Merge branch 'master' into man

[libpcap] / optimize.c

/*
 * 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.
 */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <pcap-types.h>

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <string.h>

#include <errno.h>

#include "pcap-int.h"

#include "gencode.h"

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

#ifdef BDEBUG
int pcap_optimizer_debug;
#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 fro 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.
 */
#if PCAP_IS_AT_LEAST_GNUC_VERSION(3,4)
  /*
   * GCC 3.4 and later; we have __builtin_ctz().
   */
  #define lowest_set_bit(mask) __builtin_ctz(mask)
#elif defined(_MSC_VER)
  /*
   * Visual Studio; we support only 2005 and later, so use
   * _BitScanForward().
   */
#include <intrin.h>
#pragma intrinsic(_BitScanForward)

static __forceinline 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)
                return -1;      /* mask is zero */
        return (int)bit;
}
#elif defined(MSDOS) && defined(__DJGPP__)
  /*
   * MS-DOS with DJGPP, which declares ffs() in <string.h>, which
   * we've already included.
   */
  #define lowest_set_bit(mask)  (ffs((mask)) - 1)
#elif (defined(MSDOS) && defined(__WATCOMC__)) || defined(STRINGS_H_DECLARES_FFS)
  /*
   * MS-DOS with Watcom C, which has <strings.h> and declares ffs() there,
   * or some other platform (UN*X conforming to a sufficient recent version
   * of the Single UNIX Specification).
   */
  #include <strings.h>
  #define lowest_set_bit(mask)  (ffs((mask)) - 1)
#else
/*
 * None of the above.
 * Use a perfect-hash-function-based function.
 */
static int
lowest_set_bit(int mask)
{
        unsigned int v = (unsigned int)mask;

        static const int MultiplyDeBruijnBitPosition[32] = {
                0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
                31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
        };

        /*
         * We strip off all but the lowermost set bit (v & ~v),
         * and perform a minimal perfect hash on it to look up the
         * number of low-order zero bits in a table.
         *
         * See:
         *
         *      http://7ooo.mooo.com/text/ComputingTrailingZerosHOWTO.pdf
         *
         *      http://supertech.csail.mit.edu/papers/debruijn.pdf
         */
        return (MultiplyDeBruijnBitPosition[((v & -v) * 0x077CB531U) >> 27]);
}
#endif

/*
 * 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 Shwarz 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;
        int v0, v1;
        int val;
        struct valnode *next;
};

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

struct vmapinfo {
        int is_const;
        bpf_int32 const_val;
};

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

        int n_blocks;
        struct block **blocks;
        int n_edges;
        struct edge **edges;

        /*
         * A bit vector set representation of the dominators.
         * We round up the set size to the next power of two.
         */
        int nodewords;
        int edgewords;
        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] & (1 << ((unsigned)(a) % BITS_PER_WORD)))

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

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

/*
 * a := a intersect b
 */
#define SET_INTERSECT(a, b, n)\
{\
        register bpf_u_int32 *_x = a, *_y = b;\
        register int _n = n;\
        while (--_n >= 0) *_x++ &= *_y++;\
}

/*
 * a := a - b
 */
#define SET_SUBTRACT(a, b, n)\
{\
        register bpf_u_int32 *_x = a, *_y = b;\
        register int _n = n;\
        while (--_n >= 0) *_x++ &=~ *_y++;\
}

/*
 * a := a union b
 */
#define SET_UNION(a, b, n)\
{\
        register bpf_u_int32 *_x = a, *_y = b;\
        register int _n = n;\
        while (--_n >= 0) *_x++ |= *_y++;\
}

        uset all_dom_sets;
        uset all_closure_sets;
        uset all_edge_sets;

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

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

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

static void opt_init(compiler_state_t *, opt_state_t *, struct icode *);
static void opt_cleanup(opt_state_t *);

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

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

#ifndef MAX
#define MAX(a,b) ((a)>(b)?(a):(b))
#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)
{
        int i;
        struct block *b;
        bpf_u_int32 *x;

        /*
         * Initialize sets to contain all nodes.
         */
        x = opt_state->all_dom_sets;
        i = opt_state->n_blocks * opt_state->nodewords;
        while (--i >= 0)
                *x++ = ~0;
        /* Root starts off empty. */
        for (i = opt_state->nodewords; --i >= 0;)
                root->dom[i] = 0;

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

static void
propedom(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)
{
        int i;
        uset x;
        struct block *b;

        x = opt_state->all_edge_sets;
        for (i = opt_state->n_edges * opt_state->edgewords; --i >= 0; )
                x[i] = ~0;

        /* 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 (i = root->level; i >= 0; --i) {
                for (b = opt_state->levels[i]; 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, struct block *root)
{
        int i;
        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 (i = root->level; i >= 0; --i) {
                for (b = opt_state->levels[i]; b; b = b->link) {
                        SET_INSERT(b->closure, b->id);
                        if (JT(b) == 0)
                                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.  If A and X are both
 * used, return AX_ATOM.  If no register is used, return -1.
 *
 * The implementation should probably change to an array access.
 */
static int
atomuse(struct stmt *s)
{
        register int c = s->code;

        if (c == NOP)
                return -1;

        switch (BPF_CLASS(c)) {

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

        case BPF_LD:
        case BPF_LDX:
                return (BPF_MODE(c) == BPF_IND) ? X_ATOM :
                        (BPF_MODE(c) == BPF_MEM) ? 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;
        }
        abort();
        /* NOTREACHED */
}

/*
 * 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(opt_state_t *opt_state, 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. */
static int
F(opt_state_t *opt_state, int code, int v0, int v1)
{
        u_int hash;
        int 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;

        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, int *valp, int 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(compiler_state_t *cstate, struct icode *ic, opt_state_t *opt_state,
    struct stmt *s, int v0, int 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)
                        bpf_error(cstate, "division by zero");
                a /= b;
                break;

        case BPF_MOD:
                if (b == 0)
                        bpf_error(cstate, "modulus 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:
                a <<= b;
                break;

        case BPF_RSH:
                a >>= b;
                break;

        default:
                abort();
        }
        s->k = a;
        s->code = BPF_LD|BPF_IMM;
        opt_state->done = 0;
}

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;
        int val;

        s = b->stmts;
        if (s == 0)
                return;

        last = s;
        for (/*empty*/; /*empty*/; s = next) {
                /*
                 * Skip over nops.
                 */
                s = this_op(s);
                if (s == 0)
                        break;  /* nothing left in the block */

                /*
                 * Find the next real instruction after that one
                 * (skipping nops).
                 */
                next = this_op(s->next);
                if (next == 0)
                        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_MEM) &&
                    s->s.k == next->s.k) {
                        opt_state->done = 0;
                        next->s.code = BPF_MISC|BPF_TAX;
                }
                /*
                 * ld  #k       -->     ldx  #k
                 * tax                  txa
                 */
                if (s->s.code == (BPF_LD|BPF_IMM) &&
                    next->s.code == (BPF_MISC|BPF_TAX)) {
                        s->s.code = BPF_LDX|BPF_IMM;
                        next->s.code = BPF_MISC|BPF_TXA;
                        opt_state->done = 0;
                }
                /*
                 * 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_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 ldi
                         * is an addx, or it's an ldxms with an addx
                         * following it (with 0 or more nops between the
                         * ldxms and addx).
                         */
                        if (next->s.code != (BPF_LDX|BPF_MSH|BPF_B))
                                add = next;
                        else
                                add = this_op(next->next);
                        if (add == 0 || 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 == 0 || tax->s.code != (BPF_MISC|BPF_TAX))
                                continue;

                        /*
                         * Check that an ild follows that (with 0 or more
                         * nops between them).
                         */
                        ild = this_op(tax->next);
                        if (ild == 0 || BPF_CLASS(ild->s.code) != BPF_LD ||
                            BPF_MODE(ild->s.code) != BPF_IND)
                                continue;
                        /*
                         * We want to turn this sequence:
                         *
                         * (004) ldi     #0x2           {s}
                         * (005) ldxms   [14]           {next}  -- optional
                         * (006) addx                   {add}
                         * (007) tax                    {tax}
                         * (008) ild     [x+0]          {ild}
                         *
                         * into this sequence:
                         *
                         * (004) nop
                         * (005) ldxms   [14]
                         * (006) nop
                         * (007) nop
                         * (008) ild     [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;
                }
        }
        /*
         * 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 sometime 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;
                        } 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;
                        }
                }
                /*
                 * 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;
                }
                /*
                 * 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);
                }
        }
        /*
         * jset #0        ->   never
         * jset #ffffffff ->   always
         */
        if (b->s.code == (BPF_JMP|BPF_K|BPF_JSET)) {
                if (b->s.k == 0)
                        JT(b) = JF(b);
                if ((u_int)b->s.k == 0xffffffffU)
                        JF(b) = JT(b);
        }
        /*
         * 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_int32 v = opt_state->vmap[val].const_val;
                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_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 = (unsigned)v > (unsigned)b->s.k;
                        break;

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

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

                default:
                        abort();
                }
                if (JF(b) != JT(b))
                        opt_state->done = 0;
                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(compiler_state_t *cstate, struct icode *ic, opt_state_t *opt_state,
    struct stmt *s, int val[], int alter)
{
        int op;
        int 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;
                }
                else
                        v = F(opt_state, s->code, s->k, v);
                vstore(s, &val[A_ATOM], v, alter);
                break;

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

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

        case BPF_LDX|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_IMM;
                        s->k = -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) {
                                /* don't optimize away "sub #0"
                                 * as it may be needed later to
                                 * fixup the generated math code */
                                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_IMM;
                                        val[A_ATOM] = K(s->k);
                                        break;
                                }
                        }
                        if (opt_state->vmap[val[A_ATOM]].is_const) {
                                fold_op(cstate, ic, 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(cstate, ic, 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;
                                opt_state->done = 0;
                                val[A_ATOM] =
                                        F(opt_state, s->code, val[A_ATOM], K(s->k));
                        }
                        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_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_MEM:
                v = val[s->k];
                if (alter && opt_state->vmap[v].is_const) {
                        s->code = BPF_LD|BPF_IMM;
                        s->k = opt_state->vmap[v].const_val;
                        opt_state->done = 0;
                }
                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_MEM:
                v = val[s->k];
                if (alter && opt_state->vmap[v].is_const) {
                        s->code = BPF_LDX|BPF_IMM;
                        s->k = opt_state->vmap[v].const_val;
                        opt_state->done = 0;
                }
                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, register struct stmt *s, register struct stmt *last[])
{
        register 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;
                }
                last[atom] = s;
        }
}

static void
opt_deadstores(opt_state_t *opt_state, register struct block *b)
{
        register struct slist *s;
        register 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;
                        opt_state->done = 0;
                }
}

static void
opt_blk(compiler_state_t *cstate, struct icode *ic, opt_state_t *opt_state,
    struct block *b, int do_stmts)
{
        struct slist *s;
        struct edge *p;
        int i;
        bpf_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 == 0) {
                /*
                 * 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(cstate, ic, 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?
         *
         * 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;
                }
        } 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(struct block *b, 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;
}

static struct block *
fold_edge(struct block *child, struct edge *ep)
{
        int sense;
        int aval0, aval1, oval0, oval1;
        int code = ep->code;

        if (code < 0) {
                code = -code;
                sense = 0;
        } else
                sense = 1;

        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 (aval0 != aval1)
                return 0;

        if (oval0 == oval1)
                /*
                 * The operands of the branch instructions are
                 * identical, so 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;
}

static void
opt_j(opt_state_t *opt_state, struct edge *ep)
{
        register int i, k;
        register struct block *target;

        if (JT(ep->succ) == 0)
                return;

        if (JT(ep->succ) == JF(ep->succ)) {
                /*
                 * Common branch targets can be eliminated, provided
                 * there is no data dependency.
                 */
                if (!use_conflict(ep->pred, ep->succ->et.succ)) {
                        opt_state->done = 0;
                        ep->succ = JT(ep->succ);
                }
        }
        /*
         * 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) {
                register bpf_u_int32 x = ep->edom[i];

                while (x != 0) {
                        k = lowest_set_bit(x);
                        x &=~ (1 << k);
                        k += i * BITS_PER_WORD;

                        target = fold_edge(ep->succ, opt_state->edges[k]);
                        /*
                         * Check that there is no data dependency between
                         * nodes that will be violated if we move the edge.
                         */
                        if (target != 0 && !use_conflict(ep->pred, target)) {
                                opt_state->done = 0;
                                ep->succ = target;
                                if (JT(target) != 0)
                                        /*
                                         * Start over unless we hit a leaf.
                                         */
                                        goto top;
                                return;
                        }
                }
        }
}


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

        ep = b->in_edges;
        if (ep == 0)
                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;

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

        at_top = 1;
        while (1) {
                if (*diffp == 0)
                        return;

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

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

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

                diffp = &JF(*diffp);
                at_top = 0;
        }
        samep = &JF(*diffp);
        while (1) {
                if (*samep == 0)
                        return;

                if (JT(*samep) != JT(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 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;

        opt_state->done = 0;
}

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

        ep = b->in_edges;
        if (ep == 0)
                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;
        while (1) {
                if (*diffp == 0)
                        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);
        while (1) {
                if (*samep == 0)
                        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;

        opt_state->done = 0;
}

static void
opt_blks(compiler_state_t *cstate, 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(cstate, ic, opt_state, p, do_stmts);

        if (do_stmts)
                /*
                 * No point trying to move branches; it can't possibly
                 * make a difference at this point.
                 */
                return;

        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);
                        and_pullup(opt_state, p);
                }
        }
}

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, struct block *root)
{
        int i;
        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 (i = root->level; i > 0; --i) {
                for (b = opt_state->levels[i]; 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(compiler_state_t *cstate, opt_state_t *opt_state, struct icode *ic,
    int do_stmts)
{

#ifdef BDEBUG
        if (pcap_optimizer_debug > 1) {
                printf("opt_loop(root, %d) begin\n", do_stmts);
                opt_dump(cstate, ic);
        }
#endif
        do {
                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(cstate, opt_state, ic, do_stmts);
#ifdef BDEBUG
                if (pcap_optimizer_debug > 1) {
                        printf("opt_loop(root, %d) bottom, done=%d\n", do_stmts, opt_state->done);
                        opt_dump(cstate, ic);
                }
#endif
        } while (!opt_state->done);
}

/*
 * Optimize the filter code in its dag representation.
 */
void
bpf_optimize(compiler_state_t *cstate, struct icode *ic)
{
        opt_state_t opt_state;

        opt_init(cstate, &opt_state, ic);
        opt_loop(cstate, &opt_state, ic, 0);
        opt_loop(cstate, &opt_state, ic, 1);
        intern_blocks(&opt_state, ic);
#ifdef BDEBUG
        if (pcap_optimizer_debug > 1) {
                printf("after intern_blocks()\n");
                opt_dump(cstate, ic);
        }
#endif
        opt_root(&ic->root);
#ifdef BDEBUG
        if (pcap_optimizer_debug > 1) {
                printf("after opt_root()\n");
                opt_dump(cstate, ic);
        }
#endif
        opt_cleanup(&opt_state);
}

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);
}

/*
 * 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)
{
        while (1) {
                while (x && x->s.code == NOP)
                        x = x->next;
                while (y && y->s.code == NOP)
                        y = y->next;
                if (x == 0)
                        return y == 0;
                if (y == 0)
                        return x == 0;
                if (x->s.code != y->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;
        int i, j;
        int done1; /* don't shadow global */
 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; ) {
                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) == 0)
                        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((void *)opt_state->vnode_base);
        free((void *)opt_state->vmap);
        free((void *)opt_state->edges);
        free((void *)opt_state->space);
        free((void *)opt_state->levels);
        free((void *)opt_state->blocks);
}

/*
 * 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 == 0 || 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)
{
        int n;

        if (p == 0 || isMarked(ic, p))
                return;

        Mark(ic, p);
        n = opt_state->n_blocks++;
        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 == 0 || 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(compiler_state_t *cstate, opt_state_t *opt_state, struct icode *ic)
{
        bpf_u_int32 *p;
        int i, n, max_stmts;

        /*
         * First, count the blocks, so we can malloc 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)
                bpf_error(cstate, "malloc");
        unMarkAll(ic);
        opt_state->n_blocks = 0;
        number_blks_r(opt_state, ic, ic->root);

        opt_state->n_edges = 2 * opt_state->n_blocks;
        opt_state->edges = (struct edge **)calloc(opt_state->n_edges, sizeof(*opt_state->edges));
        if (opt_state->edges == NULL)
                bpf_error(cstate, "malloc");

        /*
         * 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)
                bpf_error(cstate, "malloc");

        opt_state->edgewords = opt_state->n_edges / (8 * sizeof(bpf_u_int32)) + 1;
        opt_state->nodewords = opt_state->n_blocks / (8 * sizeof(bpf_u_int32)) + 1;

        /* XXX */
        opt_state->space = (bpf_u_int32 *)malloc(2 * opt_state->n_blocks * opt_state->nodewords * sizeof(*opt_state->space)
                                 + opt_state->n_edges * opt_state->edgewords * sizeof(*opt_state->space));
        if (opt_state->space == NULL)
                bpf_error(cstate, "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) {
                register 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));
        opt_state->vnode_base = (struct valnode *)calloc(opt_state->maxval, sizeof(*opt_state->vnode_base));
        if (opt_state->vmap == NULL || opt_state->vnode_base == NULL)
                bpf_error(cstate, "malloc");
}

/*
 * 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[1000];
#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(compiler_state_t *cstate, conv_state_t *conv_state,
    struct icode *ic, struct block *p)
{
        struct bpf_insn *dst;
        struct slist *src;
        u_int slen;
        u_int off;
        int extrajmps;          /* number of extra jumps inserted */
        struct slist **offset = NULL;

        if (p == 0 || isMarked(ic, p))
                return (1);
        Mark(ic, p);

        if (convert_code_r(cstate, conv_state, ic, JF(p)) == 0)
                return (0);
        if (convert_code_r(cstate, 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) {
                        bpf_error(cstate, "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) {
                                bpf_error(cstate, "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) {
                        bpf_error(cstate, ljerr, "no jmp destination", off);
                        /*NOTREACHED*/
                }

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

                                dst->jt = i - off - 1;
                                jt++;
                        }
                        if (offset[i] == src->s.jf) {
                                if (jf) {
                                        bpf_error(cstate, ljerr, "multiple matches", off);
                                        /*NOTREACHED*/
                                }
                                dst->jf = i - off - 1;
                                jf++;
                        }
                }
                if (!jt || !jf) {
                        bpf_error(cstate, ljerr, "no destination found", off);
                        /*NOTREACHED*/
                }
            }
filled:
                ++dst;
                ++off;
        }
        if (offset)
                free(offset);

#ifdef BDEBUG
        bids[dst - conv_state->fstart] = p->id + 1;
#endif
        dst->code = (u_short)p->s.code;
        dst->k = p->s.k;
        if (JT(p)) {
                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);
                    }
                    /* branch if T to following jump */
                    dst->jt = extrajmps;
                    extrajmps++;
                    dst[extrajmps].code = BPF_JMP|BPF_JA;
                    dst[extrajmps].k = off - extrajmps;
                }
                else
                    dst->jt = 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 = 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(compiler_state_t *cstate, struct icode *ic,
    struct block *root, u_int *lenp)
{
        u_int n;
        struct bpf_insn *fp;
        conv_state_t conv_state;

        /*
         * Loop doing convert_code_r() until no branches remain
         * with too-large offsets.
         */
        while (1) {
            unMarkAll(ic);
            n = *lenp = count_stmts(ic, root);

            fp = (struct bpf_insn *)malloc(sizeof(*fp) * n);
            if (fp == NULL)
                    bpf_error(cstate, "malloc");
            memset((char *)fp, 0, sizeof(*fp) * n);
            conv_state.fstart = fp;
            conv_state.ftail = fp + n;

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

        return fp;
}

/*
 * 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
install_bpf_program(pcap_t *p, struct bpf_program *fp)
{
        size_t prog_size;

        /*
         * Validate the program.
         */
        if (!bpf_validate(fp->bf_insns, fp->bf_len)) {
                pcap_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) {
                pcap_snprintf(p->errbuf, sizeof(p->errbuf),
                         "malloc: %s", pcap_strerror(errno));
                return (-1);
        }
        memcpy(p->fcode.bf_insns, fp->bf_insns, prog_size);
        return (0);
}

#ifdef BDEBUG
static void
dot_dump_node(struct icode *ic, struct block *block, struct bpf_program *prog,
    FILE *out)
{
        int icount, noffset;
        int i;

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

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

        fprintf(out, "\tblock%d [shape=ellipse, id=\"block-%d\" label=\"BLOCK%d\\n", block->id, block->id, block->id);
        for (i = block->offset; i < noffset; i++) {
                fprintf(out, "\\n%s", bpf_image(prog->bf_insns + i, i));
        }
        fprintf(out, "\" tooltip=\"");
        for (i = 0; i < BPF_MEMWORDS; i++)
                if (block->val[i] != VAL_UNKNOWN)
                        fprintf(out, "val[%d]=%d ", i, block->val[i]);
        fprintf(out, "val[A]=%d ", block->val[A_ATOM]);
        fprintf(out, "val[X]=%d", block->val[X_ATOM]);
        fprintf(out, "\"");
        if (JT(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(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%d\":se -> \"block%d\":n [label=\"T\"]; \n",
                                block->id, JT(block)->id);
                fprintf(out, "\t\"block%d\":sw -> \"block%d\":n [label=\"F\"]; \n",
                           block->id, JF(block)->id);
        }
        dot_dump_edge(ic, JT(block), out);
        dot_dump_edge(ic, JF(block), out);
}

/* Output the block CFG using graphviz/DOT language
 * In the CFG, block's code, value index for each registers at EXIT,
 * and the jump relationship is show.
 *
 * example DOT for BPF `ip src host 1.1.1.1' is:
    digraph BPF {
        block0 [shape=ellipse, id="block-0" label="BLOCK0\n\n(000) ldh      [12]\n(001) jeq      #0x800           jt 2  jf 5" tooltip="val[A]=0 val[X]=0"];
        block1 [shape=ellipse, id="block-1" label="BLOCK1\n\n(002) ld       [26]\n(003) jeq      #0x1010101       jt 4  jf 5" tooltip="val[A]=0 val[X]=0"];
        block2 [shape=ellipse, id="block-2" label="BLOCK2\n\n(004) ret      #68" tooltip="val[A]=0 val[X]=0", peripheries=2];
        block3 [shape=ellipse, id="block-3" label="BLOCK3\n\n(005) ret      #0" tooltip="val[A]=0 val[X]=0", peripheries=2];
        "block0":se -> "block1":n [label="T"];
        "block0":sw -> "block3":n [label="F"];
        "block1":se -> "block2":n [label="T"];
        "block1":sw -> "block3":n [label="F"];
    }
 *
 *  After install graphviz on http://www.graphviz.org/, save it as bpf.dot
 *  and run `dot -Tpng -O bpf.dot' to draw the graph.
 */
static void
dot_dump(compiler_state_t *cstate, struct icode *ic)
{
        struct bpf_program f;
        FILE *out = stdout;

        memset(bids, 0, sizeof bids);
        f.bf_insns = icode_to_fcode(cstate, ic, ic->root, &f.bf_len);

        fprintf(out, "digraph BPF {\n");
        unMarkAll(ic);
        dot_dump_node(ic, ic->root, &f, out);
        unMarkAll(ic);
        dot_dump_edge(ic, ic->root, out);
        fprintf(out, "}\n");

        free((char *)f.bf_insns);
}

static void
plain_dump(compiler_state_t *cstate, struct icode *ic)
{
        struct bpf_program f;

        memset(bids, 0, sizeof bids);
        f.bf_insns = icode_to_fcode(cstate, ic, ic->root, &f.bf_len);
        bpf_dump(&f, 1);
        putchar('\n');
        free((char *)f.bf_insns);
}

static void
opt_dump(compiler_state_t *cstate, struct icode *ic)
{
        /* if optimizer debugging is enabled, output DOT graph
         * `pcap_optimizer_debug=4' is equivalent to -dddd to follow -d/-dd/-ddd
         * convention in tcpdump command line
         */
        if (pcap_optimizer_debug > 3)
                dot_dump(cstate, ic);
        else
                plain_dump(cstate, ic);
}
#endif