text: /*	$OpenBSD: optimize.c,v 1.19 2016/02/05 16:58:39 canacar Exp $	*/
  
  /*
   * 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 tcpdump intermediate representation.
   */
  
  #include <sys/types.h>
  #include <sys/time.h>
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <stdint.h>
  #include <string.h>
  
  #include "pcap-int.h"
  
  #include "gencode.h"
  
  #ifdef HAVE_OS_PROTO_H
  #include "os-proto.h"
  #endif
  
  #ifdef BDEBUG
  extern int dflag;
  #endif
  
  #define A_ATOM BPF_MEMWORDS
  #define X_ATOM (BPF_MEMWORDS+1)
  
  #define NOP -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
  
  /*
   * A flag to indicate that further optimization is needed.
   * Iterative passes are continued until a given pass yields no
   * branch movement.
   */
  static int done;
  
  /*
   * A block is marked if only if its mark equals the current mark.
   * Rather than traverse the code array, marking each item, 'cur_mark' is
   * incremented.  This automatically makes each element unmarked.
   */
  static int cur_mark;
  #define isMarked(p) ((p)->mark == cur_mark)
  #define unMarkAll() cur_mark += 1
  #define Mark(p) ((p)->mark = cur_mark)
  
  static void opt_init(struct block *);
  static void opt_cleanup(void);
  
  static void make_marks(struct block *);
  static void mark_code(struct block *);
  
  static void intern_blocks(struct block *);
  
  static int eq_slist(struct slist *, struct slist *);
  
  static void find_levels_r(struct block *);
  
  static void find_levels(struct block *);
  static void find_dom(struct block *);
  static void propedom(struct edge *);
  static void find_edom(struct block *);
  static void find_closure(struct block *);
  static int atomuse(struct stmt *);
  static int atomdef(struct stmt *);
  static void compute_local_ud(struct block *);
  static void find_ud(struct block *);
  static void init_val(void);
  static int F(int, int, int);
  static __inline void vstore(struct stmt *, int *, int, int);
  static void opt_blk(struct block *, int);
  static int use_conflict(struct block *, struct block *);
  static void opt_j(struct edge *);
  static void or_pullup(struct block *);
  static void and_pullup(struct block *);
  static void opt_blks(struct block *, int);
  static __inline void link_inedge(struct edge *, struct block *);
  static void find_inedges(struct block *);
  static void opt_root(struct block **);
  static void opt_loop(struct block *, int);
  static void fold_op(struct stmt *, int, int);
  static __inline struct slist *this_op(struct slist *);
  static void opt_not(struct block *);
  static void opt_peep(struct block *);
  static void opt_stmt(struct stmt *, int[], int);
  static void deadstmt(struct stmt *, struct stmt *[]);
  static void opt_deadstores(struct block *);
  static void opt_blk(struct block *, int);
  static int use_conflict(struct block *, struct block *);
  static void opt_j(struct edge *);
  static struct block *fold_edge(struct block *, struct edge *);
  static __inline int eq_blk(struct block *, struct block *);
  static int slength(struct slist *);
  static int count_blocks(struct block *);
  static void number_blks_r(struct block *);
  static int count_stmts(struct block *);
  static int convert_code_r(struct block *);
  #ifdef BDEBUG
  static void opt_dump(struct block *);
  #endif
  
  static int n_blocks;
  struct block **blocks;
  static 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.
   */
  static int nodewords;
  static int edgewords;
  struct block **levels;
  bpf_u_int32 *space1;
  bpf_u_int32 *space2;
  #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)\
  {\
  	bpf_u_int32 *_x = a, *_y = b;\
  	int _n = n;\
  	while (--_n >= 0) *_x++ &= *_y++;\
  }
  
  /*
   * a := a - b
   */
  #define SET_SUBTRACT(a, b, n)\
  {\
  	bpf_u_int32 *_x = a, *_y = b;\
  	int _n = n;\
  	while (--_n >= 0) *_x++ &=~ *_y++;\
  }
  
  /*
   * a := a union b
   */
  #define SET_UNION(a, b, n)\
  {\
  	bpf_u_int32 *_x = a, *_y = b;\
  	int _n = n;\
  	while (--_n >= 0) *_x++ |= *_y++;\
  }
  
  static uset all_dom_sets;
  static uset all_closure_sets;
  static uset all_edge_sets;
  
  #ifndef MAX
  #define MAX(a,b) ((a)>(b)?(a):(b))
  #endif
  
  static void
  find_levels_r(b)
  	struct block *b;
  {
  	int level;
  
  	if (isMarked(b))
  		return;
  
  	Mark(b);
  	b->link = 0;
  
  	if (JT(b)) {
  		find_levels_r(JT(b));
  		find_levels_r(JF(b));
  		level = MAX(JT(b)->level, JF(b)->level) + 1;
  	} else
  		level = 0;
  	b->level = level;
  	b->link = levels[level];
  	levels[level] = b;
  }
  
  /*
   * Level graph.  The levels go from 0 at the leaves to
   * N_LEVELS at the root.  The 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(root)
  	struct block *root;
  {
  	memset((char *)levels, 0, n_blocks * sizeof(*levels));
  	unMarkAll();
  	find_levels_r(root);
  }
  
  /*
   * Find dominator relationships.
   * Assumes graph has been leveled.
   */
  static void
  find_dom(root)
  	struct block *root;
  {
  	int i;
  	struct block *b;
  	bpf_u_int32 *x;
  
  	/*
  	 * Initialize sets to contain all nodes.
  	 */
  	x = all_dom_sets;
  	i = n_blocks * nodewords;
  	while (--i >= 0)
  		*x++ = ~0;
  	/* Root starts off empty. */
  	for (i = nodewords; --i >= 0;)
  		root->dom[i] = 0;
  
  	/* root->level is the highest level no found. */
  	for (i = root->level; i >= 0; --i) {
  		for (b = levels[i]; b; b = b->link) {
  			SET_INSERT(b->dom, b->id);
  			if (JT(b) == 0)
  				continue;
  			SET_INTERSECT(JT(b)->dom, b->dom, nodewords);
  			SET_INTERSECT(JF(b)->dom, b->dom, nodewords);
  		}
  	}
  }
  
  static void
  propedom(ep)
  	struct edge *ep;
  {
  	SET_INSERT(ep->edom, ep->id);
  	if (ep->succ) {
  		SET_INTERSECT(ep->succ->et.edom, ep->edom, edgewords);
  		SET_INTERSECT(ep->succ->ef.edom, ep->edom, edgewords);
  	}
  }
  
  /*
   * Compute edge dominators.
   * Assumes graph has been leveled and predecessors established.
   */
  static void
  find_edom(root)
  	struct block *root;
  {
  	int i;
  	uset x;
  	struct block *b;
  
  	x = all_edge_sets;
  	for (i = n_edges * edgewords; --i >= 0; )
  		x[i] = ~0;
  
  	/* root->level is the highest level no found. */
  	memset(root->et.edom, 0, edgewords * sizeof(*(uset)0));
  	memset(root->ef.edom, 0, edgewords * sizeof(*(uset)0));
  	for (i = root->level; i >= 0; --i) {
  		for (b = levels[i]; b != 0; b = b->link) {
  			propedom(&b->et);
  			propedom(&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(root)
  	struct block *root;
  {
  	int i;
  	struct block *b;
  
  	/*
  	 * Initialize sets to contain no nodes.
  	 */
  	memset((char *)all_closure_sets, 0,
  	      n_blocks * nodewords * sizeof(*all_closure_sets));
  
  	/* root->level is the highest level no found. */
  	for (i = root->level; i >= 0; --i) {
  		for (b = levels[i]; b; b = b->link) {
  			SET_INSERT(b->closure, b->id);
  			if (JT(b) == 0)
  				continue;
  			SET_UNION(JT(b)->closure, b->closure, nodewords);
  			SET_UNION(JF(b)->closure, b->closure, 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(s)
  	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 :
  			(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(s)
  	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;
  }
  
  static void
  compute_local_ud(b)
  	struct block *b;
  {
  	struct slist *s;
  	atomset def = 0, use = 0, kill = 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))
  				kill |= ATOMMASK(atom);
  			def |= ATOMMASK(atom);
  		}
  	}
  	if (!ATOMELEM(def, A_ATOM) && BPF_CLASS(b->s.code) == BPF_JMP)
  		use |= ATOMMASK(A_ATOM);
  
  	b->def = def;
  	b->kill = kill;
  	b->in_use = use;
  }
  
  /*
   * Assume graph is already leveled.
   */
  static void
  find_ud(root)
  	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 = levels[i]; p; p = p->link) {
  			compute_local_ud(p);
  			p->out_use = 0;
  		}
  
  	for (i = 1; i <= maxlevel; ++i) {
  		for (p = 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;
  		}
  	}
  }
  
  /*
   * 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;
  };
  
  #define MODULUS 213
  static struct valnode *hashtbl[MODULUS];
  static int curval;
  static int maxval;
  
  /* Integer constants mapped with the load immediate opcode. */
  #define K(i) F(BPF_LD|BPF_IMM|BPF_W, i, 0L)
  
  struct vmapinfo {
  	int is_const;
  	bpf_int32 const_val;
  };
  
  struct vmapinfo *vmap;
  struct valnode *vnode_base;
  struct valnode *next_vnode;
  
  static void
  init_val()
  {
  	curval = 0;
  	next_vnode = vnode_base;
  	memset((char *)vmap, 0, maxval * sizeof(*vmap));
  	memset((char *)hashtbl, 0, sizeof hashtbl);
  }
  
  /* Because we really don't have an IR, this stuff is a little messy. */
  static int
  F(code, v0, v1)
  	int code;
  	int v0, v1;
  {
  	u_int hash;
  	int val;
  	struct valnode *p;
  
  	hash = (u_int)code ^ (v0 << 4) ^ (v1 << 8);
  	hash %= MODULUS;
  
  	for (p = hashtbl[hash]; p; p = p->next)
  		if (p->code == code && p->v0 == v0 && p->v1 == v1)
  			return p->val;
  
  	val = ++curval;
  	if (BPF_MODE(code) == BPF_IMM &&
  	    (BPF_CLASS(code) == BPF_LD || BPF_CLASS(code) == BPF_LDX)) {
  		vmap[val].const_val = v0;
  		vmap[val].is_const = 1;
  	}
  	p = next_vnode++;
  	p->val = val;
  	p->code = code;
  	p->v0 = v0;
  	p->v1 = v1;
  	p->next = hashtbl[hash];
  	hashtbl[hash] = p;
  
  	return val;
  }
  
  static __inline void
  vstore(s, valp, newval, alter)
  	struct stmt *s;
  	int *valp;
  	int newval;
  	int alter;
  {
  	if (alter && *valp == newval)
  		s->code = NOP;
  	else
  		*valp = newval;
  }
  
  static void
  fold_op(s, v0, v1)
  	struct stmt *s;
  	int v0, v1;
  {
  	bpf_int32 a, b;
  
  	a = vmap[v0].const_val;
  	b = 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("division by zero");
  		a /= b;
  		break;
  
  	case BPF_AND:
  		a &= b;
  		break;
  
  	case BPF_OR:
  		a |= b;
  		break;
  
  	case BPF_LSH:
  		a <<= b;
  		break;
  
  	case BPF_RSH:
  		a >>= b;
  		break;
  
  	case BPF_NEG:
  		a = -a;
  		break;
  
  	default:
  		abort();
  	}
  	s->k = a;
  	s->code = BPF_LD|BPF_IMM;
  	done = 0;
  }
  
  static __inline struct slist *
  this_op(s)
  	struct slist *s;
  {
  	while (s != 0 && s->s.code == NOP)
  		s = s->next;
  	return s;
  }
  
  static void
  opt_not(b)
  	struct block *b;
  {
  	struct block *tmp = JT(b);
  
  	JT(b) = JF(b);
  	JF(b) = tmp;
  }
  
  static void
  opt_peep(b)
  	struct block *b;
  {
  	struct slist *s;
  	struct slist *next, *last;
  	int val;
  
  	s = b->stmts;
  	if (s == 0)
  		return;
  
  	last = s;
  	while (1) {
  		s = this_op(s);
  		if (s == 0)
  			break;
  		next = this_op(s->next);
  		if (next == 0)
  			break;
  		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) {
  			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;
  			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))
  				break;
  
  			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))
  				break;
  
  			tax = this_op(add->next);
  			if (tax == 0 || tax->s.code != (BPF_MISC|BPF_TAX))
  				break;
  
  			ild = this_op(tax->next);
  			if (ild == 0 || BPF_CLASS(ild->s.code) != BPF_LD ||
  			    BPF_MODE(ild->s.code) != BPF_IND)
  				break;
  			/*
  			 * XXX We need to check that X is not
  			 * subsequently used.  We know we can eliminate the
  			 * accumulator modifications since it is defined
  			 * by the last stmt of this sequence.
  			 *
  			 * 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]
  			 *
  			 */
  			ild->s.k += s->s.k;
  			s->s.code = NOP;
  			add->s.code = NOP;
  			tax->s.code = NOP;
  			done = 0;
  		}
  		s = next;
  	}
  	/*
  	 * 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.
  	 */
  	if (last->s.code == (BPF_ALU|BPF_SUB|BPF_X) &&
  	    !ATOMELEM(b->out_use, A_ATOM)) {
  		val = b->val[X_ATOM];
  		if (vmap[val].is_const) {
  			int op;
  
  			b->s.k += vmap[val].const_val;
  			op = BPF_OP(b->s.code);
  			if (op == BPF_JGT || op == BPF_JGE) {
  				struct block *t = JT(b);
  				JT(b) = JF(b);
  				JF(b) = t;
  				b->s.k += 0x80000000;
  			}
  			last->s.code = NOP;
  			done = 0;
  		} else if (b->s.k == 0) {
  			/*
  			 * sub x  ->	nop
  			 * j  #0	j  x
  			 */
  			last->s.code = NOP;
  			b->s.code = BPF_CLASS(b->s.code) | BPF_OP(b->s.code) |
  				BPF_X;
  			done = 0;
  		}
  	}
  	/*
  	 * Likewise, a constant subtract can be simplified.
  	 */
  	else if (last->s.code == (BPF_ALU|BPF_SUB|BPF_K) &&
  		 !ATOMELEM(b->out_use, A_ATOM)) {
  		int op;
  
  		b->s.k += last->s.k;
  		last->s.code = NOP;
  		op = BPF_OP(b->s.code);
  		if (op == BPF_JGT || op == BPF_JGE) {
  			struct block *t = JT(b);
  			JT(b) = JF(b);
  			JF(b) = t;
  			b->s.k += 0x80000000;
  		}
  		done = 0;
  	}
  	/*
  	 * and #k	nop
  	 * jeq #0  ->	jset #k
  	 */
  	if (last->s.code == (BPF_ALU|BPF_AND|BPF_K) &&
  	    !ATOMELEM(b->out_use, A_ATOM) && b->s.k == 0) {
  		b->s.k = last->s.k;
  		b->s.code = BPF_JMP|BPF_K|BPF_JSET;
  		last->s.code = NOP;
  		done = 0;
  		opt_not(b);
  	}
  	/*
  	 * If the accumulator is a known constant, we can compute the
  	 * comparison result.
  	 */
  	val = b->val[A_ATOM];
  	if (vmap[val].is_const && BPF_SRC(b->s.code) == BPF_K) {
  		bpf_int32 v = 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 > b->s.k;
  			break;
  
  		case BPF_JGE:
  			v = (unsigned)v >= b->s.k;
  			break;
  
  		case BPF_JSET:
  			v &= b->s.k;
  			break;
  
  		default:
  			abort();
  		}
  		if (JF(b) != JT(b))
  			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(s, val, alter)
  	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(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 && vmap[v].is_const) {
  			s->code = BPF_LD|BPF_ABS|BPF_SIZE(s->code);
  			s->k += vmap[v].const_val;
  			v = F(s->code, s->k, 0L);
  			done = 0;
  		}
  		else
  			v = F(s->code, s->k, v);
  		vstore(s, &val[A_ATOM], v, alter);
  		break;
  
  	case BPF_LD|BPF_LEN:
  		v = F(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(s->code, s->k, 0L);
  		vstore(s, &val[X_ATOM], v, alter);
  		break;
  
  	case BPF_ALU|BPF_NEG:
  		if (alter && vmap[val[A_ATOM]].is_const) {
  			s->code = BPF_LD|BPF_IMM;
  			s->k = -vmap[val[A_ATOM]].const_val;
  			val[A_ATOM] = K(s->k);
  		}
  		else
  			val[A_ATOM] = F(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_AND|BPF_K:
  	case BPF_ALU|BPF_OR|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) {
  				if (op == BPF_ADD || op == BPF_SUB ||
  				    op == BPF_LSH || op == BPF_RSH ||
  				    op == BPF_OR) {
  					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 (vmap[val[A_ATOM]].is_const) {
  				fold_op(s, val[A_ATOM], K(s->k));
  				val[A_ATOM] = K(s->k);
  				break;
  			}
  		}
  		val[A_ATOM] = F(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_AND|BPF_X:
  	case BPF_ALU|BPF_OR|BPF_X:
  	case BPF_ALU|BPF_LSH|BPF_X:
  	case BPF_ALU|BPF_RSH|BPF_X:
  		op = BPF_OP(s->code);
  		if (alter && vmap[val[X_ATOM]].is_const) {
  			if (vmap[val[A_ATOM]].is_const) {
  				fold_op(s, val[A_ATOM], val[X_ATOM]);
  				val[A_ATOM] = K(s->k);
  			}
  			else {
  				s->code = BPF_ALU|BPF_K|op;
  				s->k = vmap[val[X_ATOM]].const_val;
  				done = 0;
  				val[A_ATOM] =
  					F(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, and -1, etc.
  		 */
  		if (alter && vmap[val[A_ATOM]].is_const
  		    && vmap[val[A_ATOM]].const_val == 0) {
  			if (op == BPF_ADD || op == BPF_OR ||
  			    op == BPF_LSH || op == BPF_RSH || op == BPF_SUB) {
  				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_AND) {
  				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(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 && vmap[v].is_const) {
  			s->code = BPF_LD|BPF_IMM;
  			s->k = vmap[v].const_val;
  			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 && vmap[v].is_const) {
  			s->code = BPF_LDX|BPF_IMM;
  			s->k = vmap[v].const_val;
  			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(s, last)
  	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]) {
  			done = 0;
  			last[atom]->code = NOP;
  		}
  		last[atom] = s;
  	}
  }
  
  static void
  opt_deadstores(b)
  	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(&s->s, last);
  	deadstmt(&b->s, last);
  
  	for (atom = 0; atom < N_ATOMS; ++atom)
  		if (last[atom] && !ATOMELEM(b->out_use, atom)) {
  			last[atom]->code = NOP;
  			done = 0;
  		}
  }
  
  static void
  opt_blk(b, do_stmts)
  	struct block *b;
  	int do_stmts;
  {
  	struct slist *s;
  	struct edge *p;
  	int i;
  	bpf_int32 aval;
  
  #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.
  	 * If we have no predecessors, everything is undefined.
  	 * Otherwise, we inherent our values from our predecessors.
  	 * If any register has an ambiguous value (i.e. control paths are
  	 * merging) give it the undefined value of 0.
  	 */
  	p = b->in_edges;
  	if (p == 0)
  		memset((char *)b->val, 0, sizeof(b->val));
  	else {
  		memcpy((char *)b->val, (char *)p->pred->val, sizeof(b->val));
  		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];
  	for (s = b->stmts; s; s = s->next)
  		opt_stmt(&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 with value that is
  	 * already there, or if this block is a return,
  	 * eliminate all the statements.
  	 */
  	if (do_stmts && 
  	    ((b->out_use == 0 && aval != 0 &&b->val[A_ATOM] == aval) ||
  	     BPF_CLASS(b->s.code) == BPF_RET)) {
  		if (b->stmts != 0) {
  			b->stmts = 0;
  			done = 0;
  		}
  	} else {
  		opt_peep(b);
  		opt_deadstores(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(b, succ)
  	struct block *b, *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(child, ep)
  	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 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.  We rely on the fact that
  		 * distinct constants have distinct value numbers.
  		 */
  		return JF(child);
  
  	return 0;
  }
  
  static void
  opt_j(ep)
  	struct edge *ep;
  {
  	int i, k;
  	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)) {
  			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 < edgewords; ++i) {
  		bpf_u_int32 x = ep->edom[i];
  
  		while (x != 0) {
  			k = ffs(x) - 1;
  			x &=~ (1 << k);
  			k += i * BITS_PER_WORD;
  
  			target = fold_edge(ep->succ, 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)) {
  				done = 0;
  				ep->succ = target;
  				if (JT(target) != 0)
  					/*
  					 * Start over unless we hit a leaf.
  					 */
  					goto top;
  				return;
  			}
  		}
  	}
  }
  
  
  static void
  or_pullup(b)
  	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;
  
  	done = 0;
  }
  
  static void
  and_pullup(b)
  	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;
  
  	done = 0;
  }
  
  static void
  opt_blks(root, do_stmts)
  	struct block *root;
  	int do_stmts;
  {
  	int i, maxlevel;
  	struct block *p;
  
  	init_val();
  	maxlevel = root->level;
  	for (i = maxlevel; i >= 0; --i)
  		for (p = levels[i]; p; p = p->link)
  			opt_blk(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 = levels[i]; p; p = p->link) {
  			opt_j(&p->et);
  			opt_j(&p->ef);
  		}
  	}
  	for (i = 1; i <= maxlevel; ++i) {
  		for (p = levels[i]; p; p = p->link) {
  			or_pullup(p);
  			and_pullup(p);
  		}
  	}
  }
  
  static __inline void
  link_inedge(parent, child)
  	struct edge *parent;
  	struct block *child;
  {
  	parent->next = child->in_edges;
  	child->in_edges = parent;
  }
  
  static void
  find_inedges(root)
  	struct block *root;
  {
  	int i;
  	struct block *b;
  
  	for (i = 0; i < n_blocks; ++i)
  		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 = levels[i]; b != 0; b = b->link) {
  			link_inedge(&b->et, JT(b));
  			link_inedge(&b->ef, JF(b));
  		}
  	}
  }
  
  static void
  opt_root(b)
  	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(root, do_stmts)
  	struct block *root;
  	int do_stmts;
  {
  
  #ifdef BDEBUG
  	if (dflag > 1)
  		opt_dump(root);
  #endif
  	do {
  		done = 1;
  		find_levels(root);
  		find_dom(root);
  		find_closure(root);
  		find_inedges(root);
  		find_ud(root);
  		find_edom(root);
  		opt_blks(root, do_stmts);
  #ifdef BDEBUG
  		if (dflag > 1)
  			opt_dump(root);
  #endif
  	} while (!done);
  }
  
  /*
   * Optimize the filter code in its dag representation.
   */
  void
  bpf_optimize(rootp)
  	struct block **rootp;
  {
  	struct block *root;
  
  	root = *rootp;
  
  	opt_init(root);
  	opt_loop(root, 0);
  	opt_loop(root, 1);
  	intern_blocks(root);
  	opt_root(rootp);
  	opt_cleanup();
  }
  
  static void
  make_marks(p)
  	struct block *p;
  {
  	if (!isMarked(p)) {
  		Mark(p);
  		if (BPF_CLASS(p->s.code) != BPF_RET) {
  			make_marks(JT(p));
  			make_marks(JF(p));
  		}
  	}
  }
  
  /*
   * Mark code array such that isMarked(i) is true
   * only for nodes that are alive.
   */
  static void
  mark_code(p)
  	struct block *p;
  {
  	cur_mark += 1;
  	make_marks(p);
  }
  
  /*
   * True iff the two stmt lists load the same value from the packet into
   * the accumulator.
   */
  static int
  eq_slist(x, y)
  	struct slist *x, *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(b0, b1)
  	struct block *b0, *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(root)
  	struct block *root;
  {
  	struct block *p;
  	int i, j;
  	int done;
   top:
  	done = 1;
  	for (i = 0; i < n_blocks; ++i)
  		blocks[i]->link = 0;
  
  	mark_code(root);
  
  	for (i = n_blocks - 1; --i >= 0; ) {
  		if (!isMarked(blocks[i]))
  			continue;
  		for (j = i + 1; j < n_blocks; ++j) {
  			if (!isMarked(blocks[j]))
  				continue;
  			if (eq_blk(blocks[i], blocks[j])) {
  				blocks[i]->link = blocks[j]->link ?
  					blocks[j]->link : blocks[j];
  				break;
  			}
  		}
  	}
  	for (i = 0; i < n_blocks; ++i) {
  		p = blocks[i];
  		if (JT(p) == 0)
  			continue;
  		if (JT(p)->link) {
  			done = 0;
  			JT(p) = JT(p)->link;
  		}
  		if (JF(p)->link) {
  			done = 0;
  			JF(p) = JF(p)->link;
  		}
  	}
  	if (!done)
  		goto top;
  }
  
  static void
  opt_cleanup()
  {
  	free((void *)vnode_base);
  	free((void *)vmap);
  	free((void *)edges);
  	free((void *)space1);
  	free((void *)space2);
  	free((void *)levels);
  	free((void *)blocks);
  }
  
  /*
   * Return the number of stmts in 's'.
   */
  static int
  slength(s)
  	struct slist *s;
  {
  	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(p)
  	struct block *p;
  {
  	if (p == 0 || isMarked(p))
  		return 0;
  	Mark(p);
  	return count_blocks(JT(p)) + count_blocks(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(p)
  	struct block *p;
  {
  	int n;
  
  	if (p == 0 || isMarked(p))
  		return;
  
  	Mark(p);
  	n = n_blocks++;
  	p->id = n;
  	blocks[n] = p;
  
  	number_blks_r(JT(p));
  	number_blks_r(JF(p));
  }
  
  /*
   * Return the number of stmts in the flowgraph reachable by 'p'.
   * The nodes should be unmarked before calling.
   */
  static int
  count_stmts(p)
  	struct block *p;
  {
  	int n;
  
  	if (p == 0 || isMarked(p))
  		return 0;
  	Mark(p);
  	n = count_stmts(JT(p)) + count_stmts(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(root)
  	struct block *root;
  {
  	bpf_u_int32 *p;
  	int i, n, max_stmts;
  	size_t size1, size2;
  
  	/*
  	 * First, count the blocks, so we can malloc an array to map
  	 * block number to block.  Then, put the blocks into the array.
  	 */
  	unMarkAll();
  	n = count_blocks(root);
  	blocks = reallocarray(NULL, n, sizeof(*blocks));
  	if (blocks == NULL)
  		bpf_error("malloc");
  
  	unMarkAll();
  	n_blocks = 0;
  	number_blks_r(root);
  
  	n_edges = 2 * n_blocks;
  	edges = reallocarray(NULL, n_edges, sizeof(*edges));
  	if (edges == NULL)
  		bpf_error("malloc");
  
  	/*
  	 * The number of levels is bounded by the number of nodes.
  	 */
  	levels = reallocarray(NULL, n_blocks, sizeof(*levels));
  	if (levels == NULL)
  		bpf_error("malloc");
  
  	edgewords = n_edges / (8 * sizeof(bpf_u_int32)) + 1;
  	nodewords = n_blocks / (8 * sizeof(bpf_u_int32)) + 1;
  
  	size1 = 2;
  	if (n_blocks > SIZE_MAX / size1)
  		goto fail1;
  	size1 *= n_blocks;
  	if (nodewords > SIZE_MAX / size1)
  		goto fail1;
  	size1 *= nodewords;
  	if (sizeof(*space1) > SIZE_MAX / size1)
  		goto fail1;
  	size1 *= sizeof(*space1);
  
  	space1 = (bpf_u_int32 *)malloc(size1);
  	if (space1 == NULL) {
  fail1:
  		bpf_error("malloc");
  	}
  
  	size2 = n_edges;
  	if (edgewords > SIZE_MAX / size2)
  		goto fail2;
  	size2 *= edgewords;
  	if (sizeof(*space2) > SIZE_MAX / size2)
  		goto fail2;
  	size2 *= sizeof(*space2);
  
  	space2 = (bpf_u_int32 *)malloc(size2);
  	if (space2 == NULL) {
  fail2:
  		free(space1);
  		bpf_error("malloc");
  	}
  	
  	p = space1;
  	all_dom_sets = p;
  	for (i = 0; i < n; ++i) {
  		blocks[i]->dom = p;
  		p += nodewords;
  	}
  	all_closure_sets = p;
  	for (i = 0; i < n; ++i) {
  		blocks[i]->closure = p;
  		p += nodewords;
  	}
  	p = space2;
  	all_edge_sets = p;
  	for (i = 0; i < n; ++i) {
  		struct block *b = blocks[i];
  
  		b->et.edom = p;
  		p += edgewords;
  		b->ef.edom = p;
  		p += edgewords;
  		b->et.id = i;
  		edges[i] = &b->et;
  		b->ef.id = n_blocks + i;
  		edges[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(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.
  	 */
  	maxval = 3 * max_stmts;
  	vmap = reallocarray(NULL, maxval, sizeof(*vmap));
  	vnode_base = reallocarray(NULL, maxval, sizeof(*vnode_base));
  	if (vmap == NULL || vnode_base == NULL)
  		bpf_error("malloc");
  }
  
  /*
   * 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.
   */
  static struct bpf_insn *fstart;
  static struct bpf_insn *ftail;
  
  #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(p)
  	struct block *p;
  {
  	struct bpf_insn *dst;
  	struct slist *src;
  	int slen;
  	u_int off;
  	int extrajmps;		/* number of extra jumps inserted */
  	struct slist **offset = NULL;
  
  	if (p == 0 || isMarked(p))
  		return (1);
  	Mark(p);
  
  	if (convert_code_r(JF(p)) == 0)
  		return (0);
  	if (convert_code_r(JT(p)) == 0)
  		return (0);
  
  	slen = slength(p->stmts);
  	dst = ftail -= (slen + 1 + p->longjt + p->longjf);
  		/* inflate length by any extra jumps */
  
  	p->offset = dst - fstart;
  
  	/* generate offset[] for convenience  */
  	if (slen) {
  		offset = calloc(slen, sizeof(struct slist *));
  		if (!offset) {
  			bpf_error("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("illegal jmp destination");
  				/*NOTREACHED*/
  			}
  #endif
  			goto filled;
  		}
  		if (off == slen - 2)	/*???*/
  			goto filled;
  
  	    {
  		int i;
  		int jt, jf;
  		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(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(ljerr, "multiple matches", off);
  					/*NOTREACHED*/
  				}
  
  				dst->jt = i - off - 1;
  				jt++;
  			}
  			if (offset[i] == src->s.jf) {
  				if (jf) {
  					bpf_error(ljerr, "multiple matches", off);
  					/*NOTREACHED*/
  				}
  				dst->jf = i - off - 1;
  				jf++;
  			}
  		}
  		if (!jt || !jf) {
  			bpf_error(ljerr, "no destination found", off);
  			/*NOTREACHED*/
  		}
  	    }
  filled:
  		++dst;
  		++off;
  	}
  	free(offset);
  
  #ifdef BDEBUG
  	bids[dst - 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.
   */
  struct bpf_insn *
  icode_to_fcode(root, lenp)
  	struct block *root;
  	int *lenp;
  {
  	int n;
  	struct bpf_insn *fp;
  
  	/*
  	 * Loop doing convert_codr_r() until no branches remain
  	 * with too-large offsets.
  	 */
  	while (1) {
  	    unMarkAll();
  	    n = *lenp = count_stmts(root);
      
  	    fp = calloc(n, sizeof(*fp));
  	    if (fp == NULL)
  		    bpf_error("calloc");
  
  	    fstart = fp;
  	    ftail = fp + n;
      
  	    unMarkAll();
  	    if (convert_code_r(root))
  		break;
  	    free(fp);
  	}
  
  	return fp;
  }
  
  #ifdef BDEBUG
  static void
  opt_dump(root)
  	struct block *root;
  {
  	struct bpf_program f;
  
  	memset(bids, 0, sizeof bids);
  	f.bf_insns = icode_to_fcode(root, &f.bf_len);
  	bpf_dump(&f, 1);
  	putchar('\n');
  	free((char *)f.bf_insns);
  }
  #endif
  