illegal instruction - create compile variants ?

[Hans de Goede]

Hi,

On 05/01/2011 09:56 AM, David Timms wrote:
> On 01/05/11 17:41, Hans de Goede wrote:
>> Erm, specifying a minimum support CPU in the package description is
>> not acceptable IMHO. The fix here is to patch the packages buildsystem,
>> so that it gets build for the minimum cpu level which is supported by
>> Fedora, and thus will work out of the box on all systems Fedora
>> supports.
> Where is that stated currently, and for older releases ?
>

I was hoping someone who knows the exact answer knows... All I can give
you is:
http://docs.fedoraproject.org/en-US/Fedora/13/html/Release_Notes/index.html#sect-Release_Notes-Hardware_Requirements

Which is not really exact...

>> No tricks / hacks with multiple compilation, cpu detect scripts, etc.
>> will be necessary then.
> Though it comes at the cost of audio lag/latency increase when we are
> trying to achieve near real time (eg live guitar input to sound effect
> generation output). Even on the my machine (4x core amd 3Ghz), the
> calculations the app performs for difficult presets (convolution) can
> cause jack to abort because rakarrack didn't finish it's calculations
> before it was required to deliver data.
>
> Should I be suggesting to upstream to attempt to detect CPU before
> running non-available instructions, eg as part of app startup ?
> Can that even be done (reliably) ?

As already answered by others ideal would be one binary, which runtime
chooses between normal C and mmx/sse#/whatever variants based
on CPU capabilities.

This is definitely something to ask to upstream, as this won't
be trivial.

In the mean time, you need to patch the build-system so that
you can control what cpu features it will enable, then you
can build a number of variants, install them as
/usr/libexec/%{name}-$cpufeaturepostfix

And make /usr/bin/%{name} a shell script which checks
/cpu/procinfo for features and then executes the right
binary (passing through all cmdline args using the
"$@" magic var for proper quoting)

For which variants it makes sense to build, you would
need to talk to someone who is more of a cpu head then me,
try starting a new thread, with an appropriate $subject.

Note that all of this is only really needed on 32 bits,
on 64 bit I believe (*) all cpu's have at least sse2, so unless
you want to use sse3 you'll only need 1 binary there.

Which brings us to a simpler alternative, build a no cpu
specific optimizations 32 bit version and use sse2 on x86_64,
which is nice and simple.

You could then add a README.fedora advising people to use
the 64 bit version.

Regards,

Hans

*) Still no CPU head, better verify this with someone who is.