1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556
|
/* ehopt.c--optimize gcc exception frame information.
Copyright (C) 1998-2015 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"
#include "struc-symbol.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
{
unsigned code_alignment;
int z_augmentation;
};
static int get_cie_info (struct cie_info *);
/* Extract information from the CIE. */
static int
get_cie_info (struct cie_info *info)
{
fragS *f;
fixS *fix;
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;
fix = seg_info (now_seg)->frchainP->fix_root;
/* Look through the frags of the section to find the code alignment. */
/* First make sure that the CIE Identifier Tag is 0/-1. */
if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0)
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 0;
/* 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 0;
/* 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 0;
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 0;
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. */
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 0;
}
else if (augmentation[0] != 'z')
return 0;
/* 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 1;
}
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,
};
/* 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
{
enum frame_state state;
int 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 frame_data eh_frame_data;
static struct frame_data debug_frame_data;
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 (strncmp (segment_name (now_seg), ".eh_frame", 9) == 0
&& segment_name (now_seg)[9] != '_')
d = &eh_frame_data;
else if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0)
d = &debug_frame_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;
}
}
break;
case state_saw_size:
case state_saw_cie_offset:
/* Assume whatever form it appears in, it appears atomically. */
d->state = (enum frame_state) (d->state + 1);
break;
case state_saw_pc_begin:
/* Decide whether we should see an augmentation. */
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);
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 (exp->X_add_symbol->bsym
&& exp->X_op_symbol->bsym
&& exp->X_add_symbol->sy_value.X_op == O_subtract
&& exp->X_op_symbol->sy_value.X_op == O_constant
&& ((exp->X_op == O_divide
? exp->X_op_symbol->sy_value.X_add_number
: (offsetT) 1 << exp->X_op_symbol->sy_value.X_add_number)
== (offsetT) d->cie_info.code_alignment))
{
/* 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 (&exp->X_add_symbol->sy_value),
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 < 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;
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;
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;
default:
md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
break;
}
frag->fr_fix += frag->fr_subtype & 7;
frag->fr_type = rs_fill;
frag->fr_subtype = 0;
frag->fr_offset = 0;
}
|