Re: [rfa/arm] Handle bx and blx

[Richard Earnshaw <rearnsha@arm.com>]

> On Mon, Mar 08, 2004 at 10:17:53AM +0000, Richard Earnshaw wrote:
> > > On Wed, Mar 03, 2004 at 04:01:55PM +0000, Richard Earnshaw wrote:
> > > > > The software single-step implementation in GDB doesn't know either BX or
> > > > > BLX.  This results in losing control of the inferior when we single-step
> > > > > over them.  I based this on the ARM ARM, so I'm pretty sure I've got the
> > > > > numbers correct.
> > > > > 
> > > > > OK to check in?
> > > > > 
> > > > > -- 
> > > > > Daniel Jacobowitz
> > > > > MontaVista Software                         Debian GNU/Linux Developer
> > > > > 
> > > > > 2004-02-28  Daniel Jacobowitz  <drow@mvista.com>
> > > > > 
> > > > > 	* arm-tdep.c (thumb_get_next_pc): Handle BX.
> > > > > 	(arm_get_next_pc): Handle BX and BLX.
> > > > 
> > > > Yikes!  Yes, this is OK.  However, Thumb has BLX (2 variants) as well.
> > > 
> > > Right you are.  I've checked in the above; if I'm reading
> > > thumb_get_next_pc and the ARM correctly, then the below is all I need
> > > for BLX.  The first form is already handled since we don't check H.
> > > The second form can be handled identically to BX by relaxing a test.
> > > 
> > > OK?
> > > 
> > > -- 
> > > Daniel Jacobowitz
> > > MontaVista Software                         Debian GNU/Linux Developer
> > > 
> > > 2004-03-07  Daniel Jacobowitz  <drow@mvista.com>
> > > 
> > > 	* arm-tdep.c (thumb_get_next_pc): Handle Thumb BLX.
> > 
> > Very close, and possibly good enough for most purposes.  But the ARM ARM 
> > says that in the blx(1) case, the resulting address should be masked with 
> > 0xfffffffc.  That means that there are two theoretical encodings for each 
> > target ARM-state instruction.  I think you need to add a test for H=01 and 
> > if so, to apply the mask to nextpc.
> 
> Except it also says:
>            Bit[0] for BLX     If H == 01, then bit[0] of the instruction must
> 			      be zero, or the instruction is UNDEFINED.
>                               The offset calculation method described
>                               in Usage above ensures that the offset
>                               calculated for a BLX instruction is a
>                               multiple of four, and that this
>                               restriction is obeyed.
> 
> So I think the mask really isn't needed, or am I reading that wrong?

Ah, missed that bit.  However, we could be starting with a pc value where 
pc[1] != 0, so we still need the mask.

R.