Hi Peter,
thanks for digging in so deeply!
First thing I'm going to do: Loop Johannes Demel in here, he's the most
knowledgeable
person on the CPU detection and build issues.
Johannes, see below. There's two things to unpack here:
1. It looks like debian disables NEON. Do we need to check with Mait on that?
Or is this
somehow "right"?
2. see the build problem; that looks like an archi/µarch mismatch, but I don't
know how to
interpret it.
Maybe you can shine some light on this?
Best regards,
Marcus
On 23.07.21 00:07, peter green wrote:
> Package: volk
> Version: 2.0.0-1
> Severity: important
> X-debbugs-cc: mmuel...@gnuradio.org
>
> Hi,
>
> While raspbian and Debian armhf have different baselines what they share in
> common is that neon is not part of the baseline configuration but is present
> on
> a large proportion of the systems people use in practice (for Debian armhf I
> suspect nearly all users have neon, for raspbian it's less because we still
> have pi1/pi0 users around) . So where upstream supports runtime detection
> of neon said runtime detection should be enabled and used.
>
> Back in 2015 I disabled neon in raspbian's volk package, I can't remember why
> but I suspect it was because at the time I had no means of determining whether
> the package had runtime CPU detection.
>
> alle_die_mit_der from gnuradio upstream came into #raspbian on irc (to ask
> about
> options for building stuff) and I took the opportunity to talk about the issue
> of runtime CPU detection. He guided me on how to test volk (quotes below) and
> I thus decided to revert our raspbian neon-disabling changes and build a
> package
> for testing on raspbian bullseye.
>
>> <plugwash> The volk package in raspbian currently has neon disabled. from
>> what you have
>> said I strongly suspect it could be re-enabled but before I actually
>> re-enable it I need
>> a test plan that I can use to make sure i'm not breaking anything.
>> <alle_die_mit_der> VOLK has a unit test for every single "kernel"
>> <alle_die_mit_der> `make test` is your friend :)
>> <plugwash> is there a way to run the tests against an installed version of
>> volk?
>> <alle_die_mit_der> yeah
>> <alle_die_mit_der> `volk_profile` essentially does the same, while
>> benchmarking them
>> <--snip-->
>> <plugwash> does volk_profile use the same runtime cpu detection as normal
>> use of volk?
>> <alle_die_mit_der> yes
>> <alle_die_mit_der> it should
>> <--snip-->
>> <alle_die_mit_der> you can query the runtime-available platforms with
>> `volk-config-info
>> --avail-machines`
>
> However to my surprise I discovered that the package built on raspbian from
> unmodified Debian sources didn't have any neon support either. I discovered
> that
> the CMake scripts were failing to detect Neon because they were not using
> -mfpu=neon when building test programs.
>
> I have confirmed this is not a raspbian specific issue and it seems to have
> been this way since version 2.0.0, this makes it a regression between buster
> and bullseye.
>
> I modified the cmake scripts to use -mfpu=neon when detecting neon support
> but then the build itself failed with.
>
>> cd /volk-2.4.1.new/obj-arm-linux-gnueabihf/lib && /usr/bin/cc -DHAVE_DLFCN_H
>> -DHAVE_FENV_H -D_GLIBCXX_USE_CXX11_ABI=1
>> -I/volk-2.4.1.new/kernels/volk/asm/neon
>> -I/usr/include/orc-0.4 -I/volk-2.4.1.new/obj-arm-linux-gnueabihf/include
>> -I/volk-2.4.1.new/include -I/volk-2.4.1.new/kernels
>> -I/volk-2.4.1.new/obj-arm-linux-gnueabihf/lib -I/volk-2.4.1.new/lib
>> -I/usr/include/cpu_features -O2 -g -DNDEBUG -fPIC -o
>> CMakeFiles/volk_obj.dir/__/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s.o
>> -c
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:
>> Assembler
>> messages:
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:11:
>> Error:
>> selected FPU does not support instruction -- `vmov.i32 q12,#0'
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:18:
>> Error:
>> selected processor does not support `vld2.16 {d16-d19},[r4]!' in ARM mode
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:22:
>> Error:
>> selected FPU does not support instruction -- `vsub.i16 q10,q8,q9'
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:23:
>> Error:
>> selected processor does not support `vcge.s16 q11,q10,#0' in ARM mode
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:24:
>> Error:
>> selected processor does not support `vcgt.s16 q10,q12,q10' in ARM mode
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:25:
>> Error:
>> selected processor does not support `vand.i16 q11,q8,q11' in ARM mode
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:26:
>> Error:
>> selected processor does not support `vand.i16 q10,q9,q10' in ARM mode
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:27:
>> Error:
>> selected FPU does not support instruction -- `vadd.i16 q10,q11,q10'
>> /volk-2.4.1.new/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s:28:
>> Error:
>> selected processor does not support `vst1.16 {d20-d21},[r12]!' in ARM mode
>> gmake[4]: *** [lib/CMakeFiles/volk_obj.dir/build.make:1780:
>> lib/CMakeFiles/volk_obj.dir/__/kernels/volk/asm/neon/volk_16i_max_star_horizontal_16i.s.o]
>> Error 1
>
> I could go digging deeper into the build scripts to see if I can figure out
> how
> to make the buildsystem build the neon kernels (but not the generic kernels)
> with -mfpu=neon, but I felt it was time to seek advice from those more
> familiar
> with the codebase than me.
