Why can't I use 32-bit ints array in ASM.js?

Question

This code works in Firefox nicely - though for some reason, changing the Uint8Array into Uint32array, breaks. Do I need to coerce numbers differently in that case?

 function Module(stdlib, foreign, heap) {
      "use asm";

      // Variable Declarations
      var els = new stdlib.Uint8Array(heap);

      // Function Declarations
      function firstn(x) {
          x = x|0 //32-bit (int)
          var i=0;
          for (; (i|0) < (x|0); i = (i+1)|0) {
              els[i] = i;
          }
      }

      return { firstn: firstn };
  }

  buf = ArrayBuffer(1024*8)

  f = Module(window,{},buf).firstn;
  f(5)
  console.log(new Uint8Array(buf));

And one more thing - is it possible to send an arraybuffer, and have a reference to the final array it produces, with C++ emscriptem-compiled asm.js too?

Answer 1

It looks like asm.js only allows indexing into an Int32Array or Uint32Array using an expression of the form (foo >> 2), though I can't find any reference to this in the spec. That is, it assumes that what you have is an address and are trying to look up the integer at that address. I guess that makes sense, kinda since C code like this:

int32_t arr[5];
arr[i];

would get compiled down to machine code that does the equivalent of *((char*)arr + 4*i)... Anyway, replacing els[i] = i in your code with:

  els[(i<<2)>>2] = i;

seems to make things work with a Uint32Array.