gas/ehopt.c

   1 /* ehopt.c--optimize gcc exception frame information.
   2    Copyright 1998, 2000, 2001 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 static int eh_frame_code_alignment PARAMS ((int));
  92
  93 /* Get the code alignment factor from the CIE.  */
  94
  95 static int
  96 eh_frame_code_alignment (in_seg)
  97      int in_seg;
  98 {
  99   /* ??? Assume .eh_frame and .debug_frame have the same alignment.  */
 100   static int code_alignment;
 101
 102   fragS *f;
 103   fixS *fix;
 104   int offset;
 105   char CIE_id;
 106   char augmentation[10];
 107   int iaug;
 108
 109   if (code_alignment != 0)
 110     return code_alignment;
 111
 112   /* Can't find the alignment if we've changed sections.  */
 113   if (! in_seg)
 114     return -1;
 115
 116   /* We should find the CIE at the start of the section.  */
 117
 118 #if defined (BFD_ASSEMBLER) || defined (MANY_SEGMENTS)
 119   f = seg_info (now_seg)->frchainP->frch_root;
 120 #else
 121   f = frchain_now->frch_root;
 122 #endif
 123 #ifdef BFD_ASSEMBLER
 124   fix = seg_info (now_seg)->frchainP->fix_root;
 125 #else
 126   fix = *seg_fix_rootP;
 127 #endif
 128
 129   /* Look through the frags of the section to find the code alignment.  */
 130
 131   /* First make sure that the CIE Identifier Tag is 0/-1.  */
 132
 133   if (strcmp (segment_name (now_seg), ".debug_frame") == 0)
 134     CIE_id = (char)0xff;
 135   else
 136     CIE_id = 0;
 137
 138   offset = 4;
 139   while (f != NULL && offset >= f->fr_fix)
 140     {
 141       offset -= f->fr_fix;
 142       f = f->fr_next;
 143     }
 144   if (f == NULL
 145       || f->fr_fix - offset < 4
 146       || f->fr_literal[offset] != CIE_id
 147       || f->fr_literal[offset + 1] != CIE_id
 148       || f->fr_literal[offset + 2] != CIE_id
 149       || f->fr_literal[offset + 3] != CIE_id)
 150     {
 151       code_alignment = -1;
 152       return -1;
 153     }
 154
 155   /* Next make sure the CIE version number is 1.  */
 156
 157   offset += 4;
 158   while (f != NULL && offset >= f->fr_fix)
 159     {
 160       offset -= f->fr_fix;
 161       f = f->fr_next;
 162     }
 163   if (f == NULL
 164       || f->fr_fix - offset < 1
 165       || f->fr_literal[offset] != 1)
 166     {
 167       code_alignment = -1;
 168       return -1;
 169     }
 170
 171   /* Skip the augmentation (a null terminated string).  */
 172
 173   iaug = 0;
 174   ++offset;
 175   while (1)
 176     {
 177       while (f != NULL && offset >= f->fr_fix)
 178         {
 179           offset -= f->fr_fix;
 180           f = f->fr_next;
 181         }
 182       if (f == NULL)
 183         {
 184           code_alignment = -1;
 185           return -1;
 186         }
 187       while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
 188         {
 189           if ((size_t) iaug < (sizeof augmentation) - 1)
 190             {
 191               augmentation[iaug] = f->fr_literal[offset];
 192               ++iaug;
 193             }
 194           ++offset;
 195         }
 196       if (offset < f->fr_fix)
 197         break;
 198     }
 199   ++offset;
 200   while (f != NULL && offset >= f->fr_fix)
 201     {
 202       offset -= f->fr_fix;
 203       f = f->fr_next;
 204     }
 205   if (f == NULL)
 206     {
 207       code_alignment = -1;
 208       return -1;
 209     }
 210
 211   augmentation[iaug] = '\0';
 212   if (augmentation[0] == '\0')
 213     {
 214       /* No augmentation.  */
 215     }
 216   else if (strcmp (augmentation, "eh") == 0)
 217     {
 218       /* We have to skip a pointer.  Unfortunately, we don't know how
 219          large it is.  We find out by looking for a matching fixup.  */
 220       while (fix != NULL
 221              && (fix->fx_frag != f || fix->fx_where != offset))
 222         fix = fix->fx_next;
 223       if (fix == NULL)
 224         offset += 4;
 225       else
 226         offset += fix->fx_size;
 227       while (f != NULL && offset >= f->fr_fix)
 228         {
 229           offset -= f->fr_fix;
 230           f = f->fr_next;
 231         }
 232       if (f == NULL)
 233         {
 234           code_alignment = -1;
 235           return -1;
 236         }
 237     }
 238   else
 239     {
 240       code_alignment = -1;
 241       return -1;
 242     }
 243
 244   /* We're now at the code alignment factor, which is a ULEB128.  If
 245      it isn't a single byte, forget it.  */
 246
 247   code_alignment = f->fr_literal[offset] & 0xff;
 248   if ((code_alignment & 0x80) != 0 || code_alignment == 0)
 249     {
 250       code_alignment = -1;
 251       return -1;
 252     }
 253
 254   return code_alignment;
 255 }
 256
 257 /* This function is called from emit_expr.  It looks for cases which
 258    we can optimize.
 259
 260    Rather than try to parse all this information as we read it, we
 261    look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
 262    difference.  We turn that into a rs_cfa_advance frag, and handle
 263    those frags at the end of the assembly.  If the gcc output changes
 264    somewhat, this optimization may stop working.
 265
 266    This function returns non-zero if it handled the expression and
 267    emit_expr should not do anything, or zero otherwise.  It can also
 268    change *EXP and *PNBYTES.  */
 269
 270 int
 271 check_eh_frame (exp, pnbytes)
 272      expressionS *exp;
 273      unsigned int *pnbytes;
 274 {
 275   struct frame_data
 276   {
 277     symbolS *size_end_sym;
 278     fragS *loc4_frag;
 279     int saw_size;
 280     int saw_advance_loc4;
 281     int loc4_fix;
 282   };
 283
 284   static struct frame_data eh_frame_data;
 285   static struct frame_data debug_frame_data;
 286   struct frame_data *d;
 287
 288   /* Don't optimize.  */
 289   if (flag_traditional_format)
 290     return 0;
 291
 292   /* Select the proper section data.  */
 293   if (strcmp (segment_name (now_seg), ".eh_frame") == 0)
 294     d = &eh_frame_data;
 295   else if (strcmp (segment_name (now_seg), ".debug_frame") == 0)
 296     d = &debug_frame_data;
 297   else
 298     return 0;
 299
 300   if (d->saw_size && S_IS_DEFINED (d->size_end_sym))
 301     {
 302       /* We have come to the end of the CIE or FDE.  See below where
 303          we set saw_size.  We must check this first because we may now
 304          be looking at the next size.  */
 305       d->saw_size = 0;
 306       d->saw_advance_loc4 = 0;
 307     }
 308
 309   if (! d->saw_size
 310       && *pnbytes == 4)
 311     {
 312       /* This might be the size of the CIE or FDE.  We want to know
 313          the size so that we don't accidentally optimize across an FDE
 314          boundary.  We recognize the size in one of two forms: a
 315          symbol which will later be defined as a difference, or a
 316          subtraction of two symbols.  Either way, we can tell when we
 317          are at the end of the FDE because the symbol becomes defined
 318          (in the case of a subtraction, the end symbol, from which the
 319          start symbol is being subtracted).  Other ways of describing
 320          the size will not be optimized.  */
 321       if ((exp->X_op == O_symbol || exp->X_op == O_subtract)
 322           && ! S_IS_DEFINED (exp->X_add_symbol))
 323         {
 324           d->saw_size = 1;
 325           d->size_end_sym = exp->X_add_symbol;
 326         }
 327     }
 328   else if (d->saw_size
 329            && *pnbytes == 1
 330            && exp->X_op == O_constant
 331            && exp->X_add_number == DW_CFA_advance_loc4)
 332     {
 333       /* This might be a DW_CFA_advance_loc4.  Record the frag and the
 334          position within the frag, so that we can change it later.  */
 335       d->saw_advance_loc4 = 1;
 336       frag_grow (1);
 337       d->loc4_frag = frag_now;
 338       d->loc4_fix = frag_now_fix ();
 339     }
 340   else if (d->saw_advance_loc4
 341            && *pnbytes == 4
 342            && exp->X_op == O_constant)
 343     {
 344       int ca;
 345
 346       /* This is a case which we can optimize.  The two symbols being
 347          subtracted were in the same frag and the expression was
 348          reduced to a constant.  We can do the optimization entirely
 349          in this function.  */
 350
 351       d->saw_advance_loc4 = 0;
 352
 353       ca = eh_frame_code_alignment (1);
 354       if (ca < 0)
 355         {
 356           /* Don't optimize.  */
 357         }
 358       else if (exp->X_add_number % ca == 0
 359                && exp->X_add_number / ca < 0x40)
 360         {
 361           d->loc4_frag->fr_literal[d->loc4_fix]
 362             = DW_CFA_advance_loc | (exp->X_add_number / ca);
 363           /* No more bytes needed.  */
 364           return 1;
 365         }
 366       else if (exp->X_add_number < 0x100)
 367         {
 368           d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;
 369           *pnbytes = 1;
 370         }
 371       else if (exp->X_add_number < 0x10000)
 372         {
 373           d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;
 374           *pnbytes = 2;
 375         }
 376     }
 377   else if (d->saw_advance_loc4
 378            && *pnbytes == 4
 379            && exp->X_op == O_subtract)
 380     {
 381       /* This is a case we can optimize.  The expression was not
 382          reduced, so we can not finish the optimization until the end
 383          of the assembly.  We set up a variant frag which we handle
 384          later.  */
 385
 386       d->saw_advance_loc4 = 0;
 387
 388       frag_var (rs_cfa, 4, 0, 0, make_expr_symbol (exp),
 389                 d->loc4_fix, (char *) d->loc4_frag);
 390
 391       return 1;
 392     }
 393   else
 394     d->saw_advance_loc4 = 0;
 395
 396   return 0;
 397 }
 398
 399 /* The function estimates the size of a rs_cfa variant frag based on
 400    the current values of the symbols.  It is called before the
 401    relaxation loop.  We set fr_subtype to the expected length.  */
 402
 403 int
 404 eh_frame_estimate_size_before_relax (frag)
 405      fragS *frag;
 406 {
 407   int ca;
 408   offsetT diff;
 409   int ret;
 410
 411   ca = eh_frame_code_alignment (0);
 412   diff = resolve_symbol_value (frag->fr_symbol, 0);
 413
 414   if (ca < 0)
 415     ret = 4;
 416   else if (diff % ca == 0 && diff / ca < 0x40)
 417     ret = 0;
 418   else if (diff < 0x100)
 419     ret = 1;
 420   else if (diff < 0x10000)
 421     ret = 2;
 422   else
 423     ret = 4;
 424
 425   frag->fr_subtype = ret;
 426
 427   return ret;
 428 }
 429
 430 /* This function relaxes a rs_cfa variant frag based on the current
 431    values of the symbols.  fr_subtype is the current length of the
 432    frag.  This returns the change in frag length.  */
 433
 434 int
 435 eh_frame_relax_frag (frag)
 436      fragS *frag;
 437 {
 438   int oldsize, newsize;
 439
 440   oldsize = frag->fr_subtype;
 441   newsize = eh_frame_estimate_size_before_relax (frag);
 442   return newsize - oldsize;
 443 }
 444
 445 /* This function converts a rs_cfa variant frag into a normal fill
 446    frag.  This is called after all relaxation has been done.
 447    fr_subtype will be the desired length of the frag.  */
 448
 449 void
 450 eh_frame_convert_frag (frag)
 451      fragS *frag;
 452 {
 453   offsetT diff;
 454   fragS *loc4_frag;
 455   int loc4_fix;
 456
 457   loc4_frag = (fragS *) frag->fr_opcode;
 458   loc4_fix = (int) frag->fr_offset;
 459
 460   diff = resolve_symbol_value (frag->fr_symbol, finalize_syms);
 461
 462   if (frag->fr_subtype == 0)
 463     {
 464       int ca;
 465
 466       ca = eh_frame_code_alignment (0);
 467       assert (ca > 0 && diff % ca == 0 && diff / ca < 0x40);
 468       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | (diff / ca);
 469     }
 470   else if (frag->fr_subtype == 1)
 471     {
 472       assert (diff < 0x100);
 473       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
 474       frag->fr_literal[frag->fr_fix] = diff;
 475     }
 476   else if (frag->fr_subtype == 2)
 477     {
 478       assert (diff < 0x10000);
 479       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
 480       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);
 481     }
 482   else
 483     md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
 484
 485   frag->fr_fix += frag->fr_subtype;
 486   frag->fr_type = rs_fill;
 487   frag->fr_offset = 0;
 488 }