Re: Does ARMEL toolchain include NEON support?
I ran into something similar once. Don't use the -neon switch for
AARCH64 because it's built in.
On 2/27/19, Steve McIntyre <steve@einval.com> wrote:
> 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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