float16 support

[igarnier]

Hi! This is a WIP PR adding float16 support to ctypes (see issue). Happy to have early feedback on the general direction. For now it uses a vendored copy of https://github.com/Leonidas-from-XIV/mucppo for preprocessing.

So far only (lightly) tested only on ocaml 5.2, on x86-64.
edit: also tested on 4.8

TODOs

• iron out naming of files to be preprocessed (use .cppo.ml instead of .frag.ml)
• add preprocessing directives for the >= 5.2-specific parts on the C files as well

[yallop]

Thank you for picking this up, @igarnier!

Some general early feedback: it would be good to avoid as much conditional compilation as possible. We currently don't depend on the OCaml version anywhere in the source code, so I'm not keen to introduce that kind of dependency for this feature.

Is it viable to expose Ctypes.float16 for all OCaml versions, but only actually implement the full set of bigarray operations on platforms that support it, failing at run-time where necessary on the others?

[igarnier]

After tinkering a bit I think it's possible to remove most of the conditional compilation directives, with the function Ctype_bigarray_stubs.kind as a notable exception: it pattern matches on Bigarray_compat.kind which does not contain the Float16 constructor for versions < 5.2. More work needed.

[igarnier]

The latest version eschews most of the conditional compilation. Here are the remaining bits:

• In ctypes_float16_availability.h, there's some code detecting whether the platform supports _Float16. If _Float16 is not available, reading or writing a field of type _Float16 will raise Failure (see type_info_stubs.c). This relies on the C preprocessor but we can easily use an if statement if you prefer. Also, calling Ctypes.float16 () will assert availability via a C stub relying on the above mechanism. Checking the soundness of this detection mechanism is important!
• There's a new binary configure/gen_bigarray_kind_conv.ml that generates the proper kind type and conversion from Bigarray_compat.kind based on the OCaml version.

I removed the tests for now.

[igarnier]

Another place where we use the preprocessor. Not sure how to avoid this one.

[igarnier]

Note: we reuse the same detection method as in ctypes_float16_availability . These should be kept in sync

[igarnier]

Copied some code from the stdlib, hope that's ok license-wise. Otherwise I'll hack something else.

[yallop]

I think it's better not to copy the code. Perhaps something like the following would do:

let split_on_char c s = Str.(split (regexp (String.make 1 c))) s

[igarnier]

Works with Str.regexp_string instead of Str.regexp. Pushed a fix.

[igarnier]

Just squashed history. Coming back to the code after a few weeks, I'm not super satisfied with the compile-time generation of the function kind : type a b. (a, b) Bigarray_compat.kind -> a kind. If someone has a better idea I'm all ears!