[Numpy-discussion] Request for review: dynamic_cpu_branch

[David Cournapeau]

David Cournapeau wrote:
> Charles R Harris wrote:
>   
>> On Mon, Dec 22, 2008 at 10:40 PM, David Cournapeau <cournape at gmail.com
>> <mailto:cournape at gmail.com>> wrote:
>>
>>     On Tue, Dec 23, 2008 at 2:35 PM, Robert Kern
>>     <robert.kern at gmail.com <mailto:robert.kern at gmail.com>> wrote:
>>
>>     >
>>     > I think he meant that it can be discovered at runtime in
>>     general, not
>>     > at numpy-run-time, so we can write a small C program that can be run
>>     > at numpy-build-time to add another entry to config.h.
>>
>>     But then we only move the problem: people who want to build universal
>>     numpy extensions will have the wrong value, no ? The fundamental point
>>     of my patch is that the value is set whenever ndarrayobject.h is
>>     included. So even if I build numpy on PPC, NPY_BIGENDIAN will not be
>>     defined when the header is included for a file build with gcc -arch
>>     i386.
>>
>>
>> We can probably set things up so the determination is at run time --
>> but we need to be sure that the ABI isn't affected. I did that once
>> for an old project that needed data portability. In any case, it
>> sounds like a project for a later release.
>>     
>
> It cannot work for numpy without breaking backward compatibility,
> because of the following lines:
>   

Actually, you could, by making the macro point to actual functions, but
that would add function call cost. I don't know if the function call
cost is significant or not in the cases where those macro are used,

David