Re: define pg_structiszero(addr, s, r)

On Tue, Nov 12, 2024 at 01:32:36PM -0300, Ranier Vilela wrote:
> See v1_allzeros_small.c attached.

In your pg_memory_is_all_zeros_v11:
while (((uintptr_t) p & (sizeof(size_t) - 1)) != 0)
{
if (p == end)
return true;

if (*p++ != 0)
return false;
}

if (len > sizeof(size_t) * 8)
{
for (; p < aligned_end - (sizeof(size_t) * 7); p += sizeof(size_t) * 8)
{
if ((((size_t *) p)[0] != 0) | (((size_t *) p)[1] != 0) |
(((size_t *) p)[2] != 0) | (((size_t *) p)[3] != 0) |
(((size_t *) p)[4] != 0) | (((size_t *) p)[5] != 0) |
(((size_t *) p)[6] != 0) | (((size_t *) p)[7] != 0))
return false;
}
}

If I'm reading that right, this could still read a couple of bytes
past the wanted memory area. For example, imagine a case of 65 bytes
with a location a bit unaligned (more than 2 bytes). You'd want to
check the remaining size after the first loop, not the initial one.

I'd be OK to have a quick loop for the less-than-64-byte case rather
than more checks depending on sizeof(size_t) spread, like Bertrand is
suggesting. I'd like to imagine that compilers would like that a bit
better, though I am not completely sure, either.
--
Michael