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 (strcmp (segment_name (now_seg), ".debug_frame") == 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 (strcmp (segment_name (now_seg), ".eh_frame") == 0)
 289     d = &eh_frame_data;
 290   else if (strcmp (segment_name (now_seg), ".debug_frame") == 0)
 291     d = &debug_frame_data;
 292   else
 293     return 0;
 294
 295   if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym))
 296     {
 297       /* We have come to the end of the CIE or FDE.  See below where
 298          we set saw_size.  We must check this first because we may now
 299          be looking at the next size.  */
 300       d->state = state_idle;
 301     }
 302
 303   switch (d->state)
 304     {
 305     case state_idle:
 306       if (*pnbytes == 4)
 307         {
 308           /* This might be the size of the CIE or FDE.  We want to know
 309              the size so that we don't accidentally optimize across an FDE
 310              boundary.  We recognize the size in one of two forms: a
 311              symbol which will later be defined as a difference, or a
 312              subtraction of two symbols.  Either way, we can tell when we
 313              are at the end of the FDE because the symbol becomes defined
 314              (in the case of a subtraction, the end symbol, from which the
 315              start symbol is being subtracted).  Other ways of describing
 316              the size will not be optimized.  */
 317           if ((exp->X_op == O_symbol || exp->X_op == O_subtract)
 318               && ! S_IS_DEFINED (exp->X_add_symbol))
 319             {
 320               d->state = state_saw_size;
 321               d->size_end_sym = exp->X_add_symbol;
 322             }
 323         }
 324       break;
 325
 326     case state_saw_size:
 327     case state_saw_cie_offset:
 328       /* Assume whatever form it appears in, it appears atomically.  */
 329       d->state = (enum frame_state) (d->state + 1);
 330       break;
 331
 332     case state_saw_pc_begin:
 333       /* Decide whether we should see an augmentation.  */
 334       if (! d->cie_info_ok
 335           && ! (d->cie_info_ok = get_cie_info (&d->cie_info)))
 336         d->state = state_error;
 337       else if (d->cie_info.z_augmentation)
 338         {
 339           d->state = state_seeing_aug_size;
 340           d->aug_size = 0;
 341           d->aug_shift = 0;
 342         }
 343       else
 344         d->state = state_wait_loc4;
 345       break;
 346
 347     case state_seeing_aug_size:
 348       /* Bytes == -1 means this comes from an leb128 directive.  */
 349       if ((int)*pnbytes == -1 && exp->X_op == O_constant)
 350         {
 351           d->aug_size = exp->X_add_number;
 352           d->state = state_skipping_aug;
 353         }
 354       else if (*pnbytes == 1 && exp->X_op == O_constant)
 355         {
 356           unsigned char byte = exp->X_add_number;
 357           d->aug_size |= (byte & 0x7f) << d->aug_shift;
 358           d->aug_shift += 7;
 359           if ((byte & 0x80) == 0)
 360             d->state = state_skipping_aug;
 361         }
 362       else
 363         d->state = state_error;
 364       if (d->state == state_skipping_aug && d->aug_size == 0)
 365         d->state = state_wait_loc4;
 366       break;
 367
 368     case state_skipping_aug:
 369       if ((int)*pnbytes < 0)
 370         d->state = state_error;
 371       else
 372         {
 373           int left = (d->aug_size -= *pnbytes);
 374           if (left == 0)
 375             d->state = state_wait_loc4;
 376           else if (left < 0)
 377             d->state = state_error;
 378         }
 379       break;
 380
 381     case state_wait_loc4:
 382       if (*pnbytes == 1
 383           && exp->X_op == O_constant
 384           && exp->X_add_number == DW_CFA_advance_loc4)
 385         {
 386           /* This might be a DW_CFA_advance_loc4.  Record the frag and the
 387              position within the frag, so that we can change it later.  */
 388           frag_grow (1);
 389           d->state = state_saw_loc4;
 390           d->loc4_frag = frag_now;
 391           d->loc4_fix = frag_now_fix ();
 392         }
 393       break;
 394
 395     case state_saw_loc4:
 396       d->state = state_wait_loc4;
 397       if (*pnbytes != 4)
 398         break;
 399       if (exp->X_op == O_constant)
 400         {
 401           /* This is a case which we can optimize.  The two symbols being
 402              subtracted were in the same frag and the expression was
 403              reduced to a constant.  We can do the optimization entirely
 404              in this function.  */
 405           if (exp->X_add_number < 0x40)
 406             {
 407               d->loc4_frag->fr_literal[d->loc4_fix]
 408                 = DW_CFA_advance_loc | exp->X_add_number;
 409               /* No more bytes needed.  */
 410               return 1;
 411             }
 412           else if (exp->X_add_number < 0x100)
 413             {
 414               d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;
 415               *pnbytes = 1;
 416             }
 417           else if (exp->X_add_number < 0x10000)
 418             {
 419               d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;
 420               *pnbytes = 2;
 421             }
 422         }
 423       else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1)
 424         {
 425           /* This is a case we can optimize.  The expression was not
 426              reduced, so we can not finish the optimization until the end
 427              of the assembly.  We set up a variant frag which we handle
 428              later.  */
 429           frag_var (rs_cfa, 4, 0, 1 << 3, make_expr_symbol (exp),
 430                     d->loc4_fix, (char *) d->loc4_frag);
 431           return 1;
 432         }
 433       else if ((exp->X_op == O_divide
 434                 || exp->X_op == O_right_shift)
 435                && d->cie_info.code_alignment > 1)
 436         {
 437           if (exp->X_add_symbol->bsym
 438               && exp->X_op_symbol->bsym
 439               && exp->X_add_symbol->sy_value.X_op == O_subtract
 440               && exp->X_op_symbol->sy_value.X_op == O_constant
 441               && ((exp->X_op == O_divide
 442                    ? exp->X_op_symbol->sy_value.X_add_number
 443                    : (offsetT) 1 << exp->X_op_symbol->sy_value.X_add_number)
 444                   == (offsetT) d->cie_info.code_alignment))
 445             {
 446               /* This is a case we can optimize as well.  The expression was
 447                  not reduced, so we can not finish the optimization until the
 448                  end of the assembly.  We set up a variant frag which we
 449                  handle later.  */
 450               frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3,
 451                         make_expr_symbol (&exp->X_add_symbol->sy_value),
 452                         d->loc4_fix, (char *) d->loc4_frag);
 453               return 1;
 454             }
 455         }
 456       break;
 457
 458     case state_error:
 459       /* Just skipping everything.  */
 460       break;
 461     }
 462
 463   return 0;
 464 }
 465
 466 /* The function estimates the size of a rs_cfa variant frag based on
 467    the current values of the symbols.  It is called before the
 468    relaxation loop.  We set fr_subtype{0:2} to the expected length.  */
 469
 470 int
 471 eh_frame_estimate_size_before_relax (fragS *frag)
 472 {
 473   offsetT diff;
 474   int ca = frag->fr_subtype >> 3;
 475   int ret;
 476
 477   diff = resolve_symbol_value (frag->fr_symbol);
 478
 479   gas_assert (ca > 0);
 480   diff /= ca;
 481   if (diff < 0x40)
 482     ret = 0;
 483   else if (diff < 0x100)
 484     ret = 1;
 485   else if (diff < 0x10000)
 486     ret = 2;
 487   else
 488     ret = 4;
 489
 490   frag->fr_subtype = (frag->fr_subtype & ~7) | ret;
 491
 492   return ret;
 493 }
 494
 495 /* This function relaxes a rs_cfa variant frag based on the current
 496    values of the symbols.  fr_subtype{0:2} is the current length of
 497    the frag.  This returns the change in frag length.  */
 498
 499 int
 500 eh_frame_relax_frag (fragS *frag)
 501 {
 502   int oldsize, newsize;
 503
 504   oldsize = frag->fr_subtype & 7;
 505   newsize = eh_frame_estimate_size_before_relax (frag);
 506   return newsize - oldsize;
 507 }
 508
 509 /* This function converts a rs_cfa variant frag into a normal fill
 510    frag.  This is called after all relaxation has been done.
 511    fr_subtype{0:2} will be the desired length of the frag.  */
 512
 513 void
 514 eh_frame_convert_frag (fragS *frag)
 515 {
 516   offsetT diff;
 517   fragS *loc4_frag;
 518   int loc4_fix, ca;
 519
 520   loc4_frag = (fragS *) frag->fr_opcode;
 521   loc4_fix = (int) frag->fr_offset;
 522
 523   diff = resolve_symbol_value (frag->fr_symbol);
 524
 525   ca = frag->fr_subtype >> 3;
 526   gas_assert (ca > 0);
 527   diff /= ca;
 528   switch (frag->fr_subtype & 7)
 529     {
 530     case 0:
 531       gas_assert (diff < 0x40);
 532       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff;
 533       break;
 534
 535     case 1:
 536       gas_assert (diff < 0x100);
 537       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
 538       frag->fr_literal[frag->fr_fix] = diff;
 539       break;
 540
 541     case 2:
 542       gas_assert (diff < 0x10000);
 543       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
 544       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);
 545       break;
 546
 547     default:
 548       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
 549       break;
 550     }
 551
 552   frag->fr_fix += frag->fr_subtype & 7;
 553   frag->fr_type = rs_fill;
 554   frag->fr_subtype = 0;
 555   frag->fr_offset = 0;
 556 }