Re: Enabling NEON instructions for fftw

[Mark Hatle <mark.hatle@windriver.com>]

On 1/4/13 9:36 AM, Philip Balister wrote:
> On 01/03/2013 02:21 PM, Mark Hatle wrote:
>> On 1/3/13 12:42 PM, Philip Balister wrote:
>>> So, recent versions of fftw have NEON support for the single precision
>>> build. In the poast, I just passed --enable-neon to configure or the
>>> armv7a case. Now, this is not entirely correct, since some armv7a's lack
>>> a NEON coprocessor.
>>>
>>> Does anyone have a suggestion for how to handle this better? Personally,
>>> I think if you want to run fftwf without NEON, you should have your head
>>> examined, but I would like to avoid generating packages that SIGILL on
>>> people.
>>
>> This says to me that either you need a machine specific package, or use
>> an alternative architecture for that package.. (that is compatible)..
>> then the package can switch neon on/off depending on arch of tuning flags.
>>
>> There is an armv7a-neon tuning defined.  This enabled the TUNE_FEATURES
>> of 'neon'.  So you should be able to check for that in the fftw recipe,
>> and enable the --enable-neon when it's set.  (Using PACKAGECONFIG is
>> likely best...)
>>
>> Then you can change the DEFAULTTUNE_<recipe> = "armv7-neon" in your
>> local.conf.
>
> I really do not want to depend on users putting entries in local.conf to
> obtain proper operation on a specific machine. It looks like the neon
> tune is already selected for the machine I am building for.
>
> In the end, this boils down to, do we care about supporting machines
> with/without NEON from one set of packages by making ones with NEON
> machine specific.

Machine specific is one answer, but my preference is to have a neon specific 
package arch selected.  Looking at the definitions, my expectation is that once 
the -neon version of armv7 is selected the package arch should similarly be 
modified to end in "-neon".

So as long as the BSP/machine uses the neon tune, then everything should be 
compiled and labeled properly.

> For now, I will purse the route of fftwf building with --enable if the
> neon tune is available for the machine.

Definitely the right way to do it, no matter how the package's arch is named.

> Philip
>
> Philip
>
> Philip
>
>>
>> --Mark
>>
>>> Philip
>>>
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