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

[Daniel Jacobowitz <dan@codesourcery.com>]

On Tue, Sep 28, 2010 at 06:04:14PM +0200, Ulrich Weigand wrote:
> 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?

Bah, you are correct.  Poor choice of example.  bx ip is a better
example; that can be an indirect call, a return, or a tail 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".

Sorry, I don't know :-(  Does GCC also only use lr for an indirect
tail call?  I can't tell - I couldn't get GCC to issue an indirect
tail call.  But I did get this out of RealView:

void (*foo)();
void bar()
{
  foo();
}

bar PROC
        LDR      r0,|L1.12|
        LDR      r0,[r0,#0]  ; foo
        BX       r0
        ENDP

> It seems to me that it is relatively harmless to return a false positive;
> the only thing that happens is that the check for watchpoint hits is
> delayed until the next instruction.  In particular, returning true in
> the epilogue of a frameless functions should definitely be harmless.
> (Returning true on a bx that implements a function call might in rare
> cases lead to a watchpoint hit being detected on the first instruction
> of the called function instead ...)

Yes, that sounds like the case I was worried about.  Of course, it's
relatively harmless either way; nothing in GDB absolutely relies on
this hook.  So I won't object to the patch as-is.  This would be a
nice thing to clean up some day.

-- 
Daniel Jacobowitz
CodeSourcery