330 lines
12 KiB
C++
330 lines
12 KiB
C++
/* Interface to prologue value handling for GDB.
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Copyright (C) 2003-2022 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef PROLOGUE_VALUE_H
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#define PROLOGUE_VALUE_H
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/* What sort of value is this? This determines the interpretation
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of subsequent fields. */
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enum prologue_value_kind
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{
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/* We don't know anything about the value. This is also used for
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values we could have kept track of, when doing so would have
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been too complex and we don't want to bother. The bottom of
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our lattice. */
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pvk_unknown,
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/* A known constant. K is its value. */
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pvk_constant,
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/* The value that register REG originally had *UPON ENTRY TO THE
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FUNCTION*, plus K. If K is zero, this means, obviously, just
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the value REG had upon entry to the function. REG is a GDB
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register number. Before we start interpreting, we initialize
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every register R to { pvk_register, R, 0 }. */
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pvk_register,
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};
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/* When we analyze a prologue, we're really doing 'abstract
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interpretation' or 'pseudo-evaluation': running the function's code
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in simulation, but using conservative approximations of the values
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it would have when it actually runs. For example, if our function
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starts with the instruction:
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addi r1, 42 # add 42 to r1
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we don't know exactly what value will be in r1 after executing this
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instruction, but we do know it'll be 42 greater than its original
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value.
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If we then see an instruction like:
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addi r1, 22 # add 22 to r1
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we still don't know what r1's value is, but again, we can say it is
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now 64 greater than its original value.
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If the next instruction were:
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mov r2, r1 # set r2 to r1's value
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then we can say that r2's value is now the original value of r1
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plus 64.
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It's common for prologues to save registers on the stack, so we'll
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need to track the values of stack frame slots, as well as the
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registers. So after an instruction like this:
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mov (fp+4), r2
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then we'd know that the stack slot four bytes above the frame
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pointer holds the original value of r1 plus 64.
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And so on.
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Of course, this can only go so far before it gets unreasonable. If
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we wanted to be able to say anything about the value of r1 after
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the instruction:
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xor r1, r3 # exclusive-or r1 and r3, place result in r1
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then things would get pretty complex. But remember, we're just
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doing a conservative approximation; if exclusive-or instructions
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aren't relevant to prologues, we can just say r1's value is now
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'unknown'. We can ignore things that are too complex, if that loss
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of information is acceptable for our application.
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So when I say "conservative approximation" here, what I mean is an
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approximation that is either accurate, or marked "unknown", but
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never inaccurate.
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Once you've reached the current PC, or an instruction that you
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don't know how to simulate, you stop. Now you can examine the
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state of the registers and stack slots you've kept track of.
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- To see how large your stack frame is, just check the value of the
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stack pointer register; if it's the original value of the SP
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minus a constant, then that constant is the stack frame's size.
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If the SP's value has been marked as 'unknown', then that means
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the prologue has done something too complex for us to track, and
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we don't know the frame size.
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- To see where we've saved the previous frame's registers, we just
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search the values we've tracked --- stack slots, usually, but
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registers, too, if you want --- for something equal to the
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register's original value. If the ABI suggests a standard place
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to save a given register, then we can check there first, but
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really, anything that will get us back the original value will
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probably work.
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Sure, this takes some work. But prologue analyzers aren't
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quick-and-simple pattern patching to recognize a few fixed prologue
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forms any more; they're big, hairy functions. Along with inferior
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function calls, prologue analysis accounts for a substantial
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portion of the time needed to stabilize a GDB port. So I think
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it's worthwhile to look for an approach that will be easier to
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understand and maintain. In the approach used here:
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- It's easier to see that the analyzer is correct: you just see
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whether the analyzer properly (albeit conservatively) simulates
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the effect of each instruction.
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- It's easier to extend the analyzer: you can add support for new
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instructions, and know that you haven't broken anything that
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wasn't already broken before.
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- It's orthogonal: to gather new information, you don't need to
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complicate the code for each instruction. As long as your domain
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of conservative values is already detailed enough to tell you
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what you need, then all the existing instruction simulations are
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already gathering the right data for you.
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A 'struct prologue_value' is a conservative approximation of the
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real value the register or stack slot will have. */
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struct prologue_value {
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/* What sort of value is this? This determines the interpretation
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of subsequent fields. */
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enum prologue_value_kind kind;
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/* The meanings of the following fields depend on 'kind'; see the
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comments for the specific 'kind' values. */
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int reg;
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CORE_ADDR k;
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};
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typedef struct prologue_value pv_t;
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/* Return the unknown prologue value --- { pvk_unknown, ?, ? }. */
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pv_t pv_unknown (void);
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/* Return the prologue value representing the constant K. */
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pv_t pv_constant (CORE_ADDR k);
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/* Return the prologue value representing the original value of
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register REG, plus the constant K. */
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pv_t pv_register (int reg, CORE_ADDR k);
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/* Return conservative approximations of the results of the following
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operations. */
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pv_t pv_add (pv_t a, pv_t b); /* a + b */
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pv_t pv_add_constant (pv_t v, CORE_ADDR k); /* a + k */
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pv_t pv_subtract (pv_t a, pv_t b); /* a - b */
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pv_t pv_logical_and (pv_t a, pv_t b); /* a & b */
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/* Return non-zero iff A and B are identical expressions.
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This is not the same as asking if the two values are equal; the
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result of such a comparison would have to be a pv_boolean, and
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asking whether two 'unknown' values were equal would give you
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pv_maybe. Same for comparing, say, { pvk_register, R1, 0 } and {
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pvk_register, R2, 0}.
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Instead, this function asks whether the two representations are the
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same. */
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int pv_is_identical (pv_t a, pv_t b);
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/* Return non-zero if A is known to be a constant. */
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int pv_is_constant (pv_t a);
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/* Return non-zero if A is the original value of register number R
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plus some constant, zero otherwise. */
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int pv_is_register (pv_t a, int r);
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/* Return non-zero if A is the original value of register R plus the
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constant K. */
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int pv_is_register_k (pv_t a, int r, CORE_ADDR k);
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/* A conservative boolean type, including "maybe", when we can't
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figure out whether something is true or not. */
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enum pv_boolean {
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pv_maybe,
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pv_definite_yes,
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pv_definite_no,
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};
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/* Decide whether a reference to SIZE bytes at ADDR refers exactly to
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an element of an array. The array starts at ARRAY_ADDR, and has
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ARRAY_LEN values of ELT_SIZE bytes each. If ADDR definitely does
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refer to an array element, set *I to the index of the referenced
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element in the array, and return pv_definite_yes. If it definitely
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doesn't, return pv_definite_no. If we can't tell, return pv_maybe.
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If the reference does touch the array, but doesn't fall exactly on
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an element boundary, or doesn't refer to the whole element, return
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pv_maybe. */
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enum pv_boolean pv_is_array_ref (pv_t addr, CORE_ADDR size,
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pv_t array_addr, CORE_ADDR array_len,
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CORE_ADDR elt_size,
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int *i);
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/* A 'pv_area' keeps track of values stored in a particular region of
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memory. */
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class pv_area
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{
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public:
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/* Create a new area, tracking stores relative to the original value
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of BASE_REG. If BASE_REG is SP, then this effectively records the
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contents of the stack frame: the original value of the SP is the
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frame's CFA, or some constant offset from it.
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Stores to constant addresses, unknown addresses, or to addresses
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relative to registers other than BASE_REG will trash this area; see
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pv_area::store_would_trash.
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To check whether a pointer refers to this area, only the low
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ADDR_BIT bits will be compared. */
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pv_area (int base_reg, int addr_bit);
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~pv_area ();
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DISABLE_COPY_AND_ASSIGN (pv_area);
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/* Store the SIZE-byte value VALUE at ADDR in AREA.
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If ADDR is not relative to the same base register we used in
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creating AREA, then we can't tell which values here the stored
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value might overlap, and we'll have to mark everything as
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unknown. */
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void store (pv_t addr,
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CORE_ADDR size,
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pv_t value);
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/* Return the SIZE-byte value at ADDR in AREA. This may return
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pv_unknown (). */
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pv_t fetch (pv_t addr, CORE_ADDR size);
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/* Return true if storing to address ADDR in AREA would force us to
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mark the contents of the entire area as unknown. This could happen
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if, say, ADDR is unknown, since we could be storing anywhere. Or,
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it could happen if ADDR is relative to a different register than
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the other stores base register, since we don't know the relative
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values of the two registers.
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If you've reached such a store, it may be better to simply stop the
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prologue analysis, and return the information you've gathered,
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instead of losing all that information, most of which is probably
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okay. */
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bool store_would_trash (pv_t addr);
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/* Search AREA for the original value of REGISTER. If we can't find
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it, return zero; if we can find it, return a non-zero value, and if
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OFFSET_P is non-zero, set *OFFSET_P to the register's offset within
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AREA. GDBARCH is the architecture of which REGISTER is a member.
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In the worst case, this takes time proportional to the number of
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items stored in AREA. If you plan to gather a lot of information
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about registers saved in AREA, consider calling pv_area::scan
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instead, and collecting all your information in one pass. */
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bool find_reg (struct gdbarch *gdbarch, int reg, CORE_ADDR *offset_p);
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/* For every part of AREA whose value we know, apply FUNC to CLOSURE,
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the value's address, its size, and the value itself. */
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void scan (void (*func) (void *closure,
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pv_t addr,
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CORE_ADDR size,
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pv_t value),
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void *closure);
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private:
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struct area_entry;
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/* Delete all entries from AREA. */
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void clear_entries ();
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/* Return a pointer to the first entry we hit in AREA starting at
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OFFSET and going forward.
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This may return zero, if AREA has no entries.
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And since the entries are a ring, this may return an entry that
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entirely precedes OFFSET. This is the correct behavior: depending
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on the sizes involved, we could still overlap such an area, with
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wrap-around. */
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struct area_entry *find_entry (CORE_ADDR offset);
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/* Return non-zero if the SIZE bytes at OFFSET would overlap ENTRY;
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return zero otherwise. AREA is the area to which ENTRY belongs. */
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int overlaps (struct area_entry *entry,
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CORE_ADDR offset,
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CORE_ADDR size);
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/* This area's base register. */
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int m_base_reg;
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/* The mask to apply to addresses, to make the wrap-around happen at
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the right place. */
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CORE_ADDR m_addr_mask;
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/* An element of the doubly-linked ring of entries, or zero if we
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have none. */
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struct area_entry *m_entry;
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};
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#endif /* PROLOGUE_VALUE_H */
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