On Friday, 3 April 2015 at 17:55:00 UTC, Dicebot wrote:
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.
Interesting.
It's true that it's not always faster to compile each module
separately, I already knew that. It seems to me, however, that
when that's actually the case, the practical difference is
negligible. Even if 10x slower, the linker will take longer
anyway. Because it'll all still be under a second. That's been my
experience anyway. i.e. It's either faster or it doesn't make
much of a difference.
All I know is I've seen a definite improvement in my
edit-compile-unittest cycle by compiling modules separately.
How would the decoupling happen? Is the user supposed to
partition the binary into suitable static libraries? Or is the
system supposed to be smart enough to figure that out?
Atila
