Further marginal hacking on generic atomic ops.

In the generic atomic ops that rely on a loop around a CAS primitive,
there's no need to force the initial read of the "old" value to be atomic.
In the typically-rare case that we get a torn value, that simply means
that the first CAS attempt will fail; but it will update "old" to the
atomically-read value, so the next attempt has a chance of succeeding.
It was already being done that way in pg_atomic_exchange_u64_impl(),
but let's duplicate the approach in the rest.

(Given the current coding of the pg_atomic_read functions, this change
is a no-op anyway on popular platforms; it only makes a difference where
pg_atomic_read_u64_impl() is implemented as a CAS.)

In passing, also remove unnecessary take-a-pointer-and-dereference-it
coding in the pg_atomic_read functions.  That seems to have been based
on a misunderstanding of what the C standard requires.  What actually
matters is that the pointer be declared as pointing to volatile, which
it is.

I don't believe this will change the assembly code at all on x86
platforms (even ignoring the likelihood that these implementations
get overridden by others); but it may help on less-mainstream CPUs.

Discussion: https://postgr.es/m/13707.1504718238@sss.pgh.pa.us

diff --git a/src/include/port/atomics/generic.h b/src/include/port/atomics/generic.h
index 75ffaf6e874891518fb71edc386e1470ae4e6267..c7566919deec1a527d014eb0669aaed2d268de19 100644 (file)
--- a/src/include/port/atomics/generic.h
+++ b/src/include/port/atomics/generic.h
@@ -45,7 +45,7 @@ typedef pg_atomic_uint32 pg_atomic_flag;
 static inline uint32
 pg_atomic_read_u32_impl(volatile pg_atomic_uint32 *ptr)
 {
-   return *(&ptr->value);
+   return ptr->value;
 }
 #endif
 
@@ -170,7 +170,7 @@ static inline uint32
 pg_atomic_exchange_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 xchg_)
 {
    uint32 old;
-   old = pg_atomic_read_u32_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, xchg_))
        /* skip */;
    return old;
@@ -183,7 +183,7 @@ static inline uint32
 pg_atomic_fetch_add_u32_impl(volatile pg_atomic_uint32 *ptr, int32 add_)
 {
    uint32 old;
-   old = pg_atomic_read_u32_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old + add_))
        /* skip */;
    return old;
@@ -205,7 +205,7 @@ static inline uint32
 pg_atomic_fetch_and_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 and_)
 {
    uint32 old;
-   old = pg_atomic_read_u32_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old & and_))
        /* skip */;
    return old;
@@ -218,7 +218,7 @@ static inline uint32
 pg_atomic_fetch_or_u32_impl(volatile pg_atomic_uint32 *ptr, uint32 or_)
 {
    uint32 old;
-   old = pg_atomic_read_u32_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u32_impl(ptr, &old, old | or_))
        /* skip */;
    return old;
@@ -249,7 +249,7 @@ static inline uint64
 pg_atomic_exchange_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 xchg_)
 {
    uint64 old;
-   old = ptr->value;
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, xchg_))
        /* skip */;
    return old;
@@ -299,12 +299,10 @@ static inline uint64
 pg_atomic_read_u64_impl(volatile pg_atomic_uint64 *ptr)
 {
    /*
-    * On this platform aligned 64bit reads are guaranteed to be atomic,
-    * except if using the fallback implementation, where can't guarantee the
-    * required alignment.
+    * On this platform aligned 64-bit reads are guaranteed to be atomic.
     */
    AssertPointerAlignment(ptr, 8);
-   return *(&ptr->value);
+   return ptr->value;
 }
 
 #else
@@ -315,10 +313,10 @@ pg_atomic_read_u64_impl(volatile pg_atomic_uint64 *ptr)
    uint64 old = 0;
 
    /*
-    * 64 bit reads aren't safe on all platforms. In the generic
+    * 64-bit reads aren't atomic on all platforms. In the generic
     * implementation implement them as a compare/exchange with 0. That'll
-    * fail or succeed, but always return the old value. Possible might store
-    * a 0, but only if the prev. value also was a 0 - i.e. harmless.
+    * fail or succeed, but always return the old value. Possibly might store
+    * a 0, but only if the previous value also was a 0 - i.e. harmless.
     */
    pg_atomic_compare_exchange_u64_impl(ptr, &old, 0);
 
@@ -342,7 +340,7 @@ static inline uint64
 pg_atomic_fetch_add_u64_impl(volatile pg_atomic_uint64 *ptr, int64 add_)
 {
    uint64 old;
-   old = pg_atomic_read_u64_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old + add_))
        /* skip */;
    return old;
@@ -364,7 +362,7 @@ static inline uint64
 pg_atomic_fetch_and_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 and_)
 {
    uint64 old;
-   old = pg_atomic_read_u64_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old & and_))
        /* skip */;
    return old;
@@ -377,7 +375,7 @@ static inline uint64
 pg_atomic_fetch_or_u64_impl(volatile pg_atomic_uint64 *ptr, uint64 or_)
 {
    uint64 old;
-   old = pg_atomic_read_u64_impl(ptr);
+   old = ptr->value;           /* ok if read is not atomic */
    while (!pg_atomic_compare_exchange_u64_impl(ptr, &old, old | or_))
        /* skip */;
    return old;