AliasSeq seems to compile slightly faster with static foreach

[Jonathan M Davis]

On Friday, January 05, 2018 13:16:52 Meta via Digitalmars-d wrote:
> On Friday, 5 January 2018 at 13:10:25 UTC, Jonathan M Davis wrote:
> > There was a recent PR for Phobos where Seb added static to a
> > bunch of foreach's that used AliasSeq. It hadn't actually
> > occurred to me that that was legal (I've basically just been
> > using static foreach where foreach with AliasSeq doesn't work),
> > but it is legal (which I suppose isn't surprising when you
> > think about it; I just hadn't). However, that got me to
> > wondering if such a change was purely aesthetic or whether it
> > might actually have an impact on build times - particularly
> > since running dub test for one of my recent projects keeps
> > taking longer and longer. So, I added static to a bunch of
> > foreach's over AliasSeqs in that project to see if it would
> > have any effect. The result was that dub test went from about
> > 16.5 seconds on my system to about 15.8 seconds - and that's
> > just by adding static to the foreach's over AliasSeqs, not
> > fundamentally changing what any of the code did. That's not a
> > huge speed up, but it's definitely something and far more than
> > I was expecting.
> >
> > Of course, you have to be careful with such a change, because
> > static foreach doesn't introduce a new scope, and double braces
> > are potentially required where they weren't before, but given
> > that I'd very much like to streamline that test build, adding
> > static to those foreach's was surprisingly worthwhile.
> >
> > Taking it a step further, I tried switching some of the static
> > foreach's over to using array literals, since they held values
> > rather than types, and that seemed to have minimal impact on
> > the time to run dub test. However, by switching to using
> > std.range.only, it suddenly was taking more like 11.8 seconds.
> > So, with a few small changes, I cut the time to run dub test
> > down by almost a third.
> >
> > - Jonathan M Davis
>
> It does not make any sense to me as to why using only instead of
> AliasSeq resulted in a speedup. I would've expected no change or
> worse performance. Any theories?

I don't know. It's probably related to however foreach over an AliasSeq is
implemented. The fact that only is faster than using an array literal
doesn't surprise me though, since CTFE probably does a bunch of extra,
unnecessary allocation when dealing with array literal.

Maybe the speed difference between AliasSeq and only is a sign of something
that could be improved in the compiler's implementation, or maybe only is
just fundamentally faster for one reason or another. I don't know. But from
what I can tell, the speed difference is large enough that it's kind of
crazy to use AliasSeq with values when static foreach and only will work
just as well.

- Jonathan M Davis