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
;
100 char augmentation
[10];
103 if (code_alignment
!= 0)
104 return code_alignment
;
106 /* We should find the CIE at the start of the .eh_frame section. */
108 current_seg
= now_seg
;
109 current_subseg
= now_subseg
;
110 subseg_new (".eh_frame", 0);
111 f
= seg_info (now_seg
)->frchainP
->frch_root
;
112 fix
= seg_info (now_seg
)->frchainP
->fix_root
;
113 subseg_set (current_seg
, current_subseg
);
115 /* Look through the frags of the section to find the code alignment. */
117 /* First make sure that the CIE Identifier Tag is 0. */
120 while (f
!= NULL
&& offset
>= f
->fr_fix
)
126 || f
->fr_fix
- offset
< 4
127 || f
->fr_literal
[offset
] != 0
128 || f
->fr_literal
[offset
+ 1] != 0
129 || f
->fr_literal
[offset
+ 2] != 0
130 || f
->fr_literal
[offset
+ 3] != 0)
136 /* Next make sure the CIE version number is 1. */
139 while (f
!= NULL
&& offset
>= f
->fr_fix
)
145 || f
->fr_fix
- offset
< 1
146 || f
->fr_literal
[offset
] != 1)
152 /* Skip the augmentation (a null terminated string). */
158 while (f
!= NULL
&& offset
>= f
->fr_fix
)
168 while (offset
< f
->fr_fix
&& f
->fr_literal
[offset
] != '\0')
170 if (iaug
< (sizeof augmentation
) - 1)
172 augmentation
[iaug
] = f
->fr_literal
[offset
];
177 if (offset
< f
->fr_fix
)
181 while (f
!= NULL
&& offset
>= f
->fr_fix
)
192 augmentation
[iaug
] = '\0';
193 if (augmentation
[0] == '\0')
195 /* No augmentation. */
197 else if (strcmp (augmentation
, "eh") == 0)
199 /* We have to skip a pointer. Unfortunately, we don't know how
200 large it is. We find out by looking for a matching fixup. */
202 && (fix
->fx_frag
!= f
|| fix
->fx_where
!= offset
))
207 offset
+= fix
->fx_size
;
208 while (f
!= NULL
&& offset
>= f
->fr_fix
)
225 /* We're now at the code alignment factor, which is a ULEB128. If
226 it isn't a single byte, forget it. */
228 code_alignment
= f
->fr_literal
[offset
] & 0xff;
229 if ((code_alignment
& 0x80) != 0 || code_alignment
== 0)
235 return code_alignment
;
238 /* This function is called from emit_expr. It looks for cases which
241 Rather than try to parse all this information as we read it, we
242 look for a single byte DW_CFA_advance_loc4 followed by a 4 byte
243 difference. We turn that into a rs_cfa_advance frag, and handle
244 those frags at the end of the assembly. If the gcc output changes
245 somewhat, this optimization may stop working.
247 This function returns non-zero if it handled the expression and
248 emit_expr should not do anything, or zero otherwise. It can also
249 change *EXP and *PNBYTES. */
252 check_eh_frame (exp
, pnbytes
)
254 unsigned int *pnbytes
;
256 static int saw_advance_loc4
;
257 static fragS
*loc4_frag
;
260 if (flag_traditional_format
)
262 /* Don't optimize. */
264 else if (strcmp (segment_name (now_seg
), ".eh_frame") != 0)
265 saw_advance_loc4
= 0;
266 else if (*pnbytes
== 1
267 && exp
->X_op
== O_constant
268 && exp
->X_add_number
== DW_CFA_advance_loc4
)
270 /* This might be a DW_CFA_advance_loc4. Record the frag and the
271 position within the frag, so that we can change it later. */
272 saw_advance_loc4
= 1;
274 loc4_frag
= frag_now
;
275 loc4_fix
= frag_now_fix ();
277 else if (saw_advance_loc4
279 && exp
->X_op
== O_constant
)
283 /* This is a case which we can optimize. The two symbols being
284 subtracted were in the same frag and the expression was
285 reduced to a constant. We can do the optimization entirely
288 saw_advance_loc4
= 0;
290 ca
= eh_frame_code_alignment ();
293 /* Don't optimize. */
295 else if (exp
->X_add_number
% ca
== 0
296 && exp
->X_add_number
/ ca
< 0x40)
298 loc4_frag
->fr_literal
[loc4_fix
]
299 = DW_CFA_advance_loc
| (exp
->X_add_number
/ ca
);
300 /* No more bytes needed. */
303 else if (exp
->X_add_number
< 0x100)
305 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc1
;
308 else if (exp
->X_add_number
< 0x10000)
310 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc2
;
314 else if (saw_advance_loc4
316 && exp
->X_op
== O_subtract
)
319 /* This is a case we can optimize. The expression was not
320 reduced, so we can not finish the optimization until the end
321 of the assembly. We set up a variant frag which we handle
324 saw_advance_loc4
= 0;
326 frag_var (rs_cfa
, 4, 0, 0, make_expr_symbol (exp
),
327 loc4_fix
, (char *) loc4_frag
);
332 saw_advance_loc4
= 0;
337 /* The function estimates the size of a rs_cfa variant frag based on
338 the current values of the symbols. It is called before the
339 relaxation loop. We set fr_subtype to the expected length. */
342 eh_frame_estimate_size_before_relax (frag
)
349 ca
= eh_frame_code_alignment ();
350 diff
= resolve_symbol_value (frag
->fr_symbol
, 0);
354 else if (diff
% ca
== 0 && diff
/ ca
< 0x40)
356 else if (diff
< 0x100)
358 else if (diff
< 0x10000)
363 frag
->fr_subtype
= ret
;
368 /* This function relaxes a rs_cfa variant frag based on the current
369 values of the symbols. fr_subtype is the current length of the
370 frag. This returns the change in frag length. */
373 eh_frame_relax_frag (frag
)
376 int oldsize
, newsize
;
378 oldsize
= frag
->fr_subtype
;
379 newsize
= eh_frame_estimate_size_before_relax (frag
);
380 return newsize
- oldsize
;
383 /* This function converts a rs_cfa variant frag into a normal fill
384 frag. This is called after all relaxation has been done.
385 fr_subtype will be the desired length of the frag. */
388 eh_frame_convert_frag (frag
)
395 loc4_frag
= (fragS
*) frag
->fr_opcode
;
396 loc4_fix
= (int) frag
->fr_offset
;
398 diff
= resolve_symbol_value (frag
->fr_symbol
, 1);
400 if (frag
->fr_subtype
== 0)
404 ca
= eh_frame_code_alignment ();
405 assert (ca
> 0 && diff
% ca
== 0 && diff
/ ca
< 0x40);
406 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc
| (diff
/ ca
);
408 else if (frag
->fr_subtype
== 1)
410 assert (diff
< 0x100);
411 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc1
;
412 frag
->fr_literal
[frag
->fr_fix
] = diff
;
414 else if (frag
->fr_subtype
== 2)
416 assert (diff
< 0x10000);
417 loc4_frag
->fr_literal
[loc4_fix
] = DW_CFA_advance_loc2
;
418 md_number_to_chars (frag
->fr_literal
+ frag
->fr_fix
, diff
, 2);
421 md_number_to_chars (frag
->fr_literal
+ frag
->fr_fix
, diff
, 4);
423 frag
->fr_fix
+= frag
->fr_subtype
;
424 frag
->fr_type
= rs_fill
;
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