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Switch from heap/stack to just a heap #1069

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Nov 30, 2018
Merged

Switch from heap/stack to just a heap #1069

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6 changes: 5 additions & 1 deletion crates/cli-support/src/js/js2rust.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -392,7 +392,11 @@ impl<'a, 'b> Js2Rust<'a, 'b> {
if arg.is_ref_anyref() {
self.js_arguments.push((name.clone(), "any".to_string()));
self.cx.expose_borrowed_objects();
self.finally("stack.pop();");
self.cx.expose_global_stack_pointer();
// the "stack-ful" nature means that we're always popping from the
// stack, and make sure that we actually clear our reference to
// allow stale values to get GC'd
self.finally("heap[stack_pointer++] = undefined;");
self.rust_arguments
.push(format!("addBorrowedObject({})", name));
return Ok(self);
Expand Down
268 changes: 76 additions & 192 deletions crates/cli-support/src/js/mod.rs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -114,7 +114,9 @@ enum Import<'a> {
},
}

const INITIAL_SLAB_VALUES: &[&str] = &["undefined", "null", "true", "false"];
const INITIAL_HEAP_VALUES: &[&str] = &["undefined", "null", "true", "false"];
// Must be kept in sync with `src/lib.rs` of the `wasm-bindgen` crate
const INITIAL_HEAP_OFFSET: usize = 32;

impl<'a> Context<'a> {
fn export(&mut self, name: &str, contents: &str, comments: Option<String>) {
Expand Down Expand Up @@ -168,44 +170,20 @@ impl<'a> Context<'a> {
self.write_classes()?;

self.bind("__wbindgen_object_clone_ref", &|me| {
me.expose_add_heap_object();
me.expose_get_object();
let bump_cnt = if me.config.debug {
String::from(
"
if (typeof(val) === 'number') throw new Error('corrupt slab');
val.cnt += 1;
",
)
} else {
String::from("val.cnt += 1;")
};
Ok(format!(
me.expose_add_heap_object();
Ok(String::from(
"
function(idx) {
return addHeapObject(getObject(idx));
}
"
function(idx) {{
// If this object is on the stack promote it to the heap.
if ((idx & 1) === 1) return addHeapObject(getObject(idx));

// Otherwise if the object is on the heap just bump the
// refcount and move on
const val = slab[idx >> 1];
{}
return idx;
}}
",
bump_cnt
))
})?;

self.bind("__wbindgen_object_drop_ref", &|me| {
me.expose_drop_ref();
Ok(String::from(
"
function(i) {
dropRef(i);
}
",
))
Ok(String::from("function(i) { dropObject(i); }"))
})?;

self.bind("__wbindgen_string_new", &|me| {
Expand All @@ -222,13 +200,7 @@ impl<'a> Context<'a> {

self.bind("__wbindgen_number_new", &|me| {
me.expose_add_heap_object();
Ok(String::from(
"
function(i) {
return addHeapObject(i);
}
",
))
Ok(String::from("function(i) { return addHeapObject(i); }"))
})?;

self.bind("__wbindgen_number_get", &|me| {
Expand Down Expand Up @@ -370,7 +342,7 @@ impl<'a> Context<'a> {
"
function(i) {
const obj = getObject(i).original;
dropRef(i);
dropObject(i);
if (obj.cnt-- == 1) {
obj.a = 0;
return 1;
Expand All @@ -383,7 +355,7 @@ impl<'a> Context<'a> {

self.bind("__wbindgen_cb_forget", &|me| {
me.expose_drop_ref();
Ok("dropRef".to_string())
Ok("dropObject".to_string())
})?;

self.bind("__wbindgen_json_parse", &|me| {
Expand Down Expand Up @@ -427,14 +399,7 @@ impl<'a> Context<'a> {
self.bind("__wbindgen_memory", &|me| {
me.expose_add_heap_object();
let mem = me.memory();
Ok(format!(
"
function() {{
return addHeapObject({});
}}
",
mem
))
Ok(format!("function() {{ return addHeapObject({}); }}", mem))
})?;

self.bind("__wbindgen_module", &|me| {
Expand Down Expand Up @@ -916,149 +881,54 @@ impl<'a> Context<'a> {
if !self.exposed_globals.insert("drop_ref") {
return;
}
self.expose_global_slab();
self.expose_global_slab_next();
let validate_owned = if self.config.debug {
String::from(
"
if ((idx & 1) === 1) throw new Error('cannot drop ref of stack objects');
",
)
} else {
String::new()
};
let dec_ref = if self.config.debug {
String::from(
"
if (typeof(obj) === 'number') throw new Error('corrupt slab');
obj.cnt -= 1;
if (obj.cnt > 0) return;
",
)
} else {
String::from(
"
obj.cnt -= 1;
if (obj.cnt > 0) return;
",
)
};
self.expose_global_heap();
self.expose_global_heap_next();

// Note that here we check if `idx` shouldn't actually be dropped. This
// is due to the fact that `JsValue::null()` and friends can be passed
// by value to JS where we'll automatically call this method. Those
// constants, however, cannot be dropped. See #1054 for removing this
// branch.
//
// Otherwise the free operation here is pretty simple, just appending to
// the linked list of heap slots that are free.
self.global(&format!(
"
function dropRef(idx) {{
{}
idx = idx >> 1;
function dropObject(idx) {{
if (idx < {}) return;
let obj = slab[idx];
{}
// If we hit 0 then free up our space in the slab
slab[idx] = slab_next;
slab_next = idx;
heap[idx] = heap_next;
heap_next = idx;
}}
",
validate_owned,
INITIAL_SLAB_VALUES.len(),
dec_ref
INITIAL_HEAP_OFFSET + INITIAL_HEAP_VALUES.len(),
));
}

fn expose_global_stack(&mut self) {
if !self.exposed_globals.insert("stack") {
fn expose_global_heap(&mut self) {
if !self.exposed_globals.insert("heap") {
return;
}
self.global(&format!(
"
const stack = [];
"
));
if self.config.debug {
self.export(
"assertStackEmpty",
"
function() {
if (stack.length === 0) return;
throw new Error('stack is not currently empty');
}
",
None,
);
}
self.global(&format!("const heap = new Array({});", INITIAL_HEAP_OFFSET));
self.global(&format!("heap.push({});", INITIAL_HEAP_VALUES.join(", ")));
}

fn expose_global_slab(&mut self) {
if !self.exposed_globals.insert("slab") {
fn expose_global_heap_next(&mut self) {
if !self.exposed_globals.insert("heap_next") {
return;
}
let initial_values = INITIAL_SLAB_VALUES
.iter()
.map(|s| format!("{{ obj: {} }}", s))
.collect::<Vec<_>>();
self.global(&format!("const slab = [{}];", initial_values.join(", ")));
if self.config.debug {
self.export(
"assertSlabEmpty",
&format!(
"
function() {{
for (let i = {}; i < slab.length; i++) {{
if (typeof(slab[i]) === 'number') continue;
throw new Error('slab is not currently empty');
}}
}}
",
initial_values.len()
),
None,
);
}
}

fn expose_global_slab_next(&mut self) {
if !self.exposed_globals.insert("slab_next") {
return;
}
self.expose_global_slab();
self.global(
"
let slab_next = slab.length;
",
);
self.expose_global_heap();
self.global("let heap_next = heap.length;");
}

fn expose_get_object(&mut self) {
if !self.exposed_globals.insert("get_object") {
return;
}
self.expose_global_stack();
self.expose_global_slab();
self.expose_global_heap();

let get_obj = if self.config.debug {
String::from(
"
if (typeof(val) === 'number') throw new Error('corrupt slab');
return val.obj;
",
)
} else {
String::from(
"
return val.obj;
",
)
};
self.global(&format!(
"
function getObject(idx) {{
if ((idx & 1) === 1) {{
return stack[idx >> 1];
}} else {{
const val = slab[idx >> 1];
{}
}}
}}
",
get_obj
));
// Accessing a heap object is just a simple index operation due to how
// the stack/heap are laid out.
self.global("function getObject(idx) { return heap[idx]; }");
}

fn expose_assert_num(&mut self) {
Expand Down Expand Up @@ -1510,18 +1380,32 @@ impl<'a> Context<'a> {
);
}

fn expose_global_stack_pointer(&mut self) {
if !self.exposed_globals.insert("stack_pointer") {
return;
}
self.global(&format!("let stack_pointer = {};", INITIAL_HEAP_OFFSET));
}

fn expose_borrowed_objects(&mut self) {
if !self.exposed_globals.insert("borrowed_objects") {
return;
}
self.expose_global_stack();
self.expose_global_heap();
self.expose_global_stack_pointer();
// Our `stack_pointer` points to where we should start writing stack
// objects, and the `stack_pointer` is incremented in a `finally` block
// after executing this. Once we've reserved stack space we write the
// value. Eventually underflow will throw an exception, but JS sort of
// just handles it today...
self.global(
"
function addBorrowedObject(obj) {
stack.push(obj);
return ((stack.length - 1) << 1) | 1;
if (stack_pointer == 1) throw new Error('out of js stack');
heap[--stack_pointer] = obj;
return stack_pointer;
}
",
"
);
}

Expand All @@ -1535,7 +1419,7 @@ impl<'a> Context<'a> {
"
function takeObject(idx) {
const ret = getObject(idx);
dropRef(idx);
dropObject(idx);
return ret;
}
",
Expand All @@ -1546,34 +1430,34 @@ impl<'a> Context<'a> {
if !self.exposed_globals.insert("add_heap_object") {
return;
}
self.expose_global_slab();
self.expose_global_slab_next();
let set_slab_next = if self.config.debug {
self.expose_global_heap();
self.expose_global_heap_next();
let set_heap_next = if self.config.debug {
String::from(
"
if (typeof(next) !== 'number') throw new Error('corrupt slab');
slab_next = next;
if (typeof(heap_next) !== 'number') throw new Error('corrupt heap');
",
)
} else {
String::from(
"
slab_next = next;
",
)
String::new()
};

// Allocating a slot on the heap first goes through the linked list
// (starting at `heap_next`). Once that linked list is exhausted we'll
// be pointing beyond the end of the array, at which point we'll reserve
// one more slot and use that.
self.global(&format!(
"
function addHeapObject(obj) {{
if (slab_next === slab.length) slab.push(slab.length + 1);
const idx = slab_next;
const next = slab[idx];
if (heap_next === heap.length) heap.push(heap.length + 1);
const idx = heap_next;
heap_next = heap[idx];
{}
slab[idx] = {{ obj, cnt: 1 }};
return idx << 1;
heap[idx] = obj;
return idx;
}}
",
set_slab_next
set_heap_next
));
}

Expand Down
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