the linaro toolchain and older arm versions

[Loïc Minier]

On Wed, Oct 06, 2010, John Rigby wrote:
> I'm really sorry to have started this, but for completeness here is
> the rest of the story.

 No need to be sorry, I think you're doing right to bring this up

>                         The hypothetical scenario is a developer that
> maintains u-boot for multiple platforms.  Using a codesourcery or eldk
> (from denx.de) toolchain one can use the appropriate -march= to get
> the right code from the compiler.  Also the libgcc.a is ARM so all is
> well.  Using a linaro toolchain using the same -march= you get the
> right code from the compiler but the result will get linked with a
> Thumb-2 libgcc.a and the resulting binary will not run on an older
> ARM.  If however libgcc.a was ARM then it would just work.  Again, I'm
> not saying this is a bug or even something for Linaro to care about.
> It just means that the Linaro toolchain is not something that this
> hypothetical u-boot maintainer would want to use as his/her one and
> only toolchain.

 So it strikes me as a toolchain bug; if I understand what you're saying
 above:
 - libgcc built with -march=armv7a can be used with ARMv4, v5, v6, v7
   CPUs (if you pass the correct -march=), so it's backwards compatible
   even if libgcc is built with -march=armv7a
 - libgcc built with -mthumb is NOT compatible with CPUs lacking thumb,
   even if you pass -marm

 It gets a bit muddier if one considers that some armv7 CPUs could
 support Thumb-2 only, but these aren't ARMv7*A*, so I don't think
 that's relevant for the discussion.

 I remember we had a similar case for a VFP libgcc back in jaunty, where
 doko had experienced a multilib VFP libgcc along the regular non-VFP
 libgcc.


 Is it purely accidental that libgcc built for -march=armv7 works on
 older CPUs?  Why can't this mechanism be extended to -marm/-mthumb and
 to VFP options?

-- 
Loïc Minier