RE: [patch] Enable dwarf unwind for AVR target

["Sivanupandi, Pitchumani" <Pitchumani.Sivanupandi@atmel.com>]

> -----Original Message-----
> From: Pedro Alves [mailto:palves@redhat.com]
> Sent: 04 January 2016 19:28
> To: Sivanupandi, Pitchumani <Pitchumani.Sivanupandi@atmel.com>;
> troth@openavr.org; brobecker@adacore.com; gdb-patches@sourceware.org
> Subject: Re: [patch] Enable dwarf unwind for AVR target
> 
> On 01/04/2016 08:01 AM, Sivanupandi, Pitchumani wrote:
> > -  AVR_NUM_PSEUDO_REGS = 1,
> > +  AVR_DWARF2_PC_REGNUM = 36 /*LR*/,
> 
> It's hard to comment without some explanation for the design choices in the
> patch, and/or some understanding of what this register is.
> Seems odd that you create the register and then avr_pseudo_register_read
> already returns <unavailable>?  Why did you need to add it?  Is this a real
> machine register?  What's LR?
> 
> > +  AVR_NUM_PSEUDO_REGS = 2,
> >
> >    AVR_PC_REG_INDEX = 35,	/* index into array of registers */
> >
> > @@ -211,7 +213,7 @@ avr_register_name (struct gdbarch *gdbarch, int
> regnum)
> >      "r16", "r17", "r18", "r19", "r20", "r21", "r22", "r23",
> >      "r24", "r25", "r26", "r27", "r28", "r29", "r30", "r31",
> >      "SREG", "SP", "PC2",
> > -    "pc"
> > +    "pc", "LR"
> >    };

AVR has only 32 registers. GDB maps status register, SP and PC to numbers 32,
33 and 34 respectively.

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) to gdb to map
return address register in .debug_frame. Register name is "LR" (link register).
When dwarf frame unwind asks for PC, target function should find return address
value (AVR_DWARF2_PC_REGNUM).

Dwarf frame reader records all registers from the .debug_frame section.
When gdb asks for PC value to unwind, avr_dwarf2_prev_register finds the frame
address recorded for AVR_DWARF2_PC_REGNUM and read value from that memory.
(avr_dwarf2_prev_register implementation is similar to existing 
avr_frame_prev_register function)

I assumed pseudo register read/write for this register not mandatory for dwarf
frame unwind. So, left it with dummy implementation as of now.

NOTE:
* Added extern function dwarf2_frame_addr to dwarf2-frame.c to find the frame
address for argument register from dwarf frame cache.
* dwarf2 address size set to 4.

Regards,
Pitchumani