当数组的类型不是 Uint8 时，如何从 Javascript 代码访问 asm.js 堆上的数组？

Question

I'm trying to use emscripten and asm.js in order to speed up my Javascript code. I need to get data from an Int32Array into my compiled C function. According to this Github site I can allocate a buffer, copy data into it, and call a function taking that buffer's pointer as input, like so:

var buf = Module._malloc(myTypedArray.length*myTypedArray.BYTES_PER_ELEMENT);
Module.HEAPU8.set(myTypedArray, buf);
Module.ccall('my_function', 'number', ['number'], [buf]);
Module._free(buf);

But it doesn't work for anything besides an Uint8Array , because Uint8Array.set "helpfully" converts the input array's data type to Uint8 instead of just doing a raw copy. In other words, if I try to copy Int32Array.of(1, -1) into the heap at address 100 using this method, I'll get

{ ... 100:1, 101:255, 102:0, 103:0, 104:0, 105:0, 106:0, 107:0 ... }

instead of

{ ... 100:1, 101:0, 102:0, 103:0, 104:255, 105:255, 106:255, 107:255 ... }

(assuming little endian)

So how am I supposed to copy the data to and from the asm.js heap? I understand that an ArrayBuffer object can be bitwise-casted to any typed array type, but it doesn't seem possible to do the reverse (correction: see Jaromanda X's comment). Also, I read, considered, and rejected the website's suggestion to prefer setValue / getValue whenever possible because I have millions of things to copy and I'd like to avoid the overhead of one function call each if at all possible.

Answer 1

So it turns out I was wrong: it is possible to convert Int32Array to an ArrayBuffer and consequently to an Uint8Array view of the original array. Here's a complete example of a function that sums up an array of int32_t implemented as a JS function passing an array of int32_t to a C function:

sum.c (compiled to sum.c.js)

#include <stdint.h>
#include <stddef.h>

double sum_i32(const int32_t* ints, size_t count) {
    double result = 0.0;
    while (count-- > 0) {
        result += *ints++;
    }
    return result;
}

sum.js

"use strict";

const cModule = require("./sum.c.js");

module.exports.sum_i32 = function sum_i32(array) {
    // Convert to array of int32_t if needed.
    if (!(array instanceof Int32Array)) {
        array = new Int32Array(array);
    }
    // Allocate a buffer to store a copy of the input array.
    const ptr = cModule._malloc(array.length * 4);
    let result = NaN;
    if (ptr === 0) {
        throw "Out of memory";
    }
    try {
        // View int32_t array as uint8_t using the int array's ArrayBuffer.
        const u8view = new Uint8Array(array.buffer);
        // Copy it to the right position in the heap and call the C function.
        cModule.HEAPU8.set(u8view, ptr);
        result = cModule._sum_i32(ptr, array.length);
    } finally {
        cModule._free(ptr);
    }
    return result;
}