src/runtime_functions.js

/**
 * @license
 * Copyright 2020 The Emscripten Authors
 * SPDX-License-Identifier: MIT
 */

// Wraps a JS function as a wasm function with a given signature.
function convertJsFunctionToWasm(func, sig) {
#if WASM2JS
  return func;
#else // WASM2JS

  // If the type reflection proposal is available, use the new
  // "WebAssembly.Function" constructor.
  // Otherwise, construct a minimal wasm module importing the JS function and
  // re-exporting it.
  if (typeof WebAssembly.Function === "function") {
    var typeNames = {
      'i': 'i32',
      'j': 'i64',
      'f': 'f32',
      'd': 'f64'
    };
    var type = {
      parameters: [],
      results: sig[0] == 'v' ? [] : [typeNames[sig[0]]]
    };
    for (var i = 1; i < sig.length; ++i) {
      type.parameters.push(typeNames[sig[i]]);
    }
    return new WebAssembly.Function(type, func);
  }

  // The module is static, with the exception of the type section, which is
  // generated based on the signature passed in.
  var typeSection = [
    0x01, // id: section,
    0x00, // length: 0 (placeholder)
    0x01, // count: 1
    0x60, // form: func
  ];
  var sigRet = sig.slice(0, 1);
  var sigParam = sig.slice(1);
  var typeCodes = {
    'i': 0x7f, // i32
    'j': 0x7e, // i64
    'f': 0x7d, // f32
    'd': 0x7c, // f64
  };

  // Parameters, length + signatures
  typeSection.push(sigParam.length);
  for (var i = 0; i < sigParam.length; ++i) {
    typeSection.push(typeCodes[sigParam[i]]);
  }

  // Return values, length + signatures
  // With no multi-return in MVP, either 0 (void) or 1 (anything else)
  if (sigRet == 'v') {
    typeSection.push(0x00);
  } else {
    typeSection = typeSection.concat([0x01, typeCodes[sigRet]]);
  }

  // Write the overall length of the type section back into the section header
  // (excepting the 2 bytes for the section id and length)
  typeSection[1] = typeSection.length - 2;

  // Rest of the module is static
  var bytes = new Uint8Array([
    0x00, 0x61, 0x73, 0x6d, // magic ("\0asm")
    0x01, 0x00, 0x00, 0x00, // version: 1
  ].concat(typeSection, [
    0x02, 0x07, // import section
      // (import "e" "f" (func 0 (type 0)))
      0x01, 0x01, 0x65, 0x01, 0x66, 0x00, 0x00,
    0x07, 0x05, // export section
      // (export "f" (func 0 (type 0)))
      0x01, 0x01, 0x66, 0x00, 0x00,
  ]));

   // We can compile this wasm module synchronously because it is very small.
  // This accepts an import (at "e.f"), that it reroutes to an export (at "f")
  var module = new WebAssembly.Module(bytes);
  var instance = new WebAssembly.Instance(module, {
    'e': {
      'f': func
    }
  });
  var wrappedFunc = instance.exports['f'];
  return wrappedFunc;
#endif // WASM2JS
}

var freeTableIndexes = [];

// Weak map of functions in the table to their indexes, created on first use.
var functionsInTableMap;

function getEmptyTableSlot() {
  // Reuse a free index if there is one, otherwise grow.
  if (freeTableIndexes.length) {
    return freeTableIndexes.pop();
  }
  // Grow the table
  try {
    wasmTable.grow(1);
  } catch (err) {
    if (!(err instanceof RangeError)) {
      throw err;
    }
    throw 'Unable to grow wasm table. Set ALLOW_TABLE_GROWTH.';
  }
  return wasmTable.length - 1;
}

// Add a wasm function to the table.
function addFunctionWasm(func, sig) {
  // Check if the function is already in the table, to ensure each function
  // gets a unique index. First, create the map if this is the first use.
  if (!functionsInTableMap) {
    functionsInTableMap = new WeakMap();
    for (var i = 0; i < wasmTable.length; i++) {
      var item = wasmTable.get(i);
      // Ignore null values.
      if (item) {
        functionsInTableMap.set(item, i);
      }
    }
  }
  if (functionsInTableMap.has(func)) {
    return functionsInTableMap.get(func);
  }

  // It's not in the table, add it now.

#if ASSERTIONS >= 2
  // Make sure functionsInTableMap is actually up to date, that is, that this
  // function is not actually in the wasm Table despite not being tracked in
  // functionsInTableMap.
  for (var i = 0; i < wasmTable.length; i++) {
    assert(wasmTable.get(i) != func, 'function in Table but not functionsInTableMap');
  }
#endif

  var ret = getEmptyTableSlot();

  // Set the new value.
  try {
    // Attempting to call this with JS function will cause of table.set() to fail
    wasmTable.set(ret, func);
  } catch (err) {
    if (!(err instanceof TypeError)) {
      throw err;
    }
#if ASSERTIONS
    assert(typeof sig !== 'undefined', 'Missing signature argument to addFunction: ' + func);
#endif
    var wrapped = convertJsFunctionToWasm(func, sig);
    wasmTable.set(ret, wrapped);
  }

  functionsInTableMap.set(func, ret);

  return ret;
}

function removeFunction(index) {
  functionsInTableMap.delete(wasmTable.get(index));
  freeTableIndexes.push(index);
}

// 'sig' parameter is required for the llvm backend but only when func is not
// already a WebAssembly function.
function addFunction(func, sig) {
#if ASSERTIONS
  assert(typeof func !== 'undefined');
#if ASSERTIONS == 2
  if (typeof sig === 'undefined') {
    err('warning: addFunction(): You should provide a wasm function signature string as a second argument. This is not necessary for asm.js and asm2wasm, but can be required for the LLVM wasm backend, so it is recommended for full portability.');
  }
#endif // ASSERTIONS == 2
#endif // ASSERTIONS

  return addFunctionWasm(func, sig);
}