On 2015-12-10 15:39:05 +0200, Martin Storsjö wrote:
> On Wed, 9 Dec 2015, Janne Grunau wrote:
>
> >The vector mode was deprecated in ARMv7-A/VFPv3 and various cpu
> >implementations do not support it in hardware. Vector mode code will
> >depending the OS either be emulated in software or result in an illegal
> >instruction on cpus which does not support it. This was not really
> >problem in practice since NEON implementations of the same functions are
> >preferred. It will however become a problem for checkasm which tests
> >every cpu flag separately.
> >
> >Since this is a cpu feature newer cpu do not support anymore the
> >behaviour of this flag differs from the other flags. It can be only
> >activated by runtime cpu feature selection.
> >---
> >libavutil/arm/cpu.h | 4 ++++
> >libavutil/cpu.h | 1 +
> >libavcodec/arm/dcadsp_init_arm.c | 4 ++--
> >libavcodec/arm/fft_init_arm.c | 2 +-
> >libavcodec/arm/fmtconvert_init_arm.c | 2 +-
> >libavutil/arm/cpu.c | 4 ++++
> >libavutil/cpu.c | 2 ++
> >tests/checkasm/checkasm.c | 1 +
> >8 files changed, 16 insertions(+), 4 deletions(-)
>
> This seems ok. Should something else be done for the cases where we
> lack runtime detection (like old ARMv6 iOS)? I.e. add something
> similar within the non-runtime version of ff_get_cpu_flags_arm?
I'm not sure if that makes much sense. We could of course add it to
configure with vfp_vm_deps="vfp !vfp3 !neon". The problem with that
approach is that it will always use the vfp vector mode asm functions
even when it runs on armv7-a cpu which doesn't support it.
This doesn't seem to be a problem in practice as it is the current
behaviour but feels wrong. One option would be not to enable it by
default but I guess that means nobody will discover and use the option.
What about HW detection for ios? I see that
sysctl({CTL_HW,HW_MACHINE},...) returns the "iPhone2,1" type model
names. Parsing them is annoying so it's worth checking if HW_MODEL or
HW_MACHINE_ARCH returns easier to parse information. sysctl() should be
allowed, we use it already to detect the number of CPUs and I found
articles that getting the process list via sysctl was specifically
disabled in ios 9. Parsing the model strings is not that much uglier
than parsing /proc/cpuinfo and the cpu flags are cached anyway.
Janne
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