Re: PATCH: Additional Thumb instructions for ARMv6K

[Zack Weinberg <zack at codesourcery dot com>]

Richard Earnshaw <rearnsha@gcc.gnu.org> writes:

> Hmm, I think we've got a problem here.  Gas is normally configured to
> default to the pseudo CPU 'all' which permits instructions from any
> architecture.  That means that unless the user has explicitly set a cpu
> (or gcc has done it for you) then the new nop will match rather than the
> old (because of the way the selection logic works as a bit-field and
> mask).
>
> I'm not sure that's desirable because it might break existing code that
> did not expect to have to supply a CPU option.  

I did notice this effect in testing, and it surprised me, but I wasn't
sure what to do about it.

> One partial solution is to pick some architecture (<ARMv6K) and to make
> 'all' mean anything up to that architecture -- anything after that must
> supply a command-line flag (and we should add some configury magic to
> permit selecting a specific cpu as the default).  We could do this and
> at the same time deprecate 'all' so that it could be ultimately removed.

This sounds like a sensible solution, but I do not know what a
reasonable default choice would be.

> The changes other than the NOP work are OK to install (if they can
> feasibly be separated out).

I can split them out and resubmit, but it may be a couple of days
before I get to it.

zw