On Friday, 3 April 2015 at 17:25:51 UTC, Ben Boeckel wrote:
On Fri, Apr 03, 2015 at 17:10:31 +0000, Dicebot via
Digitalmars-d-announce wrote:
On Friday, 3 April 2015 at 17:03:35 UTC, Atila Neves wrote:
> . Separate compilation. One file changes, only one file gets
> rebuilt
This immediately has caught my eye as huge "no" in the
description. We must ban C style separate compilation, there
is simply no way to move forward otherwise. At the very least
not endorse it in any way.
Why? Other than the -fversion=... stuff, what is really
blocking this? I
personally find unity builds to not be worth it, but I don't see
anything blocking separate compilation for D if dependencies
are set up
properly.
--Ben
There are 2 big problems with C-style separate compilation:
1)
Complicates whole-program optimization possibilities. Old school
object files are simply not good enough to preserve information
necessary to produce optimized builds and we are not in position
to create own metadata + linker combo to circumvent that. This
also applies to attribute inference which has become a really
important development direction to handle growing attribute hell.
During last D Berlin Meetup we had an interesting conversation on
attribute inference topic with Martin Nowak and dropping legacy
C-style separate compilation seemed to be recognized as
unavoidable to implement anything decent in that domain.
2)
Ironically, it is just very slow. Those who come from C world got
used to using separate compilation to speed up rebuilds but it
doesn't work that way in D. It may look better if you change only
1 or 2 module but as amount of modified modules grows,
incremental rebuild quickly becomes _slower_ than full program
build with all files processed in one go. It can sometimes result
in order of magnitude slowdown (personal experience).
Difference from C is that repeated imports are very cheap in D
(you don't copy-paste module content again and again like with
headers) but at the same time semantic analysis of imported
module is more expensive (because D semantics are more
complicated). When you do separate compilation you discard
already processed imports and repeat it again and again from the
very beginning for each new compiled file, accumulating huge
slowdown for application in total.
To get best compilation speed in D you want to process as many
modules with shared imports at one time as possible. At the same
time for really big projects it becomes not feasible at some
point, especially if CTFE is heavily used and memory consumption
explodes. In that case best approach is partial separate
compilation - decoupling parts of a program as static libraries
and doing parallel compilation of each separate library - but
still compiling each library in one go. That allows to get
parallelization without doing the same costly work again and
again.
