1 /* Interface to prologue value handling for GDB.
2 Copyright (C) 2003-2019 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 /* What sort of value is this? This determines the interpretation
23 of subsequent fields. */
24 enum prologue_value_kind
26 /* We don't know anything about the value. This is also used for
27 values we could have kept track of, when doing so would have
28 been too complex and we don't want to bother. The bottom of
32 /* A known constant. K is its value. */
35 /* The value that register REG originally had *UPON ENTRY TO THE
36 FUNCTION*, plus K. If K is zero, this means, obviously, just
37 the value REG had upon entry to the function. REG is a GDB
38 register number. Before we start interpreting, we initialize
39 every register R to { pvk_register, R, 0 }. */
43 /* When we analyze a prologue, we're really doing 'abstract
44 interpretation' or 'pseudo-evaluation': running the function's code
45 in simulation, but using conservative approximations of the values
46 it would have when it actually runs. For example, if our function
47 starts with the instruction:
49 addi r1, 42 # add 42 to r1
51 we don't know exactly what value will be in r1 after executing this
52 instruction, but we do know it'll be 42 greater than its original
55 If we then see an instruction like:
57 addi r1, 22 # add 22 to r1
59 we still don't know what r1's value is, but again, we can say it is
60 now 64 greater than its original value.
62 If the next instruction were:
64 mov r2, r1 # set r2 to r1's value
66 then we can say that r2's value is now the original value of r1
69 It's common for prologues to save registers on the stack, so we'll
70 need to track the values of stack frame slots, as well as the
71 registers. So after an instruction like this:
75 then we'd know that the stack slot four bytes above the frame
76 pointer holds the original value of r1 plus 64.
80 Of course, this can only go so far before it gets unreasonable. If
81 we wanted to be able to say anything about the value of r1 after
84 xor r1, r3 # exclusive-or r1 and r3, place result in r1
86 then things would get pretty complex. But remember, we're just
87 doing a conservative approximation; if exclusive-or instructions
88 aren't relevant to prologues, we can just say r1's value is now
89 'unknown'. We can ignore things that are too complex, if that loss
90 of information is acceptable for our application.
92 So when I say "conservative approximation" here, what I mean is an
93 approximation that is either accurate, or marked "unknown", but
96 Once you've reached the current PC, or an instruction that you
97 don't know how to simulate, you stop. Now you can examine the
98 state of the registers and stack slots you've kept track of.
100 - To see how large your stack frame is, just check the value of the
101 stack pointer register; if it's the original value of the SP
102 minus a constant, then that constant is the stack frame's size.
103 If the SP's value has been marked as 'unknown', then that means
104 the prologue has done something too complex for us to track, and
105 we don't know the frame size.
107 - To see where we've saved the previous frame's registers, we just
108 search the values we've tracked --- stack slots, usually, but
109 registers, too, if you want --- for something equal to the
110 register's original value. If the ABI suggests a standard place
111 to save a given register, then we can check there first, but
112 really, anything that will get us back the original value will
115 Sure, this takes some work. But prologue analyzers aren't
116 quick-and-simple pattern patching to recognize a few fixed prologue
117 forms any more; they're big, hairy functions. Along with inferior
118 function calls, prologue analysis accounts for a substantial
119 portion of the time needed to stabilize a GDB port. So I think
120 it's worthwhile to look for an approach that will be easier to
121 understand and maintain. In the approach used here:
123 - It's easier to see that the analyzer is correct: you just see
124 whether the analyzer properly (albiet conservatively) simulates
125 the effect of each instruction.
127 - It's easier to extend the analyzer: you can add support for new
128 instructions, and know that you haven't broken anything that
129 wasn't already broken before.
131 - It's orthogonal: to gather new information, you don't need to
132 complicate the code for each instruction. As long as your domain
133 of conservative values is already detailed enough to tell you
134 what you need, then all the existing instruction simulations are
135 already gathering the right data for you.
137 A 'struct prologue_value' is a conservative approximation of the
138 real value the register or stack slot will have. */
140 struct prologue_value
{
142 /* What sort of value is this? This determines the interpretation
143 of subsequent fields. */
144 enum prologue_value_kind kind
;
146 /* The meanings of the following fields depend on 'kind'; see the
147 comments for the specific 'kind' values. */
152 typedef struct prologue_value pv_t
;
155 /* Return the unknown prologue value --- { pvk_unknown, ?, ? }. */
156 pv_t
pv_unknown (void);
158 /* Return the prologue value representing the constant K. */
159 pv_t
pv_constant (CORE_ADDR k
);
161 /* Return the prologue value representing the original value of
162 register REG, plus the constant K. */
163 pv_t
pv_register (int reg
, CORE_ADDR k
);
166 /* Return conservative approximations of the results of the following
168 pv_t
pv_add (pv_t a
, pv_t b
); /* a + b */
169 pv_t
pv_add_constant (pv_t v
, CORE_ADDR k
); /* a + k */
170 pv_t
pv_subtract (pv_t a
, pv_t b
); /* a - b */
171 pv_t
pv_logical_and (pv_t a
, pv_t b
); /* a & b */
174 /* Return non-zero iff A and B are identical expressions.
176 This is not the same as asking if the two values are equal; the
177 result of such a comparison would have to be a pv_boolean, and
178 asking whether two 'unknown' values were equal would give you
179 pv_maybe. Same for comparing, say, { pvk_register, R1, 0 } and {
180 pvk_register, R2, 0}.
182 Instead, this function asks whether the two representations are the
184 int pv_is_identical (pv_t a
, pv_t b
);
187 /* Return non-zero if A is known to be a constant. */
188 int pv_is_constant (pv_t a
);
190 /* Return non-zero if A is the original value of register number R
191 plus some constant, zero otherwise. */
192 int pv_is_register (pv_t a
, int r
);
195 /* Return non-zero if A is the original value of register R plus the
197 int pv_is_register_k (pv_t a
, int r
, CORE_ADDR k
);
199 /* A conservative boolean type, including "maybe", when we can't
200 figure out whether something is true or not. */
208 /* Decide whether a reference to SIZE bytes at ADDR refers exactly to
209 an element of an array. The array starts at ARRAY_ADDR, and has
210 ARRAY_LEN values of ELT_SIZE bytes each. If ADDR definitely does
211 refer to an array element, set *I to the index of the referenced
212 element in the array, and return pv_definite_yes. If it definitely
213 doesn't, return pv_definite_no. If we can't tell, return pv_maybe.
215 If the reference does touch the array, but doesn't fall exactly on
216 an element boundary, or doesn't refer to the whole element, return
218 enum pv_boolean
pv_is_array_ref (pv_t addr
, CORE_ADDR size
,
219 pv_t array_addr
, CORE_ADDR array_len
,
224 /* A 'pv_area' keeps track of values stored in a particular region of
230 /* Create a new area, tracking stores relative to the original value
231 of BASE_REG. If BASE_REG is SP, then this effectively records the
232 contents of the stack frame: the original value of the SP is the
233 frame's CFA, or some constant offset from it.
235 Stores to constant addresses, unknown addresses, or to addresses
236 relative to registers other than BASE_REG will trash this area; see
237 pv_area::store_would_trash.
239 To check whether a pointer refers to this area, only the low
240 ADDR_BIT bits will be compared. */
241 pv_area (int base_reg
, int addr_bit
);
245 DISABLE_COPY_AND_ASSIGN (pv_area
);
247 /* Store the SIZE-byte value VALUE at ADDR in AREA.
249 If ADDR is not relative to the same base register we used in
250 creating AREA, then we can't tell which values here the stored
251 value might overlap, and we'll have to mark everything as
253 void store (pv_t addr
,
257 /* Return the SIZE-byte value at ADDR in AREA. This may return
259 pv_t
fetch (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 bool store_would_trash (pv_t addr
);
274 /* Search AREA for the original value of REGISTER. If we can't find
275 it, return zero; if we can find it, return a non-zero value, and if
276 OFFSET_P is non-zero, set *OFFSET_P to the register's offset within
277 AREA. GDBARCH is the architecture of which REGISTER is a member.
279 In the worst case, this takes time proportional to the number of
280 items stored in AREA. If you plan to gather a lot of information
281 about registers saved in AREA, consider calling pv_area::scan
282 instead, and collecting all your information in one pass. */
283 bool find_reg (struct gdbarch
*gdbarch
, int reg
, CORE_ADDR
*offset_p
);
286 /* For every part of AREA whose value we know, apply FUNC to CLOSURE,
287 the value's address, its size, and the value itself. */
288 void scan (void (*func
) (void *closure
,
298 /* Delete all entries from AREA. */
299 void clear_entries ();
301 /* Return a pointer to the first entry we hit in AREA starting at
302 OFFSET and going forward.
304 This may return zero, if AREA has no entries.
306 And since the entries are a ring, this may return an entry that
307 entirely precedes OFFSET. This is the correct behavior: depending
308 on the sizes involved, we could still overlap such an area, with
310 struct area_entry
*find_entry (CORE_ADDR offset
);
312 /* Return non-zero if the SIZE bytes at OFFSET would overlap ENTRY;
313 return zero otherwise. AREA is the area to which ENTRY belongs. */
314 int overlaps (struct area_entry
*entry
,
318 /* This area's base register. */
321 /* The mask to apply to addresses, to make the wrap-around happen at
323 CORE_ADDR m_addr_mask
;
325 /* An element of the doubly-linked ring of entries, or zero if we
327 struct area_entry
*m_entry
;
330 #endif /* PROLOGUE_VALUE_H */
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