Re: Performance improvements for src/port/snprintf.c

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Performance improvements for src/port/snprintf.c
Date: 2018-10-03 17:18:35
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I wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
>> - I know it's not new, but is it actually correct to use va_arg(args, int64)

> Well, the problem with just doing s/int64/long long/g is that the
> code would then fail on compilers without a "long long" type.
> We could ifdef our way around that, but I don't think the code would
> end up prettier.

I spent a bit more time thinking about that point. My complaint about
lack of long long should be moot given that we're now requiring C99.
So the two cases we need to worry about are (1) long long exists and
is 64 bits, and (2) long long exists and is wider than 64 bits. In
case (1) there's nothing actively wrong with the code as it stands.
In case (2), if we were to fix the problem by s/int64/long long/g,
the result would be that we'd be doing the arithmetic for all
integer-to-text conversions in 128 bits, which seems likely to be
pretty expensive.

So a "real" fix would probably require having separate versions of
fmtint for long and long long. I'm not terribly excited about
going there. I can see it happening some day when/if we need to
use 128-bit math more extensively than today, but I do not think
that day is close. (Are there *any* platforms where "long long"
is 128 bits today?)

Having said that, maybe there's a case for changing the type spec
in only the va_arg() call, and leaving snprintf's related local
variables as int64. (Is that what you actually meant?) Then,
if long long really is different from int64, at least we have
predictable truncation behavior after fetching the value, rather
than undefined behavior while fetching it.

regards, tom lane

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