opus - Jan 2015 - [PATCH] Add ARM cpu detection for iDevices

This adds run-time CPU detection for iOS devices. I found this page useful
for CPU support and models: http://iossupportmatrix.com/

The patch could be simplified to just set the OPUS_CPU_ARM_NEON flag if you
think it's unlikely that anyone would be running on the really old devices
that don't support NEON.

Tom
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On Jan 8, 2015, at 2:33 PM, Tom Hughes <tom at airtime.com<mailto:tom at
airtime.com>> wrote:

This adds run-time CPU detection for iOS devices. I found this page useful for
CPU support and models: http://iossupportmatrix.com/

The patch could be simplified to just set the OPUS_CPU_ARM_NEON flag if you
think it's unlikely that anyone would be running on the really old devices
that don't support NEON.

Tom

Every armv7 (and armv8) iOS device has supported Neon, and Xcode support for
armv6 was dropped with Xcode 4.5.

Even if you?re compiling with an old Xcode version to support really old iOS
devices, Apple?s armv6/armv7 selection was a compile-time switch (supported
using fat binaries).  I think the arch can be detected based on on preprocessor
defines, though I can?t find the specific settings offhand.

That said, since Apple is going (very soon) to require arm64 in all apps
submitted through the App Store, and no Xcode version supported both armv6 and
arm64, I don?t know how useful this would be.

So my inclination would be to just always set OPUS_CPU_ARM_NEON on iOS/arm.

That said, if we do want to support runtime Neon detection on iOS, rather than
checking hardware models I believe the semantically-correct way to do this is to
check ?hw.optional.neon? using sysctlbyname(), as in:

unsigned int hasNeon;
size_t length = sizeof(hasNeon);
if (sysctlbyname("hw.optional.neon", &hasNeon, &length, NULL,
0) != 0)
hasNeon = 0;
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On Thu, 8 Jan 2015, Jonathan Lennox wrote:
> Every armv7 (and armv8) iOS device has supported Neon, and Xcode support
for
> armv6 was dropped with Xcode 4.5.
> 
> Even if you?re compiling with an old Xcode version to support really old
iOS
> devices, Apple?s armv6/armv7 selection was a compile-time switch (supported
> using fat binaries). ?I think the arch can be detected based on on
> preprocessor defines, though I can?t find the specific settings offhand.
You can use e.g. the define __ARM_NEON__ - it is defined in apple 
toolchains when targeting both armv7 and arm64. Likewise, when the 
compiler is building code for armv7 it will use neon instructions in 
normal C codepaths as well (if it finds it useful).

So the only even theoretical use for runtime detection on iOS would be 
trying to include NEON instruction codepaths in armv6 slices, but there's 
little reason for that, and you can't probably even submit such binaries 
to appstore any longer even if you'd have an old xcode version around.

Thus, I wholeheartedly agree that there is no point in runtime detection 
for this on iOS - checking the __ARM_NEON__ define at compile time is 
enough.

// Martin