Re: Atomics for heap_parallelscan_nextpage()

I wrote:
> I can confirm that on dromedary, that regression test case is attempting
> to create a TOC with a not-well-aligned size: 93268 = 0x16c54 bytes.

... although, on closer look, it still seems like we have a fundamental
bit of schizophrenia here, because on this machine

$ grep ALIGN pg_config.h
#define ALIGNOF_DOUBLE 4
#define ALIGNOF_INT 4
#define ALIGNOF_LONG 4
#define ALIGNOF_LONG_LONG_INT 4
#define ALIGNOF_SHORT 2
#define MAXIMUM_ALIGNOF 4

Basically, therefore, ISTM that it is not a good thing that the atomics
code thinks it can rely on 8-byte-aligned data when the entire rest of
the system believes that 4-byte alignment is enough for anything.

I was wondering why the shm_toc code was using BUFFERALIGN and not
MAXALIGN, and I now suspect that the answer is "it's an entirely
undocumented kluge to make the atomics code not crash on 32-bit
machines, so long as nobody puts a pg_atomic_uint64 anywhere except
in a shm_toc".

I'm not sure that that's good enough, and I'm damn sure that it
shouldn't be undocumented.

regards, tom lane