[swift-evolution] Low-level Swift

Joe Groff jgroff at apple.com
Thu Jan 7 15:25:43 CST 2016

> On Jan 7, 2016, at 3:52 AM, Romain Goyet via swift-evolution <swift-evolution at swift.org> wrote:
> Hi everyone,
> In the introduction to Swift <https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/>, it is presented as an "industrial-quality systems programming language".
> According to Wikipedia <https://en.wikipedia.org/wiki/System_programming_language>, this means that I could expect to be able to write device drivers or operating systems in Swift.
> However it seems like this claim is only partially true, since several important low-level features seem to be completely missing:
> - Is it possible to generate volatile memory accesses <http://llvm.org/docs/LangRef.html#volatile-memory-accesses> in Swift? Writing device drivers is virtually impossible without a way to guarantee certain memory operations are actually made (and not optimized-out). 

We haven't designed APIs for this yet, but the LLVM instructions for volatile and atomic operations are available in the stdlib-private 'Builtin' module, should you want to experiment.

> - Is it possible to embed raw binary data in Swift? For example, an equivalent of the following C code "const int8_t foo[6] = {0x00,0x11,0x22,0x33,0x44,0x55};"? The simple Swift equivalent, "let foo: [Int8] = [0x0,0x1,0x2,0x3,0x4,0x6];" is obviously wildly different. Being able to embed raw binary data is very important in a low-level environment: for example, you may need to feed specific data to a device for an initialization sequence at a point where you don't have a filesystem available yet.

We also haven't designed a story for this yet. We have some support for optimizing constant literal initializations like this into static initializers, but need to do more work to be able to guarantee it.

> There might be other aspects that I'm overlooking right now, but I think that's enough to start a discussion: is Swift really meant to be a systems programming language?

Like Austin said, that's a long term goal, and I think you can get pretty far today if you're careful and understand the compiler and runtime, but we're a long ways away from really productizing our low-level programming support to a degree I'd feel comfortable advertising.

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