binutils-gdb/gas/gen-sframe.c

/* gen-sframe.c - Support for generating SFrame section.
   Copyright (C) 2022 Free Software Foundation, Inc.

   This file is part of GAS, the GNU Assembler.

   GAS is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3, or (at your option)
   any later version.

   GAS 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 a copy of the GNU General Public License
   along with GAS; see the file COPYING.  If not, write to the Free
   Software Foundation, 51 Franklin Street - Fifth Floor, Boston, MA
   02110-1301, USA.  */

#include "as.h"
#include "subsegs.h"
#include "sframe.h"
#include "gen-sframe.h"
#include "dw2gencfi.h"

#ifdef support_sframe_p

/* By default, use 32-bit relocations from .sframe into .text.  */
#ifndef SFRAME_RELOC_SIZE
# define SFRAME_RELOC_SIZE 4
#endif

/* Whether frame row entries track RA.

   A target may not need return address tracking for stack unwinding.  If it
   does need the same, SFRAME_CFA_RA_REG must be defined with the return
   address register number.  */

#if defined (sframe_ra_tracking_p) && defined (SFRAME_CFA_RA_REG)
# ifndef SFRAME_FRE_RA_TRACKING
# define SFRAME_FRE_RA_TRACKING 1
# endif
#endif

/* SFrame FRE type selection optimization is an optimization for size.

   There are three flavors of SFrame FRE representation in the binary format:
     - sframe_frame_row_entry_addr1 where the FRE start address is 1 byte.
     - sframe_frame_row_entry_addr2 where the FRE start address is 2 bytes.
     - sframe_frame_row_entry_addr4 where the FRE start address is 4 bytes.

   Note that in the SFrame format, all SFrame FREs of a function use one
   single representation.  The SFrame FRE type itself is identified via the
   information in the SFrame FDE function info.

   Now, to select the minimum required one from the list above, one needs to
   make a decision based on the size (in bytes) of the function.

   As a result, for this optimization, some fragments (generated with a new
   type rs_sframe) for the SFrame section are fixed up later.

   This optimization (for size) is enabled by default.  */

#ifndef SFRAME_FRE_TYPE_SELECTION_OPT
# define SFRAME_FRE_TYPE_SELECTION_OPT 1
#endif

/* Emit a single byte into the current segment.  */

static inline void
out_one (int byte)
{
  FRAG_APPEND_1_CHAR (byte);
}

/* Emit a two-byte word into the current segment.  */

static inline void
out_two (int data)
{
  md_number_to_chars (frag_more (2), data, 2);
}

/* Emit a four byte word into the current segment.  */

static inline void
out_four (int data)
{
  md_number_to_chars (frag_more (4), data, 4);
}

/* Get the start address symbol from the DWARF FDE.  */

static symbolS*
get_dw_fde_start_addrS (const struct fde_entry *dw_fde)
{
  return dw_fde->start_address;
}

/* Get the start address symbol from the DWARF FDE.  */

static symbolS*
get_dw_fde_end_addrS (const struct fde_entry *dw_fde)
{
  return dw_fde->end_address;
}

/* Get whether PAUTH B key is used.  */
static bool
get_dw_fde_pauth_b_key_p (const struct fde_entry *dw_fde ATTRIBUTE_UNUSED)
{
#ifdef tc_fde_entry_extras
  return (dw_fde->pauth_key == AARCH64_PAUTH_KEY_B);
#else
  return false;
#endif
}

/* SFrame Frame Row Entry (FRE) related functions.  */

static void
sframe_fre_set_begin_addr (struct sframe_row_entry *fre, symbolS *beginS)
{
  fre->pc_begin = beginS;
}

static void
sframe_fre_set_end_addr (struct sframe_row_entry *fre, symbolS *endS)
{
  fre->pc_end = endS;
}

static void
sframe_fre_set_cfa_base_reg (struct sframe_row_entry *fre,
			     unsigned int cfa_base_reg)
{
  fre->cfa_base_reg = cfa_base_reg;
  fre->merge_candidate = false;
}

static void
sframe_fre_set_cfa_offset (struct sframe_row_entry *fre,
			   offsetT cfa_offset)
{
  fre->cfa_offset = cfa_offset;
  fre->merge_candidate = false;
}

#ifdef SFRAME_FRE_RA_TRACKING
static void
sframe_fre_set_ra_track (struct sframe_row_entry *fre, offsetT ra_offset)
{
  fre->ra_loc = SFRAME_FRE_ELEM_LOC_STACK;
  fre->ra_offset = ra_offset;
  fre->merge_candidate = false;
}
#endif

static void
sframe_fre_set_bp_track (struct sframe_row_entry *fre, offsetT bp_offset)
{
  fre->bp_loc = SFRAME_FRE_ELEM_LOC_STACK;
  fre->bp_offset = bp_offset;
  fre->merge_candidate = false;
}

/* All stack offset values within an FRE are uniformly encoded in the same
   number of bytes.  The size of the stack offset values will, however, vary
   across FREs.  */

#define VALUE_8BIT  0x7f
#define VALUE_16BIT 0x7fff
#define VALUE_32BIT 0x7fffffff
#define VALUE_64BIT 0x7fffffffffffffff

/* Given a signed offset, return the size in bytes needed to represent it.  */

static unsigned int
get_offset_size_in_bytes (offsetT value)
{
  unsigned int size = 0;

  if (value <= VALUE_8BIT && value >= (offsetT) -VALUE_8BIT)
    size = 1;
  else if (value <= VALUE_16BIT && value >= (offsetT) -VALUE_16BIT)
    size = 2;
  else if (value <= VALUE_32BIT && value >= (offsetT) -VALUE_32BIT)
    size = 4;
  else if ((sizeof (offsetT) > 4) && (value <= (offsetT) VALUE_64BIT
				      && value >= (offsetT) -VALUE_64BIT))
    size = 8;

  return size;
}

#define SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_1B  0 /* SFRAME_FRE_OFFSET_1B.  */
#define SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_2B  1 /* SFRAME_FRE_OFFSET_2B.  */
#define SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_4B  2 /* SFRAME_FRE_OFFSET_4B.  */
#define SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_8B  3 /* Not supported in SFrame.  */
#define SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_MAX SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_8B

/* Helper struct for mapping offset size to output functions.  */

struct sframe_fre_offset_func_map
{
  unsigned int offset_size;
  void (*out_func)(int);
};

/* Given an OFFSET_SIZE, return the size in bytes needed to represent it.  */

static unsigned int
sframe_fre_offset_func_map_index (unsigned int offset_size)
{
  unsigned int idx = SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_MAX;

  switch (offset_size)
    {
      case SFRAME_FRE_OFFSET_1B:
	idx = SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_1B;
	break;
      case SFRAME_FRE_OFFSET_2B:
	idx = SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_2B;
	break;
      case SFRAME_FRE_OFFSET_4B:
	idx = SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_4B;
	break;
      default:
	/* Not supported in SFrame.  */
	break;
    }

  return idx;
}

/* Mapping from offset size to the output function to emit the value.  */

static const
struct sframe_fre_offset_func_map
fre_offset_func_map[SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_MAX+1] =
{
  { SFRAME_FRE_OFFSET_1B, out_one },
  { SFRAME_FRE_OFFSET_2B, out_two },
  { SFRAME_FRE_OFFSET_4B, out_four },
  { -1, NULL } /* Not Supported in SFrame.  */
};

/* SFrame version specific operations access.  */

static struct sframe_version_ops sframe_ver_ops;

/* SFrame (SFRAME_VERSION_1) set FRE info.  */

static unsigned char
sframe_v1_set_fre_info (unsigned int base_reg, unsigned int num_offsets,
			unsigned int offset_size, bool mangled_ra_p)
{
  unsigned char fre_info;
  fre_info = SFRAME_V1_FRE_INFO (base_reg, num_offsets, offset_size);
  fre_info = SFRAME_V1_FRE_INFO_UPDATE_MANGLED_RA_P (mangled_ra_p, fre_info);
  return fre_info;
}

/* SFrame (SFRAME_VERSION_1) set function info.  */
static unsigned char
sframe_v1_set_func_info (unsigned int fde_type, unsigned int fre_type,
			 unsigned int pauth_key)
{
  unsigned char func_info;
  func_info = SFRAME_V1_FUNC_INFO (fde_type, fre_type);
  func_info = SFRAME_V1_FUNC_INFO_UPDATE_PAUTH_KEY (pauth_key, func_info);
  return func_info;
}

/* SFrame version specific operations setup.  */

static void
sframe_set_version (uint32_t sframe_version __attribute__((unused)))
{
  sframe_ver_ops.format_version = SFRAME_VERSION_1;

  sframe_ver_ops.set_fre_info = sframe_v1_set_fre_info;

  sframe_ver_ops.set_func_info = sframe_v1_set_func_info;
}

/* SFrame set FRE info.  */

static unsigned char
sframe_set_fre_info (unsigned int base_reg, unsigned int num_offsets,
		     unsigned int offset_size, bool mangled_ra_p)
{
  return sframe_ver_ops.set_fre_info (base_reg, num_offsets,
				      offset_size, mangled_ra_p);
}

/* SFrame set func info. */

ATTRIBUTE_UNUSED static unsigned char
sframe_set_func_info (unsigned int fde_type, unsigned int fre_type,
		      unsigned int pauth_key)
{
  return sframe_ver_ops.set_func_info (fde_type, fre_type, pauth_key);
}

/* Get the number of SFrame FDEs for the current file.  */

static unsigned int
get_num_sframe_fdes (void);

/* Get the number of SFrame frame row entries for the current file.  */

static unsigned int
get_num_sframe_fres (void);

/* Get CFA base register ID as represented in SFrame Frame Row Entry.  */

static unsigned int
get_fre_base_reg_id (struct sframe_row_entry *sframe_fre)
{
  unsigned int cfi_insn_cfa_base_reg = sframe_fre->cfa_base_reg;
  unsigned fre_base_reg = SFRAME_BASE_REG_SP;

  if (cfi_insn_cfa_base_reg == SFRAME_CFA_FP_REG)
    fre_base_reg = SFRAME_BASE_REG_FP;

  /* Only one bit is reserved in SFRAME_VERSION_1.  */
  gas_assert (fre_base_reg == SFRAME_BASE_REG_SP
	      || fre_base_reg == SFRAME_BASE_REG_FP);

  return fre_base_reg;
}

/* Get number of offsets necessary for the SFrame Frame Row Entry.  */

static unsigned int
get_fre_num_offsets (struct sframe_row_entry *sframe_fre)
{
  /* Atleast 1 must always be present (to recover CFA).  */
  unsigned int fre_num_offsets = 1;

  if (sframe_fre->bp_loc == SFRAME_FRE_ELEM_LOC_STACK)
    fre_num_offsets++;
#ifdef SFRAME_FRE_RA_TRACKING
  if (sframe_fre->ra_loc == SFRAME_FRE_ELEM_LOC_STACK)
    fre_num_offsets++;
#endif
  return fre_num_offsets;
}

/* Get the minimum necessary offset size (in bytes) for this
   SFrame frame row entry.  */

static unsigned int
sframe_get_fre_offset_size (struct sframe_row_entry *sframe_fre)
{
  unsigned int max_offset_size = 0;
  unsigned int cfa_offset_size = 0;
  unsigned int bp_offset_size = 0;
  unsigned int ra_offset_size = 0;

  unsigned int fre_offset_size = 0;

  /* What size of offsets appear in this frame row entry.  */
  cfa_offset_size = get_offset_size_in_bytes (sframe_fre->cfa_offset);
  if (sframe_fre->bp_loc == SFRAME_FRE_ELEM_LOC_STACK)
    bp_offset_size = get_offset_size_in_bytes (sframe_fre->bp_offset);
#ifdef SFRAME_FRE_RA_TRACKING
  if (sframe_ra_tracking_p ()
      && sframe_fre->ra_loc == SFRAME_FRE_ELEM_LOC_STACK)
    ra_offset_size = get_offset_size_in_bytes (sframe_fre->ra_offset);
#endif

  /* Get the maximum size needed to represent the offsets.  */
  max_offset_size = cfa_offset_size;
  if (bp_offset_size > max_offset_size)
    max_offset_size = bp_offset_size;
  if (ra_offset_size > max_offset_size)
    max_offset_size = ra_offset_size;

  gas_assert (max_offset_size);

  switch (max_offset_size)
    {
    case 1:
      fre_offset_size = SFRAME_FRE_OFFSET_1B;
      break;
    case 2:
      fre_offset_size = SFRAME_FRE_OFFSET_2B;
      break;
    case 4:
      fre_offset_size = SFRAME_FRE_OFFSET_4B;
      break;
    default:
      /* Offset of size 8 bytes is not supported in SFrame format
	 version 1.  */
      as_fatal (_("SFrame unsupported offset value\n"));
      break;
    }

  return fre_offset_size;
}

#if SFRAME_FRE_TYPE_SELECTION_OPT

/* Create a composite exression CEXP (for SFrame FRE start address) such that:

      exp = <val> OP_absent <width>, where,

    - <val> and <width> are themselves expressionS.
    - <val> stores the expression which when evaluated gives the value of the
      start address offset of the FRE.
    - <width> stores the expression when evaluated gives the number of bytes
      needed to encode the start address offset of the FRE.

   The use of OP_absent as the X_op_symbol helps identify this expression
   later when fragments are fixed up.  */

static void
create_fre_start_addr_exp (expressionS *cexp, symbolS *fre_pc_begin,
			   symbolS *fde_start_address,
			   symbolS *fde_end_address)
{
  expressionS val;
  expressionS width;

  /* val expression stores the FDE start address offset from the start PC
     of function.  */
  val.X_op = O_subtract;
  val.X_add_symbol = fre_pc_begin;
  val.X_op_symbol = fde_start_address;
  val.X_add_number = 0;

  /* width expressions stores the size of the function.  This is used later
     to determine the number of bytes to be used to encode the FRE start
     address of each FRE of the function.  */
  width.X_op = O_subtract;
  width.X_add_symbol = fde_end_address;
  width.X_op_symbol = fde_start_address;
  width.X_add_number = 0;

  cexp->X_op = O_absent;
  cexp->X_add_symbol = make_expr_symbol (&val);
  cexp->X_op_symbol = make_expr_symbol (&width);
  cexp->X_add_number = 0;
}

/* Create a composite exression CEXP (for SFrame FDE function info) such that:

      exp = <rest_of_func_info> OP_modulus <width>, where,

    - <rest_of_func_info> and <width> are themselves expressionS.
    - <rest_of_func_info> stores a constant expression where X_add_number is
    used to stash away the func_info.  The upper 4-bits of the func_info are copied
    back to the resulting byte by the fragment fixup logic.
    - <width> stores the expression when evaluated gives the size of the
    funtion in number of bytes.

   The use of OP_modulus as the X_op_symbol helps identify this expression
   later when fragments are fixed up.  */

static void
create_func_info_exp (expressionS *cexp, symbolS *dw_fde_end_addrS,
		      symbolS *dw_fde_start_addrS, uint8_t func_info)
{
  expressionS width;
  expressionS rest_of_func_info;

  width.X_op = O_subtract;
  width.X_add_symbol = dw_fde_end_addrS;
  width.X_op_symbol = dw_fde_start_addrS;
  width.X_add_number = 0;

  rest_of_func_info.X_op = O_constant;
  rest_of_func_info.X_add_number = func_info;

  cexp->X_op = O_modulus;
  cexp->X_add_symbol = make_expr_symbol (&rest_of_func_info);
  cexp->X_op_symbol = make_expr_symbol (&width);
  cexp->X_add_number = 0;
}

#endif

static void
output_sframe_row_entry (symbolS *fde_start_addr,
			 symbolS *fde_end_addr,
			 struct sframe_row_entry *sframe_fre)
{
  unsigned char fre_info;
  unsigned int fre_num_offsets;
  unsigned int fre_offset_size;
  unsigned int fre_base_reg;
  expressionS exp;
  unsigned int fre_addr_size;

  unsigned int idx = 0;
  unsigned int fre_write_offsets = 0;

  fre_addr_size = 4; /* 4 bytes by default.   FIXME tie it to fre_type? */

  /* SFrame FRE Start Address.  */
#if SFRAME_FRE_TYPE_SELECTION_OPT
  create_fre_start_addr_exp (&exp, sframe_fre->pc_begin, fde_start_addr,
			     fde_end_addr);
  frag_grow (fre_addr_size);
  frag_var (rs_sframe, fre_addr_size, 0, (relax_substateT) 0,
	    make_expr_symbol (&exp), 0, (char *) frag_now);
#else
  gas_assert (fde_end_addr);
  exp.X_op = O_subtract;
  exp.X_add_symbol = sframe_fre->pc_begin; /* to.  */
  exp.X_op_symbol = fde_start_addr; /* from.  */
  exp.X_add_number = 0;
  emit_expr (&exp, fre_addr_size);
#endif

  /* Create the fre_info using the CFA base register, number of offsets and max
     size of offset in this frame row entry.  */
  fre_base_reg = get_fre_base_reg_id (sframe_fre);
  fre_num_offsets = get_fre_num_offsets (sframe_fre);
  fre_offset_size = sframe_get_fre_offset_size (sframe_fre);
  fre_info = sframe_set_fre_info (fre_base_reg, fre_num_offsets,
				  fre_offset_size, sframe_fre->mangled_ra_p);
  out_one (fre_info);

  idx = sframe_fre_offset_func_map_index (fre_offset_size);
  gas_assert (idx < SFRAME_FRE_OFFSET_FUNC_MAP_INDEX_MAX);

  /* Write out the offsets in order - cfa, bp, ra.  */
  fre_offset_func_map[idx].out_func (sframe_fre->cfa_offset);
  fre_write_offsets++;

#ifdef SFRAME_FRE_RA_TRACKING
  if (sframe_fre->ra_loc == SFRAME_FRE_ELEM_LOC_STACK)
    {
      fre_offset_func_map[idx].out_func (sframe_fre->ra_offset);
      fre_write_offsets++;
    }
#endif
  if (sframe_fre->bp_loc == SFRAME_FRE_ELEM_LOC_STACK)
    {
      fre_offset_func_map[idx].out_func (sframe_fre->bp_offset);
      fre_write_offsets++;
    }

  /* Check if the expected number offsets have been written out
     in this FRE.  */
  gas_assert (fre_write_offsets == fre_num_offsets);
}

static void
output_sframe_funcdesc (symbolS *start_of_fre_section,
			symbolS *fre_symbol,
			struct sframe_func_entry *sframe_fde)
{
  expressionS exp;
  unsigned int addr_size;
  symbolS *dw_fde_start_addrS, *dw_fde_end_addrS;
  unsigned int pauth_key;

  addr_size = SFRAME_RELOC_SIZE;
  dw_fde_start_addrS = get_dw_fde_start_addrS (sframe_fde->dw_fde);
  dw_fde_end_addrS = get_dw_fde_end_addrS (sframe_fde->dw_fde);

  /* Start address of the function.  */
  exp.X_op = O_subtract;
  exp.X_add_symbol = dw_fde_start_addrS; /* to location.  */
  exp.X_op_symbol = symbol_temp_new_now (); /* from location.  */
  exp.X_add_number = 0;
  emit_expr (&exp, addr_size);

  /* Size of the function in bytes.  */
  exp.X_op = O_subtract;
  exp.X_add_symbol = dw_fde_end_addrS;
  exp.X_op_symbol = dw_fde_start_addrS;
  exp.X_add_number = 0;
  emit_expr (&exp, addr_size);

  /* Offset to the first frame row entry.  */
  exp.X_op = O_subtract;
  exp.X_add_symbol = fre_symbol; /* Minuend.  */
  exp.X_op_symbol = start_of_fre_section; /* Subtrahend.  */
  exp.X_add_number = 0;
  emit_expr (&exp, addr_size);

  /* Number of FREs.  */
  out_four (sframe_fde->num_fres);

  /* SFrame FDE function info.  */
  unsigned char func_info;
  pauth_key = (get_dw_fde_pauth_b_key_p (sframe_fde->dw_fde)
	       ? SFRAME_AARCH64_PAUTH_KEY_B : SFRAME_AARCH64_PAUTH_KEY_A);
  func_info = sframe_set_func_info (SFRAME_FDE_TYPE_PCINC,
				    SFRAME_FRE_TYPE_ADDR4,
				    pauth_key);
#if SFRAME_FRE_TYPE_SELECTION_OPT
  expressionS cexp;
  create_func_info_exp (&cexp, dw_fde_end_addrS, dw_fde_start_addrS,
			func_info);
  frag_grow (1); /* Size of func info is unsigned char.  */
  frag_var (rs_sframe, 1, 0, (relax_substateT) 0,
	    make_expr_symbol (&cexp), 0, (char *) frag_now);
#else
  out_one (func_info);
#endif
}

static void
output_sframe_internal (void)
{
  expressionS exp;
  unsigned int i = 0;

  symbolS *end_of_frame_hdr;
  symbolS *end_of_frame_section;
  symbolS *start_of_func_desc_section;
  symbolS *start_of_fre_section;
  struct sframe_func_entry *sframe_fde;
  struct sframe_row_entry *sframe_fre;
  unsigned char abi_arch = 0;
  int fixed_bp_offset = SFRAME_CFA_FIXED_FP_INVALID;
  int fixed_ra_offset = SFRAME_CFA_FIXED_RA_INVALID;
  unsigned int addr_size;

  addr_size = SFRAME_RELOC_SIZE;

  /* The function desciptor entries as dumped by the assembler are not
     sorted on PCs.  */
  unsigned char sframe_flags = 0;
  sframe_flags |= !SFRAME_F_FDE_SORTED;

  unsigned int num_fdes = get_num_sframe_fdes ();
  unsigned int num_fres = get_num_sframe_fres ();
  symbolS **fre_symbols = XNEWVEC (symbolS *, num_fres);
  for (i = 0; i < num_fres; i++)
    fre_symbols[i] = symbol_temp_make ();

  end_of_frame_hdr = symbol_temp_make ();
  start_of_fre_section = symbol_temp_make ();
  start_of_func_desc_section = symbol_temp_make ();
  end_of_frame_section = symbol_temp_make ();

  /* Output the preamble of SFrame section.  */
  out_two (SFRAME_MAGIC);
  out_one (SFRAME_VERSION);
  out_one (sframe_flags);
  /* abi/arch.  */
#ifdef sframe_get_abi_arch
  abi_arch = sframe_get_abi_arch ();
#endif
  gas_assert (abi_arch);
  out_one (abi_arch);

  /* Offset for the BP register from CFA.  Neither of the AMD64 or AAPCS64
     ABIs have a fixed offset for the BP register from the CFA.  This may be
     useful in future (but not without additional support in the toolchain)
     for specialized handling/encoding for cases where, for example,
     -fno-omit-frame-pointer is used.  */
  out_one (fixed_bp_offset);

  /* Offset for the return address from CFA is fixed for some ABIs
     (e.g., AMD64), output a zero otherwise.  */
#ifdef sframe_ra_tracking_p
  if (!sframe_ra_tracking_p ())
    fixed_ra_offset = sframe_cfa_ra_offset ();
#endif
  out_one (fixed_ra_offset);

  /* None of the AMD64, or AARCH64 ABIs need the auxilliary header.
     When the need does arise to use this field, the appropriate backend
     must provide this information.  */
  out_one (0); /* Auxilliary SFrame header length.  */

  out_four (num_fdes); /* Number of FDEs.  */
  out_four (num_fres); /* Number of FREs.  */

  /* FRE sub-section len.  */
  exp.X_op = O_subtract;
  exp.X_add_symbol = end_of_frame_section;
  exp.X_op_symbol = start_of_fre_section;
  exp.X_add_number = 0;
  emit_expr (&exp, addr_size);

  /* Offset of Function Index sub-section.  */
  exp.X_op = O_subtract;
  exp.X_add_symbol = end_of_frame_hdr;
  exp.X_op_symbol = start_of_func_desc_section;
  exp.X_add_number = 0;
  emit_expr (&exp, addr_size);

  /* Offset of FRE sub-section.  */
  exp.X_op = O_subtract;
  exp.X_add_symbol = start_of_fre_section;
  exp.X_op_symbol = end_of_frame_hdr;
  exp.X_add_number = 0;
  emit_expr (&exp, addr_size);

  symbol_set_value_now (end_of_frame_hdr);
  symbol_set_value_now (start_of_func_desc_section);

  /* Output the SFrame function descriptor entries.  */
  i = 0;
  for (sframe_fde = all_sframe_fdes; sframe_fde; sframe_fde = sframe_fde->next)
    {
      output_sframe_funcdesc (start_of_fre_section,
			      fre_symbols[i], sframe_fde);
      i += sframe_fde->num_fres;
    }

  symbol_set_value_now (start_of_fre_section);

  /* Output the SFrame FREs.  */
  i = 0;
  sframe_fde = all_sframe_fdes;

  for (sframe_fde = all_sframe_fdes; sframe_fde; sframe_fde = sframe_fde->next)
    {
      for (sframe_fre = sframe_fde->sframe_fres;
	   sframe_fre;
	   sframe_fre = sframe_fre->next)
	{
	  symbol_set_value_now (fre_symbols[i]);
	  output_sframe_row_entry (get_dw_fde_start_addrS (sframe_fde->dw_fde),
				   get_dw_fde_end_addrS (sframe_fde->dw_fde),
				   sframe_fre);
	  i++;
	}
    }

  symbol_set_value_now (end_of_frame_section);

  gas_assert (i == num_fres);

  free (fre_symbols);
  fre_symbols = NULL;
}

/* List of SFrame FDE entries.  */

struct sframe_func_entry *all_sframe_fdes;

/* Tail of the list to add to.  */

static struct sframe_func_entry **last_sframe_fde = &all_sframe_fdes;

static unsigned int
get_num_sframe_fdes (void)
{
  struct sframe_func_entry *sframe_fde;
  unsigned int total_fdes = 0;

  for (sframe_fde = all_sframe_fdes; sframe_fde ; sframe_fde = sframe_fde->next)
    total_fdes++;

  return total_fdes;
}

/* Get the total number of SFrame row entries across the FDEs.  */

static unsigned int
get_num_sframe_fres (void)
{
  struct sframe_func_entry *sframe_fde;
  unsigned int total_fres = 0;

  for (sframe_fde = all_sframe_fdes; sframe_fde ; sframe_fde = sframe_fde->next)
    total_fres += sframe_fde->num_fres;

  return total_fres;
}

/* Allocate an SFrame FDE.  */

static struct sframe_func_entry*
sframe_fde_alloc (void)
{
  struct sframe_func_entry *sframe_fde = XCNEW (struct sframe_func_entry);
  return sframe_fde;
}

/* Link the SFrame FDE in.  */

static int
sframe_fde_link (struct sframe_func_entry *sframe_fde)
{
  *last_sframe_fde = sframe_fde;
  last_sframe_fde = &sframe_fde->next;

  return 0;
}

/* Free up the SFrame FDE.  */

static void
sframe_fde_free (struct sframe_func_entry *sframe_fde)
{
  XDELETE (sframe_fde);
  sframe_fde = NULL;
}

/* SFrame translation context functions.  */

/* Allocate a new SFrame translation context.  */

static struct sframe_xlate_ctx*
sframe_xlate_ctx_alloc (void)
{
  struct sframe_xlate_ctx* xlate_ctx = XCNEW (struct sframe_xlate_ctx);
  return xlate_ctx;
}

/* Initialize the given SFrame translation context.  */

static void
sframe_xlate_ctx_init (struct sframe_xlate_ctx *xlate_ctx)
{
  xlate_ctx->dw_fde = NULL;
  xlate_ctx->first_fre = NULL;
  xlate_ctx->last_fre = NULL;
  xlate_ctx->cur_fre = NULL;
  xlate_ctx->remember_fre = NULL;
  xlate_ctx->num_xlate_fres = 0;
}

/* Cleanup the given SFrame translation context.  */

static void
sframe_xlate_ctx_cleanup (struct sframe_xlate_ctx *xlate_ctx)
{
  struct sframe_row_entry *fre, *fre_next;

  if (xlate_ctx->num_xlate_fres)
    {
      fre = xlate_ctx->first_fre;
      while (fre)
	{
	  fre_next = fre->next;
	  XDELETE (fre);
	  fre = fre_next;
	}
    }

  sframe_xlate_ctx_init (xlate_ctx);
}

/* Transfer the state from the SFrame translation context to the SFrame FDE.  */

static void
sframe_xlate_ctx_finalize (struct sframe_xlate_ctx *xlate_ctx,
			   struct sframe_func_entry *sframe_fde)
{
  sframe_fde->dw_fde = xlate_ctx->dw_fde;
  sframe_fde->sframe_fres = xlate_ctx->first_fre;
  sframe_fde->num_fres = xlate_ctx->num_xlate_fres;
}

static struct sframe_row_entry*
sframe_row_entry_new (void)
{
  struct sframe_row_entry *fre = XCNEW (struct sframe_row_entry);
  /* Reset cfa_base_reg to -1.  A value of 0 will imply some valid register
     for the supported arches.  */
  fre->cfa_base_reg = -1;
  fre->merge_candidate = true;
  /* Reset the mangled RA status bit to zero by default.  We will initialize it in
     sframe_row_entry_initialize () with the sticky bit if set.  */
  fre->mangled_ra_p = false;

  return fre;
}

/* Add the given FRE in the list of frame row entries in the given FDE
   translation context.  */

static void
sframe_xlate_ctx_add_fre (struct sframe_xlate_ctx *xlate_ctx,
			 struct sframe_row_entry *fre)
{
  gas_assert (xlate_ctx && fre);

  /* Add the frame row entry.  */
  if (!xlate_ctx->first_fre)
    xlate_ctx->first_fre = fre;
  else if (xlate_ctx->last_fre)
    xlate_ctx->last_fre->next = fre;

  xlate_ctx->last_fre = fre;

  /* Keep track of the total number of SFrame frame row entries.  */
  xlate_ctx->num_xlate_fres++;
}

/* A SFrame Frame Row Entry is self-sufficient in terms of unwind information
   for a given PC.  It contains information assimilated from multiple CFI
   instructions, and hence, a new SFrame FRE is initialized with the data from
   the previous known FRE, if any.

   Understandably, not all information (especially the instruction begin
   and end boundaries) needs to be relayed.  Hence, the caller of this API
   must set the pc_begin and pc_end as applicable.  */

static void
sframe_row_entry_initialize (struct sframe_row_entry *cur_fre,
			     struct sframe_row_entry *prev_fre)
{
  gas_assert (prev_fre);
  cur_fre->cfa_base_reg = prev_fre->cfa_base_reg;
  cur_fre->cfa_offset = prev_fre->cfa_offset;
  cur_fre->bp_loc = prev_fre->bp_loc;
  cur_fre->bp_offset = prev_fre->bp_offset;
  cur_fre->ra_loc = prev_fre->ra_loc;
  cur_fre->ra_offset = prev_fre->ra_offset;
  /* Treat RA mangling as a sticky bit.  It retains its value until another
     .cfi_negate_ra_state is seen.  */
  cur_fre->mangled_ra_p = prev_fre->mangled_ra_p;
}

/* Translate DW_CFA_advance_loc into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_advance_loc (struct sframe_xlate_ctx *xlate_ctx,
			     struct cfi_insn_data *cfi_insn)
{
  struct sframe_row_entry *last_fre = xlate_ctx->last_fre;
  /* Get the scratchpad FRE currently being updated as the cfi_insn's
     get interpreted.  This FRE eventually gets linked in into the
     list of FREs for the specific function.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;

  if (cur_fre)
    {
      if (!cur_fre->merge_candidate)
	{
	  sframe_fre_set_end_addr (cur_fre, cfi_insn->u.ll.lab2);

	  sframe_xlate_ctx_add_fre (xlate_ctx, cur_fre);
	  last_fre = xlate_ctx->last_fre;

	  xlate_ctx->cur_fre = sframe_row_entry_new ();
	  cur_fre = xlate_ctx->cur_fre;

	  if (last_fre)
	    sframe_row_entry_initialize (cur_fre, last_fre);
	}
      else
	{
	  sframe_fre_set_end_addr (last_fre, cfi_insn->u.ll.lab2);
	  gas_assert (last_fre->merge_candidate == false);
	}
    }
  else
    {
      xlate_ctx->cur_fre = sframe_row_entry_new ();
      cur_fre = xlate_ctx->cur_fre;
    }

  gas_assert (cur_fre);
  sframe_fre_set_begin_addr (cur_fre, cfi_insn->u.ll.lab2);

  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_def_cfa into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_def_cfa (struct sframe_xlate_ctx *xlate_ctx,
			 struct cfi_insn_data *cfi_insn)

{
  /* Get the scratchpad FRE.  This FRE will eventually get linked in.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;
  if (!cur_fre)
  {
    xlate_ctx->cur_fre = sframe_row_entry_new ();
    cur_fre = xlate_ctx->cur_fre;
    sframe_fre_set_begin_addr (cur_fre,
			       get_dw_fde_start_addrS (xlate_ctx->dw_fde));
  }
  /* Define the current CFA rule to use the provided register and
     offset.  */
  sframe_fre_set_cfa_base_reg (cur_fre, cfi_insn->u.ri.reg);
  sframe_fre_set_cfa_offset (cur_fre, cfi_insn->u.ri.offset);
  cur_fre->merge_candidate = false;

  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_def_cfa_register into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_def_cfa_register (struct sframe_xlate_ctx *xlate_ctx,
				  struct cfi_insn_data *cfi_insn)
{
  struct sframe_row_entry *last_fre = xlate_ctx->last_fre;
  /* Get the scratchpad FRE.  This FRE will eventually get linked in.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;
  gas_assert (cur_fre);
  /* Define the current CFA rule to use the provided register (but to
     keep the old offset).  */
  sframe_fre_set_cfa_base_reg (cur_fre, cfi_insn->u.ri.reg);
  sframe_fre_set_cfa_offset (cur_fre, last_fre->cfa_offset);
  cur_fre->merge_candidate = false;

  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_def_cfa_offset into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_def_cfa_offset (struct sframe_xlate_ctx *xlate_ctx,
				struct cfi_insn_data *cfi_insn)
{
  /* The scratchpad FRE currently being updated with each cfi_insn
     being interpreted.  This FRE eventually gets linked in into the
     list of FREs for the specific function.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;

  gas_assert (cur_fre);
  /*  Define the current CFA rule to use the provided offset (but to keep
      the old register).  However, if the old register is not FP/SP,
      skip creating SFrame unwind info for the function. */
  if ((cur_fre->cfa_base_reg == SFRAME_CFA_FP_REG)
      || (cur_fre->cfa_base_reg == SFRAME_CFA_SP_REG))
    {
      sframe_fre_set_cfa_offset (cur_fre, cfi_insn->u.i);
      cur_fre->merge_candidate = false;
    }
  else
    return SFRAME_XLATE_ERR_NOTREPRESENTED;

  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_offset into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_offset (struct sframe_xlate_ctx *xlate_ctx,
			struct cfi_insn_data *cfi_insn)
{
  /* The scratchpad FRE currently being updated with each cfi_insn
     being interpreted.  This FRE eventually gets linked in into the
     list of FREs for the specific function.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;

  gas_assert (cur_fre);
  /* Change the rule for the register indicated by the register number to
     be the specified offset.  */
  if (cfi_insn->u.r == SFRAME_CFA_FP_REG)
    {
      gas_assert (!cur_fre->base_reg);
      sframe_fre_set_bp_track (cur_fre, cfi_insn->u.ri.offset);
      cur_fre->merge_candidate = false;
    }
#ifdef SFRAME_FRE_RA_TRACKING
  else if (sframe_ra_tracking_p ()
	   && cfi_insn->u.r == SFRAME_CFA_RA_REG)
    {
      sframe_fre_set_ra_track (cur_fre, cfi_insn->u.ri.offset);
      cur_fre->merge_candidate = false;
    }
#endif
  /* This is used to track changes to non-rsp registers, skip all others
     except FP / RA for now.  */
  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_val_offset into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_val_offset (struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
			    struct cfi_insn_data *cfi_insn)
{
  /* Previous value of register is CFA + offset.  However, if the specified
     register is not interesting (FP or RA reg), the current DW_CFA_val_offset
     instruction can be safely skipped without sacrificing the asynchonicity of
     unwind information.  */
  if (cfi_insn->u.r == SFRAME_CFA_FP_REG)
    return SFRAME_XLATE_ERR_NOTREPRESENTED; /* Not represented.  */
#ifdef SFRAME_FRE_RA_TRACKING
  else if (sframe_ra_tracking_p ()
	   && cfi_insn->u.r == SFRAME_CFA_RA_REG)
    return SFRAME_XLATE_ERR_NOTREPRESENTED; /* Not represented.  */
#endif

  /* Safe to skip.  */
  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_remember_state into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_remember_state (struct sframe_xlate_ctx *xlate_ctx)
{
  struct sframe_row_entry *last_fre = xlate_ctx->last_fre;

  /* If there is no FRE state to remember, nothing to do here.  Return
     early with non-zero error code, this will cause no SFrame unwind info
     for the function involved.  */
  if (!last_fre)
    return SFRAME_XLATE_ERR_INVAL;

  if (!xlate_ctx->remember_fre)
    xlate_ctx->remember_fre = sframe_row_entry_new ();
  sframe_row_entry_initialize (xlate_ctx->remember_fre, last_fre);

  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_restore_state into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_restore_state (struct sframe_xlate_ctx *xlate_ctx)
{
  /* The scratchpad FRE currently being updated with each cfi_insn
     being interpreted.  This FRE eventually gets linked in into the
     list of FREs for the specific function.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;

  gas_assert (xlate_ctx->remember_fre);
  gas_assert (cur_fre && cur_fre->merge_candidate);

  /* Get the CFA state from the DW_CFA_remember_state insn.  */
  sframe_row_entry_initialize (cur_fre, xlate_ctx->remember_fre);
  /* The PC boundaries of the current SFrame FRE are updated
     via other machinery.  */
  cur_fre->merge_candidate = false;
  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_restore into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_restore (struct sframe_xlate_ctx *xlate_ctx,
			 struct cfi_insn_data *cfi_insn)
{
  struct sframe_row_entry *cie_fre = xlate_ctx->first_fre;
  /* The scratchpad FRE currently being updated with each cfi_insn
     being interpreted.  This FRE eventually gets linked in into the
     list of FREs for the specific function.  */
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;

  /* Change the rule for the indicated register to the rule assigned to
     it by the initial_instructions in the CIE.  */
  gas_assert (cie_fre);
  /* SFrame FREs track only CFA and FP / RA for backtracing purposes;
     skip the other .cfi_restore directives.  */
  if (cfi_insn->u.r == SFRAME_CFA_FP_REG)
    {
      gas_assert (cur_fre);
      cur_fre->bp_loc = cie_fre->bp_loc;
      cur_fre->bp_offset = cie_fre->bp_offset;
      cur_fre->merge_candidate = false;
    }
#ifdef SFRAME_FRE_RA_TRACKING
  else if (sframe_ra_tracking_p ()
	   && cfi_insn->u.r == SFRAME_CFA_RA_REG)
    {
      gas_assert (cur_fre);
      cur_fre->ra_loc = cie_fre->ra_loc;
      cur_fre->ra_offset = cie_fre->ra_offset;
      cur_fre->merge_candidate = false;
    }
#endif
  return SFRAME_XLATE_OK;
}

/* Translate DW_CFA_GNU_window_save into SFrame context.
   Return SFRAME_XLATE_OK if success.  */

static int
sframe_xlate_do_gnu_window_save (struct sframe_xlate_ctx *xlate_ctx,
				 struct cfi_insn_data *cfi_insn ATTRIBUTE_UNUSED)
{
  struct sframe_row_entry *cur_fre = xlate_ctx->cur_fre;

  gas_assert (cur_fre);
  /* Toggle the mangled RA status bit.  */
  cur_fre->mangled_ra_p = !cur_fre->mangled_ra_p;
  cur_fre->merge_candidate = false;

  return SFRAME_XLATE_OK;
}

/* Process CFI_INSN and update the translation context with the FRE
   information.

   Returns an error code (sframe_xlate_err) if CFI_INSN is not successfully
   processed.  */

static int
sframe_do_cfi_insn (struct sframe_xlate_ctx *xlate_ctx,
		    struct cfi_insn_data *cfi_insn)
{
  int err = 0;

  /* Atleast one cfi_insn per FDE is expected.  */
  gas_assert (cfi_insn);
  int op = cfi_insn->insn;

  switch (op)
    {
    case DW_CFA_advance_loc:
      err = sframe_xlate_do_advance_loc (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_def_cfa:
      err = sframe_xlate_do_def_cfa (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_def_cfa_register:
      err = sframe_xlate_do_def_cfa_register (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_def_cfa_offset:
      err = sframe_xlate_do_def_cfa_offset (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_offset:
      err = sframe_xlate_do_offset (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_val_offset:
      err = sframe_xlate_do_val_offset (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_remember_state:
      err = sframe_xlate_do_remember_state (xlate_ctx);
      break;
    case DW_CFA_restore_state:
      err = sframe_xlate_do_restore_state (xlate_ctx);
      break;
    case DW_CFA_restore:
      err = sframe_xlate_do_restore (xlate_ctx, cfi_insn);
      break;
    /* DW_CFA_AARCH64_negate_ra_state is multiplexed with
       DW_CFA_GNU_window_save.  */
    case DW_CFA_GNU_window_save:
      err = sframe_xlate_do_gnu_window_save (xlate_ctx, cfi_insn);
      break;
    case DW_CFA_undefined:
    case DW_CFA_same_value:
      break;
    default:
      {
	/* Other CFI opcodes are not processed at this time.
	   These do not impact the coverage of the basic stack unwinding
	   information as conveyed in the SFrame format.
	    - DW_CFA_register,
	    - ...

	   Following skipped operations do, however, impact the asynchronicity:
	     - CFI_escape  */

	err = SFRAME_XLATE_ERR_NOTREPRESENTED;
	// printf (_("SFrame Unsupported or unknown Dwarf CFI number: %#x\n"), op);
      }
    }

  return err;
}


static int
sframe_do_fde (struct sframe_xlate_ctx *xlate_ctx,
	       const struct fde_entry *dw_fde)
{
  struct cfi_insn_data *cfi_insn;
  int err = SFRAME_XLATE_OK;

  xlate_ctx->dw_fde = dw_fde;

  /* If the return column is not RIP, SFrame format cannot represent it.  */
  if (xlate_ctx->dw_fde->return_column != DWARF2_DEFAULT_RETURN_COLUMN)
    return SFRAME_XLATE_ERR_NOTREPRESENTED;

  /* Iterate over the CFIs and create SFrame FREs.  */
  for (cfi_insn = dw_fde->data; cfi_insn; cfi_insn = cfi_insn->next)
    {
      /* Translate each CFI, and buffer the state in translation context.  */
      err = sframe_do_cfi_insn (xlate_ctx, cfi_insn);
      if (err != SFRAME_XLATE_OK)
	{
	  /* Skip generating SFrame unwind info for the function if any
	     offending CFI is encountered by sframe_do_cfi_insn ().  */
	  return err; /* Return the error code.  */
	}
    }

  /* No errors encountered.  */

  /* Link in the scratchpad FRE that the last few CFI insns helped create.  */
  if (xlate_ctx->cur_fre)
    {
      sframe_xlate_ctx_add_fre (xlate_ctx, xlate_ctx->cur_fre);
      xlate_ctx->cur_fre = NULL;
    }
  /* Designate the end of the last SFrame FRE.  */
  if (xlate_ctx->last_fre)
    {
      xlate_ctx->last_fre->pc_end
	= get_dw_fde_end_addrS (xlate_ctx->dw_fde);
    }

  return SFRAME_XLATE_OK;
}

/* Create SFrame unwind info for all functions.

   This function consumes the already generated FDEs (by dw2gencfi) and
   generates unwind data in SFrame format.  */

static void
create_sframe_all (void)
{
  struct fde_entry *dw_fde = NULL;
  struct sframe_func_entry *sframe_fde = NULL;

  struct sframe_xlate_ctx *xlate_ctx = sframe_xlate_ctx_alloc ();

  for (dw_fde = all_fde_data; dw_fde ; dw_fde = dw_fde->next)
    {
      sframe_fde = sframe_fde_alloc ();
      /* Initialize the translation context with information anew.  */
      sframe_xlate_ctx_init (xlate_ctx);

      /* Process and link SFrame FDEs if no error.  Also skip adding an SFrame
	 FDE if it does not contain any SFrame FREs.  There is little use of an
	 SFrame FDE if there is no unwind information about the function.  */
      int err = sframe_do_fde (xlate_ctx, dw_fde);
      if (err || xlate_ctx->num_xlate_fres == 0)
	{
	  sframe_xlate_ctx_cleanup (xlate_ctx);
	  sframe_fde_free (sframe_fde);
	}
      else
	{
	  /* All done.  Transfer the state from the SFrame translation
	     context to the SFrame FDE.  */
	  sframe_xlate_ctx_finalize (xlate_ctx, sframe_fde);
	  sframe_fde_link (sframe_fde);
	}
    }
}

void
output_sframe (segT sframe_seg)
{
  (void) sframe_seg;

  /* Setup the version specific access functions.  */
  sframe_set_version (SFRAME_VERSION_1);

  /* Process all fdes and create SFrame unwind information.  */
  create_sframe_all ();

  output_sframe_internal ();
}

#else  /*  support_sframe_p  */

void
output_sframe (segT sframe_seg __attribute__((unused)))
{
}

#endif /*  support_sframe_p  */