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git.ipfire.org Git - thirdparty/binutils-gdb.git/blob - gas/ehopt.c
1 /* ehopt.c--optimize gcc exception frame information.
2 Copyright (C) 1998 Free Software Foundation, Inc.
3 Written by Ian Lance Taylor <ian@cygnus.com>.
5 This file is part of GAS, the GNU Assembler.
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 2, or (at your option)
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.
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, 59 Temple Place - Suite 330, Boston, MA
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. */
29 #include "elf/dwarf2.h"
31 /* Try to optimize gcc 2.8 exception frame information.
33 Exception frame information is emitted for every function in the
34 .eh_frame section. Simple information for a function with no
35 exceptions looks like this:
38 .4byte .LLCIE1 / Length of Common Information Entry
40 .4byte 0x0 / CIE Identifier Tag
41 .byte 0x1 / CIE Version
42 .byte 0x0 / CIE Augmentation (none)
43 .byte 0x1 / ULEB128 0x1 (CIE Code Alignment Factor)
44 .byte 0x7c / SLEB128 -4 (CIE Data Alignment Factor)
45 .byte 0x8 / CIE RA Column
46 .byte 0xc / DW_CFA_def_cfa
47 .byte 0x4 / ULEB128 0x4
48 .byte 0x4 / ULEB128 0x4
49 .byte 0x88 / DW_CFA_offset, column 0x8
50 .byte 0x1 / ULEB128 0x1
53 .set .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol
54 .4byte .LLFDE1 / FDE Length
56 .4byte .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset
57 .4byte .LFB1 / FDE initial location
58 .4byte .LFE1-.LFB1 / FDE address range
59 .byte 0x4 / DW_CFA_advance_loc4
61 .byte 0xe / DW_CFA_def_cfa_offset
62 .byte 0x8 / ULEB128 0x8
63 .byte 0x85 / DW_CFA_offset, column 0x5
64 .byte 0x2 / ULEB128 0x2
65 .byte 0x4 / DW_CFA_advance_loc4
67 .byte 0xd / DW_CFA_def_cfa_register
68 .byte 0x5 / ULEB128 0x5
69 .byte 0x4 / DW_CFA_advance_loc4
71 .byte 0x2e / DW_CFA_GNU_args_size
72 .byte 0x4 / ULEB128 0x4
73 .byte 0x4 / DW_CFA_advance_loc4
75 .byte 0x2e / DW_CFA_GNU_args_size
76 .byte 0x0 / ULEB128 0x0
79 .set .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol
81 The immediate issue we can address in the assembler is the
82 DW_CFA_advance_loc4 followed by a four byte value. The value is
83 the difference of two addresses in the function. Since gcc does
84 not know this value, it always uses four bytes. We will know the
85 value at the end of assembly, so we can do better. */
87 static int eh_frame_code_alignment
PARAMS ((void));
89 /* Get the code alignment factor from the CIE. */
92 eh_frame_code_alignment ()
94 static int code_alignment
;
96 subsegT current_subseg
;
102 if (code_alignment
!= 0)
103 return code_alignment
;
105 /* We should find the CIE at the start of the .eh_frame section. */
107 current_seg
= now_seg
;
108 current_subseg
= now_subseg
;
109 subseg_new (".eh_frame", 0);
110 f
= seg_info (now_seg
)->frchainP
->frch_root
;
111 fix
= seg_info (now_seg
)->frchainP
->fix_root
;
112 subseg_set (current_seg
, current_subseg
);
114 /* Look through the frags of the section to find the code alignment. */
116 /* First make sure that the CIE Identifier Tag is 0. */
119 while (f
!= NULL
&& offset
>= f
->fr_fix
)
125 || f
->fr_fix
- offset
< 4
126 || f
->fr_literal
[offset
] != 0
127 || f
->fr_literal
[offset
+ 1] != 0
128 || f
->fr_literal
[offset
+ 2] != 0
129 || f
->fr_literal
[offset
+ 3] != 0)
135 /* Next make sure the CIE version number is 1. */
138 while (f
!= NULL
&& offset
>= f
->fr_fix
)
144 || f
->fr_fix
- offset
< 1
145 || f
->fr_literal
[offset
] != 1)
151 /* Skip the augmentation (a null terminated string). */
157 while (f
!= NULL
&& offset
>= f
->fr_fix
)
167 while (offset
< f
->fr_fix
&& f
->fr_literal
[offset
] != '\0')
172 if (f
->fr_literal
[offset
] == 'e')
176 if (f
->fr_literal
[offset
] == 'h')
185 if (offset
< f
->fr_fix
)
189 while (f
!= NULL
&& offset
>= f
->fr_fix
)
200 /* If the augmentation field is "eh", then we have to skip a
201 pointer. Unfortunately, we don't know how large it is. We find
202 out by looking for a matching fixup. */
206 && (fix
->fx_frag
!= f
|| fix
->fx_where
!= offset
))
211 offset
+= fix
->fx_size
;
212 while (f
!= NULL
&& offset
>= f
->fr_fix
)
224 /* We're now at the code alignment factor, which is a ULEB128. If
225 it isn't a single byte, forget it. */
227 code_alignment
= f
->fr_literal
[offset
] & 0xff;
228 if ((code_alignment
& 0x80) != 0 || code_alignment
== 0)
234 return code_alignment
;
237 /* This function is called from emit_expr. It looks for cases which
240 Rather than try to parse all this information as we read it, we
241 look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
242 difference. We turn that into a rs_cfa_advance frag, and handle
243 those frags at the end of the assembly. If the gcc output changes
244 somewhat, this optimization may stop working.
246 This function returns non-zero if it handled the expression and
247 emit_expr should not do anything, or zero otherwise. It can also
248 change *EXP and *PNBYTES. */
251 check_eh_frame (exp
, pnbytes
)
253 unsigned int *pnbytes
;
255 static int saw_advance_loc4
;
256 static fragS
*loc4_frag
;
259 if (flag_traditional_format
)
261 /* Don't optimize. */
263 else if (strcmp (segment_name (now_seg
), ".eh_frame") != 0)
264 saw_advance_loc4
= 0;
265 else if (*pnbytes
== 1
266 && exp
->X_op
== O_constant
267 && exp
->X_add_number
== DW_CFA_advance_loc4
)
269 /* This might be a DW_CFA_advance_loc4. Record the frag and the
270 position within the frag, so that we can change it later. */
271 saw_advance_loc4
= 1;
273 loc4_frag
= frag_now
;
274 loc4_fix
= frag_now_fix ();
276 else if (saw_advance_loc4
278 && exp
->X_op
== O_constant
)
282 /* This is a case which we can optimize. The two symbols being
283 subtracted were in the same frag and the expression was
284 reduced to a constant. We can do the optimization entirely
287 saw_advance_loc4
= 0;
289 ca
= eh_frame_code_alignment ();
292 /* Don't optimize. */
294 else if (exp
->X_add_number
% ca
== 0
295 && exp
->X_add_number
/ ca
< 0x40)
297 loc4_frag
->fr_literal
[loc4_fix
]
298 = DW_CFA_advance_loc
| (exp
->X_add_number
/ ca
);
299 /* No more bytes needed. */
302 else if (exp
->X_add_number
< 0x100)
304 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc1
;
307 else if (exp
->X_add_number
< 0x10000)
309 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc2
;
313 else if (saw_advance_loc4
315 && exp
->X_op
== O_subtract
)
318 /* This is a case we can optimize. The expression was not
319 reduced, so we can not finish the optimization until the end
320 of the assembly. We set up a variant frag which we handle
323 saw_advance_loc4
= 0;
325 frag_var (rs_cfa
, 4, 0, 0, make_expr_symbol (exp
),
326 loc4_fix
, (char *) loc4_frag
);
331 saw_advance_loc4
= 0;
336 /* The function estimates the size of a rs_cfa variant frag based on
337 the current values of the symbols. It is called before the
338 relaxation loop. We set fr_subtype to the expected length. */
341 eh_frame_estimate_size_before_relax (frag
)
348 ca
= eh_frame_code_alignment ();
349 diff
= resolve_symbol_value (frag
->fr_symbol
, 0);
353 else if (diff
% ca
== 0 && diff
/ ca
< 0x40)
355 else if (diff
< 0x100)
357 else if (diff
< 0x10000)
362 frag
->fr_subtype
= ret
;
367 /* This function relaxes a rs_cfa variant frag based on the current
368 values of the symbols. fr_subtype is the current length of the
369 frag. This returns the change in frag length. */
372 eh_frame_relax_frag (frag
)
375 int oldsize
, newsize
;
377 oldsize
= frag
->fr_subtype
;
378 newsize
= eh_frame_estimate_size_before_relax (frag
);
379 return newsize
- oldsize
;
382 /* This function converts a rs_cfa variant frag into a normal fill
383 frag. This is called after all relaxation has been done.
384 fr_subtype will be the desired length of the frag. */
387 eh_frame_convert_frag (frag
)
394 loc4_frag
= (fragS
*) frag
->fr_opcode
;
395 loc4_fix
= (int) frag
->fr_offset
;
397 diff
= resolve_symbol_value (frag
->fr_symbol
, 1);
399 if (frag
->fr_subtype
== 0)
403 ca
= eh_frame_code_alignment ();
404 assert (ca
> 0 && diff
% ca
== 0 && diff
/ ca
< 0x40);
405 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc
| (diff
/ ca
);
407 else if (frag
->fr_subtype
== 1)
409 assert (diff
< 0x100);
410 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc1
;
411 frag
->fr_literal
[frag
->fr_fix
] = diff
;
413 else if (frag
->fr_subtype
== 2)
415 assert (diff
< 0x10000);
416 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc2
;
417 md_number_to_chars (frag
->fr_literal
+ frag
->fr_fix
, diff
, 2);
420 md_number_to_chars (frag
->fr_literal
+ frag
->fr_fix
, diff
, 4);
422 frag
->fr_fix
+= frag
->fr_subtype
;
423 frag
->fr_type
= rs_fill
;