Building LDC runtime for a microcontroller

[IGotD-]

On Monday, 7 September 2020 at 15:23:28 UTC, Severin Teona wrote:
>
> I would also appreciate any advice regarding ways to build or 
> create a small runtime for microcontrollers (runtime that can 
> fit in the memory of a microcontroller).
> Thank you very much,
> Teona
>
> [1]: https://wiki.dlang.org/Building_LDC_runtime_libraries
> [2]: 
> https://github.com/golang/go/issues/36633#issuecomment-576411479

Use betterC, which is much better suited for microcontrollers 
than the full D. The disadvantage is that many great features are 
disabled in betterC.

I have ported druntime/phobos to my system. This is pretty large 
job because structure of druntime/phobos is not very good for 
adding/porting to new systems. It's a cascade of version(this) {} 
else version(that) {}. Some functionality must be ported, some 
others can just be stubbed.

Keep in mind that you in general have to port phobos as well 
because it contains many useful functions like parsing and 
conversion. The OS dependent stuff is mixed together with OS 
independent.

For an ARM target I get about a compiled size of about 500KB of a 
simple Hello world program when linked statically. This isn't 
really microcontroller size to begin with. The size quickly 
increases as you start to use more modules from druntime/phobos.

Another interesting observation is that druntime has a option to 
use malloc/free in a clib rather than map/VirtualAlloc for GC 
allocations. What druntime does is over allocate because it 
requires page aligned memory. The result is that this workaround 
waste a lot of memory.

The conclusion is that D as it is isn't really suitable for 
systems that are memory limited or lack an MMU (reason is that 
shared libraries don't work). D is like C++ will full STL support 
which is also very large. Embedded programmers who use C++ almost 
never use STL because of this, among other things.