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

Hi,

On 2018-10-05 11:54:59 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > [ let's use strfromd ]
> 
> So I'm having second thoughts about this, based on the fact that
> strfromd() in't strictly a glibc-ism but is defined in an ISO/IEC
> standard.  That means that we can expect to see it start showing up
> on other platforms (though a quick search did not find any evidence
> that it has yet).  And that means that we'd better consider
> quality-of-implementation issues.  We know that glibc's version is
> fractionally faster than using sprintf with "%.*g", but what are
> the odds that that will be true universally?  I don't have a warm
> feeling about it, given that strfromd's API isn't a very good impedance
> match to what we really need.
> 
> I really think that what we ought to do is apply the float[48]out hack
> I showed in <30551.1538517271@sss.pgh.pa.us> and call it good, at least
> till such time as somebody wants to propose a full-on reimplementation of
> float output.  I don't want to buy back into having platform dependencies
> in this area after having just expended a lot of sweat to get rid of them.

I'm not convinced. Because of some hypothetical platform that may
introduce strfromd() in a broken/slower manner, but where sprintf() is
correct, we should not do the minimal work to alleviate an actual
performance bottleneck in a trivial manner on linux? Our most widely
used platform?  If we find a platform where it's borked, we could just
add a small hack into their platform template file.

Greetings,

Andres Freund