/* Get previous frame state for an existing frame state.
   Copyright (C) 2016 The Qt Company Ltd.
   This file is part of elfutils.

   This file is free software; you can redistribute it and/or modify
   it under the terms of either

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at
       your option) any later version

   or

     * the GNU General Public License as published by the Free
       Software Foundation; either version 2 of the License, or (at
       your option) any later version

   or both in parallel, as here.

   elfutils is distributed in the hope that it will be useful, but
   WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   General Public License for more details.

   You should have received copies of the GNU General Public License and
   the GNU Lesser General Public License along with this program.  If
   not, see <http://www.gnu.org/licenses/>.  */

#ifdef HAVE_CONFIG_H
# include <config.h>
#endif

#include <stdlib.h>
#include <assert.h>

#ifndef BACKEND
#define BACKEND arm_

// Actually, in THUMB mode it's r7. Let's ignore this.
#define FP_REG 11
#define LR_REG 14
#define SP_REG 13

// The offsets are a mess. gcc-generated code with -mapcs-frame has FP and LR the other way around.
// We make it work with QV4.
#define FP_OFFSET 0
#define LR_OFFSET 4
#define SP_OFFSET 8
#endif

#include "libebl_CPU.h"

/* There was no CFI. Maybe we happen to have a frame pointer and can unwind from that?  */

bool
EBLHOOK(unwind) (Ebl *ebl __attribute__ ((unused)), Dwarf_Addr pc __attribute__ ((unused)),
                 ebl_tid_registers_t *setfunc, ebl_tid_registers_get_t *getfunc,
                 ebl_pid_memory_read_t *readfunc, void *arg,
                 bool *signal_framep __attribute__ ((unused)))
{
  Dwarf_Word fp = 0, sp = 0; // have to be initialized because registers are 32bit only

  if (!getfunc(FP_REG, 1, &fp, arg) || fp == 0)
    return false;

  if (!getfunc(SP_REG, 1, &sp, arg) || sp == 0)
    return false;

  // Stack frames of > 8M are rather unlikely in real programs.
  if (fp <= sp || fp - sp >= 1 << 23)
    return false;

  Dwarf_Word newLr, newFp, newSp;

  if (!readfunc(fp + LR_OFFSET, &newLr, arg) || newLr == 0)
    return false;

  if (!readfunc(fp + FP_OFFSET, &newFp, arg))
    newFp = 0;

  newSp = fp + SP_OFFSET;

  if (!setfunc(-1, 1, &newLr, arg))
    return false;

  // unset the "thumb" bit. We get LR without thumb bit, so let's also pass it on that way.
  newLr &= 0xfffffffe;

  // These are not fatal if they don't work. They will just prevent unwinding at the next frame.
  setfunc(LR_REG, 1, &newLr, arg);
  setfunc(FP_REG, 1, &newFp, arg);
  setfunc(SP_REG, 1, &newSp, arg);

  return true;
}