[Cython] Wacky idea: proper macros

Wes McKinney wesmckinn at gmail.com
Mon Apr 30 15:49:32 CEST 2012

On Sun, Apr 29, 2012 at 5:56 AM, mark florisson
<markflorisson88 at gmail.com> wrote:
> On 29 April 2012 08:42, Nathaniel Smith <njs at pobox.com> wrote:
>> On Sat, Apr 28, 2012 at 10:25 PM, mark florisson
>> <markflorisson88 at gmail.com> wrote:
>>> On 28 April 2012 22:04, Nathaniel Smith <njs at pobox.com> wrote:
>>>> Was chatting with Wes today about the usual problem many of us have
>>>> encountered with needing to use some sort of templating system to
>>>> generate code handling multiple types, operations, etc., and a wacky
>>>> idea occurred to me. So I thought I'd through it out here.
>>>> What if we added a simple macro facility to Cython, that worked at the
>>>> AST level? (I.e. I'm talking lisp-style macros, *not* C-style macros.)
>>>> Basically some way to write arbitrary Python code into a .pyx file
>>>> that gets executed at compile time and can transform the AST, plus
>>>> some nice convenience APIs for simple transformations.
>>>> E.g., if we steal the illegal token sequence @@ as our marker, we
>>>> could have something like:
>>>> @@ # alone on a line, starts a block of Python code
>>>> from Cython.MacroUtil import replace_ctype
>>>> def expand_types(placeholder, typelist):
>>>>  def my_decorator(function_name, ast):
>>>>    functions = {}
>>>>    for typename in typelist:
>>>>      new_name = "%s_%s" % (function_name, typename)
>>>>      functions[name] = replace_ctype(ast, placeholder, typename)
>>>>    return functions
>>>>  return function_decorator
>>>> @@ # this token sequence cannot occur in Python, so it's a safe end-marker
>>>> # Compile-time function decorator
>>>> # Results in two cdef functions named sum_double and sum_int
>>>> @@expand_types("T", ["double", "int"])
>>>> cdef T sum(np.ndarray[T] arr):
>>>>  cdef T start = 0;
>>>>  for i in range(arr.size):
>>>>    start += arr[i]
>>>>  return start
>>>> I don't know if this is a good idea, but it seems like it'd be very
>>>> easy to do on the Cython side, fairly clean, and be dramatically less
>>>> horrible than all the ad-hoc templating stuff people do now.
>>>> Presumably there'd be strict limits on how much backwards
>>>> compatibility we'd be willing to guarantee for code that went poking
>>>> around in the AST by hand, but a small handful of functions like my
>>>> notional "replace_ctype" would go a long way, and wouldn't impose much
>>>> of a compatibility burden.
>>>> -- Nathaniel
>>>> _______________________________________________
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>>>> cython-devel at python.org
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>>> Have you looked at http://wiki.cython.org/enhancements/metaprogramming ?
>>> In general I would like better meta-programming support, maybe even
>>> allow defining new operators (although I'm not sure any of it is very
>>> pythonic), but for templates I think fused types should be used, or
>>> improved when they fall short. Maybe a plugin system could also help
>>> people.
>> I hadn't seen that, no -- thanks for the link.
>> I have to say that the examples in that link, though, give me the
>> impression of a cool solution looking for a problem. I've never wished
>> I could symbolically differentiate Python expressions at compile time,
>> or create a mutant Python+SQL hybrid language. Actually I guess I've
>> only missed define-syntax once in maybe 10 years of hacking in
>> Python-the-language: it's neat how if you do 'plot(x, log(y))' in R it
>> will peek at the caller's syntax tree to automagically label the axes
>> as "x" and "log(y)", and that can't be done in Python. But that's not
>> exactly a convincing argument for a macro system.
>> But generating optimized code is Cython's whole selling point, and
>> people really are doing klugey tricks with string-based preprocessors
>> just to generate multiple copies of loops in Cython and C.
>> Also, fused types are great, but: (1) IIUC you can't actually do
>> ndarray[fused_type] yet, which speaks to the feature's complexity, and
> What? Yes you can do that.

I haven't been able to get ndarray[fused_t] to work as we've discussed
off-list. In your own words "Unfortunately, the automatic buffer
dispatch didn't make it into 0.16, so you need to manually
specialize". I'm a bit hamstrung by other users needing to be able to
compile pandas using the latest released Cython.

>> (2) to handle Wes's original example on his blog (duplicating a bunch
>> of code between a "sum" path and a "product" path), you'd actually
>> need something like "fused operators", which aren't even on the
>> horizon. So it seems unlikely that fused types will grow to cover all
>> these cases in the near future.
> Although it doesn't handle contiguity or dimensional differences,
> currently the efficient fused operator is a function pointer. Wouldn't
> passing in a float64_t (*reducer)(float64_t, float64_t) work in this
> case (in the face of multiple types, you can have fused parameters in
> the function pointer as well)?

I have to think that using function pointers everywhere is going to
lose out to "inlined" C. Maybe gcc is smart enough to optimize
observed code paths. In other words, you don't want

for (i = 0; i < nlabels; i++) {
    lab = labels[i];
    result[lab] = reducer(sumx[i], data[i]);

when you can have

for (i = 0; i < nlabels; i++) {
    lab = labels[i];
    result[lab] = sumx[i] + data[i];

I guess I should start writing some C code and actually measuring the
performance gap as I might completely be off-base here; what you want
eventually is to look at the array data graph and "rewrite" it to
better leverage data parallelism (parallelize pieces that you can) and
cache efficiency.

The bigger problem with all this is that I want to avoid an
accumulation of ad hoc solutions.

> I agree with Dag that Julia has nice metaprogramming support, maybe
> functions could take arbitrary compile time expressions as extra
> arguments.
>> Of course some experimentation would be needed to find the right
>> syntax and convenience functions for this feature too, so maybe I'm
>> just being over-optimistic and it would also turn out to be very
>> complicated :-). But it seems like some simple AST search/replace
>> functions would get you a long way.
>> - N
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