Re: [rfa] Fix software-watchpoint failures by adding epilogue detection

[Yao Qi <yao@codesourcery.com>]

On Tue, Sep 28, 2010 at 06:04:14PM +0200, Ulrich Weigand wrote:
> Daniel Jacobowitz wrote:
> 
> > On Fri, Sep 24, 2010 at 02:39:05PM +0200, Ulrich Weigand wrote:
> > > - I'm accepting more diverse sequences due to forward-scanning for multiple
> > >   instructions, and not requiring backward-scanning.
> > 
> > This I'm worried about.  From my patch:
> > 
> > +  /* We are in the epilogue if the previous instruction was a stack
> > +     adjustment and the next instruction is a possible return (bx, mov
> > +     pc, or pop).
> > 
> > This is definitely an epilogue:
> > 
> >   pop { r4, r5, r6, lr }
> >   bx lr
> > 
> > This could be an epilogue, but it could also be an indirect call:
> > 
> >   bx lr
> > 
> > If it's an indirect call there would be a mov lr, pc before it.
> > If it's an indirect tail call, then it's an epilogue, and the return
> > address won't be saved.
> 
> I'm wondering how "bx lr" could be an indirect call; for a call,
> lr would have to point to the return address, so it couldn't also
> contain the target address ...  Am I missing something here?
> 
> My original patch accepted only specifically "bx lr"; yours also
> accepts different registers for bx.  If we have a bx with a
> different register, this may of course well be an indirect call.
> 
> As far as I can see, GCC never uses bx with any other register but
> lr to implement a return instruction.  Do you know whether this is
> also true for other compilers?  If so, maybe the easiest fix would
> be to change this back to only accepting "bx lr".
> 

I haven't seen any examples GCC uses bx with other registers so far,
but I noticed that some one is thinking of use 'bx r3' for
optimization purpose.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40887
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19599
The patch for PR19599 hasn't been committed yet, so I still can't find a
real example that using 'bx r3' to return.

In short, there *might* be some cases now or in the future, that
registers other than lr are used with bx for return.

-- 
Yao Qi
CodeSourcery
yao@codesourcery.com
(650) 331-3385 x739