Re: Re: [PATCH] unified frontend support for pg_malloc et al and palloc/pfree mulation (was xlogreader-v4)

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
>>> It does *not* combine elog_start and elog_finish into one function if
>>> varargs are available although that brings a rather measurable
>>> size/performance benefit.

>> Since you've apparently already done the measurement: how much?
>> It would be a bit tedious to support two different infrastructures for
>> elog(), but if it's a big enough win maybe we should.

> Imo its pretty definitely a big enough win. So big I have a hard time
> believing it can be true without negative effects somewhere else.

Well, actually there's a pretty serious negative effect here, which is
that when it's implemented this way it's impossible to save errno for %m
before the elog argument list is evaluated. So I think this is a no-go.
We've always had the contract that functions in the argument list could
stomp on errno without care.

If we switch to a do-while macro expansion it'd be possible to do
something like

do {
int save_errno = errno;
int elevel = whatever;

elog_internal( save_errno, elevel, fmt, __VA__ARGS__ );
} while (0);

but this would almost certainly result in more code bloat not less,
since call sites would now be responsible for fetching errno.

regards, tom lane