Update to Bare Metal STM32F4 (ARM Cortex-M4) LCD Demo Proof of Concept

[Mike via Digitalmars-d-announce]

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!

Mike

[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/iqryqssxooypdnszmzmg@forum.dlang.org