Re: [PATCH 0/6] Correct and improve the ARM tunings

[Adrian Bunk <bunk@stusta.de>]

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.

And that's only for the legacy 32bit platforms.

armv8 already has 21 feature modifiers in gcc 9,
and while not all 2^21 or 3^21[2] combinations are
valid the old tune approach won't scale for that.

> Cheers,
> 
> Richard

cu
Adrian

[1] https://www.yoctoproject.org/docs/2.6/ref-manual/ref-manual.html#migration-2.6-miscellaneous-changes
[2] enable/disable/default

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