On 12/22/2016 05:17 AM, Joakim wrote:
On Wednesday, 21 December 2016 at 15:16:34 UTC, Andrei Alexandrescu wrote:
In first approximation, whether a file gets opened or not makes a
difference (filesystem operations (possibly including networking),
necessity to rebuild if dependent code is changed. The analysis shows
there is significant overhead remaining, on average 10.5 additional
files per unused import.
Opening a file or 10 is extremely cheap compared to all the other costs
of the compiler. Purely from a technical cost perspective, I'm not sure
even scoped imports were worth it, as my simple investigation below
suggests.
This is a misunderstanding. (I'll make the DIP clearer.) It's not about
the cost of opening the file per se; I'm using the number of files
opened as a proxy for all work involved in processing a file. Meaning,
if you import 10 files you're likely to do roughly 10x the work of
importing one file.
I just tried compiling phobos and its unittests for dmd 2.066.1 and
2.067.1, the dmd releases from right before and right after Ilya's PRs
linked above (compiling phobos from the older release with the newer dmd
didn't work and I wasn't interested in porting it). I found they took
about the same amount of time to compile their respective phobos and the
later version consistently took 15 seconds longer to compile the
unittests on a single core.
Unittesting Phobos (whether in separation or together) will instantiate
everything so they are the case when local imports do _not_ make any
difference. That's why DIP1005 uses that case as an estimate of
everything in an imported module being used.
Compiling Phobos in its entirety (one command) and measuring that time
is also of tenuous relevance; I need to think of it but at first sight
the cost of unnecessary imports is collapsed, and I suspect (and will
measure) that either way most modules are in fact imported.
One measurement of interest is: write a module that imports exactly one
stdlib module, compile it, and measure time. Looking at the generated
object and executable size would be also of interest. I'll do that.
This suggests that there has been essentially no technical benefit to
scoped imports, that it is largely superficial. That's fine, I think
it's worth it just from the standpoint of understanding where most
dependencies are coming from. I don't think an additional syntax change
is necessary for the remaining few module-scope dependencies for
template constraints.
Note on investigation: Of course, some other relevant factors could have
changed in dmd and phobos between those two releases, so the result is
certainly not conclusive. But the fact that there was no decrease in
compile times while phobos took around the same amount of time is highly
suggestive.
I'm uninterested in investigating further given the consistent
hand-waving justifications for this DIP. If somebody else had submitted
this DIP, it would have been quickly shot down, and rightly so.
Walter and I have the role of scrutinizing every addition to the
language (and reject most). It is natural that our own work is met with
increased scrutiny. Picking to death anything and everything we say or
do is a staple in this community, and a rite of passage on github. It is
of course impossible to know what would have happened if the proposal
were made by someone else. All I can say is Walter knew nothing about it
and said it is good (except for the initial syntax; he's on board with
"with").
Anyhow, not to worry. The burden of proof is on the DIP. I'll take a
look at making some more measurements.
Andrei
