Subversion Repositories openrisc_2011-10-31

/* Interface to prologue value handling for GDB.

   Copyright 2003, 2004, 2005, 2007, 2008, 2009, 2010

   Free Software Foundation, Inc.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify

   it under the terms of the GNU General Public License as published by

   the Free Software Foundation; either version 3 of the License, or

   (at your option) any later version.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program.  If not, see <http://www.gnu.org/licenses/>. */

#ifndef PROLOGUE_VALUE_H

#define PROLOGUE_VALUE_H

/* When we analyze a prologue, we're really doing 'abstract

   interpretation' or 'pseudo-evaluation': running the function's code

   in simulation, but using conservative approximations of the values

   it would have when it actually runs.  For example, if our function

   starts with the instruction:

      addi r1, 42     # add 42 to r1

   we don't know exactly what value will be in r1 after executing this

   instruction, but we do know it'll be 42 greater than its original

   value.

   If we then see an instruction like:

      addi r1, 22     # add 22 to r1

   we still don't know what r1's value is, but again, we can say it is

   now 64 greater than its original value.

   If the next instruction were:

      mov r2, r1      # set r2 to r1's value

   then we can say that r2's value is now the original value of r1

   plus 64.

   It's common for prologues to save registers on the stack, so we'll

   need to track the values of stack frame slots, as well as the

   registers.  So after an instruction like this:

      mov (fp+4), r2

   then we'd know that the stack slot four bytes above the frame

   pointer holds the original value of r1 plus 64.

   And so on.

   Of course, this can only go so far before it gets unreasonable.  If

   we wanted to be able to say anything about the value of r1 after

   the instruction:

      xor r1, r3      # exclusive-or r1 and r3, place result in r1

   then things would get pretty complex.  But remember, we're just

   doing a conservative approximation; if exclusive-or instructions

   aren't relevant to prologues, we can just say r1's value is now

   'unknown'.  We can ignore things that are too complex, if that loss

   of information is acceptable for our application.

   So when I say "conservative approximation" here, what I mean is an

   approximation that is either accurate, or marked "unknown", but

   never inaccurate.

   Once you've reached the current PC, or an instruction that you

   don't know how to simulate, you stop.  Now you can examine the

   state of the registers and stack slots you've kept track of.

   - To see how large your stack frame is, just check the value of the

     stack pointer register; if it's the original value of the SP

     minus a constant, then that constant is the stack frame's size.

     If the SP's value has been marked as 'unknown', then that means

     the prologue has done something too complex for us to track, and

     we don't know the frame size.

   - To see where we've saved the previous frame's registers, we just

     search the values we've tracked --- stack slots, usually, but

     registers, too, if you want --- for something equal to the

     register's original value.  If the ABI suggests a standard place

     to save a given register, then we can check there first, but

     really, anything that will get us back the original value will

     probably work.

   Sure, this takes some work.  But prologue analyzers aren't

   quick-and-simple pattern patching to recognize a few fixed prologue

   forms any more; they're big, hairy functions.  Along with inferior

   function calls, prologue analysis accounts for a substantial

   portion of the time needed to stabilize a GDB port.  So I think

   it's worthwhile to look for an approach that will be easier to

   understand and maintain.  In the approach used here:

   - It's easier to see that the analyzer is correct: you just see

     whether the analyzer properly (albiet conservatively) simulates

     the effect of each instruction.

   - It's easier to extend the analyzer: you can add support for new

     instructions, and know that you haven't broken anything that

     wasn't already broken before.

   - It's orthogonal: to gather new information, you don't need to

     complicate the code for each instruction.  As long as your domain

     of conservative values is already detailed enough to tell you

     what you need, then all the existing instruction simulations are

     already gathering the right data for you.

   A 'struct prologue_value' is a conservative approximation of the

   real value the register or stack slot will have.  */

struct prologue_value {

  /* What sort of value is this?  This determines the interpretation

     of subsequent fields.  */

  enum {

    /* We don't know anything about the value.  This is also used for

       values we could have kept track of, when doing so would have

       been too complex and we don't want to bother.  The bottom of

       our lattice.  */

    pvk_unknown,

    /* A known constant.  K is its value.  */

    pvk_constant,

    /* The value that register REG originally had *UPON ENTRY TO THE

       FUNCTION*, plus K.  If K is zero, this means, obviously, just

       the value REG had upon entry to the function.  REG is a GDB

       register number.  Before we start interpreting, we initialize

       every register R to { pvk_register, R, 0 }.  */

    pvk_register,

  } kind;

  /* The meanings of the following fields depend on 'kind'; see the

     comments for the specific 'kind' values.  */

  int reg;

  CORE_ADDR k;

};

typedef struct prologue_value pv_t;

/* Return the unknown prologue value --- { pvk_unknown, ?, ? }.  */

pv_t pv_unknown (void);

/* Return the prologue value representing the constant K.  */

pv_t pv_constant (CORE_ADDR k);

/* Return the prologue value representing the original value of

   register REG, plus the constant K.  */

pv_t pv_register (int reg, CORE_ADDR k);

/* Return conservative approximations of the results of the following

   operations.  */

pv_t pv_add (pv_t a, pv_t b);               /* a + b */

pv_t pv_add_constant (pv_t v, CORE_ADDR k); /* a + k */

pv_t pv_subtract (pv_t a, pv_t b);          /* a - b */

pv_t pv_logical_and (pv_t a, pv_t b);       /* a & b */

/* Return non-zero iff A and B are identical expressions.

   This is not the same as asking if the two values are equal; the

   result of such a comparison would have to be a pv_boolean, and

   asking whether two 'unknown' values were equal would give you

   pv_maybe.  Same for comparing, say, { pvk_register, R1, 0 } and {

   pvk_register, R2, 0}.

   Instead, this function asks whether the two representations are the

   same.  */

int pv_is_identical (pv_t a, pv_t b);

/* Return non-zero if A is known to be a constant.  */

int pv_is_constant (pv_t a);

/* Return non-zero if A is the original value of register number R

   plus some constant, zero otherwise.  */

int pv_is_register (pv_t a, int r);

/* Return non-zero if A is the original value of register R plus the

   constant K.  */

int pv_is_register_k (pv_t a, int r, CORE_ADDR k);

/* A conservative boolean type, including "maybe", when we can't

   figure out whether something is true or not.  */

enum pv_boolean {

  pv_maybe,

  pv_definite_yes,

  pv_definite_no,

};

/* Decide whether a reference to SIZE bytes at ADDR refers exactly to

   an element of an array.  The array starts at ARRAY_ADDR, and has

   ARRAY_LEN values of ELT_SIZE bytes each.  If ADDR definitely does

   refer to an array element, set *I to the index of the referenced

   element in the array, and return pv_definite_yes.  If it definitely

   doesn't, return pv_definite_no.  If we can't tell, return pv_maybe.

   If the reference does touch the array, but doesn't fall exactly on

   an element boundary, or doesn't refer to the whole element, return

   pv_maybe.  */

enum pv_boolean pv_is_array_ref (pv_t addr, CORE_ADDR size,

                                 pv_t array_addr, CORE_ADDR array_len,

                                 CORE_ADDR elt_size,

                                 int *i);

/* A 'struct pv_area' keeps track of values stored in a particular

   region of memory.  */

struct pv_area;

/* Create a new area, tracking stores relative to the original value

   of BASE_REG.  If BASE_REG is SP, then this effectively records the

   contents of the stack frame: the original value of the SP is the

   frame's CFA, or some constant offset from it.

   Stores to constant addresses, unknown addresses, or to addresses

   relative to registers other than BASE_REG will trash this area; see

   pv_area_store_would_trash.

   To check whether a pointer refers to this area, only the low

   ADDR_BIT bits will be compared.  */

struct pv_area *make_pv_area (int base_reg, int addr_bit);

/* Free AREA.  */

void free_pv_area (struct pv_area *area);

/* Register a cleanup to free AREA.  */

struct cleanup *make_cleanup_free_pv_area (struct pv_area *area);

/* Store the SIZE-byte value VALUE at ADDR in AREA.

   If ADDR is not relative to the same base register we used in

   creating AREA, then we can't tell which values here the stored

   value might overlap, and we'll have to mark everything as

   unknown.  */

void pv_area_store (struct pv_area *area,

                    pv_t addr,

                    CORE_ADDR size,

                    pv_t value);

/* Return the SIZE-byte value at ADDR in AREA.  This may return

   pv_unknown ().  */

pv_t pv_area_fetch (struct pv_area *area, pv_t addr, CORE_ADDR size);

/* Return true if storing to address ADDR in AREA would force us to

   mark the contents of the entire area as unknown.  This could happen

   if, say, ADDR is unknown, since we could be storing anywhere.  Or,

   it could happen if ADDR is relative to a different register than

   the other stores base register, since we don't know the relative

   values of the two registers.

   If you've reached such a store, it may be better to simply stop the

   prologue analysis, and return the information you've gathered,

   instead of losing all that information, most of which is probably

   okay.  */

int pv_area_store_would_trash (struct pv_area *area, pv_t addr);

/* Search AREA for the original value of REGISTER.  If we can't find

   it, return zero; if we can find it, return a non-zero value, and if

   OFFSET_P is non-zero, set *OFFSET_P to the register's offset within

   AREA.  GDBARCH is the architecture of which REGISTER is a member.

   In the worst case, this takes time proportional to the number of

   items stored in AREA.  If you plan to gather a lot of information

   about registers saved in AREA, consider calling pv_area_scan

   instead, and collecting all your information in one pass.  */

int pv_area_find_reg (struct pv_area *area,

                      struct gdbarch *gdbarch,

                      int reg,

                      CORE_ADDR *offset_p);

/* For every part of AREA whose value we know, apply FUNC to CLOSURE,

   the value's address, its size, and the value itself.  */

void pv_area_scan (struct pv_area *area,

                   void (*func) (void *closure,

                                 pv_t addr,

                                 CORE_ADDR size,

                                 pv_t value),

                   void *closure);

#endif /* PROLOGUE_VALUE_H */