Pointer casting support about emscripten

[kripken]

Nice stuff!

I would support adding this as an option here in upstream. If you can, please file a pull request with the proper commit(s).

Note that something very close to this is already possible, with TYPED_ARRAYS == 2. In that case, typed arrays are used with a shared buffer so that you can mix reads and writes of different types, using normal C-style heap access. The generated code is endian-dependent though, and requires a browser that supports typed arrays, which is why I am in favor of adding your approach as an option.

[opichals]

Thanks! Will prepare a proper pull request to get this into master as an option.

Eh... good that I didn't invest time to understand what the HAS_TYPED_ARRAYS did before as otherwise I might have not cared to implement anything. On the other hand I think that wouldn't work in my case anyway as there could have been unaligned heap accesses for 16 and 32 bit numbers (though I could have adjusted the C code of course - which I tried to stay away, the only thing I added is a check for not doing malloc(0)... but for compatibility I would recommend returning a valid pointer in such a case, just as libc does).

Also I think the generated code my patch is producing is endian dependent as well as it stores the multibyte datatypes in big-endian format and I am not 100% sure it would work as is when compiled from a little-endian machine LLVM output. Like for example how would the ntohl() call work in such a case. I assume it would not flip the endianess correctly and therefore the fact I use big-endian heap would not work correctly. I am thinking right here?

[kripken]

Yes, endianness is a problem with that kind of code.

I just added a runtime check for the typed arrays = 2 case, since it also depends on endianness. Your new code should do the same I guess.

[juj]

Type punning pointers is guaranteed in the typed arrays=2 mode, which asm.js is based on, and we require little endian support (out of practicality and visibility on our dev platforms, not out of necessity really), so it looks like the items discussed here are implemented. If someone wants to champion to add big endian support, let's operate on that as a separate item.