[RFC] Add and armv7hf TARGET_ARCH

[John-Mark Gurney]

Andrew Turner wrote this message on Tue, Oct 07, 2014 at 09:44 +0100:
> On Mon, 6 Oct 2014 21:24:30 -0700
> John-Mark Gurney <jmg at funkthat.com> wrote:
> 
> > Andrew Turner wrote this message on Mon, Oct 06, 2014 at 22:41 +0100:
> > > On Mon, 6 Oct 2014 10:30:45 -0700
> > > John-Mark Gurney <jmg at funkthat.com> wrote:
> > > 
> > > > Andrew Turner wrote this message on Mon, Oct 06, 2014 at 13:46
> > > > +0100:
> > > > > I'm interested in peoples opinion on creating a new TARGET_ARCH
> > > > > to target ARMv7 SoCs. This will target all the current Cortex-A
> > > > > chips we support but not the Raspberry Pi. My intention with
> > > > > this is to have it become the tier 1 arm platform.
> > > > > 
> > > > > This platform will support 32-bit Cortex-A based SoCs with a VFP
> > > > > unit. As it would be targeting ARMv7 we could look at supporting
> > > > > Thumb-2.
> > > > > 
> > > > > As the VFP unit is optional and future SoCs without it will
> > > > > only be supported by the armv6 TARGET_ARCH, however I would
> > > > > expect almost all ARMv7 designs to include it.
> > > > 
> > > > So, what are the specific pros of having a new arch?  I see you
> > > > talk about Thumb-2 support, but are there other advantages?  Will
> > > > we get significant performance boosts?  What?
> > > 
> > > We would get a significant speed improvement for anything that uses
> > > floating-point. I haven't done extensive tests, but Ian was getting
> > > around 30x-34x improvement by using the vfp on one benchmark [1].
> > > I've seen a sight improvement of around 3-5 MFlops on his numbers
> > > on my board.
> > > 
> > > I expect there to be a slight performance improvement from being
> > > able to use the newer ARMv7 instructions, however this will be less
> > > pronounced than the above floating-point improvement.
> > > 
> > > There are also a number of NEON optimised libc functions we could
> > > make use of, for example [2]. While we may be able to use them on
> > > armv6 it becomes simpler if we can assume we have a NEON unit.
> > 
> > Don't we already have armv6hf for hardware float?  What is the
> > difference between armv6hf and armv7hf?  or is this 30x-34x
> > improvement over armv6hf?
> 
> My plan is to replace the armv6hf with armv7hf. The difference between
> the two is, on armv7hf we can assume newer floating-point instructions
> including the NEON SIMD instructions.

Ahh, ok. this makes more sense then...  If we really only loose the
RPI, I don't think that it's that big of a loss...  Considering how
many other boards would get a boost...

So, the real move is switching to armv7hf is the new requirement that
NEON be present, correct?  From my understanding, NEON is common on
SoCs now, isn't it?

> The performance improvement above was changing from the soft to softfp
> ABI. Softfp allows the compiler to generate vfp code, but will pass
> floating-point data between functions in the integer registers.
> 
> It has been reported on some cores moving data between the vfp and arm
> registers can cause both to stall for at least 20 cycles [1]. While
> armv7hf doesn't remove the need to move between groups of registers
> completely it will reduce the need due to the calling convention.

Yeh, sounds like it's good to move to armv7hf considering the number of
platforms...  We could possibly leave armv6hf in the tree, but make
sure people realize that it's not a "supported" option...

So, do you envision armv7hf being the main Tier 1 arm platform?

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."