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?
