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Allow overriding the magic numbers #11

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Allow overriding the magic numbers #11

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28 changes: 23 additions & 5 deletions asyncify.mjs
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Original file line number Diff line number Diff line change
Expand Up @@ -15,15 +15,17 @@
*/

// Put `__asyncify_data` somewhere at the start.
// This address is pretty hand-wavy and we might want to make it configurable in future.
// This address is pretty hand-wavy, if the module exports a __asyncify_data symbol, we'll use that instead.
// The DATA_START and DATA_END are only used if the data where __asyncify_data points at is all 0,
// and the __stack_low and __stack_pointer symbols are not exported.
// See https://github.com/WebAssembly/binaryen/blob/6371cf63687c3f638b599e086ca668c04a26cbbb/src/passes/Asyncify.cpp#L106-L113
// for structure details.
const DATA_ADDR = 16;
// Place actual data right after the descriptor (which is 2 * sizeof(i32) = 8 bytes).
const DATA_START = DATA_ADDR + 8;
// End data at 1024 bytes. This is where the unused area by Clang ends and real stack / data begins.
// Because this might differ between languages and parameters passed to wasm-ld, ideally we would
// use `__stack_pointer` here, but, sadly, it's not exposed via exports yet.
// use `__stack_pointer` here, but it's not exposed by default.
const DATA_END = 1024;

const WRAPPED_EXPORTS = new WeakMap();
Expand Down Expand Up @@ -52,6 +54,7 @@ class Asyncify {
constructor() {
this.value = undefined;
this.exports = null;
this.update_stack_area = undefined;
}

getState() {
Expand All @@ -76,7 +79,8 @@ class Asyncify {
if (!isPromise(value)) {
return value;
}
this.exports.asyncify_start_unwind(DATA_ADDR);
this.update_stack_area?.();
this.exports.asyncify_start_unwind(this.data_addr);
this.value = value;
};
}
Expand Down Expand Up @@ -114,7 +118,7 @@ class Asyncify {
this.exports.asyncify_stop_unwind();
this.value = await this.value;
this.assertNoneState();
this.exports.asyncify_start_rewind(DATA_ADDR);
this.exports.asyncify_start_rewind(this.data_addr);
result = fn(...args);
}

Expand Down Expand Up @@ -152,7 +156,21 @@ class Asyncify {

const memory = exports.memory || (imports.env && imports.env.memory);

new Int32Array(memory.buffer, DATA_ADDR).set([DATA_START, DATA_END]);
// Allow the module to provide an area for saving the asyncify stack
if(exports.__asyncify_data){
this.data_addr = exports.__asyncify_data.value;
}else{
this.data_addr = DATA_ADDR;
}
const data_start_end = new Int32Array(memory.buffer, this.data_addr, 2);
if(!data_start_end[0] && !data_start_end[1]){
if(exports.__stack_low && exports.__stack_pointer){
data_start_end[0] = exports.__stack_low.value;
this.update_stack_area = ()=>{ data_start_end[1] = exports.__stack_pointer; };
}else{
data_start_end.set([DATA_START, DATA_END]);
}
}

this.exports = this.wrapExports(exports);

Expand Down