/src/libpcap/optimize.c

Source
/*
 * Copyright (c) 1988, 1989, 1990, 1991, 1993, 1994, 1995, 1996
 *  The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that: (1) source code distributions
 * retain the above copyright notice and this paragraph in its entirety, (2)
 * distributions including binary code include the above copyright notice and
 * this paragraph in its entirety in the documentation or other materials
 * provided with the distribution, and (3) all advertising materials mentioning
 * features or use of this software display the following acknowledgement:
 * ``This product includes software developed by the University of California,
 * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
 * the University nor the names of its contributors may be used to endorse
 * or promote products derived from this software without specific prior
 * written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 *  Optimization module for BPF code intermediate representation.
 */

#include <config.h>

#include <pcap-types.h>

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <setjmp.h>
#include <string.h>
#include <limits.h> /* for SIZE_MAX */
#include <errno.h>
#include <stdbool.h>
#include <stdint.h>

#include "pcap-int.h"

#include "gencode.h"
#include "optimize.h"
#include "diag-control.h"
#include "no_sanitize.h"

#ifdef HAVE_OS_PROTO_H
#include "os-proto.h"
#endif

#ifdef BDEBUG
/*
 * The internal "debug printout" flag for the filter expression optimizer.
 * The code to print that stuff is present only if BDEBUG is defined, so
 * the flag, and the routine to set it, are defined only if BDEBUG is
 * defined.
 */
static int pcap_optimizer_debug;

/*
 * Routine to set that flag.
 *
 * This is intended for libpcap developers, not for general use.
 * If you want to set these in a program, you'll have to declare this
 * routine yourself, with the appropriate DLL import attribute on Windows;
 * it's not declared in any header file, and won't be declared in any
 * header file provided by libpcap.
 */
PCAP_API void pcap_set_optimizer_debug(int value);

PCAP_API_DEF void
pcap_set_optimizer_debug(int value)
{
  pcap_optimizer_debug = value;
}

/*
 * The internal "print dot graph" flag for the filter expression optimizer.
 * The code to print that stuff is present only if BDEBUG is defined, so
 * the flag, and the routine to set it, are defined only if BDEBUG is
 * defined.
 */
static int pcap_print_dot_graph;

/*
 * Routine to set that flag.
 *
 * This is intended for libpcap developers, not for general use.
 * If you want to set these in a program, you'll have to declare this
 * routine yourself, with the appropriate DLL import attribute on Windows;
 * it's not declared in any header file, and won't be declared in any
 * header file provided by libpcap.
 */
PCAP_API void pcap_set_print_dot_graph(int value);

PCAP_API_DEF void
pcap_set_print_dot_graph(int value)
{
  pcap_print_dot_graph = value;
}

#endif

/*
 * lowest_set_bit().
 *
 * Takes a 32-bit integer as an argument.
 *
 * If handed a non-zero value, returns the index of the lowest set bit,
 * counting upwards from zero.
 *
 * If handed zero, the results are platform- and compiler-dependent.
 * Keep it out of the light, don't give it any water, don't feed it
 * after midnight, and don't pass zero to it.
 *
 * This is the same as the count of trailing zeroes in the word.
 *
 * Because lowest_set_bit() is intended to be used as a function, to define
 * HAVE_BUILTIN_CTZ it is sufficient to verify that __builtin_ctz() can return
 * a value (the builtin does not have to evaluate to a compile-time constant).
 */
#ifdef HAVE_BUILTIN_CTZ
  #define lowest_set_bit(mask) ((u_int)__builtin_ctz(mask))
#elif defined(_MSC_VER)
  /*
   * Visual Studio; we support only 2015 and later, so use
   * _BitScanForward().
   */
#include <intrin.h>

#ifndef __clang__
#pragma intrinsic(_BitScanForward)
#endif

static __forceinline u_int
lowest_set_bit(int mask)
{
  unsigned long bit;

  /*
   * Don't sign-extend mask if long is longer than int.
   * (It's currently not, in MSVC, even on 64-bit platforms, but....)
   */
  if (_BitScanForward(&bit, (unsigned int)mask) == 0)
    abort();  /* mask is zero */
  return (u_int)bit;
}
#else
  /*
   * POSIX.1-2001 says ffs() is in <strings.h>.  Every supported non-Windows OS
   * (including Linux with musl libc and uclibc-ng) has the header and (except
   * HP-UX) declares the function there.  HP-UX declares the function in
   * <string.h>, which has already been included.
   */
  #include <strings.h>
  #define lowest_set_bit(mask)  ((u_int)(ffs((mask)) - 1))
#endif

/*
 * This is an extern shim to be invoked from translatetest.c because
 * lowest_set_bit() can be an inline function (which can be unnecessarily
 * complicated to declare extern) or a macro.
 */
uint32_t
pcapint_lowest_set_bit(const uint32_t x)
{
  return lowest_set_bit(x);
}

/*
 * Represents a deleted instruction.
 */
#define NOP -1

/*
 * Register numbers for use-def values.
 * 0 through BPF_MEMWORDS-1 represent the corresponding scratch memory
 * location.  A_ATOM is the accumulator and X_ATOM is the index
 * register.
 */
#define A_ATOM BPF_MEMWORDS
#define X_ATOM (BPF_MEMWORDS+1)

/*
 * This define is used to represent *both* the accumulator and
 * x register in use-def computations.
 * Currently, the use-def code assumes only one definition per instruction.
 */
#define AX_ATOM N_ATOMS

/*
 * These data structures are used in a Cocke and Schwartz style
 * value numbering scheme.  Since the flowgraph is acyclic,
 * exit values can be propagated from a node's predecessors
 * provided it is uniquely defined.
 */
struct valnode {
  int code;
  bpf_u_int32 v0, v1;
  int val;    /* the value number */
  struct valnode *next;
};

/* Integer constants mapped with the load immediate opcode. */
#define K(i) F(opt_state, BPF_LD|BPF_IMM|BPF_W, i, 0U)

struct vmapinfo {
  int is_const;
  bpf_u_int32 const_val;
};

typedef struct {
  /*
   * Place to longjmp to on an error.
   */
  jmp_buf top_ctx;

  /*
   * The buffer into which to put error message.
   */
  char *errbuf;

  /*
   * A flag to indicate that further optimization is needed.
   * Iterative passes are continued until a given pass yields no
   * code simplification or branch movement.
   */
  int done;

  /*
   * XXX - detect loops that do nothing but repeated AND/OR pullups
   * and edge moves.
   * If 100 passes in a row do nothing but that, treat that as a
   * sign that we're in a loop that just shuffles in a cycle in
   * which each pass just shuffles the code and we eventually
   * get back to the original configuration.
   *
   * XXX - we need a non-heuristic way of detecting, or preventing,
   * such a cycle.
   */
  int non_branch_movement_performed;

  u_int n_blocks;   /* number of blocks in the CFG; guaranteed to be > 0, as it's a RET instruction at a minimum */
  struct block **blocks;
  u_int n_edges;    /* twice n_blocks, so guaranteed to be > 0 */
  struct edge **edges;

  /*
   * A bit vector set representation of the dominators.
   * We round up the set size to the next power of two.
   */
  u_int nodewords;  /* number of 32-bit words for a bit vector of "number of nodes" bits; guaranteed to be > 0 */
  u_int edgewords;  /* number of 32-bit words for a bit vector of "number of edges" bits; guaranteed to be > 0 */
  struct block **levels;
  bpf_u_int32 *space;

#define BITS_PER_WORD (8*sizeof(bpf_u_int32))
/*
 * True if a is in uset {p}
 */
#define SET_MEMBER(p, a) \
((p)[(unsigned)(a) / BITS_PER_WORD] & ((bpf_u_int32)1 << ((unsigned)(a) % BITS_PER_WORD)))

/*
 * Add 'a' to uset p.
 */
#define SET_INSERT(p, a) \
(p)[(unsigned)(a) / BITS_PER_WORD] |= ((bpf_u_int32)1 << ((unsigned)(a) % BITS_PER_WORD))

/*
 * Delete 'a' from uset p.
 */
#define SET_DELETE(p, a) \
(p)[(unsigned)(a) / BITS_PER_WORD] &= ~((bpf_u_int32)1 << ((unsigned)(a) % BITS_PER_WORD))

/*
 * a := a intersect b
 * n must be guaranteed to be > 0
 */
#define SET_INTERSECT(a, b, n)\
{\
  bpf_u_int32 *_x = a, *_y = b;\
  u_int _n = n;\
  do *_x++ &= *_y++; while (--_n != 0);\
}

/*
 * a := a - b
 * n must be guaranteed to be > 0
 */
#define SET_SUBTRACT(a, b, n)\
{\
  bpf_u_int32 *_x = a, *_y = b;\
  u_int _n = n;\
  do *_x++ &=~ *_y++; while (--_n != 0);\
}

/*
 * a := a union b
 * n must be guaranteed to be > 0
 */
#define SET_UNION(a, b, n)\
{\
  bpf_u_int32 *_x = a, *_y = b;\
  u_int _n = n;\
  do *_x++ |= *_y++; while (--_n != 0);\
}

  uset all_dom_sets;
  uset all_closure_sets;
  uset all_edge_sets;

#define MODULUS 213
  struct valnode *hashtbl[MODULUS];
  bpf_u_int32 curval;
  bpf_u_int32 maxval;

  struct vmapinfo *vmap;
  struct valnode *vnode_base;
  struct valnode *next_vnode;
} opt_state_t;

typedef struct {
  /*
   * Place to longjmp to on an error.
   */
  jmp_buf top_ctx;

  /*
   * The buffer into which to put error message.
   */
  char *errbuf;

  /*
   * Some pointers used to convert the basic block form of the code,
   * into the array form that BPF requires.  'fstart' will point to
   * the allocated array while 'ftail' is used during the recursive
   * traversal.
   */
  struct bpf_insn *fstart;
  struct bpf_insn *ftail;
} conv_state_t;

static void opt_init(opt_state_t *, struct icode *);
static void opt_cleanup(opt_state_t *);
static void PCAP_NORETURN opt_error(opt_state_t *, const char *, ...)
    PCAP_PRINTFLIKE(2, 3);
static void PCAP_NORETURN conv_error(conv_state_t *, const char *, ...)
    PCAP_PRINTFLIKE(2, 3);

static void intern_blocks(opt_state_t *, struct icode *);

static void find_inedges(opt_state_t *, const struct block *);
#ifdef BDEBUG
static void opt_dump(opt_state_t *, struct icode *);
#endif

static void
find_levels_r(opt_state_t *opt_state, struct icode *ic, struct block *b)
{
  int level;

  if (isMarked(ic, b))
    return;

  Mark(ic, b);
  b->link = 0;

  if (JT(b)) {
    find_levels_r(opt_state, ic, JT(b));
    find_levels_r(opt_state, ic, JF(b));
    level = max(JT(b)->level, JF(b)->level) + 1;
  } else
    level = 0;
  b->level = level;
  b->link = opt_state->levels[level];
  opt_state->levels[level] = b;
}

/*
 * Level graph.  The levels go from 0 at the leaves to
 * N_LEVELS at the root.  The opt_state->levels[] array points to the
 * first node of the level list, whose elements are linked
 * with the 'link' field of the struct block.
 */
static void
find_levels(opt_state_t *opt_state, struct icode *ic)
{
  memset((char *)opt_state->levels, 0, opt_state->n_blocks * sizeof(*opt_state->levels));
  unMarkAll(ic);
  find_levels_r(opt_state, ic, ic->root);
}

/*
 * Find dominator relationships.
 * Assumes graph has been leveled.
 */
static void
find_dom(opt_state_t *opt_state, struct block *root)
{
  u_int i;
  int level;
  struct block *b;
  bpf_u_int32 *x;

  /*
   * Initialize sets to contain all nodes.
   */
  x = opt_state->all_dom_sets;
  /*
   * In opt_init(), we've made sure the product doesn't overflow.
   */
  i = opt_state->n_blocks * opt_state->nodewords;
  while (i != 0) {
    --i;
    *x++ = 0xFFFFFFFFU;
  }
  /* Root starts off empty. */
  for (i = opt_state->nodewords; i != 0;) {
    --i;
    root->dom[i] = 0;
  }

  /* root->level is the highest level no found. */
  for (level = root->level; level >= 0; --level) {
    for (b = opt_state->levels[level]; b; b = b->link) {
      SET_INSERT(b->dom, b->id);
      if (JT(b) == NULL)
        continue;
      SET_INTERSECT(JT(b)->dom, b->dom, opt_state->nodewords);
      SET_INTERSECT(JF(b)->dom, b->dom, opt_state->nodewords);
    }
  }
}

static void
propedom(const opt_state_t *opt_state, struct edge *ep)
{
  SET_INSERT(ep->edom, ep->id);
  if (ep->succ) {
    SET_INTERSECT(ep->succ->et.edom, ep->edom, opt_state->edgewords);
    SET_INTERSECT(ep->succ->ef.edom, ep->edom, opt_state->edgewords);
  }
}

/*
 * Compute edge dominators.
 * Assumes graph has been leveled and predecessors established.
 */
static void
find_edom(opt_state_t *opt_state, struct block *root)
{
  u_int i;
  uset x;
  int level;
  struct block *b;

  x = opt_state->all_edge_sets;
  /*
   * In opt_init(), we've made sure the product doesn't overflow.
   */
  for (i = opt_state->n_edges * opt_state->edgewords; i != 0; ) {
    --i;
    x[i] = 0xFFFFFFFFU;
  }

  /* root->level is the highest level no found. */
  memset(root->et.edom, 0, opt_state->edgewords * sizeof(*(uset)0));
  memset(root->ef.edom, 0, opt_state->edgewords * sizeof(*(uset)0));
  for (level = root->level; level >= 0; --level) {
    for (b = opt_state->levels[level]; b != 0; b = b->link) {
      propedom(opt_state, &b->et);
      propedom(opt_state, &b->ef);
    }
  }
}

/*
 * Find the backwards transitive closure of the flow graph.  These sets
 * are backwards in the sense that we find the set of nodes that reach
 * a given node, not the set of nodes that can be reached by a node.
 *
 * Assumes graph has been leveled.
 */
static void
find_closure(opt_state_t *opt_state, const struct block *root)
{
  int level;
  struct block *b;

  /*
   * Initialize sets to contain no nodes.
   */
  memset((char *)opt_state->all_closure_sets, 0,
        opt_state->n_blocks * opt_state->nodewords * sizeof(*opt_state->all_closure_sets));

  /* root->level is the highest level no found. */
  for (level = root->level; level >= 0; --level) {
    for (b = opt_state->levels[level]; b; b = b->link) {
      SET_INSERT(b->closure, b->id);
      if (JT(b) == NULL)
        continue;
      SET_UNION(JT(b)->closure, b->closure, opt_state->nodewords);
      SET_UNION(JF(b)->closure, b->closure, opt_state->nodewords);
    }
  }
}

/*
 * Return the register number that is used by s.
 *
 * Returns ATOM_A if A is used, ATOM_X if X is used, AX_ATOM if both A and X
 * are used, the scratch memory location's number if a scratch memory
 * location is used (e.g., 0 for M[0]), or -1 if none of those are used.
 *
 * The implementation should probably change to an array access.
 */
static int
atomuse(const struct stmt *s)
{
  int c = s->code;

  if (c == NOP)
    return -1;

  switch (BPF_CLASS(c)) {

  case BPF_RET:
    return BPF_RVAL(c) == BPF_A ? A_ATOM : -1;

  case BPF_LD:
  case BPF_LDX:
    /*
     * As there are fewer than 2^31 memory locations,
     * s->k should be convertible to int without problems.
     */
    return (BPF_MODE(c) == BPF_IND) ? X_ATOM :
      (BPF_MODE(c) == BPF_MEM) ? (int)s->k : -1;

  case BPF_ST:
    return A_ATOM;

  case BPF_STX:
    return X_ATOM;

  case BPF_JMP:
  case BPF_ALU:
    if (BPF_SRC(c) == BPF_X)
      return AX_ATOM;
    return A_ATOM;

  case BPF_MISC:
    return BPF_MISCOP(c) == BPF_TXA ? X_ATOM : A_ATOM;
  }

  /*
   * BPF_CLASS() evaluates to [0x00, 0x07], each of which is a case
   * above, hence this function always returns from within the switch
   * statement.  At least one compiler (TinyCC) fails to see this and
   * without the mock last-resort return statement below would generate
   * a "function might return no value" warning.
   */
  /* NOTREACHED */
  return -1;
}

/*
 * Return the register number that is defined by 's'.  We assume that
 * a single stmt cannot define more than one register.  If no register
 * is defined, return -1.
 *
 * The implementation should probably change to an array access.
 */
static int
atomdef(struct stmt *s)
{
  if (s->code == NOP)
    return -1;

  switch (BPF_CLASS(s->code)) {

  case BPF_LD:
  case BPF_ALU:
    return A_ATOM;

  case BPF_LDX:
    return X_ATOM;

  case BPF_ST:
  case BPF_STX:
    return s->k;

  case BPF_MISC:
    return BPF_MISCOP(s->code) == BPF_TAX ? X_ATOM : A_ATOM;
  }
  return -1;
}

/*
 * Compute the sets of registers used, defined, and killed by 'b'.
 *
 * "Used" means that a statement in 'b' uses the register before any
 * statement in 'b' defines it, i.e. it uses the value left in
 * that register by a predecessor block of this block.
 * "Defined" means that a statement in 'b' defines it.
 * "Killed" means that a statement in 'b' defines it before any
 * statement in 'b' uses it, i.e. it kills the value left in that
 * register by a predecessor block of this block.
 */
static void
compute_local_ud(struct block *b)
{
  struct slist *s;
  atomset def = 0, use = 0, killed = 0;
  int atom;

  for (s = b->stmts; s; s = s->next) {
    if (s->s.code == NOP)
      continue;
    atom = atomuse(&s->s);
    if (atom >= 0) {
      if (atom == AX_ATOM) {
        if (!ATOMELEM(def, X_ATOM))
          use |= ATOMMASK(X_ATOM);
        if (!ATOMELEM(def, A_ATOM))
          use |= ATOMMASK(A_ATOM);
      }
      else if (atom < N_ATOMS) {
        if (!ATOMELEM(def, atom))
          use |= ATOMMASK(atom);
      }
      else
        abort();
    }
    atom = atomdef(&s->s);
    if (atom >= 0) {
      if (!ATOMELEM(use, atom))
        killed |= ATOMMASK(atom);
      def |= ATOMMASK(atom);
    }
  }
  if (BPF_CLASS(b->s.code) == BPF_JMP) {
    /*
     * XXX - what about RET?
     */
    atom = atomuse(&b->s);
    if (atom >= 0) {
      if (atom == AX_ATOM) {
        if (!ATOMELEM(def, X_ATOM))
          use |= ATOMMASK(X_ATOM);
        if (!ATOMELEM(def, A_ATOM))
          use |= ATOMMASK(A_ATOM);
      }
      else if (atom < N_ATOMS) {
        if (!ATOMELEM(def, atom))
          use |= ATOMMASK(atom);
      }
      else
        abort();
    }
  }

  b->def = def;
  b->kill = killed;
  b->in_use = use;
}

/*
 * Assume graph is already leveled.
 */
static void
find_ud(const opt_state_t *opt_state, const struct block *root)
{
  int i, maxlevel;
  struct block *p;

  /*
   * root->level is the highest level no found;
   * count down from there.
   */
  maxlevel = root->level;
  for (i = maxlevel; i >= 0; --i)
    for (p = opt_state->levels[i]; p; p = p->link) {
      compute_local_ud(p);
      p->out_use = 0;
    }

  for (i = 1; i <= maxlevel; ++i) {
    for (p = opt_state->levels[i]; p; p = p->link) {
      p->out_use |= JT(p)->in_use | JF(p)->in_use;
      p->in_use |= p->out_use &~ p->kill;
    }
  }
}
static void
init_val(opt_state_t *opt_state)
{
  opt_state->curval = 0;
  opt_state->next_vnode = opt_state->vnode_base;
  memset((char *)opt_state->vmap, 0, opt_state->maxval * sizeof(*opt_state->vmap));
  memset((char *)opt_state->hashtbl, 0, sizeof opt_state->hashtbl);
}

/*
 * Because we really don't have an IR, this stuff is a little messy.
 *
 * This routine looks in the table of existing value number for a value
 * with generated from an operation with the specified opcode and
 * the specified values.  If it finds it, it returns its value number,
 * otherwise it makes a new entry in the table and returns the
 * value number of that entry.
 */
UNSIGNED_SHIFT_OK static bpf_u_int32
F(opt_state_t *opt_state, int code, bpf_u_int32 v0, bpf_u_int32 v1)
{
  u_int hash;
  bpf_u_int32 val;
  struct valnode *p;

  hash = (u_int)code ^ (v0 << 4) ^ (v1 << 8);
  hash %= MODULUS;

  for (p = opt_state->hashtbl[hash]; p; p = p->next)
    if (p->code == code && p->v0 == v0 && p->v1 == v1)
      return p->val;

  /*
   * Not found.  Allocate a new value, and assign it a new
   * value number.
   *
   * opt_state->curval starts out as 0, which means VAL_UNKNOWN; we
   * increment it before using it as the new value number, which
   * means we never assign VAL_UNKNOWN.
   *
   * XXX - unless we overflow, but we probably won't have 2^32-1
   * values; we treat 32 bits as effectively infinite.
   */
  val = ++opt_state->curval;
  if (BPF_MODE(code) == BPF_IMM &&
      (BPF_CLASS(code) == BPF_LD || BPF_CLASS(code) == BPF_LDX)) {
    opt_state->vmap[val].const_val = v0;
    opt_state->vmap[val].is_const = 1;
  }
  p = opt_state->next_vnode++;
  p->val = val;
  p->code = code;
  p->v0 = v0;
  p->v1 = v1;
  p->next = opt_state->hashtbl[hash];
  opt_state->hashtbl[hash] = p;

  return val;
}

static inline void
vstore(struct stmt *s, bpf_u_int32 *valp, bpf_u_int32 newval, int alter)
{
  if (alter && newval != VAL_UNKNOWN && *valp == newval)
    s->code = NOP;
  else
    *valp = newval;
}

/*
 * Do constant-folding on binary operators.
 * (Unary operators are handled elsewhere.)
 */
static void
fold_op(opt_state_t *opt_state, struct stmt *s, bpf_u_int32 v0, bpf_u_int32 v1)
{
  bpf_u_int32 a, b;

  a = opt_state->vmap[v0].const_val;
  b = opt_state->vmap[v1].const_val;

  switch (BPF_OP(s->code)) {
  case BPF_ADD:
    a += b;
    break;

  case BPF_SUB:
    a -= b;
    break;

  case BPF_MUL:
    a *= b;
    break;

  case BPF_DIV:
    if (b == 0)
      opt_error(opt_state, ERRSTR_DIV_BY_ZERO);
    a /= b;
    break;

  case BPF_MOD:
    if (b == 0)
      opt_error(opt_state, ERRSTR_MOD_BY_ZERO);
    a %= b;
    break;

  case BPF_AND:
    a &= b;
    break;

  case BPF_OR:
    a |= b;
    break;

  case BPF_XOR:
    a ^= b;
    break;

  case BPF_LSH:
    // See the comment in gencode.h.
    a = UINT32_LSH(a, b);
    break;

  case BPF_RSH:
    // See the comment in gencode.h.
    a = UINT32_RSH(a, b);
    break;

  default:
    abort();
  }
  s->k = a;
  s->code = BPF_LD|BPF_W|BPF_IMM;
  opt_state->done = 0;
  /*
   * XXX - optimizer loop detection.
   */
  opt_state->non_branch_movement_performed = 1;
}

static inline struct slist *
this_op(struct slist *s)
{
  while (s != 0 && s->s.code == NOP)
    s = s->next;
  return s;
}

static void
opt_not(struct block *b)
{
  struct block *tmp = JT(b);

  JT(b) = JF(b);
  JF(b) = tmp;
}

static void
opt_peep(opt_state_t *opt_state, struct block *b)
{
  struct slist *s;
  struct slist *next, *last;
  bpf_u_int32 val;

  s = b->stmts;
  if (s == NULL)
    return;

  last = s;
  for (/*empty*/; /*empty*/; s = next) {
    /*
     * Skip over nops.
     */
    s = this_op(s);
    if (s == NULL)
      break;  /* nothing left in the block */

    /*
     * Find the next real instruction after that one
     * (skipping nops).
     */
    next = this_op(s->next);
    if (next == NULL)
      break;  /* no next instruction */
    last = next;

    /*
     * st  M[k] --> st  M[k]
     * ldx M[k]   tax
     */
    if (s->s.code == BPF_ST &&
        next->s.code == (BPF_LDX|BPF_W|BPF_MEM) &&
        s->s.k == next->s.k) {
      opt_state->done = 0;
      next->s.code = BPF_MISC|BPF_TAX;
      /*
       * The value of 'k' is still the scratch memory
       * register index from the "ldx M[k]", so if it is not
       * zero, the replacement "tax" is not identical to a
       * "tax" produced in pcap_parse().  Make it identical
       * to eliminate the need to reason whether it will be
       * equivalent in all possible contexts.
       *
       * Belt and braces: the replacement "tax" very likely
       * will have been optimised away before opt_loop()
       * returns, and even if it gets to eq_slist(), the
       * latter will ignore 'k' if it is irrelevant for the
       * opcode, but let's make bugs less likely elsewhere
       * too.
       */
      next->s.k = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    /*
     * ld  #k --> ldx  #k
     * tax      txa
     */
    if (s->s.code == (BPF_LD|BPF_W|BPF_IMM) &&
        next->s.code == (BPF_MISC|BPF_TAX)) {
      s->s.code = BPF_LDX|BPF_W|BPF_IMM;
      next->s.code = BPF_MISC|BPF_TXA;
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    /*
     * This is an ugly special case, but it happens
     * when you say tcp[k] or udp[k] where k is a constant.
     */
    if (s->s.code == (BPF_LD|BPF_W|BPF_IMM)) {
      struct slist *add, *tax, *ild;

      /*
       * Check that X isn't used on exit from this
       * block (which the optimizer might cause).
       * We know the code generator won't generate
       * any local dependencies.
       */
      if (ATOMELEM(b->out_use, X_ATOM))
        continue;

      /*
       * Check that the instruction following the
       * "ld #k" is an "add x", or it's an
       * "ldxb 4*([k]&0xf)" with an "add x"
       * following it (with 0 or more nops between the
       * "ldxb 4*([k]&0xf)" and "add x").
       */
      if (next->s.code != (BPF_LDX|BPF_MSH|BPF_B))
        add = next;
      else
        add = this_op(next->next);
      if (add == NULL || add->s.code != (BPF_ALU|BPF_ADD|BPF_X))
        continue;

      /*
       * Check that a tax follows that (with 0 or more
       * nops between them).
       */
      tax = this_op(add->next);
      if (tax == NULL || tax->s.code != (BPF_MISC|BPF_TAX))
        continue;

      /*
       * Check that an "ld [x+k]", an "ldh [x+k]" or an
       * "ldb [x+k]" follows that (with 0 or more
       * nops between them).
       */
      ild = this_op(tax->next);
      if (ild == NULL || BPF_CLASS(ild->s.code) != BPF_LD ||
          BPF_MODE(ild->s.code) != BPF_IND)
        continue;
      /*
       * We want to turn this sequence:
       *
       * (004) ld      #0x2   {s}
       * (005) ldxb    4*([14]&0xf) {next}  -- optional
       * (006) add x      {add}
       * (007) tax      {tax}
       * (008) ld      [x+0]    {ild}
       *
       * into this sequence:
       *
       * (004) nop
       * (005) ldxb    4*([14]&0xf)
       * (006) nop
       * (007) nop
       * (008) ld      [x+2]
       *
       * XXX We need to check that X is not
       * subsequently used, because we want to change
       * what'll be in it after this sequence.
       *
       * We know we can eliminate the accumulator
       * modifications earlier in the sequence since
       * it is defined by the last stmt of this sequence
       * (i.e., the last statement of the sequence loads
       * a value into the accumulator, so we can eliminate
       * earlier operations on the accumulator).
       */
      ild->s.k += s->s.k;
      s->s.code = NOP;
      add->s.code = NOP;
      tax->s.code = NOP;
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
  }
  /*
   * If the comparison at the end of a block is an equality
   * comparison against a constant, and nobody uses the value
   * we leave in the A register at the end of a block, and
   * the operation preceding the comparison is an arithmetic
   * operation, we can sometimes optimize it away.
   */
  if (b->s.code == (BPF_JMP|BPF_JEQ|BPF_K) &&
      !ATOMELEM(b->out_use, A_ATOM)) {
    /*
     * We can optimize away certain subtractions of the
     * X register.
     */
    if (last->s.code == (BPF_ALU|BPF_SUB|BPF_X)) {
      val = b->val[X_ATOM];
      if (opt_state->vmap[val].is_const) {
        /*
         * If we have a subtract to do a comparison,
         * and the X register is a known constant,
         * we can merge this value into the
         * comparison:
         *
         * sub x  ->  nop
         * jeq #y jeq #(x+y)
         */
        b->s.k += opt_state->vmap[val].const_val;
        last->s.code = NOP;
        opt_state->done = 0;
        /*
         * XXX - optimizer loop detection.
         */
        opt_state->non_branch_movement_performed = 1;
      } else if (b->s.k == 0) {
        /*
         * If the X register isn't a constant,
         * and the comparison in the test is
         * against 0, we can compare with the
         * X register, instead:
         *
         * sub x  ->  nop
         * jeq #0 jeq x
         */
        last->s.code = NOP;
        b->s.code = BPF_JMP|BPF_JEQ|BPF_X;
        opt_state->done = 0;
        /*
         * XXX - optimizer loop detection.
         */
        opt_state->non_branch_movement_performed = 1;
      }
    }
    /*
     * Likewise, a constant subtract can be simplified:
     *
     * sub #x ->  nop
     * jeq #y ->  jeq #(x+y)
     */
    else if (last->s.code == (BPF_ALU|BPF_SUB|BPF_K)) {
      last->s.code = NOP;
      b->s.k += last->s.k;
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    /*
     * And, similarly, a constant AND can be simplified
     * if we're testing against 0, i.e.:
     *
     * and #k nop
     * jeq #0  -> jset #k
     */
    else if (last->s.code == (BPF_ALU|BPF_AND|BPF_K) &&
        b->s.k == 0) {
      b->s.k = last->s.k;
      b->s.code = BPF_JMP|BPF_K|BPF_JSET;
      last->s.code = NOP;
      opt_state->done = 0;
      opt_not(b);
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
  }
  /*
   * jset #0x0         ->  never
   * jset #0xffffffff  ->  iff A != 0
   */
  if (b->s.code == (BPF_JMP|BPF_K|BPF_JSET)) {
    /*
     * This can be, but not necessarily is a result of the folding
     * into "jset #k" above.
     */
    if (b->s.k == 0)
      JT(b) = JF(b);
    else if (b->s.k == 0xffffffffU) {
      /*
       * For any A: "A has at least one bit set" means the
       * same as "A != 0".  Test the latter condition, which
       * executes slightly faster and is easier to read.
       * This is not meant to be the inverse of the folding,
       * hence do not prepend a [no-op] "and #0xffffffff".
       */
      b->s.code = BPF_JMP|BPF_JEQ|BPF_K;
      b->s.k = 0;
      opt_not(b);
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
  }
  /*
   * If we're comparing against the index register, and the index
   * register is a known constant, we can just compare against that
   * constant.
   */
  val = b->val[X_ATOM];
  if (opt_state->vmap[val].is_const && BPF_SRC(b->s.code) == BPF_X) {
    bpf_u_int32 v = opt_state->vmap[val].const_val;
    // Make "BPF_SRC(b->s.code) == BPF_K" true.
    b->s.code &= ~BPF_X;
    b->s.k = v;
  }
  /*
   * If the accumulator is a known constant, we can compute the
   * comparison result.
   */
  val = b->val[A_ATOM];
  if (opt_state->vmap[val].is_const && BPF_SRC(b->s.code) == BPF_K) {
    bpf_u_int32 v = opt_state->vmap[val].const_val;
    switch (BPF_OP(b->s.code)) {

    case BPF_JEQ:
      v = v == b->s.k;
      break;

    case BPF_JGT:
      v = v > b->s.k;
      break;

    case BPF_JGE:
      v = v >= b->s.k;
      break;

    case BPF_JSET:
      v &= b->s.k;
      break;

    default:
      abort();
    }
    if (JF(b) != JT(b)) {
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    if (v)
      JF(b) = JT(b);
    else
      JT(b) = JF(b);
  }
}

/*
 * Compute the symbolic value of expression of 's', and update
 * anything it defines in the value table 'val'.  If 'alter' is true,
 * do various optimizations.  This code would be cleaner if symbolic
 * evaluation and code transformations weren't folded together.
 */
static void
opt_stmt(opt_state_t *opt_state, struct stmt *s, bpf_u_int32 val[], int alter)
{
  int op;
  bpf_u_int32 v;

  switch (s->code) {

  case BPF_LD|BPF_ABS|BPF_W:
  case BPF_LD|BPF_ABS|BPF_H:
  case BPF_LD|BPF_ABS|BPF_B:
    v = F(opt_state, s->code, s->k, 0L);
    vstore(s, &val[A_ATOM], v, alter);
    break;

  case BPF_LD|BPF_IND|BPF_W:
  case BPF_LD|BPF_IND|BPF_H:
  case BPF_LD|BPF_IND|BPF_B:
    v = val[X_ATOM];
    if (alter && opt_state->vmap[v].is_const) {
      s->code = BPF_LD|BPF_ABS|BPF_SIZE(s->code);
      s->k += opt_state->vmap[v].const_val;
      v = F(opt_state, s->code, s->k, 0L);
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    else
      v = F(opt_state, s->code, s->k, v);
    vstore(s, &val[A_ATOM], v, alter);
    break;

  case BPF_LD|BPF_W|BPF_LEN:
    v = F(opt_state, s->code, 0L, 0L);
    vstore(s, &val[A_ATOM], v, alter);
    break;

  case BPF_LD|BPF_W|BPF_IMM:
    v = K(s->k);
    vstore(s, &val[A_ATOM], v, alter);
    break;

  case BPF_LDX|BPF_W|BPF_IMM:
    v = K(s->k);
    vstore(s, &val[X_ATOM], v, alter);
    break;

  case BPF_LDX|BPF_MSH|BPF_B:
    v = F(opt_state, s->code, s->k, 0L);
    vstore(s, &val[X_ATOM], v, alter);
    break;

  case BPF_ALU|BPF_NEG:
    if (alter && opt_state->vmap[val[A_ATOM]].is_const) {
      s->code = BPF_LD|BPF_W|BPF_IMM;
      // See the comment in gencode.h.
      s->k = UINT_NEG(opt_state->vmap[val[A_ATOM]].const_val);
      val[A_ATOM] = K(s->k);
    }
    else
      val[A_ATOM] = F(opt_state, s->code, val[A_ATOM], 0L);
    break;

  case BPF_ALU|BPF_ADD|BPF_K:
  case BPF_ALU|BPF_SUB|BPF_K:
  case BPF_ALU|BPF_MUL|BPF_K:
  case BPF_ALU|BPF_DIV|BPF_K:
  case BPF_ALU|BPF_MOD|BPF_K:
  case BPF_ALU|BPF_AND|BPF_K:
  case BPF_ALU|BPF_OR|BPF_K:
  case BPF_ALU|BPF_XOR|BPF_K:
  case BPF_ALU|BPF_LSH|BPF_K:
  case BPF_ALU|BPF_RSH|BPF_K:
    op = BPF_OP(s->code);
    if (alter) {
      if (s->k == 0) {
        /*
         * Optimize operations where the constant
         * is zero.
         *
         * Don't optimize away "sub #0"
         * as it may be needed later to
         * fixup the generated math code.
         *
         * Fail if we're dividing by zero or taking
         * a modulo by zero.
         */
        if (op == BPF_ADD ||
            op == BPF_LSH || op == BPF_RSH ||
            op == BPF_OR || op == BPF_XOR) {
          s->code = NOP;
          break;
        }
        if (op == BPF_MUL || op == BPF_AND) {
          s->code = BPF_LD|BPF_W|BPF_IMM;
          val[A_ATOM] = K(s->k);
          break;
        }
        if (op == BPF_DIV)
          opt_error(opt_state,
              ERRSTR_DIV_BY_ZERO);
        if (op == BPF_MOD)
          opt_error(opt_state,
              ERRSTR_MOD_BY_ZERO);
      }
      if (opt_state->vmap[val[A_ATOM]].is_const) {
        fold_op(opt_state, s, val[A_ATOM], K(s->k));
        val[A_ATOM] = K(s->k);
        break;
      }
    }
    val[A_ATOM] = F(opt_state, s->code, val[A_ATOM], K(s->k));
    break;

  case BPF_ALU|BPF_ADD|BPF_X:
  case BPF_ALU|BPF_SUB|BPF_X:
  case BPF_ALU|BPF_MUL|BPF_X:
  case BPF_ALU|BPF_DIV|BPF_X:
  case BPF_ALU|BPF_MOD|BPF_X:
  case BPF_ALU|BPF_AND|BPF_X:
  case BPF_ALU|BPF_OR|BPF_X:
  case BPF_ALU|BPF_XOR|BPF_X:
  case BPF_ALU|BPF_LSH|BPF_X:
  case BPF_ALU|BPF_RSH|BPF_X:
    op = BPF_OP(s->code);
    if (alter && opt_state->vmap[val[X_ATOM]].is_const) {
      if (opt_state->vmap[val[A_ATOM]].is_const) {
        fold_op(opt_state, s, val[A_ATOM], val[X_ATOM]);
        val[A_ATOM] = K(s->k);
      }
      else {
        s->code = BPF_ALU|BPF_K|op;
        s->k = opt_state->vmap[val[X_ATOM]].const_val;
        if ((op == BPF_LSH || op == BPF_RSH) &&
            s->k > 31)
          opt_error(opt_state,
              ERRSTR_SHIFT_BY_MORE);
        opt_state->done = 0;
        val[A_ATOM] =
          F(opt_state, s->code, val[A_ATOM], K(s->k));
        /*
         * XXX - optimizer loop detection.
         */
        opt_state->non_branch_movement_performed = 1;
      }
      break;
    }
    /*
     * Check if we're doing something to an accumulator
     * that is 0, and simplify.  This may not seem like
     * much of a simplification but it could open up further
     * optimizations.
     * XXX We could also check for mul by 1, etc.
     */
    if (alter && opt_state->vmap[val[A_ATOM]].is_const
        && opt_state->vmap[val[A_ATOM]].const_val == 0) {
      if (op == BPF_ADD || op == BPF_OR || op == BPF_XOR) {
        s->code = BPF_MISC|BPF_TXA;
        vstore(s, &val[A_ATOM], val[X_ATOM], alter);
        break;
      }
      else if (op == BPF_MUL || op == BPF_DIV || op == BPF_MOD ||
         op == BPF_AND || op == BPF_LSH || op == BPF_RSH) {
        s->code = BPF_LD|BPF_W|BPF_IMM;
        s->k = 0;
        vstore(s, &val[A_ATOM], K(s->k), alter);
        break;
      }
      else if (op == BPF_NEG) {
        s->code = NOP;
        break;
      }
    }
    val[A_ATOM] = F(opt_state, s->code, val[A_ATOM], val[X_ATOM]);
    break;

  case BPF_MISC|BPF_TXA:
    vstore(s, &val[A_ATOM], val[X_ATOM], alter);
    break;

  case BPF_LD|BPF_W|BPF_MEM:
    v = val[s->k];
    if (alter && opt_state->vmap[v].is_const) {
      s->code = BPF_LD|BPF_W|BPF_IMM;
      s->k = opt_state->vmap[v].const_val;
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    vstore(s, &val[A_ATOM], v, alter);
    break;

  case BPF_MISC|BPF_TAX:
    vstore(s, &val[X_ATOM], val[A_ATOM], alter);
    break;

  case BPF_LDX|BPF_W|BPF_MEM:
    v = val[s->k];
    if (alter && opt_state->vmap[v].is_const) {
      s->code = BPF_LDX|BPF_W|BPF_IMM;
      s->k = opt_state->vmap[v].const_val;
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    vstore(s, &val[X_ATOM], v, alter);
    break;

  case BPF_ST:
    vstore(s, &val[s->k], val[A_ATOM], alter);
    break;

  case BPF_STX:
    vstore(s, &val[s->k], val[X_ATOM], alter);
    break;
  }
}

static void
deadstmt(opt_state_t *opt_state, struct stmt *s, struct stmt *last[])
{
  int atom;

  atom = atomuse(s);
  if (atom >= 0) {
    if (atom == AX_ATOM) {
      last[X_ATOM] = 0;
      last[A_ATOM] = 0;
    }
    else
      last[atom] = 0;
  }
  atom = atomdef(s);
  if (atom >= 0) {
    if (last[atom]) {
      opt_state->done = 0;
      last[atom]->code = NOP;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
    last[atom] = s;
  }
}

static void
opt_deadstores(opt_state_t *opt_state, struct block *b)
{
  struct slist *s;
  int atom;
  struct stmt *last[N_ATOMS];

  memset((char *)last, 0, sizeof last);

  for (s = b->stmts; s != 0; s = s->next)
    deadstmt(opt_state, &s->s, last);
  deadstmt(opt_state, &b->s, last);

  for (atom = 0; atom < N_ATOMS; ++atom)
    if (last[atom] && !ATOMELEM(b->out_use, atom)) {
      last[atom]->code = NOP;
      /*
       * The store was removed as it's dead,
       * so the value stored into now has
       * an unknown value.
       */
      vstore(0, &b->val[atom], VAL_UNKNOWN, 0);
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
}

static void
opt_blk(opt_state_t *opt_state, struct block *b, int do_stmts)
{
  struct slist *s;
  struct edge *p;
  int i;
  bpf_u_int32 aval, xval;

#if 0
  for (s = b->stmts; s && s->next; s = s->next)
    if (BPF_CLASS(s->s.code) == BPF_JMP) {
      do_stmts = 0;
      break;
    }
#endif

  /*
   * Initialize the atom values.
   */
  p = b->in_edges;
  if (p == NULL) {
    /*
     * We have no predecessors, so everything is undefined
     * upon entry to this block.
     */
    memset((char *)b->val, 0, sizeof(b->val));
  } else {
    /*
     * Inherit values from our predecessors.
     *
     * First, get the values from the predecessor along the
     * first edge leading to this node.
     */
    memcpy((char *)b->val, (char *)p->pred->val, sizeof(b->val));
    /*
     * Now look at all the other nodes leading to this node.
     * If, for the predecessor along that edge, a register
     * has a different value from the one we have (i.e.,
     * control paths are merging, and the merging paths
     * assign different values to that register), give the
     * register the undefined value of 0.
     */
    while ((p = p->next) != NULL) {
      for (i = 0; i < N_ATOMS; ++i)
        if (b->val[i] != p->pred->val[i])
          b->val[i] = 0;
    }
  }
  aval = b->val[A_ATOM];
  xval = b->val[X_ATOM];
  for (s = b->stmts; s; s = s->next)
    opt_stmt(opt_state, &s->s, b->val, do_stmts);

  /*
   * This is a special case: if we don't use anything from this
   * block, and we load the accumulator or index register with a
   * value that is already there, or if this block is a return,
   * eliminate all the statements.
   *
   * XXX - what if it does a store?  Presumably that falls under
   * the heading of "if we don't use anything from this block",
   * i.e., if we use any memory location set to a different
   * value by this block, then we use something from this block.
   *
   * XXX - why does it matter whether we use anything from this
   * block?  If the accumulator or index register doesn't change
   * its value, isn't that OK even if we use that value?
   *
   * XXX - if we load the accumulator with a different value,
   * and the block ends with a conditional branch, we obviously
   * can't eliminate it, as the branch depends on that value.
   * For the index register, the conditional branch only depends
   * on the index register value if the test is against the index
   * register value rather than a constant; if nothing uses the
   * value we put into the index register, and we're not testing
   * against the index register's value, and there aren't any
   * other problems that would keep us from eliminating this
   * block, can we eliminate it?
   */
  if (do_stmts &&
      ((b->out_use == 0 &&
        aval != VAL_UNKNOWN && b->val[A_ATOM] == aval &&
        xval != VAL_UNKNOWN && b->val[X_ATOM] == xval) ||
       BPF_CLASS(b->s.code) == BPF_RET)) {
    if (b->stmts != 0) {
      b->stmts = 0;
      opt_state->done = 0;
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
  } else {
    opt_peep(opt_state, b);
    opt_deadstores(opt_state, b);
  }
  /*
   * Set up values for branch optimizer.
   */
  if (BPF_SRC(b->s.code) == BPF_K)
    b->oval = K(b->s.k);
  else
    b->oval = b->val[X_ATOM];
  b->et.code = b->s.code;
  b->ef.code = -b->s.code;
}

/*
 * Return true if any register that is used on exit from 'succ', has
 * an exit value that is different from the corresponding exit value
 * from 'b'.
 */
static int
use_conflict(const struct block *b, const struct block *succ)
{
  int atom;
  atomset use = succ->out_use;

  if (use == 0)
    return 0;

  for (atom = 0; atom < N_ATOMS; ++atom)
    if (ATOMELEM(use, atom))
      if (b->val[atom] != succ->val[atom])
        return 1;
  return 0;
}

/*
 * Given a block that is the successor of an edge, and an edge that
 * dominates that edge, return either a pointer to a child of that
 * block (a block to which that block jumps) if that block is a
 * candidate to replace the successor of the latter edge or NULL
 * if neither of the children of the first block are candidates.
 */
static struct block *
fold_edge(struct block *child, struct edge *ep)
{
  int sense;
  bpf_u_int32 aval0, aval1, oval0, oval1;
  int code = ep->code;

  if (code < 0) {
    /*
     * This edge is a "branch if false" edge.
     */
    code = -code;
    sense = 0;
  } else {
    /*
     * This edge is a "branch if true" edge.
     */
    sense = 1;
  }

  /*
   * If the opcode for the branch at the end of the block we
   * were handed isn't the same as the opcode for the branch
   * to which the edge we were handed corresponds, the tests
   * for those branches aren't testing the same conditions,
   * so the blocks to which the first block branches aren't
   * candidates to replace the successor of the edge.
   */
  if (child->s.code != code)
    return 0;

  aval0 = child->val[A_ATOM];
  oval0 = child->oval;
  aval1 = ep->pred->val[A_ATOM];
  oval1 = ep->pred->oval;

  /*
   * If the A register value on exit from the successor block
   * isn't the same as the A register value on exit from the
   * predecessor of the edge, the blocks to which the first
   * block branches aren't candidates to replace the successor
   * of the edge.
   */
  if (aval0 != aval1)
    return 0;

  if (oval0 == oval1)
    /*
     * The operands of the branch instructions are
     * identical, so the branches are testing the
     * same condition, and the result is true if a true
     * branch was taken to get here, otherwise false.
     */
    return sense ? JT(child) : JF(child);

  if (sense && code == (BPF_JMP|BPF_JEQ|BPF_K))
    /*
     * At this point, we only know the comparison if we
     * came down the true branch, and it was an equality
     * comparison with a constant.
     *
     * I.e., if we came down the true branch, and the branch
     * was an equality comparison with a constant, we know the
     * accumulator contains that constant.  If we came down
     * the false branch, or the comparison wasn't with a
     * constant, we don't know what was in the accumulator.
     *
     * We rely on the fact that distinct constants have distinct
     * value numbers.
     */
    return JF(child);

  return 0;
}

/*
 * If we can make this edge go directly to a child of the edge's current
 * successor, do so.
 */
static void
opt_j(opt_state_t *opt_state, struct edge *ep)
{
  u_int i, k;
  struct block *target;

  /*
   * Does this edge go to a block where, if the test
   * at the end of it succeeds, it goes to a block
   * that's a leaf node of the DAG, i.e. a return
   * statement?
   * If so, there's nothing to optimize.
   */
  if (JT(ep->succ) == NULL)
    return;

  /*
   * Does this edge go to a block that goes, in turn, to
   * the same block regardless of whether the test at the
   * end succeeds or fails?
   */
  if (JT(ep->succ) == JF(ep->succ)) {
    /*
     * Common branch targets can be eliminated, provided
     * there is no data dependency.
     *
     * Check whether any register used on exit from the
     * block to which the successor of this edge goes
     * has a value at that point that's different from
     * the value it has on exit from the predecessor of
     * this edge.  If not, the predecessor of this edge
     * can just go to the block to which the successor
     * of this edge goes, bypassing the successor of this
     * edge, as the successor of this edge isn't doing
     * any calculations whose results are different
     * from what the blocks before it did and isn't
     * doing any tests the results of which matter.
     */
    if (!use_conflict(ep->pred, JT(ep->succ))) {
      /*
       * No, there isn't.
       * Make this edge go to the block to
       * which the successor of that edge
       * goes.
       */
      opt_state->done = 0;
      ep->succ = JT(ep->succ);
      /*
       * XXX - optimizer loop detection.
       */
      opt_state->non_branch_movement_performed = 1;
    }
  }
  /*
   * For each edge dominator that matches the successor of this
   * edge, promote the edge successor to the its grandchild.
   *
   * XXX We violate the set abstraction here in favor a reasonably
   * efficient loop.
   */
 top:
  for (i = 0; i < opt_state->edgewords; ++i) {
    /* i'th word in the bitset of dominators */
    bpf_u_int32 x = ep->edom[i];

    while (x != 0) {
      /* Find the next dominator in that word and mark it as found */
      k = lowest_set_bit(x);
      x &=~ ((bpf_u_int32)1 << k);
      k += i * BITS_PER_WORD;

      target = fold_edge(ep->succ, opt_state->edges[k]);
      /*
       * We have a candidate to replace the successor
       * of ep.
       *
       * Check that there is no data dependency between
       * nodes that will be violated if we move the edge;
       * i.e., if any register used on exit from the
       * candidate has a value at that point different
       * from the value it has when we exit the
       * predecessor of that edge, there's a data
       * dependency that will be violated.
       */
      if (target != 0 && !use_conflict(ep->pred, target)) {
        /*
         * It's safe to replace the successor of
         * ep; do so, and note that we've made
         * at least one change.
         *
         * XXX - this is one of the operations that
         * happens when the optimizer gets into
         * one of those infinite loops.
         */
        opt_state->done = 0;
        ep->succ = target;
        if (JT(target) != 0)
          /*
           * Start over unless we hit a leaf.
           */
          goto top;
        return;
      }
    }
  }
}

/*
 * XXX - is this, and and_pullup(), what's described in section 6.1.2
 * "Predicate Assertion Propagation" in the BPF+ paper?
 *
 * Note that this looks at block dominators, not edge dominators.
 * Don't think so.
 *
 * "A or B" compiles into
 *
 *          A
 *       t / \ f
 *        /   B
 *       / t / \ f
 *      \   /
 *       \ /
 *        X
 *
 *
 */
static void
or_pullup(opt_state_t *opt_state, struct block *b, struct block *root)
{
  bpf_u_int32 val;
  int at_top;
  struct block *pull;
  struct block **diffp, **samep;
  struct edge *ep;

  ep = b->in_edges;
  if (ep == NULL)
    return;

  /*
   * Make sure each predecessor loads the same value.
   * XXX why?
   */
  val = ep->pred->val[A_ATOM];
  for (ep = ep->next; ep != 0; ep = ep->next)
    if (val != ep->pred->val[A_ATOM])
      return;

  /*
   * For the first edge in the list of edges coming into this block,
   * see whether the predecessor of that edge comes here via a true
   * branch or a false branch.
   */
  if (JT(b->in_edges->pred) == b)
    diffp = &JT(b->in_edges->pred); /* jt */
  else
    diffp = &JF(b->in_edges->pred);  /* jf */

  /*
   * diffp is a pointer to a pointer to the block.
   *
   * Go down the false chain looking as far as you can,
   * making sure that each jump-compare is doing the
   * same as the original block.
   *
   * If you reach the bottom before you reach a
   * different jump-compare, just exit.  There's nothing
   * to do here.  XXX - no, this version is checking for
   * the value leaving the block; that's from the BPF+
   * pullup routine.
   */
  at_top = 1;
  for (;;) {
    /*
     * Done if that's not going anywhere XXX
     */
    if (*diffp == NULL)
      return;

    /*
     * Done if that predecessor blah blah blah isn't
     * going the same place we're going XXX
     *
     * Does the true edge of this block point to the same
     * location as the true edge of b?
     */
    if (JT(*diffp) != JT(b))
      return;

    /*
     * Done if this node isn't a dominator of that
     * node blah blah blah XXX
     *
     * Does b dominate diffp?
     */
    if (!SET_MEMBER((*diffp)->dom, b->id))
      return;

    /*
     * Break out of the loop if that node's value of A
     * isn't the value of A above XXX
     */
    if ((*diffp)->val[A_ATOM] != val)
      break;

    /*
     * Get the JF for that node XXX
     * Go down the false path.
     */
    diffp = &JF(*diffp);
    at_top = 0;
  }

  /*
   * Now that we've found a different jump-compare in a chain
   * below b, search further down until we find another
   * jump-compare that looks at the original value.  This
   * jump-compare should get pulled up.  XXX again we're
   * comparing values not jump-compares.
   */
  samep = &JF(*diffp);
  for (;;) {
    /*
     * Done if that's not going anywhere XXX
     */
    if (*samep == NULL)
      return;

    /*
     * Done if that predecessor blah blah blah isn't
     * going the same place we're going XXX
     */
    if (JT(*samep) != JT(b))
      return;

    /*
     * Done if this node isn't a dominator of that
     * node blah blah blah XXX
     *
     * Does b dominate samep?
     */
    if (!SET_MEMBER((*samep)->dom, b->id))
      return;

    /*
     * Break out of the loop if that node's value of A
     * is the value of A above XXX
     */
    if ((*samep)->val[A_ATOM] == val)
      break;

    /* XXX Need to check that there are no data dependencies
       between dp0 and dp1.  Currently, the code generator
       will not produce such dependencies. */
    samep = &JF(*samep);
  }
#ifdef notdef
  /* XXX This doesn't cover everything. */
  for (i = 0; i < N_ATOMS; ++i)
    if ((*samep)->val[i] != pred->val[i])
      return;
#endif
  /* Pull up the node. */
  pull = *samep;
  *samep = JF(pull);
  JF(pull) = *diffp;

  /*
   * At the top of the chain, each predecessor needs to point at the
   * pulled up node.  Inside the chain, there is only one predecessor
   * to worry about.
   */
  if (at_top) {
    for (ep = b->in_edges; ep != 0; ep = ep->next) {
      if (JT(ep->pred) == b)
        JT(ep->pred) = pull;
      else
        JF(ep->pred) = pull;
    }
  }
  else
    *diffp = pull;

  /*
   * XXX - this is one of the operations that happens when the
   * optimizer gets into one of those infinite loops.
   */
  opt_state->done = 0;

  /*
   * Recompute dominator sets as control flow graph has changed.
   */
  find_dom(opt_state, root);
}

static void
and_pullup(opt_state_t *opt_state, struct block *b, struct block *root)
{
  bpf_u_int32 val;
  int at_top;
  struct block *pull;
  struct block **diffp, **samep;
  struct edge *ep;

  ep = b->in_edges;
  if (ep == NULL)
    return;

  /*
   * Make sure each predecessor loads the same value.
   */
  val = ep->pred->val[A_ATOM];
  for (ep = ep->next; ep != 0; ep = ep->next)
    if (val != ep->pred->val[A_ATOM])
      return;

  if (JT(b->in_edges->pred) == b)
    diffp = &JT(b->in_edges->pred);
  else
    diffp = &JF(b->in_edges->pred);

  at_top = 1;
  for (;;) {
    if (*diffp == NULL)
      return;

    if (JF(*diffp) != JF(b))
      return;

    if (!SET_MEMBER((*diffp)->dom, b->id))
      return;

    if ((*diffp)->val[A_ATOM] != val)
      break;

    diffp = &JT(*diffp);
    at_top = 0;
  }
  samep = &JT(*diffp);
  for (;;) {
    if (*samep == NULL)
      return;

    if (JF(*samep) != JF(b))
      return;

    if (!SET_MEMBER((*samep)->dom, b->id))
      return;

    if ((*samep)->val[A_ATOM] == val)
      break;

    /* XXX Need to check that there are no data dependencies
       between diffp and samep.  Currently, the code generator
       will not produce such dependencies. */
    samep = &JT(*samep);
  }
#ifdef notdef
  /* XXX This doesn't cover everything. */
  for (i = 0; i < N_ATOMS; ++i)
    if ((*samep)->val[i] != pred->val[i])
      return;
#endif
  /* Pull up the node. */
  pull = *samep;
  *samep = JT(pull);
  JT(pull) = *diffp;

  /*
   * At the top of the chain, each predecessor needs to point at the
   * pulled up node.  Inside the chain, there is only one predecessor
   * to worry about.
   */
  if (at_top) {
    for (ep = b->in_edges; ep != 0; ep = ep->next) {
      if (JT(ep->pred) == b)
        JT(ep->pred) = pull;
      else
        JF(ep->pred) = pull;
    }
  }
  else
    *diffp = pull;

  /*
   * XXX - this is one of the operations that happens when the
   * optimizer gets into one of those infinite loops.
   */
  opt_state->done = 0;

  /*
   * Recompute dominator sets as control flow graph has changed.
   */
  find_dom(opt_state, root);
}

static void
opt_blks(opt_state_t *opt_state, struct icode *ic, int do_stmts)
{
  int i, maxlevel;
  struct block *p;

  init_val(opt_state);
  maxlevel = ic->root->level;

  find_inedges(opt_state, ic->root);
  for (i = maxlevel; i >= 0; --i)
    for (p = opt_state->levels[i]; p; p = p->link)
      opt_blk(opt_state, p, do_stmts);

  if (do_stmts)
    /*
     * No point trying to move branches; it can't possibly
     * make a difference at this point.
     *
     * XXX - this might be after we detect a loop where
     * we were just looping infinitely moving branches
     * in such a fashion that we went through two or more
     * versions of the machine code, eventually returning
     * to the first version.  (We're really not doing a
     * full loop detection, we're just testing for two
     * passes in a row where we do nothing but
     * move branches.)
     */
    return;

  /*
   * Is this what the BPF+ paper describes in sections 6.1.1,
   * 6.1.2, and 6.1.3?
   */
  for (i = 1; i <= maxlevel; ++i) {
    for (p = opt_state->levels[i]; p; p = p->link) {
      opt_j(opt_state, &p->et);
      opt_j(opt_state, &p->ef);
    }
  }

  find_inedges(opt_state, ic->root);
  for (i = 1; i <= maxlevel; ++i) {
    for (p = opt_state->levels[i]; p; p = p->link) {
      or_pullup(opt_state, p, ic->root);
      and_pullup(opt_state, p, ic->root);
    }
  }
}

static inline void
link_inedge(struct edge *parent, struct block *child)
{
  parent->next = child->in_edges;
  child->in_edges = parent;
}

static void
find_inedges(opt_state_t *opt_state, const struct block *root)
{
  u_int i;
  int level;
  struct block *b;

  for (i = 0; i < opt_state->n_blocks; ++i)
    opt_state->blocks[i]->in_edges = 0;

  /*
   * Traverse the graph, adding each edge to the predecessor
   * list of its successors.  Skip the leaves (i.e. level 0).
   */
  for (level = root->level; level > 0; --level) {
    for (b = opt_state->levels[level]; b != 0; b = b->link) {
      link_inedge(&b->et, JT(b));
      link_inedge(&b->ef, JF(b));
    }
  }
}

static void
opt_root(struct block **b)
{
  struct slist *tmp, *s;

  s = (*b)->stmts;
  (*b)->stmts = 0;
  while (BPF_CLASS((*b)->s.code) == BPF_JMP && JT(*b) == JF(*b))
    *b = JT(*b);

  tmp = (*b)->stmts;
  if (tmp != 0)
    sappend(s, tmp);
  (*b)->stmts = s;

  /*
   * If the root node is a return, then there is no
   * point executing any statements (since the bpf machine
   * has no side effects).
   */
  if (BPF_CLASS((*b)->s.code) == BPF_RET)
    (*b)->stmts = 0;
}

static void
opt_loop(opt_state_t *opt_state, struct icode *ic, int do_stmts)
{

#ifdef BDEBUG
  if (pcap_optimizer_debug > 1 || pcap_print_dot_graph) {
    printf("%s(root, %d) begin\n", __func__, do_stmts);
    opt_dump(opt_state, ic);
  }
#endif

  /*
   * XXX - optimizer loop detection.
   */
  int loop_count = 0;
  for (;;) {
    /*
     * XXX - optimizer loop detection.
     */
    opt_state->non_branch_movement_performed = 0;
    opt_state->done = 1;
    find_levels(opt_state, ic);
    find_dom(opt_state, ic->root);
    find_closure(opt_state, ic->root);
    find_ud(opt_state, ic->root);
    find_edom(opt_state, ic->root);
    opt_blks(opt_state, ic, do_stmts);
#ifdef BDEBUG
    if (pcap_optimizer_debug > 1 || pcap_print_dot_graph) {
      printf("%s(root, %d) bottom, done=%d\n", __func__, do_stmts, opt_state->done);
      opt_dump(opt_state, ic);
    }
#endif

    /*
     * Was anything done in this optimizer pass?
     */
    if (opt_state->done) {
      /*
       * No, so we've reached a fixed point.
       * We're done.
       */
      break;
    }

    /*
     * XXX - was anything done other than branch movement
     * in this pass?
     */
    if (opt_state->non_branch_movement_performed) {
      /*
       * Yes.  Clear any loop-detection counter;
       * we're making some form of progress (assuming
       * we can't get into a cycle doing *other*
       * optimizations...).
       */
      loop_count = 0;
    } else {
      /*
       * No - increment the counter, and quit if
       * it's up to 100.
       */
      loop_count++;
      if (loop_count >= 100) {
        /*
         * We've done nothing but branch movement
         * for 100 passes; we're probably
         * in a cycle and will never reach a
         * fixed point.
         *
         * XXX - yes, we really need a non-
         * heuristic way of detecting a cycle.
         */
        opt_state->done = 1;
        break;
      }
    }
  }
}

/*
 * Optimize the filter code in its dag representation.
 * Return 0 on success, -1 on error.
 */
int
bpf_optimize(struct icode *ic, char *errbuf)
{
  opt_state_t opt_state;

  memset(&opt_state, 0, sizeof(opt_state));
  opt_state.errbuf = errbuf;
  if (setjmp(opt_state.top_ctx)) {
    opt_cleanup(&opt_state);
    return -1;
  }
  opt_init(&opt_state, ic);
  opt_loop(&opt_state, ic, 0);
  opt_loop(&opt_state, ic, 1);
  intern_blocks(&opt_state, ic);
#ifdef BDEBUG
  if (pcap_optimizer_debug > 1 || pcap_print_dot_graph) {
    printf("after intern_blocks()\n");
    opt_dump(&opt_state, ic);
  }
#endif
  opt_root(&ic->root);
#ifdef BDEBUG
  if (pcap_optimizer_debug > 1 || pcap_print_dot_graph) {
    printf("after opt_root()\n");
    opt_dump(&opt_state, ic);
  }
#endif
  opt_cleanup(&opt_state);
  return 0;
}

static void
make_marks(struct icode *ic, struct block *p)
{
  if (!isMarked(ic, p)) {
    Mark(ic, p);
    if (BPF_CLASS(p->s.code) != BPF_RET) {
      make_marks(ic, JT(p));
      make_marks(ic, JF(p));
    }
  }
}

/*
 * Mark code array such that isMarked(ic->cur_mark, i) is true
 * only for nodes that are alive.
 */
static void
mark_code(struct icode *ic)
{
  ic->cur_mark += 1;
  make_marks(ic, ic->root);
}

// Return 1 iff opcode is valid and uses the 'k' field.
bool
pcapint_opcode_without_k(const uint16_t opcode)
{
  static const bool without_k[UINT8_MAX + 1] = {
    [BPF_LD   | BPF_W  | BPF_LEN] = true,
    [BPF_LDX  | BPF_W  | BPF_LEN] = true,
    [BPF_JMP  | BPF_JA          ] = true, // no_optimize == 1
    [BPF_JMP  | BPF_JEQ  | BPF_X] = true, // block exit only
    [BPF_JMP  | BPF_JGT  | BPF_X] = true, // block exit only
    [BPF_JMP  | BPF_JGE  | BPF_X] = true, // block exit only
    [BPF_JMP  | BPF_JSET | BPF_X] = true, // block exit only
    [BPF_ALU  | BPF_ADD  | BPF_X] = true,
    [BPF_ALU  | BPF_SUB  | BPF_X] = true,
    [BPF_ALU  | BPF_MUL  | BPF_X] = true,
    [BPF_ALU  | BPF_DIV  | BPF_X] = true,
    [BPF_ALU  | BPF_OR   | BPF_X] = true,
    [BPF_ALU  | BPF_AND  | BPF_X] = true,
    [BPF_ALU  | BPF_LSH  | BPF_X] = true,
    [BPF_ALU  | BPF_RSH  | BPF_X] = true,
    [BPF_ALU  | BPF_NEG         ] = true,
    [BPF_ALU  | BPF_MOD  | BPF_X] = true,
    [BPF_ALU  | BPF_XOR  | BPF_X] = true,
    [BPF_RET  | BPF_A           ] = true,
    [BPF_MISC | BPF_TAX         ] = true,
    [BPF_MISC | BPF_TXA         ] = true,
  };
  return opcode <= UINT8_MAX && without_k[(uint8_t)opcode];
}

/*
 * True iff the two stmt lists load the same value from the packet into
 * the accumulator.
 */
static int
eq_slist(struct slist *x, struct slist *y)
{
  for (;;) {
    x = this_op(x);
    y = this_op(y);
    /*
     * If at least one list has been exhausted, return true iff
     * both lists have been exhausted.
     */
    if (! (x && y))
      return ! (x || y);
    /*
     * After this_op() neither of the two opcodes is NOP, so the
     * type cast is as safe as in convert_code_r().
     */
    if (x->s.code != y->s.code ||
        (! pcapint_opcode_without_k((uint16_t)x->s.code) &&
         x->s.k != y->s.k))
      return 0;
    x = x->next;
    y = y->next;
  }
}

static inline int
eq_blk(struct block *b0, struct block *b1)
{
  if (b0->s.code == b1->s.code &&
      b0->s.k == b1->s.k &&
      b0->et.succ == b1->et.succ &&
      b0->ef.succ == b1->ef.succ)
    return eq_slist(b0->stmts, b1->stmts);
  return 0;
}

static void
intern_blocks(opt_state_t *opt_state, struct icode *ic)
{
  struct block *p;
  u_int i, j;
  int done1;
 top:
  done1 = 1;
  for (i = 0; i < opt_state->n_blocks; ++i)
    opt_state->blocks[i]->link = 0;

  mark_code(ic);

  for (i = opt_state->n_blocks - 1; i != 0; ) {
    --i;
    if (!isMarked(ic, opt_state->blocks[i]))
      continue;
    for (j = i + 1; j < opt_state->n_blocks; ++j) {
      if (!isMarked(ic, opt_state->blocks[j]))
        continue;
      if (eq_blk(opt_state->blocks[i], opt_state->blocks[j])) {
        opt_state->blocks[i]->link = opt_state->blocks[j]->link ?
          opt_state->blocks[j]->link : opt_state->blocks[j];
        break;
      }
    }
  }
  for (i = 0; i < opt_state->n_blocks; ++i) {
    p = opt_state->blocks[i];
    if (JT(p) == NULL)
      continue;
    if (JT(p)->link) {
      done1 = 0;
      JT(p) = JT(p)->link;
    }
    if (JF(p)->link) {
      done1 = 0;
      JF(p) = JF(p)->link;
    }
  }
  if (!done1)
    goto top;
}

static void
opt_cleanup(opt_state_t *opt_state)
{
  free(opt_state->vnode_base);
  free(opt_state->vmap);
  free(opt_state->edges);
  free(opt_state->space);
  free(opt_state->levels);
  free(opt_state->blocks);
}

/*
 * For optimizer errors.
 */
static void PCAP_NORETURN
opt_error(opt_state_t *opt_state, const char *fmt, ...)
{
  va_list ap;

  if (opt_state->errbuf != NULL) {
    va_start(ap, fmt);
    (void)vsnprintf(opt_state->errbuf,
        PCAP_ERRBUF_SIZE, fmt, ap);
    va_end(ap);
  }
  longjmp(opt_state->top_ctx, 1);
  /* NOTREACHED */
#ifdef _AIX
  PCAP_UNREACHABLE
#endif /* _AIX */
}

/*
 * Return the number of stmts in 's'.
 */
static u_int
slength(struct slist *s)
{
  u_int n = 0;

  for (; s; s = s->next)
    if (s->s.code != NOP)
      ++n;
  return n;
}

/*
 * Return the number of nodes reachable by 'p'.
 * All nodes should be initially unmarked.
 */
static int
count_blocks(struct icode *ic, struct block *p)
{
  if (p == NULL || isMarked(ic, p))
    return 0;
  Mark(ic, p);
  return count_blocks(ic, JT(p)) + count_blocks(ic, JF(p)) + 1;
}

/*
 * Do a depth first search on the flow graph, numbering the
 * the basic blocks, and entering them into the 'blocks' array.`
 */
static void
number_blks_r(opt_state_t *opt_state, struct icode *ic, struct block *p)
{
  u_int n;

  if (p == NULL || isMarked(ic, p))
    return;

  Mark(ic, p);
  n = opt_state->n_blocks++;
  if (opt_state->n_blocks == 0) {
    /*
     * Overflow.
     */
    opt_error(opt_state, "filter is too complex to optimize");
  }
  p->id = n;
  opt_state->blocks[n] = p;

  number_blks_r(opt_state, ic, JT(p));
  number_blks_r(opt_state, ic, JF(p));
}

/*
 * Return the number of stmts in the flowgraph reachable by 'p'.
 * The nodes should be unmarked before calling.
 *
 * Note that "stmts" means "instructions", and that this includes
 *
 *  side-effect statements in 'p' (slength(p->stmts));
 *
 *  statements in the true branch from 'p' (count_stmts(JT(p)));
 *
 *  statements in the false branch from 'p' (count_stmts(JF(p)));
 *
 *  the conditional jump itself (1);
 *
 *  an extra long jump if the true branch requires it (p->longjt);
 *
 *  an extra long jump if the false branch requires it (p->longjf).
 */
static u_int
count_stmts(struct icode *ic, struct block *p)
{
  u_int n;

  if (p == NULL || isMarked(ic, p))
    return 0;
  Mark(ic, p);
  n = count_stmts(ic, JT(p)) + count_stmts(ic, JF(p));
  return slength(p->stmts) + n + 1 + p->longjt + p->longjf;
}

/*
 * Allocate memory.  All allocation is done before optimization
 * is begun.  A linear bound on the size of all data structures is computed
 * from the total number of blocks and/or statements.
 */
static void
opt_init(opt_state_t *opt_state, struct icode *ic)
{
  bpf_u_int32 *p;
  int i, n, max_stmts;
  u_int product;
  size_t block_memsize, edge_memsize;

  /*
   * First, count the blocks, so we can allocate an array to map
   * block number to block.  Then, put the blocks into the array.
   */
  unMarkAll(ic);
  n = count_blocks(ic, ic->root);
  opt_state->blocks = (struct block **)calloc(n, sizeof(*opt_state->blocks));
  if (opt_state->blocks == NULL)
    opt_error(opt_state, "calloc");
  unMarkAll(ic);
  opt_state->n_blocks = 0;
  number_blks_r(opt_state, ic, ic->root);

  /*
   * This "should not happen".
   */
  if (opt_state->n_blocks == 0)
    opt_error(opt_state, "filter has no instructions; please report this as a libpcap issue");

  opt_state->n_edges = 2 * opt_state->n_blocks;
  if ((opt_state->n_edges / 2) != opt_state->n_blocks) {
    /*
     * Overflow.
     */
    opt_error(opt_state, "filter is too complex to optimize");
  }
  opt_state->edges = (struct edge **)calloc(opt_state->n_edges, sizeof(*opt_state->edges));
  if (opt_state->edges == NULL) {
    opt_error(opt_state, "calloc");
  }

  /*
   * The number of levels is bounded by the number of nodes.
   */
  opt_state->levels = (struct block **)calloc(opt_state->n_blocks, sizeof(*opt_state->levels));
  if (opt_state->levels == NULL) {
    opt_error(opt_state, "calloc");
  }

  opt_state->edgewords = opt_state->n_edges / BITS_PER_WORD + 1;
  opt_state->nodewords = opt_state->n_blocks / BITS_PER_WORD + 1;

  /*
   * Make sure opt_state->n_blocks * opt_state->nodewords fits
   * in a u_int; we use it as a u_int number-of-iterations
   * value.
   */
  product = opt_state->n_blocks * opt_state->nodewords;
  if ((product / opt_state->n_blocks) != opt_state->nodewords) {
    /*
     * XXX - just punt and don't try to optimize?
     * In practice, this is unlikely to happen with
     * a normal filter.
     */
    opt_error(opt_state, "filter is too complex to optimize");
  }

  /*
   * Make sure the total memory required for that doesn't
   * overflow.
   */
  block_memsize = (size_t)2 * product * sizeof(*opt_state->space);
  if ((block_memsize / product) != 2 * sizeof(*opt_state->space)) {
    opt_error(opt_state, "filter is too complex to optimize");
  }

  /*
   * Make sure opt_state->n_edges * opt_state->edgewords fits
   * in a u_int; we use it as a u_int number-of-iterations
   * value.
   */
  product = opt_state->n_edges * opt_state->edgewords;
  if ((product / opt_state->n_edges) != opt_state->edgewords) {
    opt_error(opt_state, "filter is too complex to optimize");
  }

  /*
   * Make sure the total memory required for that doesn't
   * overflow.
   */
  edge_memsize = (size_t)product * sizeof(*opt_state->space);
  if (edge_memsize / product != sizeof(*opt_state->space)) {
    opt_error(opt_state, "filter is too complex to optimize");
  }

  /*
   * Make sure the total memory required for both of them doesn't
   * overflow.
   */
  if (block_memsize > SIZE_MAX - edge_memsize) {
    opt_error(opt_state, "filter is too complex to optimize");
  }

  /* XXX */
  opt_state->space = (bpf_u_int32 *)malloc(block_memsize + edge_memsize);
  if (opt_state->space == NULL) {
    opt_error(opt_state, "malloc");
  }
  p = opt_state->space;
  opt_state->all_dom_sets = p;
  for (i = 0; i < n; ++i) {
    opt_state->blocks[i]->dom = p;
    p += opt_state->nodewords;
  }
  opt_state->all_closure_sets = p;
  for (i = 0; i < n; ++i) {
    opt_state->blocks[i]->closure = p;
    p += opt_state->nodewords;
  }
  opt_state->all_edge_sets = p;
  for (i = 0; i < n; ++i) {
    struct block *b = opt_state->blocks[i];

    b->et.edom = p;
    p += opt_state->edgewords;
    b->ef.edom = p;
    p += opt_state->edgewords;
    b->et.id = i;
    opt_state->edges[i] = &b->et;
    b->ef.id = opt_state->n_blocks + i;
    opt_state->edges[opt_state->n_blocks + i] = &b->ef;
    b->et.pred = b;
    b->ef.pred = b;
  }
  max_stmts = 0;
  for (i = 0; i < n; ++i)
    max_stmts += slength(opt_state->blocks[i]->stmts) + 1;
  /*
   * We allocate at most 3 value numbers per statement,
   * so this is an upper bound on the number of valnodes
   * we'll need.
   */
  opt_state->maxval = 3 * max_stmts;
  opt_state->vmap = (struct vmapinfo *)calloc(opt_state->maxval, sizeof(*opt_state->vmap));
  if (opt_state->vmap == NULL) {
    opt_error(opt_state, "calloc");
  }
  opt_state->vnode_base = (struct valnode *)calloc(opt_state->maxval, sizeof(*opt_state->vnode_base));
  if (opt_state->vnode_base == NULL) {
    opt_error(opt_state, "calloc");
  }
}

/*
 * This is only used when supporting optimizer debugging.  It is
 * global state, so do *not* do more than one compile in parallel
 * and expect it to provide meaningful information.
 */
#ifdef BDEBUG
int bids[NBIDS];
#endif

/*
 * Returns true if successful.  Returns false if a branch has
 * an offset that is too large.  If so, we have marked that
 * branch so that on a subsequent iteration, it will be treated
 * properly.
 */
static int
convert_code_r(conv_state_t *conv_state, struct icode *ic, struct block *p)
{
  struct bpf_insn *dst;
  struct slist *src;
  u_int slen;
  u_int off;
  struct slist **offset = NULL;

  if (p == NULL || isMarked(ic, p))
    return (1);
  Mark(ic, p);

  if (convert_code_r(conv_state, ic, JF(p)) == 0)
    return (0);
  if (convert_code_r(conv_state, ic, JT(p)) == 0)
    return (0);

  slen = slength(p->stmts);
  dst = conv_state->ftail -= (slen + 1 + p->longjt + p->longjf);
    /* inflate length by any extra jumps */

  p->offset = (int)(dst - conv_state->fstart);

  /* generate offset[] for convenience  */
  if (slen) {
    offset = (struct slist **)calloc(slen, sizeof(struct slist *));
    if (!offset) {
      conv_error(conv_state, "not enough core");
      /*NOTREACHED*/
    }
  }
  src = p->stmts;
  for (off = 0; off < slen && src; off++) {
#if 0
    printf("off=%d src=%x\n", off, src);
#endif
    offset[off] = src;
    src = src->next;
  }

  off = 0;
  for (src = p->stmts; src; src = src->next) {
    if (src->s.code == NOP)
      continue;
    dst->code = (u_short)src->s.code;
    dst->k = src->s.k;

    /* fill block-local relative jump */
    if (BPF_CLASS(src->s.code) != BPF_JMP || src->s.code == (BPF_JMP|BPF_JA)) {
#if 0
      if (src->s.jt || src->s.jf) {
        free(offset);
        conv_error(conv_state, "illegal jmp destination");
        /*NOTREACHED*/
      }
#endif
      goto filled;
    }
    if (off == slen - 2)  /*???*/
      goto filled;

      {
    u_int i;
    int jt, jf;
    const char ljerr[] = "%s for block-local relative jump: off=%d";

#if 0
    printf("code=%x off=%d %x %x\n", src->s.code,
      off, src->s.jt, src->s.jf);
#endif

    if (!src->s.jt || !src->s.jf) {
      free(offset);
      conv_error(conv_state, ljerr, "no jmp destination", off);
      /*NOTREACHED*/
    }

    jt = jf = 0;
    for (i = 0; i < slen; i++) {
      if (offset[i] == src->s.jt) {
        if (jt) {
          free(offset);
          conv_error(conv_state, ljerr, "multiple matches", off);
          /*NOTREACHED*/
        }

        if (i - off - 1 >= 256) {
          free(offset);
          conv_error(conv_state, ljerr, "out-of-range jump", off);
          /*NOTREACHED*/
        }
        dst->jt = (u_char)(i - off - 1);
        jt++;
      }
      if (offset[i] == src->s.jf) {
        if (jf) {
          free(offset);
          conv_error(conv_state, ljerr, "multiple matches", off);
          /*NOTREACHED*/
        }
        if (i - off - 1 >= 256) {
          free(offset);
          conv_error(conv_state, ljerr, "out-of-range jump", off);
          /*NOTREACHED*/
        }
        dst->jf = (u_char)(i - off - 1);
        jf++;
      }
    }
    if (!jt || !jf) {
      free(offset);
      conv_error(conv_state, ljerr, "no destination found", off);
      /*NOTREACHED*/
    }
      }
filled:
    ++dst;
    ++off;
  }
  if (offset)
    free(offset);

#ifdef BDEBUG
  if (dst - conv_state->fstart < NBIDS)
    bids[dst - conv_state->fstart] = p->id + 1;
#endif
  dst->code = (u_short)p->s.code;
  dst->k = p->s.k;
  if (JT(p)) {
    /* number of extra jumps inserted */
    u_char extrajmps = 0;
    off = JT(p)->offset - (p->offset + slen) - 1;
    if (off >= 256) {
        /* offset too large for branch, must add a jump */
        if (p->longjt == 0) {
      /* mark this instruction and retry */
      p->longjt++;
      return(0);
        }
        dst->jt = extrajmps;
        extrajmps++;
        dst[extrajmps].code = BPF_JMP|BPF_JA;
        dst[extrajmps].k = off - extrajmps;
    }
    else
        dst->jt = (u_char)off;
    off = JF(p)->offset - (p->offset + slen) - 1;
    if (off >= 256) {
        /* offset too large for branch, must add a jump */
        if (p->longjf == 0) {
      /* mark this instruction and retry */
      p->longjf++;
      return(0);
        }
        /* branch if F to following jump */
        /* if two jumps are inserted, F goes to second one */
        dst->jf = extrajmps;
        extrajmps++;
        dst[extrajmps].code = BPF_JMP|BPF_JA;
        dst[extrajmps].k = off - extrajmps;
    }
    else
        dst->jf = (u_char)off;
  }
  return (1);
}


/*
 * Convert flowgraph intermediate representation to the
 * BPF array representation.  Set *lenp to the number of instructions.
 *
 * This routine does *NOT* leak the memory pointed to by fp.  It *must
 * not* do free(fp) before returning fp; doing so would make no sense,
 * as the BPF array pointed to by the return value of icode_to_fcode()
 * must be valid - it's being returned for use in a bpf_program structure.
 *
 * If it appears that icode_to_fcode() is leaking, the problem is that
 * the program using pcap_compile() is failing to free the memory in
 * the BPF program when it's done - the leak is in the program, not in
 * the routine that happens to be allocating the memory.  (By analogy, if
 * a program calls fopen() without ever calling fclose() on the FILE *,
 * it will leak the FILE structure; the leak is not in fopen(), it's in
 * the program.)  Change the program to use pcap_freecode() when it's
 * done with the filter program.  See the pcap man page.
 */
struct bpf_insn *
icode_to_fcode(struct icode *ic, struct block *root, u_int *lenp,
    char *errbuf)
{
  u_int n;
  struct bpf_insn *fp;
  conv_state_t conv_state;

  conv_state.fstart = NULL;
  conv_state.errbuf = errbuf;
  if (setjmp(conv_state.top_ctx) != 0) {
    free(conv_state.fstart);
    return NULL;
  }

  /*
   * Loop doing convert_code_r() until no branches remain
   * with too-large offsets.
   */
  for (;;) {
      unMarkAll(ic);
      n = count_stmts(ic, root);

      fp = (struct bpf_insn *)calloc(n, sizeof(*fp));
      if (fp == NULL) {
    snprintf(errbuf, PCAP_ERRBUF_SIZE, "calloc");
    return NULL;
      }
      conv_state.fstart = fp;
      conv_state.ftail = fp + n;

      unMarkAll(ic);
      if (convert_code_r(&conv_state, ic, root))
    break;
      free(fp);
  }

  *lenp = n;
  return fp;
}

/*
 * For iconv_to_fconv() errors.
 */
static void PCAP_NORETURN
conv_error(conv_state_t *conv_state, const char *fmt, ...)
{
  va_list ap;

  va_start(ap, fmt);
  (void)vsnprintf(conv_state->errbuf,
      PCAP_ERRBUF_SIZE, fmt, ap);
  va_end(ap);
  longjmp(conv_state->top_ctx, 1);
  /* NOTREACHED */
#ifdef _AIX
  PCAP_UNREACHABLE
#endif /* _AIX */
}

/*
 * Make a copy of a BPF program and put it in the "fcode" member of
 * a "pcap_t".
 *
 * If we fail to allocate memory for the copy, fill in the "errbuf"
 * member of the "pcap_t" with an error message, and return -1;
 * otherwise, return 0.
 */
int
pcapint_install_bpf_program(pcap_t *p, struct bpf_program *fp)
{
  size_t prog_size;

  /*
   * Validate the program.
   */
  if (!pcapint_validate_filter(fp->bf_insns, fp->bf_len)) {
    snprintf(p->errbuf, sizeof(p->errbuf),
      "BPF program is not valid");
    return (-1);
  }

  /*
   * Free up any already installed program.
   */
  pcap_freecode(&p->fcode);

  prog_size = sizeof(*fp->bf_insns) * fp->bf_len;
  p->fcode.bf_len = fp->bf_len;
  p->fcode.bf_insns = (struct bpf_insn *)malloc(prog_size);
  if (p->fcode.bf_insns == NULL) {
    pcapint_fmt_errmsg_for_errno(p->errbuf, sizeof(p->errbuf),
        errno, "malloc");
    return (-1);
  }
  memcpy(p->fcode.bf_insns, fp->bf_insns, prog_size);
  return (0);
}

#ifdef BDEBUG
static void
dot_dump_node(const struct icode *ic, struct block *block,
    const struct bpf_program *prog, FILE *out)
{
  if (block == NULL || isMarked(ic, block))
    return;
  Mark(ic, block);

  {
    const unsigned icount = slength(block->stmts) + 1 + block->longjt + block->longjf;
    const unsigned noffset = min(block->offset + icount, prog->bf_len);

    fprintf(out, "\tblock%u [label=\"BLOCK%u\\l\\l", block->id, block->id);
    for (unsigned i = block->offset; i < noffset; i++)
      fprintf(out, "%s\\l", bpf_image(prog->bf_insns + i, i));
    fprintf(out, "\"");
  }

  {
    bool tooltip = false;
    tooltip |= IS_KNOWN(block, A_ATOM);
    tooltip |= IS_KNOWN(block, X_ATOM);
    for (unsigned i = 0; i < BPF_MEMWORDS; i++)
      tooltip |= IS_KNOWN(block, i);

    if (tooltip) {
      const char *sep = "";
      fprintf(out, ", tooltip=\"");
      if (IS_KNOWN(block, A_ATOM)) {
        fprintf(out, "val[A]=%d", block->val[A_ATOM]);
        sep = " ";
      }
      if (IS_KNOWN(block, X_ATOM)) {
        fprintf(out, "%sval[X]=%d", sep, block->val[X_ATOM]);
        sep = " ";
      }
      for (unsigned i = 0; i < BPF_MEMWORDS; i++)
        if (IS_KNOWN(block, i)) {
          fprintf(out, "%sval[%d]=%d", sep, i, block->val[i]);
          sep = " ";
        }
      fprintf(out, "\"");
    }
  }

  if (JT(block) == NULL && JF(block) == NULL)
    fprintf(out, ", peripheries=2");
  fprintf(out, "];\n");

  dot_dump_node(ic, JT(block), prog, out);
  dot_dump_node(ic, JF(block), prog, out);
}

static void
dot_dump_edge(const struct icode *ic, struct block *block, FILE *out)
{
  if (block == NULL || isMarked(ic, block))
    return;
  Mark(ic, block);

  if (JT(block))
    fprintf(out, "\t\"block%u\":se -> \"block%u\" [label=\"T\"];\n",
            block->id, JT(block)->id);
  if (JF(block))
    fprintf(out, "\t\"block%u\":sw -> \"block%u\" [label=\"F\"];\n",
            block->id, JF(block)->id);
  dot_dump_edge(ic, JT(block), out);
  dot_dump_edge(ic, JF(block), out);
}

/*
 * Output the filter program's CFG using Graphviz DOT language.  Show each
 * block with the instructions and every known value index for the registers at
 * exit.  Show all jumps between the blocks.
 *
 * Example DOT output for DLT_EN10MB and the expression "ip src host 1.1.1.1":
 * ----------------
    digraph BPF {
  node [shape=box, fontname="Courier"];
  edge [fontname="Courier"];
  block0 [label="BLOCK0\l\l(000) ldh      [12]\l(001) jeq      #0x800           jt 2  jf 5\l", tooltip="val[A]=1"];
  block1 [label="BLOCK1\l\l(002) ld       [26]\l(003) jeq      #0x1010101       jt 4  jf 5\l", tooltip="val[A]=3"];
  block2 [label="BLOCK2\l\l(004) ret      #262144\l", tooltip="val[A]=3", peripheries=2];
  block3 [label="BLOCK3\l\l(005) ret      #0\l", peripheries=2];
  "block0":se -> "block1" [label="T"];
  "block0":sw -> "block3" [label="F"];
  "block1":se -> "block2" [label="T"];
  "block1":sw -> "block3" [label="F"];
    }
 * ----------------
 * After installing Graphviz from a package or directly from [1], save the DOT
 * output as bpf.dot and run `dot -Tpng -O bpf.dot' to produce an image.
 * Alternatively, use XDot to browse .dot files directly or BPF Exam [2] to see
 * multiple CFGs on one web page.
 *
 * 1: https://www.graphviz.org/
 * 2: https://www.tcpdump.org/bpfexam/
 */
static void
dot_dump(struct icode *ic, const struct bpf_program *f, FILE *out)
{
  /*
   * Do not specify "strict" because in opt_loop() a graph can have two
   * edges between the same pair of nodes, e.g. after "JF(b) = JT(b);".
   */
  fprintf(out, "digraph BPF {\n");
  fprintf(out, "\tnode [shape=box, fontname=\"Courier\"];\n");
  fprintf(out, "\tedge [fontname=\"Courier\"];\n");
  unMarkAll(ic);
  dot_dump_node(ic, ic->root, f, out);
  unMarkAll(ic);
  dot_dump_edge(ic, ic->root, out);
  fprintf(out, "}\n");
}

static void
opt_dump(opt_state_t *opt_state, struct icode *ic)
{
  memset(bids, 0, sizeof(bids));
  char errbuf[PCAP_ERRBUF_SIZE];
  struct bpf_program f;
  f.bf_insns = icode_to_fcode(ic, ic->root, &f.bf_len, errbuf);
  if (f.bf_insns == NULL)
    opt_error(opt_state, "%s: icode_to_fcode failed: %s", __func__, errbuf);

  /*
   * If the CFG, in DOT format, is requested, output it rather than
   * the code that would be generated from that graph.
   */
  if (pcap_print_dot_graph)
    dot_dump(ic, &f, stdout);
  else {
    bpf_dump(&f, 1);
    putchar('\n');
  }
  free(f.bf_insns);
}
#endif

Coverage Report

Created: 2026-10-28 06:57