Re: define pg_structiszero(addr, s, r) - Mailing list pgsql-hackers

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.