ref: 63f9a4fda36f916e543c8f3ea117b31c194a4db7
dir: /sys/src/cmd/7l/noop.c/
#include "l.h"
Prog *divuconst(Prog *, uvlong, int, int, int);
Prog *divconst(Prog *, vlong, int, int, int);
Prog *modconst(Prog *, vlong, int, int, int);
void excise(Prog *);
void
noops(void)
{
Prog *p, *p1, *q, *q1, *q2;
int o, curframe, curbecome, maxbecome, shift;
/*
* find leaf subroutines
* become sizes
* frame sizes
* strip NOPs
* expand RET and other macros
* expand BECOME pseudo
* use conditional moves where appropriate
*/
if(debug['v'])
Bprint(&bso, "%5.2f noops\n", cputime());
Bflush(&bso);
curframe = 0;
curbecome = 0;
maxbecome = 0;
curtext = 0;
q = P;
for(p = firstp; p != P; p = p->link) {
/* find out how much arg space is used in this TEXT */
if(p->to.type == D_OREG && p->to.reg == REGSP)
if(p->to.offset > curframe)
curframe = p->to.offset;
switch(p->as) {
case ATEXT:
if(curtext && curtext->from.sym) {
curtext->from.sym->frame = curframe;
curtext->from.sym->become = curbecome;
if(curbecome > maxbecome)
maxbecome = curbecome;
}
curframe = 0;
curbecome = 0;
p->mark |= LABEL|LEAF|SYNC;
if(p->link)
p->link->mark |= LABEL;
curtext = p;
break;
/* don't mess with what we don't understand */
case AWORD:
case ACALL_PAL:
/* etc. */
p->mark |= LABEL;
for(q1=p->link; q1 != P; q1 = q1->link) {
q1->mark |= LABEL;
if(q1->as != AXORNOT) /* used as NOP in PALcode */
break;
}
break;
case ARET:
/* special form of RET is BECOME */
if(p->from.type == D_CONST)
if(p->from.offset > curbecome)
curbecome = p->from.offset;
if(p->link != P)
p->link->mark |= LABEL;
break;
case ANOP:
q1 = p->link;
q->link = q1; /* q is non-nop */
q1->mark |= p->mark;
continue;
case AJSR:
if(curtext != P)
curtext->mark &= ~LEAF;
case ABEQ:
case ABNE:
case ABGE:
case ABGT:
case ABLE:
case ABLT:
case ABLBC:
case ABLBS:
case AFBEQ:
case AFBNE:
case AFBGE:
case AFBGT:
case AFBLE:
case AFBLT:
case AJMP:
p->mark |= BRANCH;
q1 = p->cond;
if(q1 != P) {
while(q1->as == ANOP) {
q1 = q1->link;
p->cond = q1;
}
if(!(q1->mark & LEAF)) {
if (q1->mark & LABEL)
q1->mark |= LABEL2;
else
q1->mark |= LABEL;
}
} else
p->mark |= LABEL;
q1 = p->link;
if(q1 != P) {
if (q1->mark & LABEL)
q1->mark |= LABEL2;
else
q1->mark |= LABEL;
}
else
p->mark |= LABEL; /* ??? */
break;
case ADIVQ:
case ADIVQU:
case AMODQ:
case AMODQU:
case ADIVL:
case ADIVLU:
case AMODL:
case AMODLU:
if(p->from.type == D_CONST /*&& !debug['d']*/)
continue;
if(prog_divq == P)
initdiv();
if(curtext != P)
curtext->mark &= ~LEAF;
break;
}
q = p;
}
if(curtext && curtext->from.sym) {
curtext->from.sym->frame = curframe;
curtext->from.sym->become = curbecome;
if(curbecome > maxbecome)
maxbecome = curbecome;
}
if(debug['b'])
print("max become = %d\n", maxbecome);
xdefine("ALEFbecome", STEXT, maxbecome);
curtext = 0;
for(p = firstp; p != P; p = p->link) {
switch(p->as) {
case ATEXT:
curtext = p;
break;
case AJSR:
if(curtext != P && curtext->from.sym != S && curtext->to.offset >= 0) {
o = maxbecome - curtext->from.sym->frame;
if(o <= 0)
break;
/* calling a become or calling a variable */
if(p->to.sym == S || p->to.sym->become) {
curtext->to.offset += o;
if(debug['b']) {
curp = p;
print("%D calling %D increase %d\n",
&curtext->from, &p->to, o);
}
}
}
break;
}
}
for(p = firstp; p != P; p = p->link) {
o = p->as;
switch(o) {
case ATEXT:
curtext = p;
autosize = p->to.offset + 8;
if(autosize <= 8)
if(curtext->mark & LEAF) {
p->to.offset = -8;
autosize = 0;
}
if (autosize & 4)
autosize += 4;
q = p;
if(autosize)
q = genIRR(p, ASUBQ, autosize, NREG, REGSP);
else if(!(curtext->mark & LEAF)) {
if(debug['v'])
Bprint(&bso, "save suppressed in: %s\n",
curtext->from.sym->name);
Bflush(&bso);
curtext->mark |= LEAF;
}
if(curtext->mark & LEAF) {
if(curtext->from.sym)
curtext->from.sym->type = SLEAF;
break;
}
genstore(q, AMOVL, REGLINK, 0LL, REGSP);
break;
case ARET:
nocache(p);
if(p->from.type == D_CONST)
goto become;
if(curtext->mark & LEAF) {
if(!autosize) {
p->as = AJMP;
p->from = zprg.from;
p->to.type = D_OREG;
p->to.offset = 0;
p->to.reg = REGLINK;
break;
}
p->as = AADDQ;
p->from.type = D_CONST;
p->from.offset = autosize;
p->to.type = D_REG;
p->to.reg = REGSP;
q = prg();
q->as = AJMP;
q->line = p->line;
q->to.type = D_OREG;
q->to.offset = 0;
q->to.reg = REGLINK;
q->mark |= BRANCH;
q->link = p->link;
p->link = q;
break;
}
p->as = AMOVL;
p->from.type = D_OREG;
p->from.offset = 0;
p->from.reg = REGSP;
p->to.type = D_REG;
p->to.reg = REGLINK;
q = p;
if(autosize)
q = genIRR(p, AADDQ, autosize, NREG, REGSP);
q1 = prg();
q1->as = AJMP;
q1->line = p->line;
q1->to.type = D_OREG;
q1->to.offset = 0;
q1->to.reg = REGLINK;
q1->mark |= BRANCH;
q1->link = q->link;
q->link = q1;
break;
become:
if(curtext->mark & LEAF) {
q = prg();
q->line = p->line;
q->as = AJMP;
q->from = zprg.from;
q->to = p->to;
q->cond = p->cond;
q->link = p->link;
q->mark |= BRANCH;
p->link = q;
p->as = AADDQ;
p->from = zprg.from;
p->from.type = D_CONST;
p->from.offset = autosize;
p->to = zprg.to;
p->to.type = D_REG;
p->to.reg = REGSP;
break;
}
q = prg();
q->line = p->line;
q->as = AJMP;
q->from = zprg.from;
q->to = p->to;
q->cond = p->cond;
q->link = p->link;
q->mark |= BRANCH;
p->link = q;
q = genIRR(p, AADDQ, autosize, NREG, REGSP);
p->as = AMOVL;
p->from = zprg.from;
p->from.type = D_OREG;
p->from.offset = 0;
p->from.reg = REGSP;
p->to = zprg.to;
p->to.type = D_REG;
p->to.reg = REGLINK;
break;
/* All I wanted was a MOVB... */
case AMOVB:
case AMOVW:
/* rewrite sign extend; could use v3 extension in asmout case 1 */
if (p->to.type == D_REG) {
nocache(p);
shift = (p->as == AMOVB) ? (64-8) : (64-16);
if (p->from.type == D_REG) {
p->as = ASLLQ;
p->reg = p->from.reg;
p->from.type = D_CONST;
p->from.offset = shift;
q = genIRR(p, ASRAQ, shift, p->to.reg, p->to.reg);
break;
}
else {
p->as = (p->as == AMOVB) ? AMOVBU : AMOVWU;
q = genIRR(p, ASLLQ, shift, p->to.reg, p->to.reg);
q = genIRR(q, ASRAQ, shift, p->to.reg, p->to.reg);
}
}
/* fall through... */
case AMOVBU:
case AMOVWU:
if(!debug['x'])
break; /* use BWX extension */
o = p->as;
nocache(p);
if (p->from.type == D_OREG) {
if (p->to.type != D_REG)
break;
p->as = AMOVQU;
q = genXXX(p, AEXTBL, &p->to, REGTMP2, &p->to);
if (o == AMOVW || o == AMOVWU)
q->as = AEXTWL;
p->to.reg = REGTMP2;
if ((p->from.offset & 7) != 0 || aclass(&p->from) != C_SOREG) {
q1 = genXXX(p, AMOVA, &p->from, NREG, &q->to);
q1->from.offset &= 7;
q->from = q->to;
}
else
q->from.reg = p->from.reg;
if (o == AMOVB || o == AMOVW)
genXXX(q, o, &q->to, NREG, &q->to);
}
else if (p->to.type == D_OREG) {
if (aclass(&p->from) == C_ZCON) {
p->from.type = D_REG;
p->from.reg = REGZERO;
}
else if (p->from.type != D_REG)
break;
p->as = AMOVQU;
q = genRRR(p, AMSKBL, p->to.reg, REGTMP2, REGTMP2);
q1 = genRRR(q, AINSBL, p->to.reg, p->from.reg, REGTMP);
if (o == AMOVW || o == AMOVWU) {
q->as = AMSKWL;
q1->as = AINSWL;
}
q2 = genXXX(q1, AOR, &q->to, REGTMP, &q->to);
genXXX(q2, AMOVQU, &q->to, NREG, &p->to);
p->from = p->to;
p->to = q->to;
if ((p->from.offset & 7) != 0 || aclass(&p->from) != C_SOREG) {
q->from.reg = REGTMP;
q1->from.reg = REGTMP;
q = genXXX(p, AMOVA, &p->from, NREG, &q->from);
q->from.offset &= 7;
}
}
break;
case ASLLL:
p->as = ASLLQ;
p = genXXX(p, AADDL, &p->to, REGZERO, &p->to);
break;
case ASRLL:
if (p->to.type != D_REG) {
diag("illegal dest type in %P", p);
break;
}
if (p->reg == NREG)
p->reg = p->to.reg;
q = genXXX(p, ASRLQ, &p->from, REGTMP, &p->to);
p->as = AZAP;
p->from.type = D_CONST;
p->from.offset = 0xf0;
p->to.reg = REGTMP;
p = q;
p = genXXX(p, AADDL, &p->to, REGZERO, &p->to);
break;
case ASRAL:
p->as = ASRAQ;
break;
case ADIVQ:
case ADIVQU:
case AMODQ:
case AMODQU:
case ADIVL:
case ADIVLU:
case AMODL:
case AMODLU:
/* if (debug['d'])
print("%P\n", p); */
if(p->to.type != D_REG)
break;
/*if(debug['d'] && p->from.type == D_CONST) {
q = genRRR(p, p->as, REGTMP, p->reg, p->to.reg);
p->as = AMOVQ;
p->reg = NREG;
p->to.reg = REGTMP;
p = q;
}*/
if(p->from.type == D_CONST) {
if (p->reg == NREG)
p->reg = p->to.reg;
switch (p->as) {
case ADIVQ:
q = divconst(p, p->from.offset, p->reg, p->to.reg, 64);
break;
case ADIVQU:
q = divuconst(p, p->from.offset, p->reg, p->to.reg, 64);
break;
case AMODQ:
q = modconst(p, p->from.offset, p->reg, p->to.reg, 64);
break;
case AMODQU:
q = divuconst(p, p->from.offset, p->reg, REGTMP2, 64);
q = genIRR(q, AMULQ, p->from.offset, REGTMP2, REGTMP2);
q = genRRR(q, ASUBQ, REGTMP2, p->reg, p->to.reg);
break;
case ADIVL:
q = divconst(p, p->from.offset, p->reg, p->to.reg, 32);
break;
case ADIVLU:
q = divuconst(p, p->from.offset, p->reg, p->to.reg, 32);
break;
case AMODL:
q = modconst(p, p->from.offset, p->reg, p->to.reg, 32);
break;
case AMODLU:
q = divuconst(p, p->from.offset, p->reg, REGTMP2, 32);
q = genIRR(q, AMULQ, p->from.offset, REGTMP2, REGTMP2);
q = genRRR(q, ASUBQ, REGTMP2, p->reg, p->to.reg);
break;
}
excise(p);
p = q;
break;
}
if(p->from.type != D_REG){
diag("bad instruction %P", p);
break;
}
o = p->as;
q = genIRR(p, ASUBQ, 16LL, NREG, REGSP);
q = genstore(q, AMOVQ, p->from.reg, 8LL, REGSP);
if (o == ADIVL || o == ADIVL || o == AMODL || o == AMODLU)
q->as = AMOVL;
q = genRRR(q, AMOVQ, p->reg, NREG, REGTMP);
if (p->reg == NREG)
q->from.reg = p->to.reg;
/* CALL appropriate */
q1 = prg();
q1->link = q->link;
q->link = q1;
q1->as = AJSR;
q1->line = p->line;
q1->to.type = D_BRANCH;
q1->cond = divsubr(o);
q1->mark |= BRANCH;
q = q1;
q = genRRR(q, AMOVQ, REGTMP, NREG, p->to.reg);
q = genIRR(q, AADDQ, 16LL, NREG, REGSP);
excise(p);
p = q;
break;
/* Attempt to replace {cond. branch, mov} with a cmov */
/* XXX warning: this is all a bit experimental */
case ABEQ:
case ABNE:
case ABGE:
case ABGT:
case ABLE:
case ABLT:
case ABLBC:
case ABLBS:
q = p->link;
if (q == P)
break;
q1 = q->link;
if (q1 != p->cond || q1 == P)
break;
/*print("%P\n", q); /* */
if (q->to.type != D_REG)
break;
if (q->from.type != D_REG && (q->from.type != D_CONST || q->from.name != D_NONE))
break;
if (q->mark&LABEL2)
break;
/* print("%P\n", q); /* */
if (q->as != AMOVQ) /* XXX can handle more than this! */
break;
q->as = (p->as^1) + ACMOVEQ-ABEQ; /* sleazy hack */
q->reg = p->from.reg; /* XXX check CMOVx operand order! */
excise(p); /* XXX p's LABEL? */
if (!(q1->mark&LABEL2))
q1->mark &= ~LABEL;
break;
case AFBEQ:
case AFBNE:
case AFBGE:
case AFBGT:
case AFBLE:
case AFBLT:
q = p->link;
if (q == P)
break;
q1 = q->link;
if (q1 != p->cond || q1 == P)
break;
if (q->from.type != D_FREG || q->to.type != D_FREG)
break;
/* print("%P\n", q); /* */
if (q->mark&LABEL2)
break;
if (q->as != AMOVT) /* XXX can handle more than this! */
break;
q->as = (p->as^1) + AFCMOVEQ-AFBEQ; /* sleazy hack */
q->reg = p->from.reg; /* XXX check CMOVx operand order! */
excise(p); /* XXX p's LABEL? */
if (!(q1->mark&LABEL2))
q1->mark &= ~LABEL;
break;
}
}
curtext = P;
q = P; /* p - 1 */
q1 = firstp; /* top of block */
o = 0; /* count of instructions */
for(p = firstp; p != P; p = p1) {
p1 = p->link;
o++;
if(p->mark & NOSCHED){
if(q1 != p){
sched(q1, q);
}
for(; p != P; p = p->link){
if(!(p->mark & NOSCHED))
break;
q = p;
}
p1 = p;
q1 = p;
o = 0;
continue;
}
if(p->mark & (LABEL|SYNC)) {
if(q1 != p)
sched(q1, q);
q1 = p;
o = 1;
}
if(p->mark & (BRANCH|SYNC)) {
sched(q1, p);
q1 = p1;
o = 0;
}
if(o >= NSCHED) {
sched(q1, p);
q1 = p1;
o = 0;
}
q = p;
}
}
void
nocache(Prog *p)
{
p->optab = 0;
p->from.class = 0;
p->to.class = 0;
}
/* XXX use of this may lose important LABEL flags, check that this isn't happening (or fix) */
void
excise(Prog *p)
{
Prog *q;
q = p->link;
*p = *q;
}
void
initdiv(void)
{
Sym *s1, *s2, *s3, *s4, *s5, *s6, *s7, *s8;
Prog *p;
s1 = lookup("_divq", 0);
s2 = lookup("_divqu", 0);
s3 = lookup("_modq", 0);
s4 = lookup("_modqu", 0);
s5 = lookup("_divl", 0);
s6 = lookup("_divlu", 0);
s7 = lookup("_modl", 0);
s8 = lookup("_modlu", 0);
for(p = firstp; p != P; p = p->link)
if(p->as == ATEXT) {
if(p->from.sym == s1)
prog_divq = p;
if(p->from.sym == s2)
prog_divqu = p;
if(p->from.sym == s3)
prog_modq = p;
if(p->from.sym == s4)
prog_modqu = p;
if(p->from.sym == s5)
prog_divl = p;
if(p->from.sym == s6)
prog_divlu = p;
if(p->from.sym == s7)
prog_modl = p;
if(p->from.sym == s8)
prog_modlu = p;
}
if(prog_divq == P) {
diag("undefined: %s", s1->name);
prog_divq = curtext;
}
if(prog_divqu == P) {
diag("undefined: %s", s2->name);
prog_divqu = curtext;
}
if(prog_modq == P) {
diag("undefined: %s", s3->name);
prog_modq = curtext;
}
if(prog_modqu == P) {
diag("undefined: %s", s4->name);
prog_modqu = curtext;
}
if(prog_divl == P) {
diag("undefined: %s", s5->name);
prog_divl = curtext;
}
if(prog_divlu == P) {
diag("undefined: %s", s6->name);
prog_divlu = curtext;
}
if(prog_modl == P) {
diag("undefined: %s", s7->name);
prog_modl = curtext;
}
if(prog_modlu == P) {
diag("undefined: %s", s8->name);
prog_modlu = curtext;
}
}
Prog *
divsubr(int o)
{
switch(o) {
case ADIVQ:
return prog_divq;
case ADIVQU:
return prog_divqu;
case AMODQ:
return prog_modq;
case AMODQU:
return prog_modqu;
case ADIVL:
return prog_divl;
case ADIVLU:
return prog_divlu;
case AMODL:
return prog_modl;
case AMODLU:
return prog_modlu;
default:
diag("bad op %A in divsubr", o);
return prog_modlu;
}
}
Prog*
divuconst(Prog *p, uvlong y, int num, int quot, int bits)
{
int logy, i, shift;
uvlong k, m, n, mult, tmp, msb;
if(num == NREG)
num = quot;
if(y == 0) {
diag("division by zero");
return p;
}
if(y == 1)
return genRRR(p, AMOVQ, num, NREG, quot);
if(num == REGTMP || quot == REGTMP)
diag("bad register in divuconst");
tmp = y;
for(logy = -1; tmp != 0; logy++)
tmp >>= 1;
msb = (1LL << (bits-1));
if((y & (y-1)) == 0) /* power of 2 */
return genIRR(p, ASRLQ, logy, num, quot);
if(y > msb)
return genIRR(p, ACMPUGE, y, num, quot);
/* k = (-2^(bits+logy)) % y */
m = msb/y;
n = msb%y;
if(debug['d'])
Bprint(&bso, "divuconst: y=%lld msb=%lld m=%lld n=%lld\n",
y, msb, m, n);
for(i = 0; i <= logy; i++) {
m *= 2LL;
n *= 2LL;
if(n > y) {
m += 1LL;
n -= y;
}
}
if(debug['d'])
Bprint(&bso, "divuconst: y=%lld msb=%lld m=%lld n=%lld\n",
y, msb, m, n);
k = y - n;
if(k > (1LL << logy)) {
mult = 2LL*m + 1LL;
bits++;
} else
mult = m + 1LL;
shift = bits + logy;
if(debug['d'])
Bprint(&bso, "divuconst: y=%lld mult=%lld shift=%d bits=%d k=%lld\n",
y, mult, shift, bits, k);
if(bits <= 32) {
p = genIRR(p, AMOVQ, mult, NREG, REGTMP);
p = genRRR(p, AEXTLL, REGZERO, num, quot);
p = genRRR(p, AMULQ, REGTMP, quot, quot);
p = genIRR(p, ASRLQ, shift, quot, quot);
p = genRRR(p, AADDL, quot, REGZERO, quot);
return p;
}
if(bits == 33) {
if(shift < 64) {
mult <<= (64-shift);
shift = 64;
}
p = genIRR(p, AMOVQ, mult, NREG, REGTMP);
p = genRRR(p, AEXTLL, REGZERO, num, quot);
p = genRRR(p, AUMULH, REGTMP, quot, quot);
if(shift != 64)
p = genIRR(p, ASRLQ, shift-64, quot, quot);
p = genRRR(p, AADDL, quot, REGZERO, quot);
return p;
}
if(bits <= 64) {
if(shift < 64) {
mult <<= (64-shift);
shift = 64;
}
p = genIRR(p, AMOVQ, mult, NREG, REGTMP);
p = genRRR(p, AUMULH, REGTMP, num, quot);
if(shift != 64)
p = genIRR(p, ASRLQ, shift-64, quot, quot);
return p;
}
p = genIRR(p, AMOVQ, mult, NREG, REGTMP);
p = genRRR(p, AUMULH, REGTMP, num, REGTMP);
p = genRRR(p, AADDQ, num, REGTMP, quot);
p = genRRR(p, ACMPUGT, REGTMP, quot, REGTMP);
p = genIRR(p, ASLLQ, 128-shift, REGTMP, REGTMP);
p = genIRR(p, ASRLQ, shift-64, quot, quot);
p = genRRR(p, AADDQ, REGTMP, quot, quot);
return p;
}
Prog *
divconst(Prog *p, vlong y, int num, int quot, int bits)
{
vlong yabs;
Prog *q;
yabs = y;
if (y < 0)
yabs = -y;
q = genRRR(p, ASUBQ, num, REGZERO, REGTMP2);
if (num != quot)
q = genRRR(q, AMOVQ, num, NREG, quot);
q = genRRR(q, ACMOVGT, REGTMP2, REGTMP2, quot);
q = divuconst(q, yabs, quot, quot, bits-1);
q = genRRR(q, ASUBQ, quot, REGZERO, REGTMP);
q = genRRR(q, (y < 0)? ACMOVLT: ACMOVGT, REGTMP, REGTMP2, quot);
return q;
}
Prog *
modconst(Prog *p, vlong y, int num, int quot, int bits)
{
vlong yabs;
Prog *q;
yabs = y;
if (y < 0)
yabs = -y;
q = genRRR(p, ASUBQ, num, REGZERO, REGTMP2);
q = genRRR(q, ACMOVLT, num, REGTMP2, REGTMP2);
q = divuconst(q, yabs, REGTMP2, REGTMP2, bits-1);
q = genRRR(q, ASUBQ, REGTMP2, REGZERO, REGTMP);
q = genRRR(q, ACMOVLT, REGTMP, num, REGTMP2);
q = genIRR(q, AMULQ, yabs, REGTMP2, REGTMP2);
q = genRRR(q, ASUBQ, REGTMP2, num, quot);
return q;
}
Prog *
genXXX(Prog *q, int op, Adr *from, int reg, Adr *to)
{
Prog *p;
p = prg();
p->as = op;
p->line = q->line;
p->from = *from;
p->to = *to;
p->reg = reg;
p->link = q->link;
q->link = p;
return p;
}
Prog *
genRRR(Prog *q, int op, int from, int reg, int to)
{
Prog *p;
p = prg();
p->as = op;
p->line = q->line;
p->from.type = D_REG;
p->from.reg = from;
p->to.type = D_REG;
p->to.reg = to;
p->reg = reg;
p->link = q->link;
q->link = p;
return p;
}
Prog *
genIRR(Prog *q, int op, vlong v, int reg, int to)
{
Prog *p;
p = prg();
p->as = op;
p->line = q->line;
p->from.type = D_CONST;
p->from.offset = v;
p->to.type = D_REG;
p->to.reg = to;
p->reg = reg;
p->link = q->link;
q->link = p;
return p;
}
Prog *
genstore(Prog *q, int op, int from, vlong offset, int to)
{
Prog *p;
p = prg();
p->as = op;
p->line = q->line;
p->from.type = D_REG;
p->from.reg = from;
p->to.type = D_OREG;
p->to.reg = to;
p->to.offset = offset;
p->reg = NREG;
p->link = q->link;
q->link = p;
return p;
}
Prog *
genload(Prog *q, int op, vlong offset, int from, int to)
{
Prog *p;
p = prg();
p->as = op;
p->line = q->line;
p->from.type = D_OREG;
p->from.offset = offset;
p->from.reg = from;
p->to.type = D_REG;
p->to.reg = to;
p->reg = NREG;
p->link = q->link;
q->link = p;
return p;
}