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

Here's a version of this patch rebased over commit 625b38ea0.

That commit's fix for the possibly-expensive memset means that we need
to reconsider performance numbers for this patch. I re-ran my previous
tests, and it's still looking like this is a substantial win, as it makes
snprintf.c faster than the native snprintf for most non-float cases.
We're still stuck at something like 10% penalty for float cases.

While there might be value in implementing our own float printing code,
I have a pretty hard time getting excited about the cost/benefit ratio
of that. I think that what we probably really ought to do here is hack
float4out/float8out to bypass the extra overhead, as in the 0002 patch
below.

For reference, I attach the testbed I'm using now plus some results.
I wasn't able to get my cranky NetBSD system up today, so I don't
have results for that. However, I did add recent glibc (Fedora 28)
to the mix, and I was interested to discover that they seem to have
added a fast-path for format strings that are exactly "%s", just as
NetBSD did. I wonder if we should reconsider our position on doing
that. It'd be a simple enough addition...

regards, tom lane