[Numpy-discussion] A bite of C++

[Sebastian Berg]

On Wed, 2021-08-25 at 17:48 +0200, Serge Guelton wrote:
> Hi folks,
> 
> https://github.com/numpy/numpy/pull/19713 showcases what *could* be a
> first step
> toward getting rid of generated C code within numpy, in favor of some
> C++ code,
> coupled with a single macro trick.
> 
> Basically, templated code is an easy and robust way to replace
> generated code
> (the C++ compiler becomes the code generator when instantiating
> code), and a
> single X-macro takes care of the glue with the C world.

I am not a C++ export, and really have to get to used to this code.  So
I would prefer if some C++ experts can look at it and give feedback.

This will be a bit harder to read for me than our `.c.src` code for a
while.  But on the up-side, I am frustrated by my IDE not being able to
deal with the `c.src` templating.

One reaction reading the X-macro trick is that I would be more
comfortable with a positive list rather than block-listing.  It just
felt a bit like too much magic and I am not sure how good it is to
assume we usually want to export everything (for one, datetimes are
pretty special).

Even if it is verbose, I would not mind if we just list everything, so
long we have short-hands for all-integers, all-float, all-inexact, etc.


> 
> Some changes in distutils were needed to cope with C++-specific
> flags, and
> extensions that consist in mixed C and C++ code.

<snip>

> Potential follow-ups :
> 
> - do we want to use -nostdlib, to be sure we don't bring any C++
> runtime dep?

What does this mean for compatibility?  It sounds reasonable to me for
now if it increases systems we can run on, but I really don't know.

> - what about -fno-exception, -fno-rtti?

How do C++ exceptions work at run-time?  What if I store a C++ function
pointer that raises an exception and use it from a C program?  Does it
add run-time overhead, do we need that `no-exception` to define that
our functions are actually C "calling convention" in this regard?!

Run-time calling convention changes worry me, because I am not sure C++
exception have a place in the current or even future ABI.  All our
current API use a `-1` return value for exceptions.

This is just like Python's type slots, so there must be "off the
shelve" approaches for this?

Embracing C++ exceptions seems a bit steep to me right now, unless I am
missing something awesome?

I will note that a lot of the functions that we want to template like
this, are – and should be – accessible as public API (i.e. you can ask
NumPy to give you the function pointer).

Cheers,

Sebastian


> - coding style?
> - (I'm-not-a-farseer-I-don-t-know-all-topics)
> 
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