Re: define pg_structiszero(addr, s, r)

On Wed, Nov 06, 2024 at 03:53:48PM +1300, David Rowley wrote:
> I'm not sure if I'm clear on what works for you. The latest patch I
> saw did 1 size_t per iteration. Are you saying we should do just
> size_t per loop? or we should form the code in a way that allows the
> compiler to use SIMD instructions?

Oh, sorry. I thought that you wanted to keep the size_t checks as
done in [1] anyway.

But your suggestion is different, and instructions like xmmword would
be enough to show up as you group more the checks 8 at a time:
bool
pg_memory_is_all_zeros_size_t_times_8(const void *ptr, size_t len)
{
const char *p = (const char *) ptr;
const char *end = &p[len];
const char *aligned_end = (const char *) ((uintptr_t) end & (~(sizeof(size_t) - 1)));

while (((uintptr_t) p & (sizeof(size_t) - 1)) != 0)
{
if (p == end)
return true;
if (*p++ != 0)
return false;
}
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;
}
while (p < end)
{
if (*p++ != 0)
return false;
}
return true;
}

That's smart for large areas to cover. The patch should document why
we are doing it this way. This should have a few more parenthesis in
the second loop, or -Wparentheses would complain.

Should the last loop check only 1 byte at a time or should this stuff
include one more step before the last one you wrote to do a couple of
checks with size_t? That may matter for areas small enough (len <
sizeof(size_t) * 8) causing the second step to not be taken, but large
enough (len > sizeof(size_t)) to apply a couple of size_t checks per
loop.

[1]: https://www.postgresql.org/message-id/Zym0UvNM+fswQsIZ@ip-10-97-1-34.eu-west-3.compute.internal
--
Michael