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// Serves a generated wasm module large enough to be compiled off the main
// thread.
//
// ScriptLoader only compiles a wasm module off-main-thread when its bytecode is
// at least OffThreadMinimumWasmLength (16 KiB, see dom/script/ScriptLoader.cpp).
//
// The module is a sequence of uniquely-named exported functions taking two i32s
// and returning one. f0 uses i32.add, so f0(2, 3) == 5.
const CC = Components.Constructor;
const BinaryOutputStream = CC(
"@mozilla.org/binaryoutputstream;1",
"nsIBinaryOutputStream",
"setOutputStream"
);
const MINIMUM_BYTES = 20 * 1024;
// i32.add, i32.sub, i32.mul, i32.and, i32.or, i32.xor. Rotating the operation
// keeps the function bodies from being byte-identical.
const OPS = [0x6a, 0x6b, 0x6c, 0x71, 0x72, 0x73];
const I32 = 0x7f;
const FUNCTYPE = 0x60;
const SECTION_TYPE = 1;
const SECTION_FUNCTION = 3;
const SECTION_EXPORT = 7;
const SECTION_CODE = 10;
function uleb128(value) {
const out = [];
for (;;) {
const byte = value & 0x7f;
value >>>= 7;
if (value) {
out.push(byte | 0x80);
} else {
out.push(byte);
return out;
}
}
}
function vec(items) {
return uleb128(items.length).concat(...items);
}
function section(sectionId, payload) {
return [sectionId].concat(uleb128(payload.length), payload);
}
function buildModule(numFunctions) {
const functype = [FUNCTYPE].concat(vec([[I32], [I32]]), vec([[I32]]));
const exports = [];
const codes = [];
for (let i = 0; i < numFunctions; i++) {
const name = Array.from(`f${i}`, c => c.charCodeAt(0));
exports.push(uleb128(name.length).concat(name, [0x00], uleb128(i)));
// local.get 0; local.get 1; <op>; end
const body = [0x00, 0x20, 0x00, 0x20, 0x01, OPS[i % OPS.length], 0x0b];
codes.push(uleb128(body.length).concat(body));
}
return [0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00].concat(
section(SECTION_TYPE, vec([functype])),
section(SECTION_FUNCTION, vec(new Array(numFunctions).fill(uleb128(0)))),
section(SECTION_EXPORT, vec(exports)),
section(SECTION_CODE, vec(codes))
);
}
function buildModuleOfAtLeast(minimumBytes) {
// We currently average ~17 bytes per function, so this is safe.
let numFunctions = Math.floor(minimumBytes / 16);
return buildModule(numFunctions);
}
function handleRequest(request, response) {
response.setHeader("Content-Type", "application/wasm", false);
response.setHeader("Cache-Control", "no-store", false);
const bos = new BinaryOutputStream(response.bodyOutputStream);
bos.writeByteArray(buildModuleOfAtLeast(MINIMUM_BYTES));
}