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bpf_optimize.c
2039 lines (1793 loc) · 41.3 KB
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bpf_optimize.c
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/* $OpenBSD: optimize.c,v 1.13 2007/09/02 15:19:18 deraadt Exp $ */
/* $Id: bpf_optimize.c,v 1.2 2007/10/01 02:35:41 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 <memory.h>
#include <pcap/pcap.h>
#include "sf-gencode.h"
#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 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 *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++;\
}
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(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(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(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(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(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(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(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;
}
static void
compute_local_ud(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(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(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 = 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(struct stmt *s, int *valp, int newval, int alter)
{
if (alter && *valp == newval)
s->code = NOP;
else
*valp = newval;
}
static void
fold_op(struct stmt *s, int v0, int 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)
sf_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(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(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(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];