Use more relaxed atomics

Turns out all uses of _cairo_atomic_ptr_get can be turned
to relaxed loads. So we rename _cairo_atomic_ptr_get to
_cairo_atomic_ptr_get_relaxed and drop its memory-ordering
constraints.

In addition, most uses of _cairo_atomic_int_get can be turned
to relaxed loads. We replace these calls of _cairo_atomic_int_get
to _cairo_atomic_int_get_relaxed (which is already implemented)

src/cairo-atomic-private.h

@@ -75,9 +75,9 @@ _cairo_atomic_int_set_relaxed (cairo_atomic_int_t *x, int val)
 }
 
 static cairo_always_inline void *
-_cairo_atomic_ptr_get (cairo_atomic_intptr_t *x)
+_cairo_atomic_ptr_get_relaxed (cairo_atomic_intptr_t *x)
 {
-    return atomic_load_explicit (x, memory_order_seq_cst);
+    return atomic_load_explicit (x, memory_order_relaxed);
 }
 
 # define _cairo_atomic_int_inc(x) ((void) atomic_fetch_add_explicit(x, 1, memory_order_seq_cst))
@@ -169,9 +169,9 @@ _cairo_atomic_int_set_relaxed (cairo_atomic_int_t *x, int val)
 }
 
 static cairo_always_inline void *
-_cairo_atomic_ptr_get (cairo_atomic_intptr_t *x)
+_cairo_atomic_ptr_get_relaxed (cairo_atomic_intptr_t *x)
 {
-    return (void*)__atomic_load_n(x, __ATOMIC_SEQ_CST);
+    return (void*)__atomic_load_n(x, __ATOMIC_RELAXED);
 }
 
 # define _cairo_atomic_int_inc(x) ((void) __atomic_fetch_add(x, 1, __ATOMIC_SEQ_CST))
@@ -253,10 +253,9 @@ _cairo_atomic_int_set_relaxed (cairo_atomic_int_t *x, int val)
 }
 
 static cairo_always_inline void *
-_cairo_atomic_ptr_get (cairo_atomic_intptr_t *x)
+_cairo_atomic_ptr_get_relaxed (cairo_atomic_intptr_t *x)
 {
-    __sync_synchronize ();
-    return (void*)*x;
+    return *(void * volatile *)x;
 }
 
 # define _cairo_atomic_int_inc(x) ((void) __sync_fetch_and_add(x, 1))
@@ -300,7 +299,7 @@ typedef AO_t cairo_atomic_intptr_t;
 
 static_assert (sizeof (AO_t) >= sizeof (void *), "AO_t cannot be used for pointers");
 
-# define _cairo_atomic_ptr_get(x) _cairo_atomic_intptr_to_voidptr (AO_load_full (x))
+# define _cairo_atomic_ptr_get_relaxed(x) _cairo_atomic_intptr_to_voidptr (AO_load (x))
 # define _cairo_atomic_ptr_cmpxchg(x, oldv, newv) AO_compare_and_swap_full(x, (AO_t)oldv, (AO_t)newv)
 # define _cairo_atomic_ptr_cmpxchg_return_old(x, oldv, newv) AO_fetch_compare_and_swap_full(x, (AO_t)oldv, (AO_t)newv)
 
@@ -335,7 +334,7 @@ typedef intptr_t cairo_atomic_intptr_t;
 #error No matching integer pointer type
 #endif
 
-# define _cairo_atomic_ptr_get(x) (OSMemoryBarrier(), *(x))
+# define _cairo_atomic_ptr_get_relaxed(x) (*(void * volatile *)(x))
 
 #endif /* HAVE_OS_ATOMIC_OPS */
 
@@ -386,10 +385,15 @@ _cairo_atomic_int_cmpxchg_return_old_impl (cairo_atomic_int_t *x,
   _cairo_atomic_int_cmpxchg_return_old_impl(x, oldv, newv)
 
 static cairo_always_inline void *
-_cairo_atomic_ptr_get (cairo_atomic_intptr_t *x)
+_cairo_atomic_ptr_get_relaxed (cairo_atomic_intptr_t *x)
 {
-    MemoryBarrier ();
-    return (void *) *x;
+#if SIZEOF_VOID_P == 4
+    return (void *) __iso_volatile_load32 ((__int32 *) (void *) x);
+#elif SIZEOF_VOID_P == 8
+    return (void *) __iso_volatile_load64 ((__int64 *) (void *) x);
+#else
+#error "unknown pointer size"
+#endif
 }
 
 static cairo_always_inline cairo_bool_t
@@ -445,7 +449,7 @@ void
 _cairo_atomic_int_set_relaxed (cairo_atomic_int_t *x, int val);
 
 cairo_private void*
-_cairo_atomic_ptr_get(cairo_atomic_intptr_t *x);
+_cairo_atomic_ptr_get_relaxed(cairo_atomic_intptr_t *x);
 
 #else
 
@@ -462,7 +466,7 @@ _cairo_atomic_int_cmpxchg_return_old_fallback(cairo_atomic_int_t *x, int oldv, i
     int curr;
 
     do {
-        curr = _cairo_atomic_int_get (x);
+        curr = _cairo_atomic_int_get_relaxed (x);
     } while (curr == oldv && !_cairo_atomic_int_cmpxchg (x, oldv, newv));
 
     return curr;
@@ -474,7 +478,7 @@ _cairo_atomic_ptr_cmpxchg_return_old_fallback(cairo_atomic_intptr_t *x, void *ol
     void *curr;
 
     do {
-        curr = _cairo_atomic_ptr_get (x);
+        curr = _cairo_atomic_ptr_get_relaxed (x);
     } while (curr == oldv && !_cairo_atomic_ptr_cmpxchg (x, oldv, newv));
 
     return curr;

src/cairo-atomic.c

@@ -116,7 +116,7 @@ _cairo_atomic_int_set_relaxed (cairo_atomic_int_t *x, int val)
 }
 
 void*
-_cairo_atomic_ptr_get (void **x)
+_cairo_atomic_ptr_get_relaxed (void **x)
 {
     void *ret;
 

src/cairo-freed-pool-private.h

@@ -60,7 +60,7 @@ _atomic_fetch (cairo_atomic_intptr_t *slot)
     void *ptr;
 
     do {
-        ptr = _cairo_atomic_ptr_get (slot);
+        ptr = _cairo_atomic_ptr_get_relaxed (slot);
     } while (! _cairo_atomic_ptr_cmpxchg (slot, ptr, NULL));
 
     return ptr;

src/cairo-reference-count-private.h

@@ -50,7 +50,7 @@ typedef struct {
 
 #define CAIRO_REFERENCE_COUNT_INIT(RC, VALUE) ((RC)->ref_count = (VALUE))
 
-#define CAIRO_REFERENCE_COUNT_GET_VALUE(RC) _cairo_atomic_int_get (&(RC)->ref_count)
+#define CAIRO_REFERENCE_COUNT_GET_VALUE(RC) _cairo_atomic_int_get_relaxed (&(RC)->ref_count)
 
 #define CAIRO_REFERENCE_COUNT_INVALID_VALUE ((int) -1)
 #define CAIRO_REFERENCE_COUNT_INVALID {CAIRO_REFERENCE_COUNT_INVALID_VALUE}