Tip: Calling C functions directly with "noalloc"

In OCaml you can call a C function by writing:

external call_1 : float -> float = "call_1"

then just using call_1. However these calls are not direct. They go via an OCaml runtime function called caml_c_call. This is a tiny bit of assembler, so the overhead isn't large, but it does use a computed jump which on many processors is quite slow.

Luckily this indirection is only needed in order to set up the garbage collector. If your C function won't perform any OCaml allocations, then you don't need this, and you can tell OCaml to jump directly to your C function like this:

external call_2 : float -> float = "call_2" "noalloc"

Let's compare the generated assembly code for the calls in both cases:

Normal                                    "noalloc"       

pushl   %eax                              pushl   %eax
movl    $call_1, %eax                     call    call_2
call    caml_c_call                       addl    $4, %esp
addl    $4, %esp
   ...
caml_c_call:
movl    (%esp), %edx
movl    %edx, G(caml_last_return_address)
leal    4(%esp), %edx
movl    %edx, G(caml_bottom_of_stack)
jmp     *%eax

As you can see, the "noalloc" version is much shorter.