Re: Reducing the chunk header sizes on all memory context types

I wrote:
> I also avoided using 001: based on my work with converting guc.c to use
> palloc [1], it seems that pfree'ing a malloc-provided pointer is likely
> to see 001 a lot, at least on 64-bit glibc platforms.

I poked at this some more by creating a function that intentionally
does pfree(malloc(N)) for various values of N.

RHEL8, x86_64: the low-order nibble of the header is consistently 0001.

macOS 12.6, arm64: the low-order nibble is consistently 0000.

FreeBSD 13.0, arm64: Usually the low-order nibble is 0000 or 1111,
but for some smaller values of N it sometimes comes up as 0010.

NetBSD 9.2, amd64: results similar to FreeBSD.

I still haven't looked into anybody's source code, but based on these
results I'm inclined to leave both 001 and 010 IDs unused for now.
That'll help the GUC malloc -> palloc transition tremendously, because
people will get fairly clear errors rather than weird assertions
and/or memory corruption. That'll leave us in a situation where only
one more context ID can be assigned without risk of reducing our error
detection ability, but I'm content with that for now.

regards, tom lane