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Improve the emscripten backend (#172) 

* Use the js! macro from stdweb

* Rework the Buffer::finish method

* Use references from stdweb

* Fix emscripten warnings

* Rework the run() method to use stdweb

* Adjust timings

* Add entry in CHANGELOG
This commit is contained in:
tomaka 2017-10-22 14:17:25 +02:00 committed by GitHub
parent f7c503ff05
commit c524f63000
4 changed files with 126 additions and 123 deletions
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2
CHANGELOG.md
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@ -1,5 +1,7 @@
# Unreleased
- Changed the emscripten backend to consume less CPU.
# Version 0.5.1 (2017-10-21)
- Added `Sample::to_i16()`, `Sample::to_u16()` and `Sample::from`.

3
Cargo.toml
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@ -22,3 +22,6 @@ libc = "0.2"
[target.'cfg(any(target_os = "macos", target_os = "ios"))'.dependencies]
coreaudio-rs = "0.7.0"
[target.'cfg(target_os = "emscripten")'.dependencies]
stdweb = { version = "0.1.3", default-features = false }

237
src/emscripten/mod.rs
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@ -1,7 +1,12 @@
use std::marker::PhantomData;
use std::os::raw::c_char;
use std::os::raw::c_int;
use std::mem;
use std::os::raw::c_void;
use std::slice::from_raw_parts;
use std::sync::Mutex;
use stdweb;
use stdweb::Reference;
use stdweb::unstable::TryInto;
use stdweb::web::set_timeout;
use stdweb::web::TypedArray;
use CreationError;
use Format;
@ -10,12 +15,6 @@ use Sample;
use SupportedFormat;
use UnknownTypeBuffer;
extern {
fn emscripten_set_main_loop_arg(_: extern fn(*mut c_void), _: *mut c_void, _: c_int, _: c_int);
fn emscripten_run_script(script: *const c_char);
fn emscripten_run_script_int(script: *const c_char) -> c_int;
}
// The emscripten backend works by having a global variable named `_cpal_audio_contexts`, which
// is an array of `AudioContext` objects. A voice ID corresponds to an entry in this array.
//
@ -25,121 +24,115 @@ extern {
// that is in each buffer ; this is obviously bad, and also the schedule is too tight and there may
// be underflows
pub struct EventLoop;
pub struct EventLoop {
voices: Mutex<Vec<Option<Reference>>>,
}
impl EventLoop {
#[inline]
pub fn new() -> EventLoop {
EventLoop
}
stdweb::initialize();
#[inline]
pub fn run<F>(&self, mut callback: F) -> !
where F: FnMut(VoiceId, UnknownTypeBuffer)
{
unsafe {
// The `run` function uses `emscripten_set_main_loop_arg` to invoke a Rust callback
// repeatidely. The job of this callback is to fill the content of the audio buffers.
// The first argument of the callback function (a `void*`) is a casted pointer to the
// `callback` parameter that was passed to `run`.
extern "C" fn callback_fn<F>(callback_ptr: *mut c_void)
where F: FnMut(VoiceId, UnknownTypeBuffer)
{
unsafe {
let num_contexts = emscripten_run_script_int("(function() {
if (window._cpal_audio_contexts)
return window._cpal_audio_contexts.length;
else
return 0;
})()\0".as_ptr() as *const _);
// TODO: this processes all the voices, even those from maybe other event loops
// this is not a problem yet, but may become one in the future?
for voice_id in 0 .. num_contexts {
let callback_ptr = &mut *(callback_ptr as *mut F);
let buffer = Buffer {
temporary_buffer: vec![0.0; 44100 * 2 / 3],
voice_id: voice_id,
marker: PhantomData,
};
callback_ptr(VoiceId(voice_id), ::UnknownTypeBuffer::F32(::Buffer { target: Some(buffer) }));
}
}
}
let callback_ptr = &mut callback as *mut F as *mut c_void;
emscripten_set_main_loop_arg(callback_fn::<F>, callback_ptr, 3, 1);
unreachable!()
EventLoop {
voices: Mutex::new(Vec::new()),
}
}
#[inline]
pub fn build_voice(&self, _: &Endpoint, format: &Format)
pub fn run<F>(&self, callback: F) -> !
where F: FnMut(VoiceId, UnknownTypeBuffer)
{
// The `run` function uses `set_timeout` to invoke a Rust callback repeatidely. The job
// of this callback is to fill the content of the audio buffers.
// The first argument of the callback function (a `void*`) is a casted pointer to `self`
// and to the `callback` parameter that was passed to `run`.
fn callback_fn<F>(user_data_ptr: *mut c_void)
where F: FnMut(VoiceId, UnknownTypeBuffer)
{
unsafe {
let user_data_ptr2 = user_data_ptr as *mut (&EventLoop, F);
let user_data = &mut *user_data_ptr2;
let user_cb = &mut user_data.1;
let voices = user_data.0.voices.lock().unwrap().clone();
for (voice_id, voice) in voices.iter().enumerate() {
let voice = match voice.as_ref() {
Some(v) => v,
None => continue,
};
let buffer = Buffer {
temporary_buffer: vec![0.0; 44100 * 2 / 3],
voice: &voice,
};
user_cb(VoiceId(voice_id), ::UnknownTypeBuffer::F32(::Buffer { target: Some(buffer) }));
}
set_timeout(|| callback_fn::<F>(user_data_ptr), 330);
}
}
let mut user_data = (self, callback);
let user_data_ptr = &mut user_data as *mut (_, _);
set_timeout(|| callback_fn::<F>(user_data_ptr as *mut _), 10);
stdweb::event_loop();
}
#[inline]
pub fn build_voice(&self, _: &Endpoint, _format: &Format)
-> Result<VoiceId, CreationError>
{
// TODO: find an empty element in the array first, instead of pushing at the end, in case
// the user creates and destroys lots of voices?
let voice = js!(return new AudioContext()).into_reference().unwrap();
let num = unsafe {
emscripten_run_script_int(concat!(r#"(function() {
if (!window._cpal_audio_contexts)
window._cpal_audio_contexts = new Array();
window._cpal_audio_contexts.push(new AudioContext());
return window._cpal_audio_contexts.length - 1;
})()"#, "\0").as_ptr() as *const _)
let mut voices = self.voices.lock().unwrap();
let voice_id = if let Some(pos) = voices.iter().position(|v| v.is_none()) {
voices[pos] = Some(voice);
pos
} else {
let l = voices.len();
voices.push(Some(voice));
l
};
Ok(VoiceId(num))
Ok(VoiceId(voice_id))
}
#[inline]
pub fn destroy_voice(&self, voice_id: VoiceId) {
unsafe {
let script = format!("
if (window._cpal_audio_contexts)
window._cpal_audio_contexts[{}] = null;\0", voice_id.0);
emscripten_run_script(script.as_ptr() as *const _)
}
self.voices.lock().unwrap()[voice_id.0] = None;
}
#[inline]
pub fn play(&self, voice_id: VoiceId) {
unsafe {
let script = format!("
if (window._cpal_audio_contexts)
if (window._cpal_audio_contexts[{v}])
window._cpal_audio_contexts[{v}].resume();\0", v = voice_id.0);
emscripten_run_script(script.as_ptr() as *const _)
}
let voices = self.voices.lock().unwrap();
let voice = voices.get(voice_id.0).and_then(|v| v.as_ref()).expect("invalid voice ID");
js!(@{voice}.resume());
}
#[inline]
pub fn pause(&self, voice_id: VoiceId) {
unsafe {
let script = format!("
if (window._cpal_audio_contexts)
if (window._cpal_audio_contexts[{v}])
window._cpal_audio_contexts[{v}].suspend();\0", v = voice_id.0);
emscripten_run_script(script.as_ptr() as *const _)
}
let voices = self.voices.lock().unwrap();
let voice = voices.get(voice_id.0).and_then(|v| v.as_ref()).expect("invalid voice ID");
js!(@{voice}.suspend());
}
}
// Index within the `_cpal_audio_contexts` global variable in Javascript.
// Index within the `voices` array of the events loop.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct VoiceId(c_int);
pub struct VoiceId(usize);
// Detects whether the `AudioContext` global variable is available.
fn is_webaudio_available() -> bool {
unsafe {
emscripten_run_script_int(concat!(r#"(function() {
if (!AudioContext) { return 0; } else { return 1; }
})()"#, "\0").as_ptr() as *const _) != 0
}
stdweb::initialize();
js!(
if (!AudioContext) { return false; } else { return true; }
).try_into().unwrap()
}
// Content is false if the iterator is empty.
@ -201,8 +194,7 @@ pub type SupportedFormatsIterator = ::std::vec::IntoIter<SupportedFormat>;
pub struct Buffer<'a, T: 'a> where T: Sample {
temporary_buffer: Vec<T>,
voice_id: c_int,
marker: PhantomData<&'a mut T>,
voice: &'a Reference,
}
impl<'a, T> Buffer<'a, T> where T: Sample {
@ -218,37 +210,36 @@ impl<'a, T> Buffer<'a, T> where T: Sample {
#[inline]
pub fn finish(self) {
unsafe {
// TODO: **very** slow
let src_data = self.temporary_buffer.iter().map(|&b| b.to_f32().to_string() + ", ").fold(String::new(), |mut a, b| { a.push_str(&b); a });
// TODO: directly use a TypedArray<f32> once this is supported by stdweb
debug_assert_eq!(self.temporary_buffer.len() % 2, 0); // TODO: num channels
let typed_array = {
let t_slice: &[T] = self.temporary_buffer.as_slice();
let u8_slice: &[u8] = unsafe { from_raw_parts(t_slice.as_ptr() as *const _, t_slice.len() * mem::size_of::<T>()) };
let typed_array: TypedArray<u8> = u8_slice.into();
typed_array
};
let script = format!("(function() {{
if (!window._cpal_audio_contexts)
return;
var context = window._cpal_audio_contexts[{voice_id}];
if (!context)
return;
var buffer = context.createBuffer({num_channels}, {buf_len} / {num_channels}, 44100);
var src = [{src_data}];
for (var channel = 0; channel < {num_channels}; ++channel) {{
var buffer_content = buffer.getChannelData(channel);
for (var i = 0; i < {buf_len} / {num_channels}; ++i) {{
buffer_content[i] = src[i * {num_channels} + channel];
}}
}}
var node = context.createBufferSource();
node.buffer = buffer;
node.connect(context.destination);
node.start();
}})()\0",
num_channels = 2,
voice_id = self.voice_id,
buf_len = self.temporary_buffer.len(),
src_data = src_data);
let num_channels = 2u32; // TODO: correct value
debug_assert_eq!(self.temporary_buffer.len() % num_channels as usize, 0);
emscripten_run_script(script.as_ptr() as *const _)
}
js!(
var src_buffer = new Float32Array(@{typed_array}.buffer);
var context = @{self.voice};
var buf_len = @{self.temporary_buffer.len() as u32};
var num_channels = @{num_channels};
var buffer = context.createBuffer(num_channels, buf_len / num_channels, 44100);
for (var channel = 0; channel < num_channels; ++channel) {
var buffer_content = buffer.getChannelData(channel);
for (var i = 0; i < buf_len / num_channels; ++i) {
buffer_content[i] = src_buffer[i * num_channels + channel];
}
}
var node = context.createBufferSource();
node.buffer = buffer;
node.connect(context.destination);
node.start();
);
}
}

7
src/lib.rs
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@ -38,9 +38,16 @@ from time to time.
*/
#![recursion_limit = "512"]
#[macro_use]
extern crate lazy_static;
// Extern crate declarations with `#[macro_use]` must unfortunately be at crate root.
#[cfg(target_os = "emscripten")]
#[macro_use]
extern crate stdweb;
pub use samples_formats::{Sample, SampleFormat};
#[cfg(all(not(windows), not(target_os = "linux"), not(target_os = "freebsd"),