[swift-users] Swift in bare-metal embedded programming/Swift runtime

[Slava Pestov]

Even if you take care not to create class instances, the compiler emits many calls to runtime functions to implement features such as generics, casts and existentials. It is possible to write code where a large number of runtime calls are optimized away, but I don’t think they can be eliminated completely.

If anyone is interested in taking this on as a community project, it would be a fair amount of work, but I think the first step could be to add compiler flags where calls to runtime functions become diagnostics. Again though, I’m not sure how much effort it would take to eliminate them completely.

Slava

> On Aug 10, 2016, at 3:28 AM, David Sweeris via swift-users <swift-users at swift.org> wrote:
> 
> There's definitely a runtime, but I *think* you can avoid actually using it by being very careful with your data structures. ARC means that classes obviously trigger it, and I think it *might* be involved resizing arrays and strings (they do some tricks behind the scenes, but I can't remember what).
> 
> So... only use structs and don't resize anything? I'm not sure... I think there might be some cases where protocols or indirect enums get stored as references, and that might involve the runtime as well.
> 
> Maybe you should go over to the evolution list and suggest a "no runtime" compiler flag or source code annotation, which disallows anything which would use the runtime. I believe there could be advantages outside of running on bare-metal, since you could use it to get the compiler to yell at you for doing overhead-inducing stuff in a loop, for example. 
> 
> Anyway, best of luck :-)
> 
> - Dave Sweeris
> 
>> On Aug 9, 2016, at 15:10, Rick Mann via swift-users <swift-users at swift.org> wrote:
>> 
>> Is it possible to use Swift for bare-metal programming on embedded devices? These devices usually have memory-mapped registers that are read and written to affect the operation of the device. Some can be quite small (e.g. 8-bit registers, simple single physical memory address space), and others quite robust (full 32- or 64-bit machines with MMUs, etc.).
>> 
>> But bare metal development for all of them starts with emitting code with "raw" packaging (no Mach or ELF headers, etc.), and the ability to read and write specific memory addresses.
>> 
>> For the smaller devices, runtime library overhead is a concern (mostly due to code size). Is it possible to write swift code with no runtime library? I think this is possible in Rust (came up on another list).
>> 
>> Most such development is done in C, and there is always some form of library to take on some of the standard library tasks and stub out basic IO, as well as filling in gaps for larger variable sizes not directly supported by the hardware.
>> 
>> I imagine there's some runtime support for ARC, although maybe that's handled entirely in the compilation phase?
>> 
>> Anyway, I'd appreciate someone more knowledgable letting me know if this is something I should experiment with. Thanks!
>> 
>> -- 
>> Rick Mann
>> rmann at latencyzero.com
>> 
>> 
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