1 /* ehopt.c--optimize gcc exception frame information.
2 Copyright (C) 1998-2024 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 3, 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, 51 Franklin Street - Fifth Floor, Boston, MA
20 02110-1301, 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 "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 struct cie_info
92 {
93 unsigned code_alignment;
94 int z_augmentation;
95 };
96
97 /* Extract information from the CIE. */
98
99 static int
get_cie_info(struct cie_info * info)100 get_cie_info (struct cie_info *info)
101 {
102 fragS *f;
103 fixS *fix;
104 unsigned int offset;
105 char CIE_id;
106 char augmentation[10];
107 int iaug;
108 int code_alignment = 0;
109
110 /* We should find the CIE at the start of the section. */
111
112 f = seg_info (now_seg)->frchainP->frch_root;
113 fix = seg_info (now_seg)->frchainP->fix_root;
114
115 /* Look through the frags of the section to find the code alignment. */
116
117 /* First make sure that the CIE Identifier Tag is 0/-1. */
118
119 if (startswith (segment_name (now_seg), ".debug_frame"))
120 CIE_id = (char)0xff;
121 else
122 CIE_id = 0;
123
124 offset = 4;
125 while (f != NULL && offset >= f->fr_fix)
126 {
127 offset -= f->fr_fix;
128 f = f->fr_next;
129 }
130 if (f == NULL
131 || f->fr_fix - offset < 4
132 || f->fr_literal[offset] != CIE_id
133 || f->fr_literal[offset + 1] != CIE_id
134 || f->fr_literal[offset + 2] != CIE_id
135 || f->fr_literal[offset + 3] != CIE_id)
136 return 0;
137
138 /* Next make sure the CIE version number is 1. */
139
140 offset += 4;
141 while (f != NULL && offset >= f->fr_fix)
142 {
143 offset -= f->fr_fix;
144 f = f->fr_next;
145 }
146 if (f == NULL
147 || f->fr_fix - offset < 1
148 || f->fr_literal[offset] != 1)
149 return 0;
150
151 /* Skip the augmentation (a null terminated string). */
152
153 iaug = 0;
154 ++offset;
155 while (1)
156 {
157 while (f != NULL && offset >= f->fr_fix)
158 {
159 offset -= f->fr_fix;
160 f = f->fr_next;
161 }
162 if (f == NULL)
163 return 0;
164
165 while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
166 {
167 if ((size_t) iaug < (sizeof augmentation) - 1)
168 {
169 augmentation[iaug] = f->fr_literal[offset];
170 ++iaug;
171 }
172 ++offset;
173 }
174 if (offset < f->fr_fix)
175 break;
176 }
177 ++offset;
178 while (f != NULL && offset >= f->fr_fix)
179 {
180 offset -= f->fr_fix;
181 f = f->fr_next;
182 }
183 if (f == NULL)
184 return 0;
185
186 augmentation[iaug] = '\0';
187 if (augmentation[0] == '\0')
188 {
189 /* No augmentation. */
190 }
191 else if (strcmp (augmentation, "eh") == 0)
192 {
193 /* We have to skip a pointer. Unfortunately, we don't know how
194 large it is. We find out by looking for a matching fixup. */
195 while (fix != NULL
196 && (fix->fx_frag != f || fix->fx_where != offset))
197 fix = fix->fx_next;
198 if (fix == NULL)
199 offset += 4;
200 else
201 offset += fix->fx_size;
202 while (f != NULL && offset >= f->fr_fix)
203 {
204 offset -= f->fr_fix;
205 f = f->fr_next;
206 }
207 if (f == NULL)
208 return 0;
209 }
210 else if (augmentation[0] != 'z')
211 return 0;
212
213 /* We're now at the code alignment factor, which is a ULEB128. If
214 it isn't a single byte, forget it. */
215
216 code_alignment = f->fr_literal[offset] & 0xff;
217 if ((code_alignment & 0x80) != 0)
218 code_alignment = 0;
219
220 info->code_alignment = code_alignment;
221 info->z_augmentation = (augmentation[0] == 'z');
222
223 return 1;
224 }
225
226 enum frame_state
227 {
228 state_idle,
229 state_saw_size,
230 state_saw_cie_offset,
231 state_saw_pc_begin,
232 state_seeing_aug_size,
233 state_skipping_aug,
234 state_wait_loc4,
235 state_saw_loc4,
236 state_error,
237 };
238
239 struct frame_data
240 {
241 enum frame_state state;
242
243 int cie_info_ok;
244 struct cie_info cie_info;
245
246 symbolS *size_end_sym;
247 fragS *loc4_frag;
248 int loc4_fix;
249
250 int aug_size;
251 int aug_shift;
252 };
253
254 static struct eh_state
255 {
256 struct frame_data eh_data;
257 struct frame_data debug_data;
258 } frame;
259
260 /* This function is called from emit_expr. It looks for cases which
261 we can optimize.
262
263 Rather than try to parse all this information as we read it, we
264 look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
265 difference. We turn that into a rs_cfa_advance frag, and handle
266 those frags at the end of the assembly. If the gcc output changes
267 somewhat, this optimization may stop working.
268
269 This function returns non-zero if it handled the expression and
270 emit_expr should not do anything, or zero otherwise. It can also
271 change *EXP and *PNBYTES. */
272
273 int
check_eh_frame(expressionS * exp,unsigned int * pnbytes)274 check_eh_frame (expressionS *exp, unsigned int *pnbytes)
275 {
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 (startswith (segment_name (now_seg), ".eh_frame")
289 && segment_name (now_seg)[9] != '_')
290 d = &frame.eh_data;
291 else if (startswith (segment_name (now_seg), ".debug_frame"))
292 d = &frame.debug_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 + 4);
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 (symbol_symbolS (exp->X_add_symbol)
439 && symbol_constant_p (exp->X_op_symbol)
440 && S_GET_SEGMENT (exp->X_op_symbol) == absolute_section
441 && ((exp->X_op == O_divide
442 ? *symbol_X_add_number (exp->X_op_symbol)
443 : (offsetT) 1 << *symbol_X_add_number (exp->X_op_symbol))
444 == (offsetT) d->cie_info.code_alignment))
445 {
446 expressionS *symval;
447
448 symval = symbol_get_value_expression (exp->X_add_symbol);
449 if (symval->X_op == O_subtract)
450 {
451 /* This is a case we can optimize as well. The
452 expression was not reduced, so we can not finish
453 the optimization until the end of the assembly.
454 We set up a variant frag which we handle later. */
455 frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3,
456 make_expr_symbol (symval),
457 d->loc4_fix, (char *) d->loc4_frag);
458 return 1;
459 }
460 }
461 }
462 break;
463
464 case state_error:
465 /* Just skipping everything. */
466 break;
467 }
468
469 return 0;
470 }
471
472 /* The function estimates the size of a rs_cfa variant frag based on
473 the current values of the symbols. It is called before the
474 relaxation loop. We set fr_subtype{0:2} to the expected length. */
475
476 int
eh_frame_estimate_size_before_relax(fragS * frag)477 eh_frame_estimate_size_before_relax (fragS *frag)
478 {
479 offsetT diff;
480 int ca = frag->fr_subtype >> 3;
481 int ret;
482
483 diff = resolve_symbol_value (frag->fr_symbol);
484
485 gas_assert (ca > 0);
486 diff /= ca;
487 if (diff == 0)
488 ret = -1;
489 else if (diff < 0x40)
490 ret = 0;
491 else if (diff < 0x100)
492 ret = 1;
493 else if (diff < 0x10000)
494 ret = 2;
495 else
496 ret = 4;
497
498 frag->fr_subtype = (frag->fr_subtype & ~7) | (ret & 7);
499
500 return ret;
501 }
502
503 /* This function relaxes a rs_cfa variant frag based on the current
504 values of the symbols. fr_subtype{0:2} is the current length of
505 the frag. This returns the change in frag length. */
506
507 int
eh_frame_relax_frag(fragS * frag)508 eh_frame_relax_frag (fragS *frag)
509 {
510 int oldsize, newsize;
511
512 oldsize = frag->fr_subtype & 7;
513 if (oldsize == 7)
514 oldsize = -1;
515 newsize = eh_frame_estimate_size_before_relax (frag);
516 return newsize - oldsize;
517 }
518
519 /* This function converts a rs_cfa variant frag into a normal fill
520 frag. This is called after all relaxation has been done.
521 fr_subtype{0:2} will be the desired length of the frag. */
522
523 void
eh_frame_convert_frag(fragS * frag)524 eh_frame_convert_frag (fragS *frag)
525 {
526 offsetT diff;
527 fragS *loc4_frag;
528 int loc4_fix, ca;
529
530 loc4_frag = (fragS *) frag->fr_opcode;
531 loc4_fix = (int) frag->fr_offset;
532
533 diff = resolve_symbol_value (frag->fr_symbol);
534
535 ca = frag->fr_subtype >> 3;
536 gas_assert (ca > 0);
537 diff /= ca;
538 switch (frag->fr_subtype & 7)
539 {
540 case 0:
541 gas_assert (diff < 0x40);
542 loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff;
543 break;
544
545 case 1:
546 gas_assert (diff < 0x100);
547 loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1;
548 frag->fr_literal[frag->fr_fix] = diff;
549 break;
550
551 case 2:
552 gas_assert (diff < 0x10000);
553 loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2;
554 md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2);
555 break;
556
557 case 4:
558 md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
559 break;
560
561 case 7:
562 gas_assert (diff == 0);
563 frag->fr_fix -= 8;
564 break;
565
566 default:
567 abort ();
568 }
569
570 frag->fr_fix += frag->fr_subtype & 7;
571 frag->fr_type = rs_fill;
572 frag->fr_subtype = 0;
573 frag->fr_offset = 0;
574 }
575
576 void
eh_begin(void)577 eh_begin (void)
578 {
579 memset (&frame, 0, sizeof (frame));
580 }
581