Re: [rfa/arm] Handle bx and blx

Daniel Jacobowitz
2004-03-08T14:09:51Z

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?