gdb/prologue-value.h

   1 /* Interface to prologue value handling for GDB.
   2    Copyright (C) 2003-2017 Free Software Foundation, Inc.
   3
   4    This file is part of GDB.
   5
   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.
  10
  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.
  15
  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/>.  */
  18
  19 #ifndef PROLOGUE_VALUE_H
  20 #define PROLOGUE_VALUE_H
  21
  22 /* What sort of value is this?  This determines the interpretation
  23    of subsequent fields.  */
  24 enum prologue_value_kind
  25 {
  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
  29      our lattice.  */
  30   pvk_unknown,
  31
  32   /* A known constant.  K is its value.  */
  33   pvk_constant,
  34
  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 }.  */
  40   pvk_register,
  41 };
  42
  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:
  48
  49       addi r1, 42     # add 42 to r1
  50
  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
  53    value.
  54
  55    If we then see an instruction like:
  56
  57       addi r1, 22     # add 22 to r1
  58
  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.
  61
  62    If the next instruction were:
  63
  64       mov r2, r1      # set r2 to r1's value
  65
  66    then we can say that r2's value is now the original value of r1
  67    plus 64.
  68
  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:
  72
  73       mov (fp+4), r2
  74
  75    then we'd know that the stack slot four bytes above the frame
  76    pointer holds the original value of r1 plus 64.
  77
  78    And so on.
  79
  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
  82    the instruction:
  83
  84       xor r1, r3      # exclusive-or r1 and r3, place result in r1
  85
  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.
  91
  92    So when I say "conservative approximation" here, what I mean is an
  93    approximation that is either accurate, or marked "unknown", but
  94    never inaccurate.
  95
  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.
  99
 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.
 106
 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
 113      probably work.
 114
 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:
 122
 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.
 126
 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.
 130
 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.
 136
 137    A 'struct prologue_value' is a conservative approximation of the
 138    real value the register or stack slot will have.  */
 139
 140 struct prologue_value {
 141
 142   /* What sort of value is this?  This determines the interpretation
 143      of subsequent fields.  */
 144   enum prologue_value_kind kind;
 145
 146   /* The meanings of the following fields depend on 'kind'; see the
 147      comments for the specific 'kind' values.  */
 148   int reg;
 149   CORE_ADDR k;
 150 };
 151
 152 typedef struct prologue_value pv_t;
 153
 154
 155 /* Return the unknown prologue value --- { pvk_unknown, ?, ? }.  */
 156 pv_t pv_unknown (void);
 157
 158 /* Return the prologue value representing the constant K.  */
 159 pv_t pv_constant (CORE_ADDR k);
 160
 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);
 164
 165
 166 /* Return conservative approximations of the results of the following
 167    operations.  */
 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 */
 172
 173
 174 /* Return non-zero iff A and B are identical expressions.
 175
 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}.
 181
 182    Instead, this function asks whether the two representations are the
 183    same.  */
 184 int pv_is_identical (pv_t a, pv_t b);
 185
 186
 187 /* Return non-zero if A is known to be a constant.  */
 188 int pv_is_constant (pv_t a);
 189
 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);
 193
 194
 195 /* Return non-zero if A is the original value of register R plus the
 196    constant K.  */
 197 int pv_is_register_k (pv_t a, int r, CORE_ADDR k);
 198
 199 /* A conservative boolean type, including "maybe", when we can't
 200    figure out whether something is true or not.  */
 201 enum pv_boolean {
 202   pv_maybe,
 203   pv_definite_yes,
 204   pv_definite_no,
 205 };
 206
 207
 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.
 214
 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
 217    pv_maybe.  */
 218 enum pv_boolean pv_is_array_ref (pv_t addr, CORE_ADDR size,
 219                                  pv_t array_addr, CORE_ADDR array_len,
 220                                  CORE_ADDR elt_size,
 221                                  int *i);
 222
 223
 224 /* A 'struct pv_area' keeps track of values stored in a particular
 225    region of memory.  */
 226 struct pv_area;
 227
 228 /* Create a new area, tracking stores relative to the original value
 229    of BASE_REG.  If BASE_REG is SP, then this effectively records the
 230    contents of the stack frame: the original value of the SP is the
 231    frame's CFA, or some constant offset from it.
 232
 233    Stores to constant addresses, unknown addresses, or to addresses
 234    relative to registers other than BASE_REG will trash this area; see
 235    pv_area_store_would_trash.
 236
 237    To check whether a pointer refers to this area, only the low
 238    ADDR_BIT bits will be compared.  */
 239 struct pv_area *make_pv_area (int base_reg, int addr_bit);
 240
 241 /* Free AREA.  */
 242 void free_pv_area (struct pv_area *area);
 243
 244
 245 /* Register a cleanup to free AREA.  */
 246 struct cleanup *make_cleanup_free_pv_area (struct pv_area *area);
 247
 248
 249 /* Store the SIZE-byte value VALUE at ADDR in AREA.
 250
 251    If ADDR is not relative to the same base register we used in
 252    creating AREA, then we can't tell which values here the stored
 253    value might overlap, and we'll have to mark everything as
 254    unknown.  */
 255 void pv_area_store (struct pv_area *area,
 256                     pv_t addr,
 257                     CORE_ADDR size,
 258                     pv_t value);
 259
 260 /* Return the SIZE-byte value at ADDR in AREA.  This may return
 261    pv_unknown ().  */
 262 pv_t pv_area_fetch (struct pv_area *area, pv_t addr, CORE_ADDR size);
 263
 264 /* Return true if storing to address ADDR in AREA would force us to
 265    mark the contents of the entire area as unknown.  This could happen
 266    if, say, ADDR is unknown, since we could be storing anywhere.  Or,
 267    it could happen if ADDR is relative to a different register than
 268    the other stores base register, since we don't know the relative
 269    values of the two registers.
 270
 271    If you've reached such a store, it may be better to simply stop the
 272    prologue analysis, and return the information you've gathered,
 273    instead of losing all that information, most of which is probably
 274    okay.  */
 275 int pv_area_store_would_trash (struct pv_area *area, pv_t addr);
 276
 277
 278 /* Search AREA for the original value of REGISTER.  If we can't find
 279    it, return zero; if we can find it, return a non-zero value, and if
 280    OFFSET_P is non-zero, set *OFFSET_P to the register's offset within
 281    AREA.  GDBARCH is the architecture of which REGISTER is a member.
 282
 283    In the worst case, this takes time proportional to the number of
 284    items stored in AREA.  If you plan to gather a lot of information
 285    about registers saved in AREA, consider calling pv_area_scan
 286    instead, and collecting all your information in one pass.  */
 287 int pv_area_find_reg (struct pv_area *area,
 288                       struct gdbarch *gdbarch,
 289                       int reg,
 290                       CORE_ADDR *offset_p);
 291
 292
 293 /* For every part of AREA whose value we know, apply FUNC to CLOSURE,
 294    the value's address, its size, and the value itself.  */
 295 void pv_area_scan (struct pv_area *area,
 296                    void (*func) (void *closure,
 297                                  pv_t addr,
 298                                  CORE_ADDR size,
 299                                  pv_t value),
 300                    void *closure);
 301
 302
 303 #endif /* PROLOGUE_VALUE_H */