Re: RFC: Unpredictable register set operations

[Daniel Jacobowitz <drow at mvista dot com>]

On Tue, Jul 15, 2003 at 11:21:18PM -0400, Daniel Jacobowitz wrote:
> On Tue, Jul 15, 2003 at 10:46:44PM -0400, Andrew Cagney wrote:
> > >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?
> 
> Where's the invalidation supposed to happen presently?  Perhaps I need
> to retest.

I don't know if we're talking about the same thing, but this definitely
doesn't happen now.  I believe we invalidate the frame cache, not
the current regcache... try it, for example on a remote ARM target:

set debug remote 1
info registers
[see the g packet]
set $r0 = 2
[see the G packet]
info registers
[from the cache, no packets on the wire]

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer