64
 
 #endif /* HAVE_ATOMIC_H */
 
+#endif /* defined(HAVE_ATOMICS) */
+
 
 #if defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS)
 
+#if defined(HAVE_ATOMICS)
+
 #ifdef HAVE_ATOMIC_H
 
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
@@ -71,4 +77,6 @@ pg_atomic_compare_exchange_u64_impl(volatile pg_atomic_uint64 *ptr,
 
 #endif /* HAVE_ATOMIC_H */
 
+#endif /* defined(HAVE_ATOMICS) */
+
 #endif /* defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS) */

src/include/port/atomics/generic-xlc.h

@@ -16,6 +16,8 @@
  * -------------------------------------------------------------------------
  */
 
+#if defined(HAVE_ATOMICS)
+
 #include <atomic.h>
 
 #define PG_HAVE_ATOMIC_U32_SUPPORT
@@ -35,8 +37,12 @@ typedef struct pg_atomic_uint64
 
 #endif /* __64BIT__ */
 
+#endif /* defined(HAVE_ATOMICS) */
+
 #if defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS)
 
+#if defined(HAVE_ATOMICS)
+
 #define PG_HAVE_ATOMIC_COMPARE_EXCHANGE_U32
 static inline bool
 pg_atomic_compare_exchange_u32_impl(volatile pg_atomic_uint32 *ptr,
@@ -69,7 +75,6 @@ pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
 {
 	return __fetch_and_add(&ptr->value, add_);
 }
-#endif
 
 #ifdef PG_HAVE_ATOMIC_U64_SUPPORT
 
@@ -96,8 +101,9 @@ pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
 {
 	return __fetch_and_addlp(&ptr->value, add_);
 }
-#endif
 
 #endif /* PG_HAVE_ATOMIC_U64_SUPPORT */
 
+#endif /* defined(HAVE_ATOMICS) */
+
 #endif /* defined(PG_USE_INLINE) || defined(ATOMICS_INCLUDE_DEFINITIONS) */