Suggestion - use local imports in Phobos

[Idan Arye]

On Tuesday, 4 June 2013 at 19:20:26 UTC, H. S. Teoh wrote:
> On Tue, Jun 04, 2013 at 08:59:15PM +0200, Idan Arye wrote:
> [...]
>> Due to these characteristics of Phobos, I believe making the 
>> imports
>> local to the unit tests and templates that use them will 
>> reduce the
>> number of imports the compiler has to do.
>
> This breaks DRY because some imports are used by multiple 
> unittests. As
> long as the imports are wrapped in version(unittest) blocks, I 
> don't see
> this as a problem.

That's like saying that defining a local `i` variable for using 
in a `for` loop breaks DRY, because you could have defined it 
once at global scope.

Importing the same module over and over does not break DRY, just 
like calling the same function in multiple places does not break 
DRY. Breaking DRY means to write the internal code of that 
function in several places - or implementing the same things in 
several modules.

> [...]
>> Also, I'm not really familiar with the internals of dmd - how 
>> much
>> impact will importing the same module many times have on the
>> compilation performance? Will it be more or less than what we 
>> save
>> by reducing the number of imported modules?
> [...]
>
> I may be wrong, but I seem to recall hearing/seeing somewhere 
> that a
> repeated import is just ignored by DMD, since it has already 
> loaded the
> imported module. If not, then DMD should be fixed to do that. :)

I'm pretty sure dmd ignores already loaded modules - if it 
didn't, trying to compile anything that uses Phobos would enter 
an infinite loop/recursion - and I'm not sure about ignoring 
already imported modules, but it probably does that too. Not 
doing it would have a major impact on compilation performance, 
considering that most Phobos modules import many of the other 
modules.

The problem is that ignoring an import, while cheaper than doing 
the import and much much much cheaper than loading the module, is 
not a free action. Before the compiler decides to ignore an 
import, it needs to perform a lookup to check that it was already 
imported. If an import is local to a template, this lookup will 
be performed every time the template is instantiated - and if the 
lookup is not fast enough, this can add up to harm performance.

I believe that lookup is pretty fast, but as I'm not familiar 
with the dmd implementation I can't know this for sure, so I 
thought it would be better to bring it up.