Re: Does ARMEL toolchain include NEON support?
On Wed, Feb 27, 2019 at 06:30:36PM -0500, Jeffrey Walton wrote:
>On Wed, Feb 27, 2019 at 5:46 PM Steve McIntyre <steve@einval.com> wrote:
>>
>> So, I've got to ask - what hardware are you likely targeting here
>> where it matters to build stuff for armel yet also use NEON if it's
>> available? Most people with hardware that *can* do NEON should be
>> using armhf, surely?
>
>Yeah, I know what you are saying.
>
>The problem in practice with mainstream compilers is (1) ARM and the
>ACLE defines are a mess, (2) -march=native does not work like on i686
>or x86_64, and (3) RTFM does not work.
>
>For a regular user who wants to use Debian on ARM we need to figure
>out how to build to the least capable machine (like ARMv5 or ARMv6)
>while making more capable features (like NEON) available.
So this is a place where the world is just *different* compared to x86
- the different versions of the ARM architectures have signficantly
different capabilities. If you're looking to build something that
performs well on a modern v7 CPU, compilling for v5 is a
mistake. You'll be using the wrong locking primitives, barriers,
etc. Equally, the features you're going to be looking for (like SMP,
NEON) just don't make sense / don't exist on v5 CPUs.
>User's don't want to RTFM to figure out what compiler switches to use.
>They just want things to work. The compilers don't make it any easier
>because -march=native does not work on ARM.
It's a much wider world out there. :-/
>So the use case we target is, user want the most from their hardware
>without reading the manual to configure properly. That means we have
>to go through extra gyrations when building.
>
>(I know we bring it on ourselves. We could easily say fuck it - the
>user did not bother to read a man page so its the user's problem. But
>I'm in the camp that common cases should just work for folks. Folks
>should not need to read a manual to make the common case work).
Agreed. But it might not actually be *possible* to do some of the
optimisation stuff you're looking at, depending on the CPUs
involved. This is basically one of the reasons we started the armhf
port - it's a higher baseline that makes more sense for modern ARMv7
CPUs, while still making it possible for people to use the older ARMv5
cores out there.
--
Steve McIntyre, Cambridge, UK. steve@einval.com
"Yes, of course duct tape works in a near-vacuum. Duct tape works
anywhere. Duct tape is magic and should be worshipped."
-― Andy Weir, "The Martian"
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