aboutsummaryrefslogtreecommitdiff
path: root/gas/ehopt.c
blob: 56756ffdc965404f2e683f9a74dd3d1bccd9d1a2 (plain)
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) 1998-2014 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;
}