Add support for setting the default allocator and deallocator functions in Halide::Runtime::Buffer.

[mcourteaux]

This uses static inline to use inline linkage, as suggested by @abadams, to deduplicate upon link time the storage of the struct holding the two function pointers.

Outdated:

Additionally, the struct is templated and parameterized over the Runtime::Buffer template argument to make sure that the storage is unique to the Buffer<void> struct (and thus does not gets linked in for every type/dim combo of Buffer.

One option is to change the setters from this:

    static void set_default_allocate_fn(void *(*allocate_fn)(size_t)) {
        Buffer<>::default_allocator_fns.default_allocate_fn = allocate_fn;
    }
    static void set_default_deallocate_fn(void (*deallocate_fn)(void *)) {
        Buffer<>::default_allocator_fns.default_deallocate_fn = deallocate_fn;
    }

to this:

    static void set_default_allocate_fn(void *(*allocate_fn)(size_t)) {
        default_allocator_fns.default_allocate_fn = allocate_fn;
    }
    static void set_default_deallocate_fn(void (*deallocate_fn)(void *)) {
        default_allocator_fns.default_deallocate_fn = deallocate_fn;
    }

This way, it will only be valid to try to set them on the Buffer<> or Buffer<void> template instance. I.e., it will fail on Buffer<float>::set_default_allocate_fn(), due to the specialization of the DefaultAllocatorFn<> struct.

[steven-johnson]

Would be nice to add a test :-)

[abadams]

The place to add it would be in test/correctness/halide_buffer.cpp

[mcourteaux]

Was also thinking that I don't see a reason to embed this struct in the Buffer struct. We can make those two fields static inline right in the auxiliary struct.

[mcourteaux]

It's ready for merge, IMO.