Though CloudABI is strongly inspired by POSIX, its absence of features
that don't work well with capability-based sandboxing makes it different
enough that adding bits to sys/unix will make things a mess. This change
therefore adds CloudABI specific platform code under sys/cloudabi and
borrows parts from sys/unix that can be used without changes.
One of the goals of this implementation is to build as much as possible
directly on top of CloudABI's system call layer, as opposed to using the
C library. This is preferred, as the system call layer is supposed to be
stable, whereas the C library ABI technically is not. An advantage of
this approach is that it allows us to implement certain interfaces, such
as mutexes and condition variables more optimally. They can be lighter
than the ones provided by pthreads.
This change disables some modules that cannot realistically be
implemented right now. For example, libstd's pathname abstraction is not
designed with POSIX *at() (e.g., openat()) in mind. The *at() functions
are the only set of file system APIs available on CloudABI. There is no
global file system namespace, nor a process working directory.
Discussions on how to port these modules over are outside the scope of
this change.
Apart from this change, there are still some other minor fixups that
need to be made to platform independent code to make things build. These
will be sent out separately, so they can be reviewed more thoroughly.
This commit alters how we compile LLVM by default enabling the WebAssembly
backend. This then also adds the wasm32-unknown-unknown target to get compiled
on the `cross` builder and distributed through rustup. Tests are not yet enabled
for this target but that should hopefully be coming soon!
This commit adds a new target to the compiler: wasm32-unknown-unknown. This
target is a reimagining of what it looks like to generate WebAssembly code from
Rust. Instead of using Emscripten which can bring with it a weighty runtime this
instead is a target which uses only the LLVM backend for WebAssembly and a
"custom linker" for now which will hopefully one day be direct calls to lld.
Notable features of this target include:
* There is zero runtime footprint. The target assumes nothing exists other than
the wasm32 instruction set.
* There is zero toolchain footprint beyond adding the target. No custom linker
is needed, rustc contains everything.
* Very small wasm modules can be generated directly from Rust code using this
target.
* Most of the standard library is stubbed out to return an error, but anything
related to allocation works (aka `HashMap`, `Vec`, etc).
* Naturally, any `#[no_std]` crate should be 100% compatible with this new
target.
This target is currently somewhat janky due to how linking works. The "linking"
is currently unconditional whole program LTO (aka LLVM is being used as a
linker). Naturally that means compiling programs is pretty slow! Eventually
though this target should have a linker.
This target is also intended to be quite experimental. I'm hoping that this can
act as a catalyst for further experimentation in Rust with WebAssembly. Breaking
changes are very likely to land to this target, so it's not recommended to rely
on it in any critical capacity yet. We'll let you know when it's "production
ready".
---
Currently testing-wise this target is looking pretty good but isn't complete.
I've got almost the entire `run-pass` test suite working with this target (lots
of tests ignored, but many passing as well). The `core` test suite is still
getting LLVM bugs fixed to get that working and will take some time. Relatively
simple programs all seem to work though!
---
It's worth nothing that you may not immediately see the "smallest possible wasm
module" for the input you feed to rustc. For various reasons it's very difficult
to get rid of the final "bloat" in vanilla rustc (again, a real linker should
fix all this). For now what you'll have to do is:
cargo install --git https://github.com/alexcrichton/wasm-gc
wasm-gc foo.wasm bar.wasm
And then `bar.wasm` should be the smallest we can get it!
---
In any case for now I'd love feedback on this, particularly on the various
integration points if you've got better ideas of how to approach them!
