1 /* ehopt.c--optimize gcc exception frame information.
2    Copyright (C) 1998-2016 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 #include "struc-symbol.h"
25 
26 /* We include this ELF file, even though we may not be assembling for
27    ELF, since the exception frame information is always in a format
28    derived from DWARF.  */
29 
30 #include "dwarf2.h"
31 
32 /* Try to optimize gcc 2.8 exception frame information.
33 
34    Exception frame information is emitted for every function in the
35    .eh_frame or .debug_frame sections.  Simple information for a function
36    with no exceptions looks like this:
37 
38 __FRAME_BEGIN__:
39 	.4byte	.LLCIE1	/ Length of Common Information Entry
40 .LSCIE1:
41 #if .eh_frame
42 	.4byte	0x0	/ CIE Identifier Tag
43 #elif .debug_frame
44 	.4byte	0xffffffff / CIE Identifier Tag
45 #endif
46 	.byte	0x1	/ CIE Version
47 	.byte	0x0	/ CIE Augmentation (none)
48 	.byte	0x1	/ ULEB128 0x1 (CIE Code Alignment Factor)
49 	.byte	0x7c	/ SLEB128 -4 (CIE Data Alignment Factor)
50 	.byte	0x8	/ CIE RA Column
51 	.byte	0xc	/ DW_CFA_def_cfa
52 	.byte	0x4	/ ULEB128 0x4
53 	.byte	0x4	/ ULEB128 0x4
54 	.byte	0x88	/ DW_CFA_offset, column 0x8
55 	.byte	0x1	/ ULEB128 0x1
56 	.align 4
57 .LECIE1:
58 	.set	.LLCIE1,.LECIE1-.LSCIE1	/ CIE Length Symbol
59 	.4byte	.LLFDE1	/ FDE Length
60 .LSFDE1:
61 	.4byte	.LSFDE1-__FRAME_BEGIN__	/ FDE CIE offset
62 	.4byte	.LFB1	/ FDE initial location
63 	.4byte	.LFE1-.LFB1	/ FDE address range
64 	.byte	0x4	/ DW_CFA_advance_loc4
65 	.4byte	.LCFI0-.LFB1
66 	.byte	0xe	/ DW_CFA_def_cfa_offset
67 	.byte	0x8	/ ULEB128 0x8
68 	.byte	0x85	/ DW_CFA_offset, column 0x5
69 	.byte	0x2	/ ULEB128 0x2
70 	.byte	0x4	/ DW_CFA_advance_loc4
71 	.4byte	.LCFI1-.LCFI0
72 	.byte	0xd	/ DW_CFA_def_cfa_register
73 	.byte	0x5	/ ULEB128 0x5
74 	.byte	0x4	/ DW_CFA_advance_loc4
75 	.4byte	.LCFI2-.LCFI1
76 	.byte	0x2e	/ DW_CFA_GNU_args_size
77 	.byte	0x4	/ ULEB128 0x4
78 	.byte	0x4	/ DW_CFA_advance_loc4
79 	.4byte	.LCFI3-.LCFI2
80 	.byte	0x2e	/ DW_CFA_GNU_args_size
81 	.byte	0x0	/ ULEB128 0x0
82 	.align 4
83 .LEFDE1:
84 	.set	.LLFDE1,.LEFDE1-.LSFDE1	/ FDE Length Symbol
85 
86    The immediate issue we can address in the assembler is the
87    DW_CFA_advance_loc4 followed by a four byte value.  The value is
88    the difference of two addresses in the function.  Since gcc does
89    not know this value, it always uses four bytes.  We will know the
90    value at the end of assembly, so we can do better.  */
91 
92 struct cie_info
93 {
94   unsigned code_alignment;
95   int z_augmentation;
96 };
97 
98 static int get_cie_info (struct cie_info *);
99 
100 /* Extract information from the CIE.  */
101 
102 static int
get_cie_info(struct cie_info * info)103 get_cie_info (struct cie_info *info)
104 {
105   fragS *f;
106   fixS *fix;
107   int offset;
108   char CIE_id;
109   char augmentation[10];
110   int iaug;
111   int code_alignment = 0;
112 
113   /* We should find the CIE at the start of the section.  */
114 
115   f = seg_info (now_seg)->frchainP->frch_root;
116   fix = seg_info (now_seg)->frchainP->fix_root;
117 
118   /* Look through the frags of the section to find the code alignment.  */
119 
120   /* First make sure that the CIE Identifier Tag is 0/-1.  */
121 
122   if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0)
123     CIE_id = (char)0xff;
124   else
125     CIE_id = 0;
126 
127   offset = 4;
128   while (f != NULL && offset >= f->fr_fix)
129     {
130       offset -= f->fr_fix;
131       f = f->fr_next;
132     }
133   if (f == NULL
134       || f->fr_fix - offset < 4
135       || f->fr_literal[offset] != CIE_id
136       || f->fr_literal[offset + 1] != CIE_id
137       || f->fr_literal[offset + 2] != CIE_id
138       || f->fr_literal[offset + 3] != CIE_id)
139     return 0;
140 
141   /* Next make sure the CIE version number is 1.  */
142 
143   offset += 4;
144   while (f != NULL && offset >= f->fr_fix)
145     {
146       offset -= f->fr_fix;
147       f = f->fr_next;
148     }
149   if (f == NULL
150       || f->fr_fix - offset < 1
151       || f->fr_literal[offset] != 1)
152     return 0;
153 
154   /* Skip the augmentation (a null terminated string).  */
155 
156   iaug = 0;
157   ++offset;
158   while (1)
159     {
160       while (f != NULL && offset >= f->fr_fix)
161 	{
162 	  offset -= f->fr_fix;
163 	  f = f->fr_next;
164 	}
165       if (f == NULL)
166 	return 0;
167 
168       while (offset < f->fr_fix && f->fr_literal[offset] != '\0')
169 	{
170 	  if ((size_t) iaug < (sizeof augmentation) - 1)
171 	    {
172 	      augmentation[iaug] = f->fr_literal[offset];
173 	      ++iaug;
174 	    }
175 	  ++offset;
176 	}
177       if (offset < f->fr_fix)
178 	break;
179     }
180   ++offset;
181   while (f != NULL && offset >= f->fr_fix)
182     {
183       offset -= f->fr_fix;
184       f = f->fr_next;
185     }
186   if (f == NULL)
187     return 0;
188 
189   augmentation[iaug] = '\0';
190   if (augmentation[0] == '\0')
191     {
192       /* No augmentation.  */
193     }
194   else if (strcmp (augmentation, "eh") == 0)
195     {
196       /* We have to skip a pointer.  Unfortunately, we don't know how
197 	 large it is.  We find out by looking for a matching fixup.  */
198       while (fix != NULL
199 	     && (fix->fx_frag != f || fix->fx_where != offset))
200 	fix = fix->fx_next;
201       if (fix == NULL)
202 	offset += 4;
203       else
204 	offset += fix->fx_size;
205       while (f != NULL && offset >= f->fr_fix)
206 	{
207 	  offset -= f->fr_fix;
208 	  f = f->fr_next;
209 	}
210       if (f == NULL)
211 	return 0;
212     }
213   else if (augmentation[0] != 'z')
214     return 0;
215 
216   /* We're now at the code alignment factor, which is a ULEB128.  If
217      it isn't a single byte, forget it.  */
218 
219   code_alignment = f->fr_literal[offset] & 0xff;
220   if ((code_alignment & 0x80) != 0)
221     code_alignment = 0;
222 
223   info->code_alignment = code_alignment;
224   info->z_augmentation = (augmentation[0] == 'z');
225 
226   return 1;
227 }
228 
229 enum frame_state
230 {
231   state_idle,
232   state_saw_size,
233   state_saw_cie_offset,
234   state_saw_pc_begin,
235   state_seeing_aug_size,
236   state_skipping_aug,
237   state_wait_loc4,
238   state_saw_loc4,
239   state_error,
240 };
241 
242 /* This function is called from emit_expr.  It looks for cases which
243    we can optimize.
244 
245    Rather than try to parse all this information as we read it, we
246    look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
247    difference.  We turn that into a rs_cfa_advance frag, and handle
248    those frags at the end of the assembly.  If the gcc output changes
249    somewhat, this optimization may stop working.
250 
251    This function returns non-zero if it handled the expression and
252    emit_expr should not do anything, or zero otherwise.  It can also
253    change *EXP and *PNBYTES.  */
254 
255 int
check_eh_frame(expressionS * exp,unsigned int * pnbytes)256 check_eh_frame (expressionS *exp, unsigned int *pnbytes)
257 {
258   struct frame_data
259   {
260     enum frame_state state;
261 
262     int cie_info_ok;
263     struct cie_info cie_info;
264 
265     symbolS *size_end_sym;
266     fragS *loc4_frag;
267     int loc4_fix;
268 
269     int aug_size;
270     int aug_shift;
271   };
272 
273   static struct frame_data eh_frame_data;
274   static struct frame_data debug_frame_data;
275   struct frame_data *d;
276 
277   /* Don't optimize.  */
278   if (flag_traditional_format)
279     return 0;
280 
281 #ifdef md_allow_eh_opt
282   if (! md_allow_eh_opt)
283     return 0;
284 #endif
285 
286   /* Select the proper section data.  */
287   if (strncmp (segment_name (now_seg), ".eh_frame", 9) == 0
288       && segment_name (now_seg)[9] != '_')
289     d = &eh_frame_data;
290   else if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0)
291     d = &debug_frame_data;
292   else
293     return 0;
294 
295   if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym))
296     {
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;
301     }
302 
303   switch (d->state)
304     {
305     case state_idle:
306       if (*pnbytes == 4)
307 	{
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))
319 	    {
320 	      d->state = state_saw_size;
321 	      d->size_end_sym = exp->X_add_symbol;
322 	    }
323 	}
324       break;
325 
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;
331 
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)
338 	{
339 	  d->state = state_seeing_aug_size;
340 	  d->aug_size = 0;
341 	  d->aug_shift = 0;
342 	}
343       else
344 	d->state = state_wait_loc4;
345       break;
346 
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)
350 	{
351 	  d->aug_size = exp->X_add_number;
352 	  d->state = state_skipping_aug;
353 	}
354       else if (*pnbytes == 1 && exp->X_op == O_constant)
355 	{
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;
361 	}
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;
367 
368     case state_skipping_aug:
369       if ((int)*pnbytes < 0)
370 	d->state = state_error;
371       else
372 	{
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;
378 	}
379       break;
380 
381     case state_wait_loc4:
382       if (*pnbytes == 1
383 	  && exp->X_op == O_constant
384 	  && exp->X_add_number == DW_CFA_advance_loc4)
385 	{
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 ();
392 	}
393       break;
394 
395     case state_saw_loc4:
396       d->state = state_wait_loc4;
397       if (*pnbytes != 4)
398 	break;
399       if (exp->X_op == O_constant)
400 	{
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)
406 	    {
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;
411 	    }
412 	  else if (exp->X_add_number < 0x100)
413 	    {
414 	      d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1;
415 	      *pnbytes = 1;
416 	    }
417 	  else if (exp->X_add_number < 0x10000)
418 	    {
419 	      d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2;
420 	      *pnbytes = 2;
421 	    }
422 	}
423       else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1)
424 	{
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;
432 	}
433       else if ((exp->X_op == O_divide
434 		|| exp->X_op == O_right_shift)
435 	       && d->cie_info.code_alignment > 1)
436 	{
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))
445 	    {
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;
454 	    }
455 	}
456       break;
457 
458     case state_error:
459       /* Just skipping everything.  */
460       break;
461     }
462 
463   return 0;
464 }
465 
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.  */
469 
470 int
eh_frame_estimate_size_before_relax(fragS * frag)471 eh_frame_estimate_size_before_relax (fragS *frag)
472 {
473   offsetT diff;
474   int ca = frag->fr_subtype >> 3;
475   int ret;
476 
477   diff = resolve_symbol_value (frag->fr_symbol);
478 
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;
489 
490   frag->fr_subtype = (frag->fr_subtype & ~7) | ret;
491 
492   return ret;
493 }
494 
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.  */
498 
499 int
eh_frame_relax_frag(fragS * frag)500 eh_frame_relax_frag (fragS *frag)
501 {
502   int oldsize, newsize;
503 
504   oldsize = frag->fr_subtype & 7;
505   newsize = eh_frame_estimate_size_before_relax (frag);
506   return newsize - oldsize;
507 }
508 
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.  */
512 
513 void
eh_frame_convert_frag(fragS * frag)514 eh_frame_convert_frag (fragS *frag)
515 {
516   offsetT diff;
517   fragS *loc4_frag;
518   int loc4_fix, ca;
519 
520   loc4_frag = (fragS *) frag->fr_opcode;
521   loc4_fix = (int) frag->fr_offset;
522 
523   diff = resolve_symbol_value (frag->fr_symbol);
524 
525   ca = frag->fr_subtype >> 3;
526   gas_assert (ca > 0);
527   diff /= ca;
528   switch (frag->fr_subtype & 7)
529     {
530     case 0:
531       gas_assert (diff < 0x40);
532       loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff;
533       break;
534 
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;
540 
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;
546 
547     default:
548       md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4);
549       break;
550     }
551 
552   frag->fr_fix += frag->fr_subtype & 7;
553   frag->fr_type = rs_fill;
554   frag->fr_subtype = 0;
555   frag->fr_offset = 0;
556 }
557