Re: RFA/ARM: Switch mode when setting PC

[Daniel Jacobowitz <drow at mvista dot com>]

On Fri, Jan 16, 2004 at 02:34:23PM +0000, Richard Earnshaw wrote:
> 
> > 
> > Unless the "Thumb bit" is being stripped out by GDB, then I suspect that 
> > this is a bug in the gdb/simulator binding layer.  Any attempt to force 
> > the PC value by the debugger should be taken as a potential state change.  
> > If that is not happening, then all sorts of things may not work.
> > 
> > I've suspected that there is a problem in the way that gdb drives the 
> > simulator for a while now.
> > 
> 
> sim_store_register in sim/arm/wrapper.c is currently usring ARMul_SetReg 
> to set the PC.  I think this is wrong.
> 
> RDI_CPUwrite in sim/arm/armrdi.c uses ARMul_SetPC in a similar context.
> 
> ARMul_SetReg should only be used on the PC in specific circumstances, 
> specifically from within the main simulation loop.  Even then it should 
> probably be using ARMul_SetR15.
> 
> I suspect that this is why several rather gross hacks were introduced over 
> the years to make single stepping work, and what you are seeing now may be 
> another artifact of this general problem.

OK, but I don't think that's relevant.  Take a look at what those
functions do:

void
ARMul_SetR15 (ARMul_State * state, ARMword value)
{
  if (ARMul_MODE32BIT)
    state->Reg[15] = value & PCBITS;
  else
    {
      state->Reg[15] = value;
      ARMul_R15Altered (state);
    }
  FLUSHPIPE;
}

void
ARMul_SetPC (ARMul_State * state, ARMword value)
{ 
  if (ARMul_MODE32BIT)
    state->Reg[15] = value & PCBITS;
  else
    state->Reg[15] = R15CCINTMODE | (value & R15PCBITS);
  FLUSHPIPE;
}

As I said in my other message, none of these have the potential to
switch the simulator from Thumb to ARM mode.  I spent some time looking
yesterday and I believe that none of the functions used to set the PC
do, with the exception of WriteR15Branch; and it's pretty obvious to me
that the sim interface should not be using WriteR15Branch, so the
simulator's client needs to handle setting the CPSR.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer