Re: A qsort template

On Sat, Mar 13, 2021 at 3:49 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Fri, Mar 12, 2021 at 7:58 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > I wish we had the same for bsearch... :)
>
> Glibc already has the definition of the traditional void-based
> function in /usr/include/bits/stdlib-bsearch.h, so the generated code
> when the compiler can see the comparator definition is already good in
> eg lazy_tid_reaped() and eg some nbtree search routines. We could
> probably expose more trivial comparators in headers to get more of
> that, and we could perhaps put our own bsearch definition in a header
> for other platforms that didn't think of that...
>
> It might be worth doing type-safe macro templates as well, though (as
> I already did in an earlier proposal[1]), just to have nice type safe
> code though, not sure, I'm thinking about that...

I remembered a very good reason to do this: the ability to do
branch-free comparators in more places by introducing optional wider
results. That's good for TIDs (needs 49 bits), and places that want
to "reverse" a traditional comparator (just doing -result on an int
comparator that might theoretically return INT_MIN requires at least
33 bits). So I rebased the relevant parts of my earlier version, and
went through and wrote a bunch of examples to demonstrate all this
stuff actually working.

There are two categories of change in these patches:

0002-0005: Places that sort/unique/search OIDs, BlockNumbers and TIDs,
which can reuse a small set of typed functions (a few more could be
added, if useful). See sortitemptr.h and sortscalar.h. Mostly this
is just a notational improvement, and an excuse to drop a bunch of
duplicated code. In a few places this might really speed something
important up! Like VACUUM's lazy_tid_reaped().

0006-0009. Places where a specialised function is generated for one
special purpose, such as ANALYZE's HeapTuple sort, tidbitmap.c's
pagetable sort, some places in nbtree code etc. These may require
some case-by-case research on whether the extra executable size is
worth the speedup, and there are surely more opportunities like that;
I just picked on these arbitrarily.