Who can make Phobos faster to import?

[Andrei Alexandrescu]

On 12/15/2017 02:10 PM, Seb wrote:
> Following the discussion at https://github.com/dlang/phobos/pull/5916, 
> we realized that there's quite some overhead in importing some Phobos 
> modules. Ideally importing a Phobos module without using it, should be 
> cost-free (i.e. pay for what you use).
> 
> Test case: consider a file with `import std.X` and compile it with `dmd 
> -c -o-`.
> 
> A quick overview of the worst of the worst:
> 
> std.regex    1.03s
> std.net.curl    0.28s
> std.concurrency    0.23s
> std.encoding    0.18s
> std.zip            0.16s
> std.path    0.14s
> std.file    0.14s
> std.mmfile    0.14s
> std.datetime    0.14s
> std.socket    0.12s
> std.string    0.10s
> std.uni            0.09s
> std.process    0.05s
> 
> This following discussions have already brought a huge improvements to 
> its light:
> 
> https://github.com/dlang/phobos/pull/5931
> 
> Can you do a another one?

Dmitry wrote a nice PR for that, and I wrote two:

https://github.com/dlang/phobos/pull/5942
https://github.com/dlang/phobos/pull/5931
https://github.com/dlang/phobos/pull/5948

My approach has been the following:

* First I tried to clarify which module imports what by means of 
replacing "import std.xxx;" with "import std.xxx : yyy, zzz, ...;" That 
has been of tremendous help in assessing each module's dependency liability.

* Then I tried to figure which imports are most costly. I inserted 
__EOF__ in the module at different parts and measure how long it takes 
to just import that module. After a few attempts it becomes clear which 
imports are heaviest.

* Then I pushed those heavy imports down where they're needed. An import 
placed in a function will not be opened while that function's module 
gets imported. Same goes about template types, but not structs or 
classes. (That should be fixed; see Seb's comment in my last PR.)

Not only such work reduces import times, but it's illuminating for 
maintenance because it represents dependencies in a simple, fine-grained 
matter.

A tool (call it depend - heh) to automate that would be awesome. For 
example, this run would make all imported names explicit:

depend --explicit *.d

This run would push all imports down to the innermost scope of usage:

depend --pushdown *.d


Andrei