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[binutils.git] / gas / ehopt.c
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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>.
6 This file is part of GAS, the GNU Assembler.
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.
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.
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. */
23 #include "as.h"
24 #include "subsegs.h"
25 #include "struc-symbol.h"
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. */
31 #include "dwarf2.h"
33 /* Try to optimize gcc 2.8 exception frame information.
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:
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
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. */
93 struct cie_info
95 unsigned code_alignment;
96 int z_augmentation;
99 static int get_cie_info (struct cie_info *);
101 /* Extract information from the CIE. */
103 static int
104 get_cie_info (struct cie_info *info)
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;
114 /* We should find the CIE at the start of the section. */
116 f = seg_info (now_seg)->frchainP->frch_root;
117 fix = seg_info (now_seg)->frchainP->fix_root;
119 /* Look through the frags of the section to find the code alignment. */
121 /* First make sure that the CIE Identifier Tag is 0/-1. */
123 if (strcmp (segment_name (now_seg), ".debug_frame") == 0)
124 CIE_id = (char)0xff;
125 else
126 CIE_id = 0;
128 offset = 4;
129 while (f != NULL && offset >= f->fr_fix)
131 offset -= f->fr_fix;
132 f = f->fr_next;
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;
142 /* Next make sure the CIE version number is 1. */
144 offset += 4;
145 while (f != NULL && offset >= f->fr_fix)
147 offset -= f->fr_fix;
148 f = f->fr_next;
150 if (f == NULL
151 || f->fr_fix - offset < 1
152 || f->fr_literal[offset] != 1)
153 return 0;
155 /* Skip the augmentation (a null terminated string). */
157 iaug = 0;
158 ++offset;
159 while (1)
161 while (f != NULL && offset >= f->fr_fix)
163 offset -= f->fr_fix;
164 f = f->fr_next;
166 if (f == NULL)
167 return 0;
169 while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
171 if ((size_t) iaug < (sizeof augmentation) - 1)
173 augmentation[iaug] = f->fr_literal[offset];
174 ++iaug;
176 ++offset;
178 if (offset < f->fr_fix)
179 break;
181 ++offset;
182 while (f != NULL && offset >= f->fr_fix)
184 offset -= f->fr_fix;
185 f = f->fr_next;
187 if (f == NULL)
188 return 0;
190 augmentation[iaug] = '\0';
191 if (augmentation[0] == '\0')
193 /* No augmentation. */
195 else if (strcmp (augmentation, "eh") == 0)
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)
208 offset -= f->fr_fix;
209 f = f->fr_next;
211 if (f == NULL)
212 return 0;
214 else if (augmentation[0] != 'z')
215 return 0;
217 /* We're now at the code alignment factor, which is a ULEB128. If
218 it isn't a single byte, forget it. */
220 code_alignment = f->fr_literal[offset] & 0xff;
221 if ((code_alignment & 0x80) != 0)
222 code_alignment = 0;
224 info->code_alignment = code_alignment;
225 info->z_augmentation = (augmentation[0] == 'z');
227 return 1;
230 enum frame_state
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,
243 /* This function is called from emit_expr. It looks for cases which
244 we can optimize.
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.
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. */
257 check_eh_frame (expressionS *exp, unsigned int *pnbytes)
259 struct frame_data
261 enum frame_state state;
263 int cie_info_ok;
264 struct cie_info cie_info;
266 symbolS *size_end_sym;
267 fragS *loc4_frag;
268 int loc4_fix;
270 int aug_size;
271 int aug_shift;
274 static struct frame_data eh_frame_data;
275 static struct frame_data debug_frame_data;
276 struct frame_data *d;
278 /* Don't optimize. */
279 if (flag_traditional_format)
280 return 0;
282 #ifdef md_allow_eh_opt
283 if (! md_allow_eh_opt)
284 return 0;
285 #endif
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;
295 if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym))
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;
303 switch (d->state)
305 case state_idle:
306 if (*pnbytes == 4)
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))
320 d->state = state_saw_size;
321 d->size_end_sym = exp->X_add_symbol;
324 break;
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;
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)
339 d->state = state_seeing_aug_size;
340 d->aug_size = 0;
341 d->aug_shift = 0;
343 else
344 d->state = state_wait_loc4;
345 break;
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)
351 d->aug_size = exp->X_add_number;
352 d->state = state_skipping_aug;
354 else if (*pnbytes == 1 && exp->X_op == O_constant)
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;
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;
368 case state_skipping_aug:
369 if ((int)*pnbytes < 0)
370 d->state = state_error;
371 else
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;
379 break;
381 case state_wait_loc4:
382 if (*pnbytes == 1
383 && exp->X_op == O_constant
384 && exp->X_add_number == DW_CFA_advance_loc4)
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 ();
393 break;
395 case state_saw_loc4:
396 d->state = state_wait_loc4;
397 if (*pnbytes != 4)
398 break;
399 if (exp->X_op == O_constant)
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)
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;
412 else if (exp->X_add_number < 0x100)
414 d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;
415 *pnbytes = 1;
417 else if (exp->X_add_number < 0x10000)
419 d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;
420 *pnbytes = 2;
423 else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1)
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;
433 else if ((exp->X_op == O_divide
434 || exp->X_op == O_right_shift)
435 && d->cie_info.code_alignment > 1)
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))
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;
456 break;
458 case state_error:
459 /* Just skipping everything. */
460 break;
463 return 0;
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. */
471 eh_frame_estimate_size_before_relax (fragS *frag)
473 offsetT diff;
474 int ca = frag->fr_subtype >> 3;
475 int ret;
477 diff = resolve_symbol_value (frag->fr_symbol);
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;
490 frag->fr_subtype = (frag->fr_subtype & ~7) | ret;
492 return ret;
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. */
500 eh_frame_relax_frag (fragS *frag)
502 int oldsize, newsize;
504 oldsize = frag->fr_subtype & 7;
505 newsize = eh_frame_estimate_size_before_relax (frag);
506 return newsize - oldsize;
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. */
513 void
514 eh_frame_convert_frag (fragS *frag)
516 offsetT diff;
517 fragS *loc4_frag;
518 int loc4_fix, ca;
520 loc4_frag = (fragS *) frag->fr_opcode;
521 loc4_fix = (int) frag->fr_offset;
523 diff = resolve_symbol_value (frag->fr_symbol);
525 ca = frag->fr_subtype >> 3;
526 gas_assert (ca > 0);
527 diff /= ca;
528 switch (frag->fr_subtype & 7)
530 case 0:
531 gas_assert (diff < 0x40);
532 loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff;
533 break;
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;
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;
547 default:
548 md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
549 break;
552 frag->fr_fix += frag->fr_subtype & 7;
553 frag->fr_type = rs_fill;
554 frag->fr_subtype = 0;
555 frag->fr_offset = 0;