gas/ehopt.c

   1 /* ehopt.c--optimize gcc exception frame information.
   2    Copyright 1998, 2000, 2001, 2003 Free Software Foundation, Inc.
   3    Written by Ian Lance Taylor <ian@cygnus.com>.
   4
   5 This file is part of GAS, the GNU Assembler.
   6
   7 GAS is free software; you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation; either version 2, or (at your option)
  10 any later version.
  11
  12 GAS is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GAS; see the file COPYING.  If not, write to the Free
  19 Software Foundation, 59 Temple Place - Suite 330, Boston, MA
  20 02111-1307, USA.  */
  21
  22 #include "as.h"
  23 #include "subsegs.h"
  24
  25 /* We include this ELF file, even though we may not be assembling for
  26    ELF, since the exception frame information is always in a format
  27    derived from DWARF.  */
  28
  29 #include "elf/dwarf2.h"
  30
  31 /* Try to optimize gcc 2.8 exception frame information.
  32
  33    Exception frame information is emitted for every function in the
  34    .eh_frame or .debug_frame sections.  Simple information for a function
  35    with no exceptions looks like this:
  36
  37 __FRAME_BEGIN__:
  38         .4byte  .LLCIE1 / Length of Common Information Entry
  39 .LSCIE1:
  40 #if .eh_frame
  41         .4byte  0x0     / CIE Identifier Tag
  42 #elif .debug_frame
  43         .4byte  0xffffffff / CIE Identifier Tag
  44 #endif
  45         .byte   0x1     / CIE Version
  46         .byte   0x0     / CIE Augmentation (none)
  47         .byte   0x1     / ULEB128 0x1 (CIE Code Alignment Factor)
  48         .byte   0x7c    / SLEB128 -4 (CIE Data Alignment Factor)
  49         .byte   0x8     / CIE RA Column
  50         .byte   0xc     / DW_CFA_def_cfa
  51         .byte   0x4     / ULEB128 0x4
  52         .byte   0x4     / ULEB128 0x4
  53         .byte   0x88    / DW_CFA_offset, column 0x8
  54         .byte   0x1     / ULEB128 0x1
  55         .align 4
  56 .LECIE1:
  57         .set    .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol
  58         .4byte  .LLFDE1 / FDE Length
  59 .LSFDE1:
  60         .4byte  .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset
  61         .4byte  .LFB1   / FDE initial location
  62         .4byte  .LFE1-.LFB1     / FDE address range
  63         .byte   0x4     / DW_CFA_advance_loc4
  64         .4byte  .LCFI0-.LFB1
  65         .byte   0xe     / DW_CFA_def_cfa_offset
  66         .byte   0x8     / ULEB128 0x8
  67         .byte   0x85    / DW_CFA_offset, column 0x5
  68         .byte   0x2     / ULEB128 0x2
  69         .byte   0x4     / DW_CFA_advance_loc4
  70         .4byte  .LCFI1-.LCFI0
  71         .byte   0xd     / DW_CFA_def_cfa_register
  72         .byte   0x5     / ULEB128 0x5
  73         .byte   0x4     / DW_CFA_advance_loc4
  74         .4byte  .LCFI2-.LCFI1
  75         .byte   0x2e    / DW_CFA_GNU_args_size
  76         .byte   0x4     / ULEB128 0x4
  77         .byte   0x4     / DW_CFA_advance_loc4
  78         .4byte  .LCFI3-.LCFI2
  79         .byte   0x2e    / DW_CFA_GNU_args_size
  80         .byte   0x0     / ULEB128 0x0
  81         .align 4
  82 .LEFDE1:
  83         .set    .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol
  84
  85    The immediate issue we can address in the assembler is the
  86    DW_CFA_advance_loc4 followed by a four byte value.  The value is
  87    the difference of two addresses in the function.  Since gcc does
  88    not know this value, it always uses four bytes.  We will know the
  89    value at the end of assembly, so we can do better.  */
  90
  91 struct cie_info
  92 {
  93   unsigned code_alignment;
  94   int z_augmentation;
  95 };
  96
  97 static int get_cie_info PARAMS ((struct cie_info *));
  98
  99 /* Extract information from the CIE.  */
 100
 101 static int
 102 get_cie_info (info)
 103      struct cie_info *info;
 104 {
 105   fragS *f;
 106   fixS *fix;
 107   int offset;
 108   char CIE_id;
 109   char augmentation[10];
 110   int iaug;
 111   int code_alignment = 0;
 112
 113   /* We should find the CIE at the start of the section.  */
 114
 115 #if defined (BFD_ASSEMBLER) || defined (MANY_SEGMENTS)
 116   f = seg_info (now_seg)->frchainP->frch_root;
 117 #else
 118   f = frchain_now->frch_root;
 119 #endif
 120 #ifdef BFD_ASSEMBLER
 121   fix = seg_info (now_seg)->frchainP->fix_root;
 122 #else
 123   fix = *seg_fix_rootP;
 124 #endif
 125
 126   /* Look through the frags of the section to find the code alignment.  */
 127
 128   /* First make sure that the CIE Identifier Tag is 0/-1.  */
 129
 130   if (strcmp (segment_name (now_seg), ".debug_frame") == 0)
 131     CIE_id = (char)0xff;
 132   else
 133     CIE_id = 0;
 134
 135   offset = 4;
 136   while (f != NULL && offset >= f->fr_fix)
 137     {
 138       offset -= f->fr_fix;
 139       f = f->fr_next;
 140     }
 141   if (f == NULL
 142       || f->fr_fix - offset < 4
 143       || f->fr_literal[offset] != CIE_id
 144       || f->fr_literal[offset + 1] != CIE_id
 145       || f->fr_literal[offset + 2] != CIE_id
 146       || f->fr_literal[offset + 3] != CIE_id)
 147     return 0;
 148
 149   /* Next make sure the CIE version number is 1.  */
 150
 151   offset += 4;
 152   while (f != NULL && offset >= f->fr_fix)
 153     {
 154       offset -= f->fr_fix;
 155       f = f->fr_next;
 156     }
 157   if (f == NULL
 158       || f->fr_fix - offset < 1
 159       || f->fr_literal[offset] != 1)
 160     return 0;
 161
 162   /* Skip the augmentation (a null terminated string).  */
 163
 164   iaug = 0;
 165   ++offset;
 166   while (1)
 167     {
 168       while (f != NULL && offset >= f->fr_fix)
 169         {
 170           offset -= f->fr_fix;
 171           f = f->fr_next;
 172         }
 173       if (f == NULL)
 174         return 0;
 175
 176       while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
 177         {
 178           if ((size_t) iaug < (sizeof augmentation) - 1)
 179             {
 180               augmentation[iaug] = f->fr_literal[offset];
 181               ++iaug;
 182             }
 183           ++offset;
 184         }
 185       if (offset < f->fr_fix)
 186         break;
 187     }
 188   ++offset;
 189   while (f != NULL && offset >= f->fr_fix)
 190     {
 191       offset -= f->fr_fix;
 192       f = f->fr_next;
 193     }
 194   if (f == NULL)
 195     return 0;
 196
 197   augmentation[iaug] = '\0';
 198   if (augmentation[0] == '\0')
 199     {
 200       /* No augmentation.  */
 201     }
 202   else if (strcmp (augmentation, "eh") == 0)
 203     {
 204       /* We have to skip a pointer.  Unfortunately, we don't know how
 205          large it is.  We find out by looking for a matching fixup.  */
 206       while (fix != NULL
 207              && (fix->fx_frag != f || fix->fx_where != offset))
 208         fix = fix->fx_next;
 209       if (fix == NULL)
 210         offset += 4;
 211       else
 212         offset += fix->fx_size;
 213       while (f != NULL && offset >= f->fr_fix)
 214         {
 215           offset -= f->fr_fix;
 216           f = f->fr_next;
 217         }
 218       if (f == NULL)
 219         return 0;
 220     }
 221   else if (augmentation[0] != 'z')
 222     return 0;
 223
 224   /* We're now at the code alignment factor, which is a ULEB128.  If
 225      it isn't a single byte, forget it.  */
 226
 227   code_alignment = f->fr_literal[offset] & 0xff;
 228   if ((code_alignment & 0x80) != 0)
 229     code_alignment = 0;
 230
 231   info->code_alignment = code_alignment;
 232   info->z_augmentation = (augmentation[0] == 'z');
 233
 234   return 1;
 235 }
 236
 237 /* This function is called from emit_expr.  It looks for cases which
 238    we can optimize.
 239
 240    Rather than try to parse all this information as we read it, we
 241    look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
 242    difference.  We turn that into a rs_cfa_advance frag, and handle
 243    those frags at the end of the assembly.  If the gcc output changes
 244    somewhat, this optimization may stop working.
 245
 246    This function returns non-zero if it handled the expression and
 247    emit_expr should not do anything, or zero otherwise.  It can also
 248    change *EXP and *PNBYTES.  */
 249
 250 int
 251 check_eh_frame (exp, pnbytes)
 252      expressionS *exp;
 253      unsigned int *pnbytes;
 254 {
 255   struct frame_data
 256   {
 257     enum frame_state
 258     {
 259       state_idle,
 260       state_saw_size,
 261       state_saw_cie_offset,
 262       state_saw_pc_begin,
 263       state_seeing_aug_size,
 264       state_skipping_aug,
 265       state_wait_loc4,
 266       state_saw_loc4,
 267       state_error,
 268     } state;
 269
 270     int cie_info_ok;
 271     struct cie_info cie_info;
 272
 273     symbolS *size_end_sym;
 274     fragS *loc4_frag;
 275     int loc4_fix;
 276
 277     int aug_size;
 278     int aug_shift;
 279   };
 280
 281   static struct frame_data eh_frame_data;
 282   static struct frame_data debug_frame_data;
 283   struct frame_data *d;
 284
 285   /* Don't optimize.  */
 286   if (flag_traditional_format)
 287     return 0;
 288
 289   /* Select the proper section data.  */
 290   if (strcmp (segment_name (now_seg), ".eh_frame") == 0)
 291     d = &eh_frame_data;
 292   else if (strcmp (segment_name (now_seg), ".debug_frame") == 0)
 293     d = &debug_frame_data;
 294   else
 295     return 0;
 296
 297   if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym))
 298     {
 299       /* We have come to the end of the CIE or FDE.  See below where
 300          we set saw_size.  We must check this first because we may now
 301          be looking at the next size.  */
 302       d->state = state_idle;
 303     }
 304
 305   switch (d->state)
 306     {
 307     case state_idle:
 308       if (*pnbytes == 4)
 309         {
 310           /* This might be the size of the CIE or FDE.  We want to know
 311              the size so that we don't accidentally optimize across an FDE
 312              boundary.  We recognize the size in one of two forms: a
 313              symbol which will later be defined as a difference, or a
 314              subtraction of two symbols.  Either way, we can tell when we
 315              are at the end of the FDE because the symbol becomes defined
 316              (in the case of a subtraction, the end symbol, from which the
 317              start symbol is being subtracted).  Other ways of describing
 318              the size will not be optimized.  */
 319           if ((exp->X_op == O_symbol || exp->X_op == O_subtract)
 320               && ! S_IS_DEFINED (exp->X_add_symbol))
 321             {
 322               d->state = state_saw_size;
 323               d->size_end_sym = exp->X_add_symbol;
 324             }
 325         }
 326       break;
 327
 328     case state_saw_size:
 329     case state_saw_cie_offset:
 330       /* Assume whatever form it appears in, it appears atomically.  */
 331       d->state += 1;
 332       break;
 333
 334     case state_saw_pc_begin:
 335       /* Decide whether we should see an augmentation.  */
 336       if (! d->cie_info_ok
 337           && ! (d->cie_info_ok = get_cie_info (&d->cie_info)))
 338         d->state = state_error;
 339       else if (d->cie_info.z_augmentation)
 340         {
 341           d->state = state_seeing_aug_size;
 342           d->aug_size = 0;
 343           d->aug_shift = 0;
 344         }
 345       else
 346         d->state = state_wait_loc4;
 347       break;
 348
 349     case state_seeing_aug_size:
 350       /* Bytes == -1 means this comes from an leb128 directive.  */
 351       if ((int)*pnbytes == -1 && exp->X_op == O_constant)
 352         {
 353           d->aug_size = exp->X_add_number;
 354           d->state = state_skipping_aug;
 355         }
 356       else if (*pnbytes == 1 && exp->X_op == O_constant)
 357         {
 358           unsigned char byte = exp->X_add_number;
 359           d->aug_size |= (byte & 0x7f) << d->aug_shift;
 360           d->aug_shift += 7;
 361           if ((byte & 0x80) == 0)
 362             d->state = state_skipping_aug;
 363         }
 364       else
 365         d->state = state_error;
 366       if (d->state == state_skipping_aug && d->aug_size == 0)
 367         d->state = state_wait_loc4;
 368       break;
 369
 370     case state_skipping_aug:
 371       if ((int)*pnbytes < 0)
 372         d->state = state_error;
 373       else
 374         {
 375           int left = (d->aug_size -= *pnbytes);
 376           if (left == 0)
 377             d->state = state_wait_loc4;
 378           else if (left < 0)
 379             d->state = state_error;
 380         }
 381       break;
 382
 383     case state_wait_loc4:
 384       if (*pnbytes == 1
 385           && exp->X_op == O_constant
 386           && exp->X_add_number == DW_CFA_advance_loc4)
 387         {
 388           /* This might be a DW_CFA_advance_loc4.  Record the frag and the
 389              position within the frag, so that we can change it later.  */
 390           frag_grow (1);
 391           d->state = state_saw_loc4;
 392           d->loc4_frag = frag_now;
 393           d->loc4_fix = frag_now_fix ();
 394         }
 395       break;
 396
 397     case state_saw_loc4:
 398       d->state = state_wait_loc4;
 399       if (*pnbytes != 4)
 400         break;
 401       if (exp->X_op == O_constant)
 402         {
 403           /* This is a case which we can optimize.  The two symbols being
 404              subtracted were in the same frag and the expression was
 405              reduced to a constant.  We can do the optimization entirely
 406              in this function.  */
 407           if (d->cie_info.code_alignment > 0
 408               && exp->X_add_number % d->cie_info.code_alignment == 0
 409               && exp->X_add_number / d->cie_info.code_alignment < 0x40)
 410             {
 411               d->loc4_frag->fr_literal[d->loc4_fix]
 412                 = DW_CFA_advance_loc
 413                   | (exp->X_add_number / d->cie_info.code_alignment);
 414               /* No more bytes needed.  */
 415               return 1;
 416             }
 417           else if (exp->X_add_number < 0x100)
 418             {
 419               d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;
 420               *pnbytes = 1;
 421             }
 422           else if (exp->X_add_number < 0x10000)
 423             {
 424               d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;
 425               *pnbytes = 2;
 426             }
 427         }
 428       else if (exp->X_op == O_subtract)
 429         {
 430           /* This is a case we can optimize.  The expression was not
 431              reduced, so we can not finish the optimization until the end
 432              of the assembly.  We set up a variant frag which we handle
 433              later.  */
 434           int fr_subtype;
 435
 436           if (d->cie_info.code_alignment > 0)
 437             fr_subtype = d->cie_info.code_alignment << 3;
 438           else
 439             fr_subtype = 0;
 440
 441           frag_var (rs_cfa, 4, 0, fr_subtype, make_expr_symbol (exp),
 442                     d->loc4_fix, (char *) d->loc4_frag);
 443           return 1;
 444         }
 445       break;
 446
 447     case state_error:
 448       /* Just skipping everything.  */
 449       break;
 450     }
 451
 452   return 0;
 453 }
 454
 455 /* The function estimates the size of a rs_cfa variant frag based on
 456    the current values of the symbols.  It is called before the
 457    relaxation loop.  We set fr_subtype{0:2} to the expected length.  */
 458
 459 int
 460 eh_frame_estimate_size_before_relax (frag)
 461      fragS *frag;
 462 {
 463   offsetT diff;
 464   int ca = frag->fr_subtype >> 3;
 465   int ret;
 466
 467   diff = resolve_symbol_value (frag->fr_symbol);
 468
 469   if (ca > 0 && diff % ca == 0 && diff / ca < 0x40)
 470     ret = 0;
 471   else if (diff < 0x100)
 472     ret = 1;
 473   else if (diff < 0x10000)
 474     ret = 2;
 475   else
 476     ret = 4;
 477
 478   frag->fr_subtype = (frag->fr_subtype & ~7) | ret;
 479
 480   return ret;
 481 }
 482
 483 /* This function relaxes a rs_cfa variant frag based on the current
 484    values of the symbols.  fr_subtype{0:2} is the current length of
 485    the frag.  This returns the change in frag length.  */
 486
 487 int
 488 eh_frame_relax_frag (frag)
 489      fragS *frag;
 490 {
 491   int oldsize, newsize;
 492
 493   oldsize = frag->fr_subtype & 7;
 494   newsize = eh_frame_estimate_size_before_relax (frag);
 495   return newsize - oldsize;
 496 }
 497
 498 /* This function converts a rs_cfa variant frag into a normal fill
 499    frag.  This is called after all relaxation has been done.
 500    fr_subtype{0:2} will be the desired length of the frag.  */
 501
 502 void
 503 eh_frame_convert_frag (frag)
 504      fragS *frag;
 505 {
 506   offsetT diff;
 507   fragS *loc4_frag;
 508   int loc4_fix;
 509
 510   loc4_frag = (fragS *) frag->fr_opcode;
 511   loc4_fix = (int) frag->fr_offset;
 512
 513   diff = resolve_symbol_value (frag->fr_symbol);
 514
 515   switch (frag->fr_subtype & 7)
 516     {
 517     case 0:
 518       {
 519         int ca = frag->fr_subtype >> 3;
 520         assert (ca > 0 && diff % ca == 0 && diff / ca < 0x40);
 521         loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | (diff / ca);
 522       }
 523       break;
 524
 525     case 1:
 526       assert (diff < 0x100);
 527       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
 528       frag->fr_literal[frag->fr_fix] = diff;
 529       break;
 530
 531     case 2:
 532       assert (diff < 0x10000);
 533       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
 534       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);
 535       break;
 536
 537     default:
 538       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
 539       break;
 540     }
 541
 542   frag->fr_fix += frag->fr_subtype & 7;
 543   frag->fr_type = rs_fill;
 544   frag->fr_subtype = 0;
 545   frag->fr_offset = 0;
 546 }