1 /* Interface to prologue value handling for GDB.
2 Copyright (C) 2003-2014 Free Software Foundation, Inc.
4 This file is part of GDB.
6 This program is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 3 of the License, or
9 (at your option) any later version.
11 This program is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>. */
19 #ifndef PROLOGUE_VALUE_H
20 #define PROLOGUE_VALUE_H
22 /* When we analyze a prologue, we're really doing 'abstract
23 interpretation' or 'pseudo-evaluation': running the function's code
24 in simulation, but using conservative approximations of the values
25 it would have when it actually runs. For example, if our function
26 starts with the instruction:
28 addi r1, 42 # add 42 to r1
30 we don't know exactly what value will be in r1 after executing this
31 instruction, but we do know it'll be 42 greater than its original
34 If we then see an instruction like:
36 addi r1, 22 # add 22 to r1
38 we still don't know what r1's value is, but again, we can say it is
39 now 64 greater than its original value.
41 If the next instruction were:
43 mov r2, r1 # set r2 to r1's value
45 then we can say that r2's value is now the original value of r1
48 It's common for prologues to save registers on the stack, so we'll
49 need to track the values of stack frame slots, as well as the
50 registers. So after an instruction like this:
54 then we'd know that the stack slot four bytes above the frame
55 pointer holds the original value of r1 plus 64.
59 Of course, this can only go so far before it gets unreasonable. If
60 we wanted to be able to say anything about the value of r1 after
63 xor r1, r3 # exclusive-or r1 and r3, place result in r1
65 then things would get pretty complex. But remember, we're just
66 doing a conservative approximation; if exclusive-or instructions
67 aren't relevant to prologues, we can just say r1's value is now
68 'unknown'. We can ignore things that are too complex, if that loss
69 of information is acceptable for our application.
71 So when I say "conservative approximation" here, what I mean is an
72 approximation that is either accurate, or marked "unknown", but
75 Once you've reached the current PC, or an instruction that you
76 don't know how to simulate, you stop. Now you can examine the
77 state of the registers and stack slots you've kept track of.
79 - To see how large your stack frame is, just check the value of the
80 stack pointer register; if it's the original value of the SP
81 minus a constant, then that constant is the stack frame's size.
82 If the SP's value has been marked as 'unknown', then that means
83 the prologue has done something too complex for us to track, and
84 we don't know the frame size.
86 - To see where we've saved the previous frame's registers, we just
87 search the values we've tracked --- stack slots, usually, but
88 registers, too, if you want --- for something equal to the
89 register's original value. If the ABI suggests a standard place
90 to save a given register, then we can check there first, but
91 really, anything that will get us back the original value will
94 Sure, this takes some work. But prologue analyzers aren't
95 quick-and-simple pattern patching to recognize a few fixed prologue
96 forms any more; they're big, hairy functions. Along with inferior
97 function calls, prologue analysis accounts for a substantial
98 portion of the time needed to stabilize a GDB port. So I think
99 it's worthwhile to look for an approach that will be easier to
100 understand and maintain. In the approach used here:
102 - It's easier to see that the analyzer is correct: you just see
103 whether the analyzer properly (albiet conservatively) simulates
104 the effect of each instruction.
106 - It's easier to extend the analyzer: you can add support for new
107 instructions, and know that you haven't broken anything that
108 wasn't already broken before.
110 - It's orthogonal: to gather new information, you don't need to
111 complicate the code for each instruction. As long as your domain
112 of conservative values is already detailed enough to tell you
113 what you need, then all the existing instruction simulations are
114 already gathering the right data for you.
116 A 'struct prologue_value' is a conservative approximation of the
117 real value the register or stack slot will have. */
119 struct prologue_value
{
121 /* What sort of value is this? This determines the interpretation
122 of subsequent fields. */
125 /* We don't know anything about the value. This is also used for
126 values we could have kept track of, when doing so would have
127 been too complex and we don't want to bother. The bottom of
131 /* A known constant. K is its value. */
134 /* The value that register REG originally had *UPON ENTRY TO THE
135 FUNCTION*, plus K. If K is zero, this means, obviously, just
136 the value REG had upon entry to the function. REG is a GDB
137 register number. Before we start interpreting, we initialize
138 every register R to { pvk_register, R, 0 }. */
143 /* The meanings of the following fields depend on 'kind'; see the
144 comments for the specific 'kind' values. */
149 typedef struct prologue_value pv_t
;
152 /* Return the unknown prologue value --- { pvk_unknown, ?, ? }. */
153 pv_t
pv_unknown (void);
155 /* Return the prologue value representing the constant K. */
156 pv_t
pv_constant (CORE_ADDR k
);
158 /* Return the prologue value representing the original value of
159 register REG, plus the constant K. */
160 pv_t
pv_register (int reg
, CORE_ADDR k
);
163 /* Return conservative approximations of the results of the following
165 pv_t
pv_add (pv_t a
, pv_t b
); /* a + b */
166 pv_t
pv_add_constant (pv_t v
, CORE_ADDR k
); /* a + k */
167 pv_t
pv_subtract (pv_t a
, pv_t b
); /* a - b */
168 pv_t
pv_logical_and (pv_t a
, pv_t b
); /* a & b */
171 /* Return non-zero iff A and B are identical expressions.
173 This is not the same as asking if the two values are equal; the
174 result of such a comparison would have to be a pv_boolean, and
175 asking whether two 'unknown' values were equal would give you
176 pv_maybe. Same for comparing, say, { pvk_register, R1, 0 } and {
177 pvk_register, R2, 0}.
179 Instead, this function asks whether the two representations are the
181 int pv_is_identical (pv_t a
, pv_t b
);
184 /* Return non-zero if A is known to be a constant. */
185 int pv_is_constant (pv_t a
);
187 /* Return non-zero if A is the original value of register number R
188 plus some constant, zero otherwise. */
189 int pv_is_register (pv_t a
, int r
);
192 /* Return non-zero if A is the original value of register R plus the
194 int pv_is_register_k (pv_t a
, int r
, CORE_ADDR k
);
196 /* A conservative boolean type, including "maybe", when we can't
197 figure out whether something is true or not. */
205 /* Decide whether a reference to SIZE bytes at ADDR refers exactly to
206 an element of an array. The array starts at ARRAY_ADDR, and has
207 ARRAY_LEN values of ELT_SIZE bytes each. If ADDR definitely does
208 refer to an array element, set *I to the index of the referenced
209 element in the array, and return pv_definite_yes. If it definitely
210 doesn't, return pv_definite_no. If we can't tell, return pv_maybe.
212 If the reference does touch the array, but doesn't fall exactly on
213 an element boundary, or doesn't refer to the whole element, return
215 enum pv_boolean
pv_is_array_ref (pv_t addr
, CORE_ADDR size
,
216 pv_t array_addr
, CORE_ADDR array_len
,
221 /* A 'struct pv_area' keeps track of values stored in a particular
225 /* Create a new area, tracking stores relative to the original value
226 of BASE_REG. If BASE_REG is SP, then this effectively records the
227 contents of the stack frame: the original value of the SP is the
228 frame's CFA, or some constant offset from it.
230 Stores to constant addresses, unknown addresses, or to addresses
231 relative to registers other than BASE_REG will trash this area; see
232 pv_area_store_would_trash.
234 To check whether a pointer refers to this area, only the low
235 ADDR_BIT bits will be compared. */
236 struct pv_area
*make_pv_area (int base_reg
, int addr_bit
);
239 void free_pv_area (struct pv_area
*area
);
242 /* Register a cleanup to free AREA. */
243 struct cleanup
*make_cleanup_free_pv_area (struct pv_area
*area
);
246 /* Store the SIZE-byte value VALUE at ADDR in AREA.
248 If ADDR is not relative to the same base register we used in
249 creating AREA, then we can't tell which values here the stored
250 value might overlap, and we'll have to mark everything as
252 void pv_area_store (struct pv_area
*area
,
257 /* Return the SIZE-byte value at ADDR in AREA. This may return
259 pv_t
pv_area_fetch (struct pv_area
*area
, pv_t addr
, CORE_ADDR size
);
261 /* Return true if storing to address ADDR in AREA would force us to
262 mark the contents of the entire area as unknown. This could happen
263 if, say, ADDR is unknown, since we could be storing anywhere. Or,
264 it could happen if ADDR is relative to a different register than
265 the other stores base register, since we don't know the relative
266 values of the two registers.
268 If you've reached such a store, it may be better to simply stop the
269 prologue analysis, and return the information you've gathered,
270 instead of losing all that information, most of which is probably
272 int pv_area_store_would_trash (struct pv_area
*area
, pv_t addr
);
275 /* Search AREA for the original value of REGISTER. If we can't find
276 it, return zero; if we can find it, return a non-zero value, and if
277 OFFSET_P is non-zero, set *OFFSET_P to the register's offset within
278 AREA. GDBARCH is the architecture of which REGISTER is a member.
280 In the worst case, this takes time proportional to the number of
281 items stored in AREA. If you plan to gather a lot of information
282 about registers saved in AREA, consider calling pv_area_scan
283 instead, and collecting all your information in one pass. */
284 int pv_area_find_reg (struct pv_area
*area
,
285 struct gdbarch
*gdbarch
,
287 CORE_ADDR
*offset_p
);
290 /* For every part of AREA whose value we know, apply FUNC to CLOSURE,
291 the value's address, its size, and the value itself. */
292 void pv_area_scan (struct pv_area
*area
,
293 void (*func
) (void *closure
,
300 #endif /* PROLOGUE_VALUE_H */
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