6g etc: groundwork for eliminating redundant bounds checks.

	drop check in range over array.
	drop check in [256]array indexed by byte.

R=ken2
https://golang.org/cl/163088

src/cmd/5g/cgen.c

@@ -547,7 +547,7 @@ agen(Node *n, Node *res)
 			v = mpgetfix(nr->val.u.xval);
 			if(isslice(nl->type)) {
 
-				if(!debug['B']) {
+				if(!debug['B'] && !n->etype) {
 					n1 = n3;
 					n1.op = OINDREG;
 					n1.type = types[tptr];
@@ -599,7 +599,7 @@ agen(Node *n, Node *res)
 		gmove(&n1, &n2);
 		regfree(&n1);
 
-		if(!debug['B']) {
+		if(!debug['B'] && !n->etype) {
 			// check bounds
 			regalloc(&n4, types[TUINT32], N);
 			if(isslice(nl->type)) {

src/cmd/5g/gsubr.c

@@ -1767,7 +1767,7 @@ oindex_const:
 	v = mpgetfix(r->val.u.xval);
 	if(o & ODynam) {
 
-		if(!debug['B']) {
+		if(!debug['B'] && !n->etype) {
 			n1 = *reg;
 			n1.op = OINDREG;
 			n1.type = types[tptr];

src/cmd/6g/cgen.c

@@ -504,7 +504,7 @@ agen(Node *n, Node *res)
 			v = mpgetfix(nr->val.u.xval);
 			if(isslice(nl->type)) {
 
-				if(!debug['B']) {
+				if(!debug['B'] && !n->etype) {
 					n1 = n3;
 					n1.op = OINDREG;
 					n1.type = types[tptr];
@@ -547,7 +547,7 @@ agen(Node *n, Node *res)
 		gmove(&n1, &n2);
 		regfree(&n1);
 
-		if(!debug['B']) {
+		if(!debug['B'] && !n->etype) {
 			// check bounds
 			if(isslice(nl->type)) {
 				n1 = n3;

src/cmd/6g/gsubr.c

@@ -1827,23 +1827,24 @@ oindex:
 		agen(l, reg);
 	}
 
+	if(!(o & ODynam) && l->type->width >= unmappedzero && l->op == OIND) {
+		// cannot rely on page protections to
+		// catch array ptr == 0, so dereference.
+		n2 = *reg;
+		n2.op = OINDREG;
+		n2.type = types[TUINT8];
+		n2.xoffset = 0;
+		gins(ATESTB, nodintconst(0), &n2);
+	}
+
 	// check bounds
-	if(!debug['B']) {
+	if(!debug['B'] && !n->etype) {
 		if(o & ODynam) {
 			n2 = *reg;
 			n2.op = OINDREG;
 			n2.type = types[tptr];
 			n2.xoffset = Array_nel;
 		} else {
-			if(l->type->width >= unmappedzero && l->op == OIND) {
-				// cannot rely on page protections to
-				// catch array ptr == 0, so dereference.
-				n2 = *reg;
-				n2.op = OINDREG;
-				n2.type = types[TUINT8];
-				n2.xoffset = 0;
-				gins(ATESTB, nodintconst(0), &n2);
-			}
 			nodconst(&n2, types[TUINT64], l->type->bound);
 		}
 		gins(optoas(OCMP, types[TUINT32]), reg1, &n2);
@@ -1879,7 +1880,7 @@ oindex_const:
 	v = mpgetfix(r->val.u.xval);
 	if(o & ODynam) {
 
-		if(!debug['B']) {
+		if(!debug['B'] && !n->etype) {
 			n1 = *reg;
 			n1.op = OINDREG;
 			n1.type = types[tptr];

src/cmd/8g/cgen.c

@@ -540,7 +540,7 @@ agen(Node *n, Node *res)
 			v = mpgetfix(nr->val.u.xval);
 			if(isslice(nl->type)) {
 
-				if(!debug['B']) {
+				if(!debug['B'] && !n->etype) {
 					n1 = n3;
 					n1.op = OINDREG;
 					n1.type = types[tptr];
@@ -558,7 +558,7 @@ agen(Node *n, Node *res)
 				n1.xoffset = Array_array;
 				gmove(&n1, &n3);
 			} else
-			if(!debug['B']) {
+			if(!debug['B'] && !n->etype) {
 				if(v < 0)
 					yyerror("out of bounds on array");
 				else
@@ -583,7 +583,7 @@ agen(Node *n, Node *res)
 		gmove(&n1, &n2);
 		regfree(&n1);
 
-		if(!debug['B']) {
+		if(!debug['B'] && !n->etype) {
 			// check bounds
 			if(isslice(nl->type)) {
 				n1 = n3;

src/cmd/gc/range.c

@@ -91,7 +91,7 @@ walkrange(Node *n)
 	Node *ohv1, *hv1, *hv2;	// hidden (old) val 1, 2
 	Node *ha, *hit;	// hidden aggregate, iterator
 	Node *a, *v1, *v2;	// not hidden aggregate, val 1, 2
-	Node *fn;
+	Node *fn, *tmp;
 	NodeList *body, *init;
 	Type *th, *t;
 
@@ -128,8 +128,11 @@ walkrange(Node *n)
 		n->nincr = nod(OASOP, hv1, nodintconst(1));
 		n->nincr->etype = OADD;
 		body = list1(nod(OAS, v1, hv1));
-		if(v2)
-			body = list(body, nod(OAS, v2, nod(OINDEX, ha, hv1)));
+		if(v2) {
+			tmp = nod(OINDEX, ha, hv1);
+			tmp->etype = 1;	// no bounds check
+			body = list(body, nod(OAS, v2, tmp));
+		}
 		break;
 
 	case TMAP:

src/cmd/gc/walk.c

@@ -787,6 +787,14 @@ walkexpr(Node **np, NodeList **init)
 	case OINDEX:
 		walkexpr(&n->left, init);
 		walkexpr(&n->right, init);
+		
+		// if range of type cannot exceed static array bound,
+		// disable bounds check
+		if(!isslice(n->left->type))
+		if(n->right->type->width < 4)
+		if((1<<(8*n->right->type->width)) <= n->left->type->bound)
+			n->etype = 1;
+
 		goto ret;
 
 	case OINDEXMAP: