Hi,
On May 19, 2020 8:05:00 PM PDT, Noah Misch <noah@leadboat.com> wrote:
>On Tue, May 19, 2020 at 04:07:29PM +0300, Konstantin Knizhnik wrote:
>> Definition of pg_atomic_compare_exchange_u64 requires alignment of
>expected
>> pointer on 8-byte boundary.
>>
>> pg_atomic_compare_exchange_u64(volatile pg_atomic_uint64 *ptr,
>> uint64 *expected, uint64 newval)
>> {
>> #ifndef PG_HAVE_ATOMIC_U64_SIMULATION
>> AssertPointerAlignment(ptr, 8);
>> AssertPointerAlignment(expected, 8);
>> #endif
>>
>>
>> I wonder if there are platforms where such restriction is actually
>needed.
>
>In general, sparc Linux does SIGBUS on unaligned access. Other
>platforms
>function but suffer performance penalties.
Indeed. Cross cacheline atomics are e.g. really expensive on x86. Essentially requiring a full blown bus lock iirc.
>> And if so, looks like our ./src/test/regress/regress.c is working
>only
>> occasionally:
>>
>> static void
>> test_atomic_uint64(void)
>> {
>> pg_atomic_uint64 var;
>> uint64 expected;
>> ...
>> if (!pg_atomic_compare_exchange_u64(&var, &expected, 1))
>>
>> because there is no warranty that "expected" variable will be aligned
>on
>> stack at 8 byte boundary (at least at Win32).
>
>src/tools/msvc sets ALIGNOF_LONG_LONG_INT=8, so it believes that win32
>does
>guarantee 8-byte alignment of both automatic variables. Is it wrong?
Generally the definition of the atomics should ensure the required alignment. E.g. using alignment attributes to the
struct.
Andres
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