[Cython] `cdef inline` and typed memory views

[mark florisson]

On 21 April 2012 20:17, Dimitri Tcaciuc <dtcaciuc at gmail.com> wrote:
> Hey everyone,
>
> Congratulations on shipping 0.16! I think I found a problem which
> seems pretty straight forward. Say I want to factor out inner part of
> some N^2 loops over a flow array, I write something like
>
>  cdef inline float _inner(size_t i, size_t j, float[:] x):
>     cdef float d = x[i] - x[j]
>     return sqrtf(d * d)
>
> In 0.16, this actually compiles (as opposed to 0.15 with ndarray) and
> function is declared as inline, which is great. However, the
> memoryview structure is passed by value:
>
>  static CYTHON_INLINE float __pyx_f_3foo__inner(size_t __pyx_v_i,
> size_t __pyx_v_j, __Pyx_memviewslice __pyx_v_x) {
>     ...
>
> This seems to hinder compiler's (in my case, GCC 4.3.4) ability to
> perform efficient inlining (although function does in fact get
> inlined). If I manually inline that distance calculation, I get 3x
> speedup. (in my case 0.324020147324 vs 1.43209195137 seconds for 10k
> elements). When I manually modified generated .c file to pass memory
> view slice by pointer, slowdown was eliminated completely.
>
> On a somewhat relevant node, have you considered enabling Issues page on Github?
>
>
> Thanks!
>
>
> Dimitri.
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Although it is neither documented nor tested, it works if you just
take the address of the memoryview. You can then index it using
memoryview_pointer[0][i]. One should be careful, as taking the pointer
and passing that around means that pointer is not acquisition counted,
and will point to invalid memory if the memoryview goes out of scope
(e.g. if it's a local variable, when you return).
Cython could manually inline functions though, which could greatly
reduce argument passing and unpacking overhead in some situations
(like buffers).