Re: [patch] [v2] Enable dwarf unwind for AVR target

[Kevin Buettner <kevinb@redhat.com>]

On Fri, 8 Jan 2016 08:52:33 +0000
"Sivanupandi, Pitchumani" <Pitchumani.Sivanupandi@atmel.com> wrote:

> Dwarf debug info generated by avr-gcc denotes the return address by register
> 36 which is not an actual register.
> e.g. .debug_frame
> (--snip--)
> 00000000 00000010 ffffffff CIE
>   Version:               1
>   Augmentation:          ""
>   Code alignment factor: 2
>   Data alignment factor: -1
>   Return address column: 36
> 
>   DW_CFA_def_cfa: r32 ofs 3
>   DW_CFA_offset: r36 at cfa-2
> (--snip--)
> 
> The fix is to add a pseudo register (36 - AVR_DWARF2_PC_REGNUM/LR) to gdb to
> map return address register. Register name is "LR" (link register). When dwarf
> frame unwind asks for PC, target function will read return address value from
> AVR_DWARF2_PC_REGNUM's CFA address.

The usual way to handle this problem is to define a dwarf2_reg_to_regnum
method which maps the index of DWARF's return address column to GDB's PC 
register.  So, for the AVR, you'd map 36 to 35.

If you do this, I think you can dispense with all of the stuff
which adds and manipulates the pseudo-register.

Here's an example (from rx-tdep.c) where this is done:

static int
rx_dwarf_reg_to_regnum (struct gdbarch *gdbarch, int reg)
{
  if (0 <= reg && reg <= 15)
    return reg;
  else if (reg == 16)
    return RX_PSW_REGNUM;
  else if (reg == 17)
    return RX_PC_REGNUM;
  else
    return -1;
}

Then, in rx_gdbarch_init, this function is registered as follows:

  set_gdbarch_dwarf2_reg_to_regnum (gdbarch, rx_dwarf_reg_to_regnum);

Hope this helps...

Kevin