/* ehopt.c--optimize gcc exception frame information.

   Copyright (C) 1998-2025 Free Software Foundation, Inc.

   Written by Ian Lance Taylor <ian@cygnus.com>.

   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"

/* We include this ELF file, even though we may not be assembling for

   ELF, since the exception frame information is always in a format

   derived from DWARF.  */

#include "dwarf2.h"

/* Try to optimize gcc 2.8 exception frame information.

   Exception frame information is emitted for every function in the

   .eh_frame or .debug_frame sections.  Simple information for a function

   with no exceptions looks like this:

__FRAME_BEGIN__:

  .4byte  .LLCIE1 / Length of Common Information Entry

.LSCIE1:

#if .eh_frame

  .4byte  0x0 / CIE Identifier Tag

#elif .debug_frame

  .4byte  0xffffffff / CIE Identifier Tag

#endif

  .byte 0x1 / CIE Version

  .byte 0x0 / CIE Augmentation (none)

  .byte 0x1 / ULEB128 0x1 (CIE Code Alignment Factor)

  .byte 0x7c  / SLEB128 -4 (CIE Data Alignment Factor)

  .byte 0x8 / CIE RA Column

  .byte 0xc / DW_CFA_def_cfa

  .byte 0x4 / ULEB128 0x4

  .byte 0x4 / ULEB128 0x4

  .byte 0x88  / DW_CFA_offset, column 0x8

  .byte 0x1 / ULEB128 0x1

  .align 4

.LECIE1:

  .set  .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol

  .4byte  .LLFDE1 / FDE Length

.LSFDE1:

  .4byte  .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset

  .4byte  .LFB1 / FDE initial location

  .4byte  .LFE1-.LFB1 / FDE address range

  .byte 0x4 / DW_CFA_advance_loc4

  .4byte  .LCFI0-.LFB1

  .byte 0xe / DW_CFA_def_cfa_offset

  .byte 0x8 / ULEB128 0x8

  .byte 0x85  / DW_CFA_offset, column 0x5

  .byte 0x2 / ULEB128 0x2

  .byte 0x4 / DW_CFA_advance_loc4

  .4byte  .LCFI1-.LCFI0

  .byte 0xd / DW_CFA_def_cfa_register

  .byte 0x5 / ULEB128 0x5

  .byte 0x4 / DW_CFA_advance_loc4

  .4byte  .LCFI2-.LCFI1

  .byte 0x2e  / DW_CFA_GNU_args_size

  .byte 0x4 / ULEB128 0x4

  .byte 0x4 / DW_CFA_advance_loc4

  .4byte  .LCFI3-.LCFI2

  .byte 0x2e  / DW_CFA_GNU_args_size

  .byte 0x0 / ULEB128 0x0

  .align 4

.LEFDE1:

  .set  .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol

   The immediate issue we can address in the assembler is the

   DW_CFA_advance_loc4 followed by a four byte value.  The value is

   the difference of two addresses in the function.  Since gcc does

   not know this value, it always uses four bytes.  We will know the

   value at the end of assembly, so we can do better.  */

struct cie_info

{

  fragS *f;

  unsigned code_alignment;

  int z_augmentation;

};

/* Extract information from the CIE.  */

static bool

get_cie_info (struct cie_info *info)

{

  fragS *f;

  unsigned int offset;

  char CIE_id;

  char augmentation[10];

  int iaug;

  int code_alignment = 0;

  /* We should find the CIE at the start of the section.  */

  f = seg_info (now_seg)->frchainP->frch_root;

  while (f != NULL && f->fr_fix == 0)

    f = f->fr_next;

  if (f != info->f)

    return false;

  /* First make sure that the CIE Identifier Tag is 0/-1.  */

  if (startswith (segment_name (now_seg), ".debug_frame"))

    CIE_id = (char)0xff;

  else

    CIE_id = 0;

  offset = 4;

  while (f != NULL && offset >= f->fr_fix)

    {

      offset -= f->fr_fix;

      f = f->fr_next;

    }

  if (f == NULL

      || f->fr_fix - offset < 4

      || f->fr_literal[offset] != CIE_id

      || f->fr_literal[offset + 1] != CIE_id

      || f->fr_literal[offset + 2] != CIE_id

      || f->fr_literal[offset + 3] != CIE_id)

    return false;

  /* Next make sure the CIE version number is 1.  */

  offset += 4;

  while (f != NULL && offset >= f->fr_fix)

    {

      offset -= f->fr_fix;

      f = f->fr_next;

    }

  if (f == NULL

      || f->fr_fix - offset < 1

      || f->fr_literal[offset] != 1)

    return false;

  /* Skip the augmentation (a null terminated string).  */

  iaug = 0;

  ++offset;

  while (1)

    {

      while (f != NULL && offset >= f->fr_fix)

  {

    offset -= f->fr_fix;

    f = f->fr_next;

  }

      if (f == NULL)

  return false;

      while (offset < f->fr_fix && f->fr_literal[offset] != '\0')

  {

    if ((size_t) iaug < (sizeof augmentation) - 1)

      {

        augmentation[iaug] = f->fr_literal[offset];

        ++iaug;

      }

    ++offset;

  }

      if (offset < f->fr_fix)

  break;

    }

  ++offset;

  while (f != NULL && offset >= f->fr_fix)

    {

      offset -= f->fr_fix;

      f = f->fr_next;

    }

  if (f == NULL)

    return false;

  augmentation[iaug] = '\0';

  if (augmentation[0] == '\0')

    {

      /* No augmentation.  */

    }

  else if (strcmp (augmentation, "eh") == 0)

    {

      /* We have to skip a pointer.  Unfortunately, we don't know how

   large it is.  We find out by looking for a matching fixup.  */

      fixS *fix = seg_info (now_seg)->frchainP->fix_root;

      while (fix != NULL

       && (fix->fx_frag != f || fix->fx_where != offset))

  fix = fix->fx_next;

      if (fix == NULL)

  offset += 4;

      else

  offset += fix->fx_size;

      while (f != NULL && offset >= f->fr_fix)

  {

    offset -= f->fr_fix;

    f = f->fr_next;

  }

      if (f == NULL)

  return false;

    }

  else if (augmentation[0] != 'z')

    return false;

  /* We're now at the code alignment factor, which is a ULEB128.  If

     it isn't a single byte, forget it.  */

  code_alignment = f->fr_literal[offset] & 0xff;

  if ((code_alignment & 0x80) != 0)

    code_alignment = 0;

  info->code_alignment = code_alignment;

  info->z_augmentation = (augmentation[0] == 'z');

  return true;

}

enum frame_state

{

  state_idle,

  state_saw_size,

  state_saw_cie_offset,

  state_saw_pc_begin,

  state_seeing_aug_size,

  state_skipping_aug,

  state_wait_loc4,

  state_saw_loc4,

  state_error,

};

struct frame_data

{

  enum frame_state state;

  bool cie_info_ok;

  struct cie_info cie_info;

  symbolS *size_end_sym;

  fragS *loc4_frag;

  int loc4_fix;

  int aug_size;

  int aug_shift;

};

static struct eh_state

{

  struct frame_data eh_data;

  struct frame_data debug_data;

} frame;

/* This function is called from emit_expr.  It looks for cases which

   we can optimize.

   Rather than try to parse all this information as we read it, we

   look for a single byte DW_CFA_advance_loc4 followed by a 4 byte

   difference.  We turn that into a rs_cfa_advance frag, and handle

   those frags at the end of the assembly.  If the gcc output changes

   somewhat, this optimization may stop working.

   This function returns non-zero if it handled the expression and

   emit_expr should not do anything, or zero otherwise.  It can also

   change *EXP and *PNBYTES.  */

int

check_eh_frame (expressionS *exp, unsigned int *pnbytes)

{

  struct frame_data *d;

  /* Don't optimize.  */

  if (flag_traditional_format)

    return 0;

#ifdef md_allow_eh_opt

  if (! md_allow_eh_opt)

    return 0;

#endif

  /* Select the proper section data.  */

  if (startswith (segment_name (now_seg), ".eh_frame")

      && segment_name (now_seg)[9] != '_')

    d = &frame.eh_data;

  else if (startswith (segment_name (now_seg), ".debug_frame"))

    d = &frame.debug_data;

  else

    return 0;

  if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym))

    {

      /* We have come to the end of the CIE or FDE.  See below where

         we set saw_size.  We must check this first because we may now

         be looking at the next size.  */

      d->state = state_idle;

    }

  switch (d->state)

    {

    case state_idle:

      if (*pnbytes == 4)

  {

    /* This might be the size of the CIE or FDE.  We want to know

       the size so that we don't accidentally optimize across an FDE

       boundary.  We recognize the size in one of two forms: a

       symbol which will later be defined as a difference, or a

       subtraction of two symbols.  Either way, we can tell when we

       are at the end of the FDE because the symbol becomes defined

       (in the case of a subtraction, the end symbol, from which the

       start symbol is being subtracted).  Other ways of describing

       the size will not be optimized.  */

    if ((exp->X_op == O_symbol || exp->X_op == O_subtract)

        && ! S_IS_DEFINED (exp->X_add_symbol))

      {

        d->state = state_saw_size;

        d->size_end_sym = exp->X_add_symbol;

        if (!d->cie_info.f)

    d->cie_info.f = frag_now;

      }

  }

      break;

    case state_saw_size:

    case state_saw_cie_offset:

      if (!(*pnbytes == 4 || *pnbytes == 8))

  /* Stop scanning if we don't see the expected FDE fields.  */

  d->state = state_error;

      else

  d->state = (enum frame_state) (d->state + 1);

      break;

    case state_saw_pc_begin:

      /* Decide whether we should see an augmentation.  */

      if (!(*pnbytes == 4 || *pnbytes == 8))

  d->state = state_error;

      else if (!d->cie_info_ok

         && !(d->cie_info_ok = get_cie_info (&d->cie_info)))

  d->state = state_error;

      else if (d->cie_info.z_augmentation)

  {

    d->state = state_seeing_aug_size;

    d->aug_size = 0;

    d->aug_shift = 0;

  }

      else

  d->state = state_wait_loc4;

      break;

    case state_seeing_aug_size:

      /* Bytes == -1 means this comes from an leb128 directive.  */

      if ((int) *pnbytes == -1 && exp->X_op == O_constant)

  {

    d->aug_size = exp->X_add_number;

    d->state = state_skipping_aug;

  }

      else if (*pnbytes == 1 && exp->X_op == O_constant)

  {

    unsigned char byte = exp->X_add_number;

    d->aug_size |= (byte & 0x7f) << d->aug_shift;

    d->aug_shift += 7;

    if ((byte & 0x80) == 0)

      d->state = state_skipping_aug;

  }

      else

  d->state = state_error;

      if (d->state == state_skipping_aug && d->aug_size == 0)

  d->state = state_wait_loc4;

      break;

    case state_skipping_aug:

      if ((int) *pnbytes < 0)

  d->state = state_error;

      else

  {

    int left = (d->aug_size -= *pnbytes);

    if (left == 0)

      d->state = state_wait_loc4;

    else if (left < 0)

      d->state = state_error;

  }

      break;

    case state_wait_loc4:

      if (*pnbytes == 1

    && exp->X_op == O_constant

    && exp->X_add_number == DW_CFA_advance_loc4)

  {

    /* This might be a DW_CFA_advance_loc4.  Record the frag and the

       position within the frag, so that we can change it later.  */

    frag_grow (1 + 4);

    d->state = state_saw_loc4;

    d->loc4_frag = frag_now;

    d->loc4_fix = frag_now_fix ();

  }

      break;

    case state_saw_loc4:

      d->state = state_wait_loc4;

      if (*pnbytes != 4)

  break;

      if (exp->X_op == O_constant)

  {

    /* This is a case which we can optimize.  The two symbols being

       subtracted were in the same frag and the expression was

       reduced to a constant.  We can do the optimization entirely

       in this function.  */

    if (exp->X_add_number < 0x40)

      {

        d->loc4_frag->fr_literal[d->loc4_fix]

    = DW_CFA_advance_loc | exp->X_add_number;

        /* No more bytes needed.  */

        return 1;

      }

    else if (exp->X_add_number < 0x100)

      {

        d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;

        *pnbytes = 1;

      }

    else if (exp->X_add_number < 0x10000)

      {

        d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;

        *pnbytes = 2;

      }

  }

      else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1)

  {

    /* This is a case we can optimize.  The expression was not

       reduced, so we can not finish the optimization until the end

       of the assembly.  We set up a variant frag which we handle

       later.  */

    frag_var (rs_cfa, 4, 0, 1 << 3, make_expr_symbol (exp),

        d->loc4_fix, (char *) d->loc4_frag);

    return 1;

  }

      else if ((exp->X_op == O_divide

    || exp->X_op == O_right_shift)

         && d->cie_info.code_alignment > 1)

  {

    if (symbol_symbolS (exp->X_add_symbol)

        && symbol_constant_p (exp->X_op_symbol)

        && S_GET_SEGMENT (exp->X_op_symbol) == absolute_section

        && ((exp->X_op == O_divide

       ? *symbol_X_add_number (exp->X_op_symbol)

       : (offsetT) 1 << *symbol_X_add_number (exp->X_op_symbol))

      == (offsetT) d->cie_info.code_alignment))

      {

        expressionS *symval;

        symval = symbol_get_value_expression (exp->X_add_symbol);

        if (symval->X_op == O_subtract)

    {

      /* This is a case we can optimize as well.  The

         expression was not reduced, so we can not finish

         the optimization until the end of the assembly.

         We set up a variant frag which we handle later.  */

      frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3,

          make_expr_symbol (symval),

          d->loc4_fix, (char *) d->loc4_frag);

      return 1;

    }

      }

  }

      break;

    case state_error:

      /* Just skipping everything.  */

      break;

    }

  return 0;

}

/* The function estimates the size of a rs_cfa variant frag based on

   the current values of the symbols.  It is called before the

   relaxation loop.  We set fr_subtype{0:2} to the expected length.  */

int

eh_frame_estimate_size_before_relax (fragS *frag)

{

  offsetT diff;

  int ca = frag->fr_subtype >> 3;

  int ret;

  diff = resolve_symbol_value (frag->fr_symbol);

  gas_assert (ca > 0);

  diff /= ca;

  if (diff == 0)

    ret = -1;

  else if (diff < 0x40)

    ret = 0;

  else if (diff < 0x100)

    ret = 1;

  else if (diff < 0x10000)

    ret = 2;

  else

    ret = 4;

  frag->fr_subtype = (frag->fr_subtype & ~7) | (ret & 7);

  return ret;

}

/* This function relaxes a rs_cfa variant frag based on the current

   values of the symbols.  fr_subtype{0:2} is the current length of

   the frag.  This returns the change in frag length.  */

int

eh_frame_relax_frag (fragS *frag)

{

  int oldsize, newsize;

  oldsize = frag->fr_subtype & 7;

  if (oldsize == 7)

    oldsize = -1;

  newsize = eh_frame_estimate_size_before_relax (frag);

  return newsize - oldsize;

}

/* This function converts a rs_cfa variant frag into a normal fill

   frag.  This is called after all relaxation has been done.

   fr_subtype{0:2} will be the desired length of the frag.  */

void

eh_frame_convert_frag (fragS *frag)

{

  offsetT diff;

  fragS *loc4_frag;

  int loc4_fix, ca;

  loc4_frag = (fragS *) frag->fr_opcode;

  loc4_fix = (int) frag->fr_offset;

  diff = resolve_symbol_value (frag->fr_symbol);

  ca = frag->fr_subtype >> 3;

  gas_assert (ca > 0);

  diff /= ca;

  switch (frag->fr_subtype & 7)

    {

    case 0:

      gas_assert (diff < 0x40);

      loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff;

      break;

    case 1:

      gas_assert (diff < 0x100);

      loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;

      frag->fr_literal[frag->fr_fix] = diff;

      break;

    case 2:

      gas_assert (diff < 0x10000);

      loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;

      md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);

      break;

    case 4:

      md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);

      break;

    case 7:

      gas_assert (diff == 0);

      frag->fr_fix -= 8;

      break;

    default:

      abort ();

    }

  frag->fr_fix += frag->fr_subtype & 7;

  frag->fr_type = rs_fill;

  frag->fr_subtype = 0;

  frag->fr_offset = 0;

}

void

eh_begin (void)

{

  memset (&frame, 0, sizeof (frame));

}