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Further marginal hacking on generic atomic ops.

Thu, 7 Sep 2017 12:50:01 +0000 (08:50 -0400)

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

src/include/port/atomics/generic.h patch | blob | blame | history

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;

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