Those intrinsics have the correct semantics for the desired fcvtz instruction,
without any undefined behaviour. The previous simd_cast was undefined for
infinite and NaN which could cause issues.
This lint is allow by default, which is why this wasn't spotted earlier.
It's denied by rust-lang/rust, so it's good to warn about it here so it
can be fixed more quickly.
Lots of time and lots of things have happened since the simd128 support
was first added to this crate. Things are starting to settle down now so
this commit syncs the Rust intrinsic definitions with the current
specification (https://github.com/WebAssembly/simd). Unfortuantely not
everything can be enabled just yet but everything is in the pipeline for
getting enabled soon.
This commit also applies a major revamp to how intrinsics are tested.
The intention is that the setup should be much more lightweight and/or
easy to work with after this commit.
At a high-level, the changes here are:
* Testing with node.js and `#[wasm_bindgen]` has been removed. Instead
intrinsics are tested with Wasmtime which has a nearly complete
implementation of the SIMD spec (and soon fully complete!)
* Testing is switched to `wasm32-wasi` to make idiomatic Rust bits a bit
easier to work with (e.g. `panic!)`
* Testing of this crate's simd128 feature for wasm is re-enabled. This
will run on CI and both compile and execute intrinsics. This should
bring wasm intrinsics to the same level of parity as x86 intrinsics,
for example.
* New wasm intrinsics have been added:
* `iNNxMM_loadAxA_{s,u}`
* `vNNxMM_load_splat`
* `v8x16_swizzle`
* `v128_andnot`
* `iNNxMM_abs`
* `iNNxMM_narrow_*_{u,s}`
* `iNNxMM_bitmask` - commented out until LLVM is updated to LLVM 11
* `iNNxMM_widen_*_{u,s}` - commented out until
bytecodealliance/wasmtime#1994 lands
* `iNNxMM_{max,min}_{u,s}`
* `iNNxMM_avgr_u`
* Some wasm intrinsics have been removed:
* `i64x2_trunc_*`
* `f64x2_convert_*`
* `i8x16_mul`
* The `v8x16.shuffle` instruction is exposed. This is done through a
`macro` (not `macro_rules!`, but `macro`). This is intended to be
somewhat experimental and unstable until we decide otherwise. This
instruction has 16 immediate-mode expressions and is as a result
unsuited to the existing `constify_*` logic of this crate. I'm hoping
that we can game out over time what a macro might look like and/or
look for better solutions. For now, though, what's implemented is the
first of its kind in this crate (an architecture-specific macro), so
some extra scrutiny looking at it would be appreciated.
* Lots of `assert_instr` annotations have been fixed for wasm.
* All wasm simd128 tests are uncommented and passing now.
This is still missing tests for new intrinsics and it's also missing
tests for various corner cases. I hope to get to those later as the
upstream spec itself gets closer to stabilization.
In the meantime, however, I went ahead and updated the `hex.rs` example
with a wasm implementation using intrinsics. With it I got some very
impressive speedups using Wasmtime:
test benches::large_default ... bench: 213,961 ns/iter (+/- 5,108) = 4900 MB/s
test benches::large_fallback ... bench: 3,108,434 ns/iter (+/- 75,730) = 337 MB/s
test benches::small_default ... bench: 52 ns/iter (+/- 0) = 2250 MB/s
test benches::small_fallback ... bench: 358 ns/iter (+/- 0) = 326 MB/s
or otherwise using Wasmtime hex encoding using SIMD is 15x faster on 1MB
chunks or 7x faster on small <128byte chunks.
All of these intrinsics are still unstable and will continue to be so
presumably until the simd proposal in wasm itself progresses to a later
stage. Additionaly we'll still want to sync with clang on intrinsic
names (or decide not to) at some point in the future.
* wasm: Unconditionally expose SIMD functions
This commit unconditionally exposes SIMD functions from the `wasm32`
module. This is done in such a way that the standard library does not
need to be recompiled to access SIMD intrinsics and use them. This,
hopefully, is the long-term story for SIMD in WebAssembly in Rust.
It's unlikely that all WebAssembly runtimes will end up implementing
SIMD so the standard library is unlikely to use SIMD any time soon, but
we want to make sure it's easily available to folks! This commit enables
all this by ensuring that SIMD is available to the standard library,
regardless of compilation flags.
This'll come with the same caveats as x86 support, where it doesn't make
sense to call these functions unless you're enabling simd support one
way or another locally. Additionally, as with x86, if you don't call
these functions then the instructions won't show up in your binary.
While I was here I went ahead and expanded the WebAssembly-specific
documentation for the wasm32 module as well, ensuring that the current
state of SIMD/Atomics are documented.
