[OE-core] [PATCH 0/6] Correct and improve the ARM tunings

[Andre McCurdy]

On Wed, Apr 3, 2019 at 1:24 PM Adrian Bunk <bunk at stusta.de> wrote:
> On Wed, Apr 03, 2019 at 12:29:29PM -0700, Andre McCurdy wrote:
> > On Tue, Apr 2, 2019 at 11:23 PM Adrian Bunk <bunk at stusta.de> wrote:
> > > On Tue, Apr 02, 2019 at 09:26:46PM +0100, Richard Purdie wrote:
> > > >...
> > > > "armv4t" is defined in the arm tune files to mean "add -march=armv4t"
> > > > which is the convention used throughout all the tune files.
> > > >...
> > >
> > > Unfortunately this is not true.
> > >
> > > OE has both armv7a and armv7at tunes.
> > >
> > > There is no armv7a without Thumb support,
> > > so no -march=armv7-at exists in gcc.
> > >
> > > Both armv7a and armv7at tunes pass the same march to gcc,
> > > but [1] is not true:
> > >   Default to using the Thumb-2 instruction set for armv7a and above.
> > >
> > > The hardware supports Thumb-2 in any case, the actual difference between
> > > the armv7a and armv7at OE tunes is whether OE tells the compiler to
> > > generate ARM or Thumb-2 code.
> > >
> > > OE has both armv6 and armv6t tunes.
> > >
> > > There is no armv6 without Thumb support
> > > so no -march=armv6t exists in gcc.
> > >
> > > Some v6 support only Thumb-1 and some v6 support also Thumb-2,
> > > so what gcc does have is an -march=armv6t2.
> > > But OE lacks tunes for that.
> > >
> > > For matching the gcc options it would be correct to remove all
> > > armv6t and armv7at tunes that have no coresponding gcc options,
> > > and add armv6t2 tunes.
> >
> > Aligning the tuning options exposed via the machine config files to
> > those supported by gcc seems like a worthy goal... but would be a big
> > upheaval at this point.
> >
> > Note that the problem isn't specific to ARM. There are similar issues
> > for x86, but there we seem happy to provide a very minimal abstraction
> > with no attempt to track gcc. e.g. "corei7" hasn't been a documented
> > -march option since gcc 4.8 and we (somewhat arbitrarily) map it to
> > -march=nehalem to hide that fact from end users.
> >
> > So the high level question seems to be: should DEFAULTTUNE even
> > attempt to provide a full featured mapping to the options provided by
> > gcc? Are we happy to expose a limited subset without a 1:1 mapping for
> > the options we do expose (current ARM approach) or is it better for
> > DEFAULTTUNE to hide away all the complexity of the options provided by
> > gcc (current x86 approach).
>
> The current 32bit ARM[1] approach seems to be an attempt
> of a 1:1 mapping.
>
> For ARMv8 it is already obvious that DEFAULTTUNE is not long-term
> maintainable, and duplicating all the gcc rules regarding feature
> flags[2] also sounds like a pointless exercise.
>
> What are actually the benefits of DEFAULTTUNE with all the tune files,
> compared to just let the user provide a string that is passed to -march?

Restricting the user to a certain set of DEFAULTTUNE options means
there will always be a valid mapping from DEFAULTTUNE to
PACKAGE_EXTRA_ARCHS.

If we let the user pass an arbitrary string to -march then we lose the
ability to determine that (for example) it's safe for a
"armv7athf-vfpv3" machine to pull from a "armv7athf-vfpv3d16" package
feed.

Whether or not anyone in the real world actually maintains a generic
package feed and pulls from it from multiple machine specific builds
(verses setting up separate package feeds for each DEFAULTTUNE they
care about) would be an interesting question...

> cu
> Adrian
>
> [1] ARM <= v7, not the differing 32bit ABI of ARMv8
> [2] example:
> 'fp16fml'
>      Enable FP16 fmla extension.  This also enables FP16 extensions and
>      floating-point instructions.  This option is enabled by default for
>      '-march=armv8.4-a'.  Use of this option with architectures prior to
>      Armv8.2-A is not supported.
>
>
> --
>
>        "Is there not promise of rain?" Ling Tan asked suddenly out
>         of the darkness. There had been need of rain for many days.
>        "Only a promise," Lao Er said.
>                                        Pearl S. Buck - Dragon Seed
>