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.
> 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?