A few years ago I created a bare metal demo on an ARM Cortex-M4 microcontroller entirely in D. It was just a demonstration that one could do bare metal programming for microcontrollers in D without any dependencies on C or assembly. It was also a proof of some ideas I had about leveraging compile-time features of D to generate highly-optimized code (both small and fast) for these resource constrained systems. I hit a wall, however, with Issue 14758[0], and ultimately abandoned D for other alternatives.

Well, that issue was recently fixed in GDC [1]. In addition, he GDC developers did some work to reduce the number of phony stubs one had to add to the runtime to get a build [2], removed the "shared is volatile" hack, and implemented the `volatileLoad/Store` intrinsics so I no longer need to do volatile access in assembly. So, I decided to give it another try, and updated that demo. You can see the results at https://github.com/JinShil/stm32f42_discovery_demo

A few observations:
* It is a better experience than it was a few years ago. Fewer dirty hacks are required, and the resulting binary is small and fast. * Everything is in D (inline assembly is D). There's no need for any C or assembly startup code, and no need for silly things like -betterC (i.e. -worseD). If you don't want the overhead from a feature of D, don't use it. * Compile times are quite slow (about 1 minute to get a 3kB binary). Some discussion about that is taking place on the GDC forum [4]. * -O2 and -O3 give me a broken binary, but -O0, -O1, and -Os work well.

I'm not sure where I'll go from here. I'm interested in helping an amputee play the drums again, and building my own mechanical keyboard, so I probably won't be spending much more time on this LCD demo, except maybe to help compiler devs debug some issues. However, I might spend some more time with D in the near future, and see how far I can take this. I'm still not as excited as I once was, but it's nice to see these improvements.

Anyway, Well done, GDC!


[0] - TypeInfo causes excessive binary bloat - https://issues.dlang.org/show_bug.cgi?id=14758 [1] - Put the TypeInfo name field into a static var - https://github.com/D-Programming-GDC/GDC/pull/505 [2] - Refactor and reformat typeinfo.cc - https://github.com/D-Programming-GDC/GDC/pull/456 [3] - Slow compile-time discussion at GDC forum - http://forum.dlang.org/post/iqryqssxooypdnszm...@forum.dlang.org

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