_malloc/stackAlloc

[cemalgnlts]

Hi,
I was looking at the stringToUTF8OnStack function and it was sending the pointer from stackAlloc to the stringToUTF8 function. Just like the stringToNewUTF8 function does, but the stringToNewUTF8 function does it with _malloc.

My question is that the stringToUTF8 function redirects the incoming pointer to HEAPU8. So here the pointer value from stackAlloc and _malloc seems to be no different.

If this is the case, why do we have the responsibility to free the _malloc pointer but not the pointer from stackAlloc?

[kripken]

Technically you can also free the pointer from stackAlloc (using stackRestore). However, the ABI used by clang and Emscripten for the wasm stack ensures that the stack is cleaned up when the parent unwinds anyhow.

Concretely, imagine we enter a function. It stores the current stack pointer as temp. We can then do various small allocations in that function, and do not need to free anything there, because when the function exits it will do stackRestore(temp), which in effect frees all allocations in the middle.

[cemalgnlts]

I am trying to port a graphics library. When I examined the code of someone who did this, he was using cwrap to add text to the screen. If I understand correctly, this function uses stringToUTF8OnStack and the part that confuses me is that this person uses requestAnimationFrame. So ~60 times per second it would run cwrap, add the string to the stack and send it to the C function.

Now I understand that this will not be a problem since space is allocated in the stack, but wouldn't it be more useful in the long run to keep the string in memory by using malloc instead?

Because I want to change my usage according to this answer.

[kripken]

If you can do a single malloc up front, and keep reusing that between calls, then yes, that sounds more efficient.

[cemalgnlts]

Thanks a lot. I understand better now.

[sbc100]

The cost of stack allocation is effectively zero for native code and very small for JS coded. Doing ~60 stack allocations per frame should be fine.

So I would way either pre-allocating using malloc or using stack allocation should both be fine. The thing you might want to avoid is calling malloc and free each frame.

[cemalgnlts]

@sbc100 Great, thanks for the explanation.