cmd/gc: eliminate some pointer arithmetic

In mparith, all the a1-- are problematic. Rewrite it all without pointers.
It's clearer anyway.

In popt, v is problematic because it is used both as a fixed pointer
(v = byvar[i]) and as a moving pointer (v = var; v++) aka slice.
Eliminate pointer movement.

Tested that this still produces bit-for-bit output for 'go build -a std'
compared to d260756 (current master).

Change-Id: I1a1bed0f98b594c3864fe95075dd95f9b52113e0
Reviewed-on: https://go-review.googlesource.com/4645Reviewed-by: Austin Clements <austin@google.com>

src/cmd/gc/mparith1.c

@@ -235,9 +235,9 @@ mppow10flt(Mpflt *a, int p)
 static void
 mphextofix(Mpint *a, char *s, int n)
 {
-	char *hexdigitp, *end, c;
+	char c;
 	long d;
-	int bit;
+	int bit, hexdigitp, end;
 
 	while(*s == '0') {
 		s++;
@@ -250,9 +250,9 @@ mphextofix(Mpint *a, char *s, int n)
 		return;
 	}
 
-	end = s+n-1;
-	for(hexdigitp=end; hexdigitp>=s; hexdigitp--) {
-		c = *hexdigitp;
+	end = n-1;
+	for(hexdigitp=end; hexdigitp>=0; hexdigitp--) {
+		c = s[hexdigitp];
 		if(c >= '0' && c <= '9')
 			d = c-'0';
 		else if(c >= 'A' && c <= 'F')
@@ -348,7 +348,8 @@ mpatoflt(Mpflt *a, char *as)
 		mpnorm(a);
 	}
 	for(;;) {
-		switch(c = *s++) {
+		c = *s++;
+		switch(c) {
 		default:
 			yyerror("malformed constant: %s (at %c)", as, c);
 			goto bad;
@@ -542,7 +543,8 @@ bad:
 int
 Bconv(Fmt *fp)
 {
-	char buf[500], *p;
+	char buf[500];
+	int p;
 	Mpint *xval, q, r, ten, sixteen;
 	int f, digit;
 
@@ -554,8 +556,8 @@ Bconv(Fmt *fp)
 		mpnegfix(&q);
 	}
 
-	p = &buf[sizeof(buf)];
-	*--p = 0;
+	p = sizeof(buf);
+	buf[--p] = 0;
 	if(fp->flags & FmtSharp) {
 		// Hexadecimal
 		mpmovecfix(&sixteen, 16);
@@ -563,27 +565,27 @@ Bconv(Fmt *fp)
 			mpdivmodfixfix(&q, &r, &q, &sixteen);
 			digit = mpgetfix(&r);
 			if(digit < 10)
-				*--p = digit + '0';
+				buf[--p] = digit + '0';
 			else
-				*--p = digit - 10 + 'A';
+				buf[--p] = digit - 10 + 'A';
 			if(mptestfix(&q) <= 0)
 				break;
 		}
-		*--p = 'x';
-		*--p = '0';
+		buf[--p] = 'x';
+		buf[--p] = '0';
 	} else {
 		// Decimal
 		mpmovecfix(&ten, 10);
 		for(;;) {
 			mpdivmodfixfix(&q, &r, &q, &ten);
-			*--p = mpgetfix(&r) + '0';
+			buf[--p] = mpgetfix(&r) + '0';
 			if(mptestfix(&q) <= 0)
 				break;
 		}
 	}
 	if(f)
-		*--p = '-';
-	return fmtstrcpy(fp, p);
+		buf[--p] = '-';
+	return fmtstrcpy(fp, &buf[p]);
 }
 
 int

src/cmd/gc/mparith2.c

@@ -14,12 +14,10 @@ static int
 mplen(Mpint *a)
 {
 	int i, n;
-	long *a1;
 
 	n = -1;
-	a1 = &a->a[0];
 	for(i=0; i<Mpprec; i++) {
-		if(*a1++ != 0)
+		if(a->a[i] != 0)
 			n = i;
 	}
 	return n+1;
@@ -32,19 +30,18 @@ mplen(Mpint *a)
 static void
 mplsh(Mpint *a, int quiet)
 {
-	long *a1, x;
+	long x;
 	int i, c;
 
 	c = 0;
-	a1 = &a->a[0];
 	for(i=0; i<Mpprec; i++) {
-		x = (*a1 << 1) + c;
+		x = (a->a[i] << 1) + c;
 		c = 0;
 		if(x >= Mpbase) {
 			x -= Mpbase;
 			c = 1;
 		}
-		*a1++ = x;
+		a->a[i] = x;
 	}
 	a->ovf = c;
 	if(a->ovf && !quiet)
@@ -58,20 +55,17 @@ mplsh(Mpint *a, int quiet)
 static void
 mplshw(Mpint *a, int quiet)
 {
-	long *a1;
 	int i;
 
-	a1 = &a->a[Mpprec-1];
-	if(*a1) {
+	i = Mpprec-1;
+	if(a->a[i]) {
 		a->ovf = 1;
 		if(!quiet)
 			yyerror("constant shift overflow");
 	}
-	for(i=1; i<Mpprec; i++) {
-		a1[0] = a1[-1];
-		a1--;
-	}
-	a1[0] = 0;
+	for(; i > 0; i--)
+		a->a[i] = a->a[i-1];
+	a->a[i] = 0;
 }
 
 //
@@ -81,15 +75,14 @@ mplshw(Mpint *a, int quiet)
 static void
 mprsh(Mpint *a)
 {
-	long *a1, x, lo;
+	long x, lo;
 	int i, c;
 
 	c = 0;
 	lo = a->a[0] & 1;
-	a1 = &a->a[Mpprec];
-	for(i=0; i<Mpprec; i++) {
-		x = *--a1;
-		*a1 = (x + c) >> 1;
+	for(i=Mpprec-1; i>=0; i--) {
+		x = a->a[i];
+		a->a[i] = (x + c) >> 1;
 		c = 0;
 		if(x & 1)
 			c = Mpbase;
@@ -105,16 +98,14 @@ mprsh(Mpint *a)
 static void
 mprshw(Mpint *a)
 {
-	long *a1, lo;
+	long lo;
 	int i;
 
 	lo = a->a[0];
-	a1 = &a->a[0];
-	for(i=1; i<Mpprec; i++) {
-		a1[0] = a1[1];
-		a1++;
+	for(i=0; i<Mpprec-1; i++) {
+		a->a[i] = a->a[i+1];
 	}
-	a1[0] = 0;
+	a->a[i] = 0;
 	if(a->neg && lo != 0)
 		mpaddcfix(a, -1);
 }
@@ -125,7 +116,7 @@ mprshw(Mpint *a)
 static int
 mpcmp(Mpint *a, Mpint *b)
 {
-	long x, *a1, *b1;
+	long x;
 	int i;
 
 	if(a->ovf || b->ovf) {
@@ -134,11 +125,8 @@ mpcmp(Mpint *a, Mpint *b)
 		return 0;
 	}
 
-	a1 = &a->a[0] + Mpprec;
-	b1 = &b->a[0] + Mpprec;
-
-	for(i=0; i<Mpprec; i++) {
-		x = *--a1 - *--b1;
+	for(i=Mpprec-1; i>=0; i--) {
+		x = a->a[i] - b->a[i];
 		if(x > 0)
 			return +1;
 		if(x < 0)
@@ -154,19 +142,18 @@ mpcmp(Mpint *a, Mpint *b)
 static void
 mpneg(Mpint *a)
 {
-	long x, *a1;
+	long x;
 	int i, c;
 
-	a1 = &a->a[0];
 	c = 0;
 	for(i=0; i<Mpprec; i++) {
-		x = -*a1 -c;
+		x = -a->a[i] -c;
 		c = 0;
 		if(x < 0) {
 			x += Mpbase;
 			c = 1;
 		}
-		*a1++ = x;
+		a->a[i] = x;
 	}
 }
 
@@ -202,7 +189,7 @@ void
 mpaddfixfix(Mpint *a, Mpint *b, int quiet)
 {
 	int i, c;
-	long x, *a1, *b1;
+	long x;
 
 	if(a->ovf || b->ovf) {
 		if(nsavederrors+nerrors == 0)
@@ -212,20 +199,18 @@ mpaddfixfix(Mpint *a, Mpint *b, int quiet)
 	}
 
 	c = 0;
-	a1 = &a->a[0];
-	b1 = &b->a[0];
 	if(a->neg != b->neg)
 		goto sub;
 
 	// perform a+b
 	for(i=0; i<Mpprec; i++) {
-		x = *a1 + *b1++ + c;
+		x = a->a[i] + b->a[i] + c;
 		c = 0;
 		if(x >= Mpbase) {
 			x -= Mpbase;
 			c = 1;
 		}
-		*a1++ = x;
+		a->a[i] = x;
 	}
 	a->ovf = c;
 	if(a->ovf && !quiet)
@@ -242,26 +227,26 @@ sub:
 
 	case 1:
 		for(i=0; i<Mpprec; i++) {
-			x = *a1 - *b1++ - c;
+			x = a->a[i] - b->a[i] - c;
 			c = 0;
 			if(x < 0) {
 				x += Mpbase;
 				c = 1;
 			}
-			*a1++ = x;
+			a->a[i] = x;
 		}
 		break;
 
 	case -1:
 		a->neg ^= 1;
 		for(i=0; i<Mpprec; i++) {
-			x = *b1++ - *a1 - c;
+			x = b->a[i] - a->a[i] - c;
 			c = 0;
 			if(x < 0) {
 				x += Mpbase;
 				c = 1;
 			}
-			*a1++ = x;
+			a->a[i] = x;
 		}
 		break;
 	}
@@ -272,8 +257,9 @@ mpmulfixfix(Mpint *a, Mpint *b)
 {
 
 	int i, j, na, nb;
-	long *a1, x;
+	long x;
 	Mpint s, q;
+	Mpint *c;
 
 	if(a->ovf || b->ovf) {
 		if(nsavederrors+nerrors == 0)
@@ -288,17 +274,17 @@ mpmulfixfix(Mpint *a, Mpint *b)
 	nb = mplen(b);
 	if(na > nb) {
 		mpmovefixfix(&s, a);
-		a1 = &b->a[0];
+		c = b;
 		na = nb;
 	} else {
 		mpmovefixfix(&s, b);
-		a1 = &a->a[0];
+		c = a;
 	}
 	s.neg = 0;
 
 	mpmovecfix(&q, 0);
 	for(i=0; i<na; i++) {
-		x = *a1++;
+		x = c->a[i];
 		for(j=0; j<Mpscale; j++) {
 			if(x & 1) {
 				if(s.ovf) {
@@ -326,7 +312,7 @@ mpmulfract(Mpint *a, Mpint *b)
 {
 
 	int i, j;
-	long *a1, x;
+	long x;
 	Mpint s, q;
 
 	if(a->ovf || b->ovf) {
@@ -337,16 +323,16 @@ mpmulfract(Mpint *a, Mpint *b)
 	}
 
 	mpmovefixfix(&s, b);
-	a1 = &a->a[Mpprec];
 	s.neg = 0;
 	mpmovecfix(&q, 0);
 
-	x = *--a1;
+	i = Mpprec-1;
+	x = a->a[i];
 	if(x != 0)
 		yyerror("mpmulfract not normal");
 
-	for(i=0; i<Mpprec-1; i++) {
-		x = *--a1;
+	for(i--; i >= 0; i--) {
+		x = a->a[i];
 		if(x == 0) {
 			mprshw(&s);
 			continue;
@@ -369,7 +355,7 @@ void
 mporfixfix(Mpint *a, Mpint *b)
 {
 	int i;
-	long x, *a1, *b1;
+	long x;
 
 	x = 0;
 	if(a->ovf || b->ovf) {
@@ -386,11 +372,9 @@ mporfixfix(Mpint *a, Mpint *b)
 	if(b->neg)
 		mpneg(b);
 
-	a1 = &a->a[0];
-	b1 = &b->a[0];
 	for(i=0; i<Mpprec; i++) {
-		x = *a1 | *b1++;
-		*a1++ = x;
+		x = a->a[i] | b->a[i];
+		a->a[i] = x;
 	}
 
 	if(b->neg)
@@ -405,7 +389,7 @@ void
 mpandfixfix(Mpint *a, Mpint *b)
 {
 	int i;
-	long x, *a1, *b1;
+	long x;
 
 	x = 0;
 	if(a->ovf || b->ovf) {
@@ -422,11 +406,9 @@ mpandfixfix(Mpint *a, Mpint *b)
 	if(b->neg)
 		mpneg(b);
 
-	a1 = &a->a[0];
-	b1 = &b->a[0];
 	for(i=0; i<Mpprec; i++) {
-		x = *a1 & *b1++;
-		*a1++ = x;
+		x = a->a[i] & b->a[i];
+		a->a[i] = x;
 	}
 
 	if(b->neg)
@@ -441,7 +423,7 @@ void
 mpandnotfixfix(Mpint *a, Mpint *b)
 {
 	int i;
-	long x, *a1, *b1;
+	long x;
 
 	x = 0;
 	if(a->ovf || b->ovf) {
@@ -458,11 +440,9 @@ mpandnotfixfix(Mpint *a, Mpint *b)
 	if(b->neg)
 		mpneg(b);
 
-	a1 = &a->a[0];
-	b1 = &b->a[0];
 	for(i=0; i<Mpprec; i++) {
-		x = *a1 & ~*b1++;
-		*a1++ = x;
+		x = a->a[i] & ~b->a[i];
+		a->a[i] = x;
 	}
 
 	if(b->neg)
@@ -477,7 +457,7 @@ void
 mpxorfixfix(Mpint *a, Mpint *b)
 {
 	int i;
-	long x, *a1, *b1;
+	long x;
 
 	x = 0;
 	if(a->ovf || b->ovf) {
@@ -494,11 +474,9 @@ mpxorfixfix(Mpint *a, Mpint *b)
 	if(b->neg)
 		mpneg(b);
 
-	a1 = &a->a[0];
-	b1 = &b->a[0];
 	for(i=0; i<Mpprec; i++) {
-		x = *a1 ^ *b1++;
-		*a1++ = x;
+		x = a->a[i] ^ b->a[i];
+		a->a[i] = x;
 	}
 
 	if(b->neg)
@@ -585,7 +563,6 @@ void
 mpmovecfix(Mpint *a, vlong c)
 {
 	int i;
-	long *a1;
 	vlong x;
 
 	a->neg = 0;
@@ -597,9 +574,8 @@ mpmovecfix(Mpint *a, vlong c)
 		x = -(uvlong)x;
 	}
 
-	a1 = &a->a[0];
 	for(i=0; i<Mpprec; i++) {
-		*a1++ = x&Mpmask;
+		a->a[i] = x&Mpmask;
 		x >>= Mpscale;
 	}
 }
@@ -658,13 +634,11 @@ mpdivmodfixfix(Mpint *q, Mpint *r, Mpint *n, Mpint *d)
 static int
 mpiszero(Mpint *a)
 {
-	long *a1;
 	int i;
-	a1 = &a->a[0] + Mpprec;
-	for(i=0; i<Mpprec; i++) {
-		if(*--a1 != 0)
+
+	for(i=Mpprec-1; i>=0; i--)
+		if(a->a[i] != 0)
 			return 0;
-	}
 	return 1;
 }
 
@@ -673,16 +647,15 @@ mpdivfract(Mpint *a, Mpint *b)
 {
 	Mpint n, d;
 	int i, j, neg;
-	long *a1, x;
+	long x;
 
 	mpmovefixfix(&n, a);	// numerator
 	mpmovefixfix(&d, b);	// denominator
-	a1 = &a->a[Mpprec];	// quotient
 
 	neg = n.neg ^ d.neg;
 	n.neg = 0;
 	d.neg = 0;
-	for(i=0; i<Mpprec; i++) {
+	for(i=Mpprec-1; i >= 0; i--) {
 		x = 0;
 		for(j=0; j<Mpscale; j++) {
 			x <<= 1;
@@ -693,7 +666,7 @@ mpdivfract(Mpint *a, Mpint *b)
 			}
 			mprsh(&d);
 		}
-		*--a1 = x;
+		a->a[i] = x;
 	}
 	a->neg = neg;
 }

src/cmd/gc/popt.c

@@ -618,7 +618,8 @@ mergetemp(Prog *firstp)
 	nkill = 0;
 
 	// Special case.
-	for(v = var; v < var+nvar; v++) {
+	for(i = 0; i < nvar; i++) {
+		v = &var[i];
 		if(v->addr)
 			continue;
 		// Used in only one instruction, which had better be a write.
@@ -665,7 +666,8 @@ mergetemp(Prog *firstp)
 	// Each flood uses a new value of gen so that we don't have
 	// to clear all the r->active words after each variable.
 	gen = 0;
-	for(v = var; v < var+nvar; v++) {
+	for(i = 0; i < nvar; i++) {
+		v = &var[i];
 		gen++;
 		for(f = v->use; f != nil; f = (Flow*)f->data)
 			mergewalk(v, f, gen);
@@ -740,7 +742,8 @@ mergetemp(Prog *firstp)
 
 	if(debugmerge > 0 && debug['v']) {
 		print("%S [%d - %d]\n", curfn->nname->sym, nvar, nkill);
-		for(v=var; v<var+nvar; v++) {
+		for(i = 0; i < nvar; i++) {
+			v = &var[i];
 			print("var %#N %T %lld-%lld", v->node, v->node->type, v->start, v->end);
 			if(v->addr)
 				print(" addr=1");
@@ -779,8 +782,10 @@ mergetemp(Prog *firstp)
 	}
 
 	// Clear aux structures.
-	for(v=var; v<var+nvar; v++)
+	for(i = 0; i < nvar; i++) {
+		v = &var[i];
 		v->node->opt = nil;
+	}
 	free(var);
 	free(bystart);
 	free(inuse);

src/cmd/gc/reg.c

@@ -103,6 +103,7 @@ regopt(Prog *firstp)
 	int nreg;
 	char **regnames;
 	Bits bit;
+	Rgn *rgp;
 
 	if(first) {
 		fmtinstall('Q', Qconv);
@@ -198,7 +199,8 @@ regopt(Prog *firstp)
 	}
 
 	for(i=0; i<nvar; i++) {
-		Var *v = var+i;
+		Var *v;
+		v = var+i;
 		if(v->addr) {
 			bit = blsh(i);
 			for(z=0; z<BITS; z++)
@@ -206,7 +208,7 @@ regopt(Prog *firstp)
 		}
 
 		if(debug['R'] && debug['v'])
-			print("bit=%2d addr=%d et=%-6E w=%-2d s=%N + %lld\n",
+			print("bit=%2d addr=%d et=%E w=%-2d s=%N + %lld\n",
 				i, v->addr, v->etype, v->width, v->node, v->offset);
 	}
 
@@ -334,7 +336,6 @@ loop2:
 	}
 	for(f = firstf; f != nil; f = f->link)
 		((Reg*)f->data)->act = zbits;
-	rgp = region;
 	nregion = 0;
 	for(f = firstf; f != nil; f = f->link) {
 		r = (Reg*)f->data;
@@ -361,10 +362,10 @@ loop2:
 					print("too many regions\n");
 				goto brk;
 			}
+			rgp = &region[nregion];
 			rgp->enter = f;
 			rgp->varno = i;
 			rgp->cost = change;
-			rgp++;
 			nregion++;
 		}
 	}
@@ -379,10 +380,10 @@ brk:
 	 * determine used registers (paint2)
 	 * replace code (paint3)
 	 */
-	rgp = region;
 	if(debug['R'] && debug['v'])
 		print("\nregisterizing\n");
 	for(i=0; i<nregion; i++) {
+		rgp = &region[i];
 		if(debug['R'] && debug['v'])
 			print("region %d: cost %d varno %d enter %lld\n", i, rgp->cost, rgp->varno, rgp->enter->prog->pc);
 		bit = blsh(rgp->varno);
@@ -393,12 +394,11 @@ brk:
 				Var *v;
 
 				v = var + rgp->varno;
-				print("registerize %N+%lld (bit=%2d et=%2E) in %R usedreg=%#llx vreg=%#llx\n",
+				print("registerize %N+%lld (bit=%2d et=%E) in %R usedreg=%#llx vreg=%#llx\n",
 						v->node, v->offset, rgp->varno, v->etype, rgp->regno, usedreg, vreg);
 			}
 			paint3(rgp->enter, rgp->varno, vreg, rgp->regno);
 		}
-		rgp++;
 	}
 
 	/*
@@ -477,7 +477,7 @@ walkvardef(Node *n, Flow *f, int active)
 		if(f1->prog->as == AVARKILL && f1->prog->to.node == n)
 			break;
 		for(v=n->opt; v!=nil; v=v->nextinnode) {
-			bn = v - var;
+			bn = v->id;
 			biset(&((Reg*)f1->data)->act, bn);
 		}
 		if(f1->prog->as == ACALL)
@@ -680,6 +680,7 @@ mkvar(Flow *f, Adr *a)
 	i = nvar;
 	nvar++;
 	v = var+i;
+	v->id = i;
 	v->offset = o;
 	v->name = n;
 	v->etype = et;
@@ -739,7 +740,7 @@ mkvar(Flow *f, Adr *a)
 		v->addr = 1;
 
 	if(debug['R'])
-		print("bit=%2d et=%2E w=%lld+%lld %#N %D flag=%d\n", i, et, o, w, node, a, v->addr);
+		print("bit=%2d et=%E w=%lld+%lld %#N %D flag=%d\n", i, et, o, w, node, a, v->addr);
 	ostats.nvar++;
 
 	return bit;
@@ -753,7 +754,7 @@ prop(Flow *f, Bits ref, Bits cal)
 {
 	Flow *f1, *f2;
 	Reg *r, *r1;
-	int z, i, j;
+	int z, i;
 	Var *v, *v1;
 
 	for(f1 = f; f1 != nil; f1 = f1->p1) {
@@ -820,11 +821,9 @@ prop(Flow *f, Bits ref, Bits cal)
 					// v is the head of the list or if the head's bit is not yet turned on.
 					// This will set the bits at most twice, keeping the overall loop linear.
 					v1 = v->node->opt;
-					j = v1 - var;
-					if(v == v1 || !btest(&cal, j)) {
+					if(v == v1 || !btest(&cal, v1->id)) {
 						for(; v1 != nil; v1 = v1->nextinnode) {
-							j = v1 - var;
-							biset(&cal, j);
+							biset(&cal, v1->id);
 						}
 					}
 				}