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

["Sivanupandi, Pitchumani" <Pitchumani dot Sivanupandi at atmel dot com>]

Previous patch and discussion is here:
https://sourceware.org/ml/gdb-patches/2016-01/msg00027.html

Test case: step over on function call statements (e.g. break.exp)

Need for dwarf unwind:
Current AVR frame unwind analyzes only the prologue and stack unwind becomes
unreliable. CFA info from dwarf debug information can be used to unwind the
stack pointer and PC reliably.

Attached patch (updated, v2) enables the dwarf unwinder for avr target.

Fix:
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.

Target function avr_dwarf2_prev_register implementation is similar to existing
avr_frame_prev_register function.

Note:
* AVR_DWARF2_PC_REGNUM is meant only to unwind PC. Also we can't expect stack
at all times (e.g. startup code) to read/write into that pseudo register. So,
the pseudo register read will return that register unavailable and write will
not do anything.
* 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 (Ref: DWARF2_ADDR_SIZE from avr-gcc).

Ran GDB regression tests with Atmel internal simulator (atmega2560). No new
regressions found.

Is this patch OK?

Regards,
Pitchumani

gdb/ChangeLog
* avr-tdep.c: Include dwarf2-frame.h
  (enum): Add new pseudo register AVR_DWARF2_PC_REGNUM (36).
  Update number of pseudo registers (AVR_NUM_PSEUDO_REGS).
  (avr_register_name): Add LR as register name for new pseudo register.
  (avr_register_type): return pc type for new register.
  (avr_pseudo_register_read): return that register unavailable for new
  pseudo register.
  (avr_pseudo_register_write): do nothing as new pseudo register is read-only.
  (avr_dwarf2_prev_register): New function to unwind prev register.
  (avr_dwarf_reg_to_regnum): Allow all valid pseudo registers.
  (avr_dwarf2_frame_init_reg): Initialize pseudo registers handler.
  (avr_gdbarch_init): Set dwarf2 address size.
  Set register state init function.
  Add dwarf2 unwinders to the unwinders list.
* dwarf2-frame.c (dwarf2_frame_addr): New function that returns CFA for
  argument register.
* dwarf2-frame.h: Add prototype for new function.

Attachment: avr-enable-dwarf-unwind-v2.patch
Description: avr-enable-dwarf-unwind-v2.patch