RFC: Unpredictable register set operations

[Andrew Cagney]

> I'm sure this has come up before, but I couldn't find a discussion anywhere
> so I'll just have to ask again...

It come up before:
http://sources.redhat.com/ml/gdb/2003-06/msg00108.html

> Consider PowerPC and the $ps register (MSR).  When debugging a kernel or
> embedded application, GDB has pretty complete control (?) over this
> register.  In GNU/Linux userspace, however, only two bits of it can be set. 
> The rest are read-only.
> 
> So what happens if you "set $ps = 0"?  Well, the right thing happens, but
> until the next time the target stops "print $ps" will print 0.  Which is not
> actually the value of the $ps register.
> 
> Here's the options that I see:
>   - Ignore and document this.
>   - Refetch registers after storing them.
>   - Invalidate registers for lazy re-fetch after storing them.
>   - Add a target hook for might-be-volatile registers, and invalidate
>     only those registers after storing them - or don't cache them at
>     all.
> 
> Thoughts?  Is this a problem worth fixing?

This is a straight bug.  The register cache should be marked as invalid 
after the store.   What puzzles me is why store.exp doesn't tickle this, 
or is this a hangover from lval_register vs lval_reg_frame_relative?

Andrew