gas/ehopt.c

   1 /* ehopt.c--optimize gcc exception frame information.
   2    Copyright 1998, 2000, 2001, 2003, 2005, 2007, 2008, 2009
   3    Free Software Foundation, Inc.
   4    Written by Ian Lance Taylor <ian@cygnus.com>.
   5
   6    This file is part of GAS, the GNU Assembler.
   7
   8    GAS is free software; you can redistribute it and/or modify
   9    it under the terms of the GNU General Public License as published by
  10    the Free Software Foundation; either version 3, or (at your option)
  11    any later version.
  12
  13    GAS is distributed in the hope that it will be useful,
  14    but WITHOUT ANY WARRANTY; without even the implied warranty of
  15    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16    GNU General Public License for more details.
  17
  18    You should have received a copy of the GNU General Public License
  19    along with GAS; see the file COPYING.  If not, write to the Free
  20    Software Foundation, 51 Franklin Street - Fifth Floor, Boston, MA
  21    02110-1301, USA.  */
  22
  23 #include "as.h"
  24 #include "subsegs.h"
  25 #include "struc-symbol.h"
  26
  27 /* We include this ELF file, even though we may not be assembling for
  28    ELF, since the exception frame information is always in a format
  29    derived from DWARF.  */
  30
  31 #include "dwarf2.h"
  32
  33 /* Try to optimize gcc 2.8 exception frame information.
  34
  35    Exception frame information is emitted for every function in the
  36    .eh_frame or .debug_frame sections.  Simple information for a function
  37    with no exceptions looks like this:
  38
  39 __FRAME_BEGIN__:
  40         .4byte  .LLCIE1 / Length of Common Information Entry
  41 .LSCIE1:
  42 #if .eh_frame
  43         .4byte  0x0     / CIE Identifier Tag
  44 #elif .debug_frame
  45         .4byte  0xffffffff / CIE Identifier Tag
  46 #endif
  47         .byte   0x1     / CIE Version
  48         .byte   0x0     / CIE Augmentation (none)
  49         .byte   0x1     / ULEB128 0x1 (CIE Code Alignment Factor)
  50         .byte   0x7c    / SLEB128 -4 (CIE Data Alignment Factor)
  51         .byte   0x8     / CIE RA Column
  52         .byte   0xc     / DW_CFA_def_cfa
  53         .byte   0x4     / ULEB128 0x4
  54         .byte   0x4     / ULEB128 0x4
  55         .byte   0x88    / DW_CFA_offset, column 0x8
  56         .byte   0x1     / ULEB128 0x1
  57         .align 4
  58 .LECIE1:
  59         .set    .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol
  60         .4byte  .LLFDE1 / FDE Length
  61 .LSFDE1:
  62         .4byte  .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset
  63         .4byte  .LFB1   / FDE initial location
  64         .4byte  .LFE1-.LFB1     / FDE address range
  65         .byte   0x4     / DW_CFA_advance_loc4
  66         .4byte  .LCFI0-.LFB1
  67         .byte   0xe     / DW_CFA_def_cfa_offset
  68         .byte   0x8     / ULEB128 0x8
  69         .byte   0x85    / DW_CFA_offset, column 0x5
  70         .byte   0x2     / ULEB128 0x2
  71         .byte   0x4     / DW_CFA_advance_loc4
  72         .4byte  .LCFI1-.LCFI0
  73         .byte   0xd     / DW_CFA_def_cfa_register
  74         .byte   0x5     / ULEB128 0x5
  75         .byte   0x4     / DW_CFA_advance_loc4
  76         .4byte  .LCFI2-.LCFI1
  77         .byte   0x2e    / DW_CFA_GNU_args_size
  78         .byte   0x4     / ULEB128 0x4
  79         .byte   0x4     / DW_CFA_advance_loc4
  80         .4byte  .LCFI3-.LCFI2
  81         .byte   0x2e    / DW_CFA_GNU_args_size
  82         .byte   0x0     / ULEB128 0x0
  83         .align 4
  84 .LEFDE1:
  85         .set    .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol
  86
  87    The immediate issue we can address in the assembler is the
  88    DW_CFA_advance_loc4 followed by a four byte value.  The value is
  89    the difference of two addresses in the function.  Since gcc does
  90    not know this value, it always uses four bytes.  We will know the
  91    value at the end of assembly, so we can do better.  */
  92
  93 struct cie_info
  94 {
  95   unsigned code_alignment;
  96   int z_augmentation;
  97 };
  98
  99 static int get_cie_info (struct cie_info *);
 100
 101 /* Extract information from the CIE.  */
 102
 103 static int
 104 get_cie_info (struct cie_info *info)
 105 {
 106   fragS *f;
 107   fixS *fix;
 108   int offset;
 109   char CIE_id;
 110   char augmentation[10];
 111   int iaug;
 112   int code_alignment = 0;
 113
 114   /* We should find the CIE at the start of the section.  */
 115
 116   f = seg_info (now_seg)->frchainP->frch_root;
 117   fix = seg_info (now_seg)->frchainP->fix_root;
 118
 119   /* Look through the frags of the section to find the code alignment.  */
 120
 121   /* First make sure that the CIE Identifier Tag is 0/-1.  */
 122
 123   if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0)
 124     CIE_id = (char)0xff;
 125   else
 126     CIE_id = 0;
 127
 128   offset = 4;
 129   while (f != NULL && offset >= f->fr_fix)
 130     {
 131       offset -= f->fr_fix;
 132       f = f->fr_next;
 133     }
 134   if (f == NULL
 135       || f->fr_fix - offset < 4
 136       || f->fr_literal[offset] != CIE_id
 137       || f->fr_literal[offset + 1] != CIE_id
 138       || f->fr_literal[offset + 2] != CIE_id
 139       || f->fr_literal[offset + 3] != CIE_id)
 140     return 0;
 141
 142   /* Next make sure the CIE version number is 1.  */
 143
 144   offset += 4;
 145   while (f != NULL && offset >= f->fr_fix)
 146     {
 147       offset -= f->fr_fix;
 148       f = f->fr_next;
 149     }
 150   if (f == NULL
 151       || f->fr_fix - offset < 1
 152       || f->fr_literal[offset] != 1)
 153     return 0;
 154
 155   /* Skip the augmentation (a null terminated string).  */
 156
 157   iaug = 0;
 158   ++offset;
 159   while (1)
 160     {
 161       while (f != NULL && offset >= f->fr_fix)
 162         {
 163           offset -= f->fr_fix;
 164           f = f->fr_next;
 165         }
 166       if (f == NULL)
 167         return 0;
 168
 169       while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
 170         {
 171           if ((size_t) iaug < (sizeof augmentation) - 1)
 172             {
 173               augmentation[iaug] = f->fr_literal[offset];
 174               ++iaug;
 175             }
 176           ++offset;
 177         }
 178       if (offset < f->fr_fix)
 179         break;
 180     }
 181   ++offset;
 182   while (f != NULL && offset >= f->fr_fix)
 183     {
 184       offset -= f->fr_fix;
 185       f = f->fr_next;
 186     }
 187   if (f == NULL)
 188     return 0;
 189
 190   augmentation[iaug] = '\0';
 191   if (augmentation[0] == '\0')
 192     {
 193       /* No augmentation.  */
 194     }
 195   else if (strcmp (augmentation, "eh") == 0)
 196     {
 197       /* We have to skip a pointer.  Unfortunately, we don't know how
 198          large it is.  We find out by looking for a matching fixup.  */
 199       while (fix != NULL
 200              && (fix->fx_frag != f || fix->fx_where != offset))
 201         fix = fix->fx_next;
 202       if (fix == NULL)
 203         offset += 4;
 204       else
 205         offset += fix->fx_size;
 206       while (f != NULL && offset >= f->fr_fix)
 207         {
 208           offset -= f->fr_fix;
 209           f = f->fr_next;
 210         }
 211       if (f == NULL)
 212         return 0;
 213     }
 214   else if (augmentation[0] != 'z')
 215     return 0;
 216
 217   /* We're now at the code alignment factor, which is a ULEB128.  If
 218      it isn't a single byte, forget it.  */
 219
 220   code_alignment = f->fr_literal[offset] & 0xff;
 221   if ((code_alignment & 0x80) != 0)
 222     code_alignment = 0;
 223
 224   info->code_alignment = code_alignment;
 225   info->z_augmentation = (augmentation[0] == 'z');
 226
 227   return 1;
 228 }
 229
 230 enum frame_state
 231 {
 232   state_idle,
 233   state_saw_size,
 234   state_saw_cie_offset,
 235   state_saw_pc_begin,
 236   state_seeing_aug_size,
 237   state_skipping_aug,
 238   state_wait_loc4,
 239   state_saw_loc4,
 240   state_error,
 241 };
 242
 243 /* This function is called from emit_expr.  It looks for cases which
 244    we can optimize.
 245
 246    Rather than try to parse all this information as we read it, we
 247    look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
 248    difference.  We turn that into a rs_cfa_advance frag, and handle
 249    those frags at the end of the assembly.  If the gcc output changes
 250    somewhat, this optimization may stop working.
 251
 252    This function returns non-zero if it handled the expression and
 253    emit_expr should not do anything, or zero otherwise.  It can also
 254    change *EXP and *PNBYTES.  */
 255
 256 int
 257 check_eh_frame (expressionS *exp, unsigned int *pnbytes)
 258 {
 259   struct frame_data
 260   {
 261     enum frame_state state;
 262
 263     int cie_info_ok;
 264     struct cie_info cie_info;
 265
 266     symbolS *size_end_sym;
 267     fragS *loc4_frag;
 268     int loc4_fix;
 269
 270     int aug_size;
 271     int aug_shift;
 272   };
 273
 274   static struct frame_data eh_frame_data;
 275   static struct frame_data debug_frame_data;
 276   struct frame_data *d;
 277
 278   /* Don't optimize.  */
 279   if (flag_traditional_format)
 280     return 0;
 281
 282 #ifdef md_allow_eh_opt
 283   if (! md_allow_eh_opt)
 284     return 0;
 285 #endif
 286
 287   /* Select the proper section data.  */
 288   if (strncmp (segment_name (now_seg), ".eh_frame", 9) == 0
 289       && segment_name (now_seg)[9] != '_')
 290     d = &eh_frame_data;
 291   else if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0)
 292     d = &debug_frame_data;
 293   else
 294     return 0;
 295
 296   if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym))
 297     {
 298       /* We have come to the end of the CIE or FDE.  See below where
 299          we set saw_size.  We must check this first because we may now
 300          be looking at the next size.  */
 301       d->state = state_idle;
 302     }
 303
 304   switch (d->state)
 305     {
 306     case state_idle:
 307       if (*pnbytes == 4)
 308         {
 309           /* This might be the size of the CIE or FDE.  We want to know
 310              the size so that we don't accidentally optimize across an FDE
 311              boundary.  We recognize the size in one of two forms: a
 312              symbol which will later be defined as a difference, or a
 313              subtraction of two symbols.  Either way, we can tell when we
 314              are at the end of the FDE because the symbol becomes defined
 315              (in the case of a subtraction, the end symbol, from which the
 316              start symbol is being subtracted).  Other ways of describing
 317              the size will not be optimized.  */
 318           if ((exp->X_op == O_symbol || exp->X_op == O_subtract)
 319               && ! S_IS_DEFINED (exp->X_add_symbol))
 320             {
 321               d->state = state_saw_size;
 322               d->size_end_sym = exp->X_add_symbol;
 323             }
 324         }
 325       break;
 326
 327     case state_saw_size:
 328     case state_saw_cie_offset:
 329       /* Assume whatever form it appears in, it appears atomically.  */
 330       d->state = (enum frame_state) (d->state + 1);
 331       break;
 332
 333     case state_saw_pc_begin:
 334       /* Decide whether we should see an augmentation.  */
 335       if (! d->cie_info_ok
 336           && ! (d->cie_info_ok = get_cie_info (&d->cie_info)))
 337         d->state = state_error;
 338       else if (d->cie_info.z_augmentation)
 339         {
 340           d->state = state_seeing_aug_size;
 341           d->aug_size = 0;
 342           d->aug_shift = 0;
 343         }
 344       else
 345         d->state = state_wait_loc4;
 346       break;
 347
 348     case state_seeing_aug_size:
 349       /* Bytes == -1 means this comes from an leb128 directive.  */
 350       if ((int)*pnbytes == -1 && exp->X_op == O_constant)
 351         {
 352           d->aug_size = exp->X_add_number;
 353           d->state = state_skipping_aug;
 354         }
 355       else if (*pnbytes == 1 && exp->X_op == O_constant)
 356         {
 357           unsigned char byte = exp->X_add_number;
 358           d->aug_size |= (byte & 0x7f) << d->aug_shift;
 359           d->aug_shift += 7;
 360           if ((byte & 0x80) == 0)
 361             d->state = state_skipping_aug;
 362         }
 363       else
 364         d->state = state_error;
 365       if (d->state == state_skipping_aug && d->aug_size == 0)
 366         d->state = state_wait_loc4;
 367       break;
 368
 369     case state_skipping_aug:
 370       if ((int)*pnbytes < 0)
 371         d->state = state_error;
 372       else
 373         {
 374           int left = (d->aug_size -= *pnbytes);
 375           if (left == 0)
 376             d->state = state_wait_loc4;
 377           else if (left < 0)
 378             d->state = state_error;
 379         }
 380       break;
 381
 382     case state_wait_loc4:
 383       if (*pnbytes == 1
 384           && exp->X_op == O_constant
 385           && exp->X_add_number == DW_CFA_advance_loc4)
 386         {
 387           /* This might be a DW_CFA_advance_loc4.  Record the frag and the
 388              position within the frag, so that we can change it later.  */
 389           frag_grow (1);
 390           d->state = state_saw_loc4;
 391           d->loc4_frag = frag_now;
 392           d->loc4_fix = frag_now_fix ();
 393         }
 394       break;
 395
 396     case state_saw_loc4:
 397       d->state = state_wait_loc4;
 398       if (*pnbytes != 4)
 399         break;
 400       if (exp->X_op == O_constant)
 401         {
 402           /* This is a case which we can optimize.  The two symbols being
 403              subtracted were in the same frag and the expression was
 404              reduced to a constant.  We can do the optimization entirely
 405              in this function.  */
 406           if (exp->X_add_number < 0x40)
 407             {
 408               d->loc4_frag->fr_literal[d->loc4_fix]
 409                 = DW_CFA_advance_loc | exp->X_add_number;
 410               /* No more bytes needed.  */
 411               return 1;
 412             }
 413           else if (exp->X_add_number < 0x100)
 414             {
 415               d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;
 416               *pnbytes = 1;
 417             }
 418           else if (exp->X_add_number < 0x10000)
 419             {
 420               d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;
 421               *pnbytes = 2;
 422             }
 423         }
 424       else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1)
 425         {
 426           /* This is a case we can optimize.  The expression was not
 427              reduced, so we can not finish the optimization until the end
 428              of the assembly.  We set up a variant frag which we handle
 429              later.  */
 430           frag_var (rs_cfa, 4, 0, 1 << 3, make_expr_symbol (exp),
 431                     d->loc4_fix, (char *) d->loc4_frag);
 432           return 1;
 433         }
 434       else if ((exp->X_op == O_divide
 435                 || exp->X_op == O_right_shift)
 436                && d->cie_info.code_alignment > 1)
 437         {
 438           if (exp->X_add_symbol->bsym
 439               && exp->X_op_symbol->bsym
 440               && exp->X_add_symbol->sy_value.X_op == O_subtract
 441               && exp->X_op_symbol->sy_value.X_op == O_constant
 442               && ((exp->X_op == O_divide
 443                    ? exp->X_op_symbol->sy_value.X_add_number
 444                    : (offsetT) 1 << exp->X_op_symbol->sy_value.X_add_number)
 445                   == (offsetT) d->cie_info.code_alignment))
 446             {
 447               /* This is a case we can optimize as well.  The expression was
 448                  not reduced, so we can not finish the optimization until the
 449                  end of the assembly.  We set up a variant frag which we
 450                  handle later.  */
 451               frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3,
 452                         make_expr_symbol (&exp->X_add_symbol->sy_value),
 453                         d->loc4_fix, (char *) d->loc4_frag);
 454               return 1;
 455             }
 456         }
 457       break;
 458
 459     case state_error:
 460       /* Just skipping everything.  */
 461       break;
 462     }
 463
 464   return 0;
 465 }
 466
 467 /* The function estimates the size of a rs_cfa variant frag based on
 468    the current values of the symbols.  It is called before the
 469    relaxation loop.  We set fr_subtype{0:2} to the expected length.  */
 470
 471 int
 472 eh_frame_estimate_size_before_relax (fragS *frag)
 473 {
 474   offsetT diff;
 475   int ca = frag->fr_subtype >> 3;
 476   int ret;
 477
 478   diff = resolve_symbol_value (frag->fr_symbol);
 479
 480   gas_assert (ca > 0);
 481   diff /= ca;
 482   if (diff < 0x40)
 483     ret = 0;
 484   else if (diff < 0x100)
 485     ret = 1;
 486   else if (diff < 0x10000)
 487     ret = 2;
 488   else
 489     ret = 4;
 490
 491   frag->fr_subtype = (frag->fr_subtype & ~7) | ret;
 492
 493   return ret;
 494 }
 495
 496 /* This function relaxes a rs_cfa variant frag based on the current
 497    values of the symbols.  fr_subtype{0:2} is the current length of
 498    the frag.  This returns the change in frag length.  */
 499
 500 int
 501 eh_frame_relax_frag (fragS *frag)
 502 {
 503   int oldsize, newsize;
 504
 505   oldsize = frag->fr_subtype & 7;
 506   newsize = eh_frame_estimate_size_before_relax (frag);
 507   return newsize - oldsize;
 508 }
 509
 510 /* This function converts a rs_cfa variant frag into a normal fill
 511    frag.  This is called after all relaxation has been done.
 512    fr_subtype{0:2} will be the desired length of the frag.  */
 513
 514 void
 515 eh_frame_convert_frag (fragS *frag)
 516 {
 517   offsetT diff;
 518   fragS *loc4_frag;
 519   int loc4_fix, ca;
 520
 521   loc4_frag = (fragS *) frag->fr_opcode;
 522   loc4_fix = (int) frag->fr_offset;
 523
 524   diff = resolve_symbol_value (frag->fr_symbol);
 525
 526   ca = frag->fr_subtype >> 3;
 527   gas_assert (ca > 0);
 528   diff /= ca;
 529   switch (frag->fr_subtype & 7)
 530     {
 531     case 0:
 532       gas_assert (diff < 0x40);
 533       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff;
 534       break;
 535
 536     case 1:
 537       gas_assert (diff < 0x100);
 538       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
 539       frag->fr_literal[frag->fr_fix] = diff;
 540       break;
 541
 542     case 2:
 543       gas_assert (diff < 0x10000);
 544       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
 545       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);
 546       break;
 547
 548     default:
 549       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
 550       break;
 551     }
 552
 553   frag->fr_fix += frag->fr_subtype & 7;
 554   frag->fr_type = rs_fill;
 555   frag->fr_subtype = 0;
 556   frag->fr_offset = 0;
 557 }