Re: Use C11 alignas instead of palloc/malloc for alignment

On 08/09/2026 13:08, Peter Eisentraut wrote:
> There are a number of places where palloc()/malloc()/etc. was used
> solely to obtain an aligned buffer.  We can do these much simpler by
> using alignas with a local variable instead.  See attached patch.

In XLogRecordAssemble, you could use a local variable in XLogInsert()
for the scratch buffer, and pass a pointer to that to
XLogRecordAssemble(). That would avoid the a global variable, which is
nice for multi-threading.

Not new with this patch, but the calculations for HEADER_SCRATCH_SIZE
are a little scary. I'd also suggest adding an assertion somewhere that
you don't overrun the HEADER_SCRATCH_SIZE buffer, and maybe some extra
comments. I don't see any bugs there, but it's very subtle that if you
e.g. add a new "special" block ID similar to XLR_BLOCK_ID_TOPLEVEL_XID,
you need to remember to adjust HEADER_SCRATCH_SIZE accordingly.

For the BLCKSZ-sized buffers, how about using PGAlignedBlock?

- Heikki