Building the compiler in 2 seconds with `dmd -i`

[RazvanN]

On Friday, 19 May 2023 at 09:51:39 UTC, Dennis wrote:
> In theory, dmd is really easy to build: it's just a collection 
> of D files in a single repository with no external 
> dependencies. Why have there always been such complex 
> makefiles/scripts to build it?
>
> Well, after replacing most of dmd's backend's C-style extern 
> function declarations with proper imports [1], there's a simple 
> way to build the compiler. Instead of needing to list every 
> source file, you can leverage the `-i` flag which automatically 
> compiles imports, and create a build script as short as this:
>
> ```
> echo -n "/etc" >> SYSCONFDIR.imp
> cd compiler/src
> dmd -i dmd/mars.d -of=../../dmd dmd/eh.d dmd/backend/dtype.d 
> -version=MARS -Jdmd/res -J../..
> ```
>
> On my linux pc, these 3 lines take 2.0 seconds to complete and 
> it uses max 680MB RAM.
>
> The official way to build a non-release compiler is:
>
> ```
> cd compiler/src
> rdmd build.d BUILD=debug
> ```
>
> build.d is 2400 lines, takes 2.3 seconds to complete a clean 
> build, and uses max 613MB RAM.
>

I would suggest dropping the build.d file. Its main advantage is 
that you have the same script to compile the code regardless of 
the platform you are running on, however, we are now stuck 
between 2 worlds: dmd has build.d but druntime and phobos use 
makefiles.

Also, build.d is much more complicated and requires more time for 
handling than a measly makefile which anyone can understand. 
Every time I add a new file to the repo I waste time to read and 
understand how build.d works.

> The build script does separate compilation, multi-threading, 
> includes debug info (`-g`), and builds other things as well. 
> Still, my takeaways are:
>
> - The build script is overly complex
> - `dmd -i` is awesome
> - Without too much templates/CTFE, dmd compiles pretty fast 
> (150 KLOC/second in this case), reducing the need for 
> incremental/parallel compilation
>
> Eventually I hope to make just `dmd -i dmd/mars.d` work, I'll 
> see how close I can get to that.
>

Good luck!

> [1] 
> https://github.com/dlang/dmd/pulls?q=is%3Aclosed+is%3Apr+author%3Adkorpel+label%3ABackend+label%3ARefactoring+prototypes