x[y:] for strings

R=ken2
https://golang.org/cl/157114

src/cmd/gc/builtin.c.boot

@@ -19,6 +19,7 @@ char *runtimeimport =
 	"func runtime.catstring (? string, ? string) (? string)\n"
 	"func runtime.cmpstring (? string, ? string) (? int)\n"
 	"func runtime.slicestring (? string, ? int, ? int) (? string)\n"
+	"func runtime.slicestring1 (? string, ? int) (? string)\n"
 	"func runtime.indexstring (? string, ? int) (? uint8)\n"
 	"func runtime.intstring (? int64) (? string)\n"
 	"func runtime.slicebytetostring (? []uint8) (? string)\n"

src/cmd/gc/const.c

@@ -55,7 +55,7 @@ truncfltlit(Mpflt *oldv, Type *t)
 void
 convlit(Node **np, Type *t)
 {
-	return convlit1(np, t, 0);
+	convlit1(np, t, 0);
 }
 
 /*

src/cmd/gc/runtime.go

@@ -27,6 +27,7 @@ func printsp()
 func catstring(string, string) string
 func cmpstring(string, string) int
 func slicestring(string, int, int) string
+func slicestring1(string, int) string
 func indexstring(string, int) byte
 func intstring(int64) string
 func slicebytetostring([]byte) string

src/cmd/gc/walk.c

@@ -918,10 +918,16 @@ walkexpr(Node **np, NodeList **init)
 
 	case OSLICESTR:
 		// sys_slicestring(s, lb, hb)
-		n = mkcall("slicestring", n->type, init,
-			conv(n->left, types[TSTRING]),
-			conv(n->right->left, types[TINT]),
-			conv(n->right->right, types[TINT]));
+		if(n->right->right) {
+			n = mkcall("slicestring", n->type, init,
+				conv(n->left, types[TSTRING]),
+				conv(n->right->left, types[TINT]),
+				conv(n->right->right, types[TINT]));
+		} else {
+			n = mkcall("slicestring1", n->type, init,
+				conv(n->left, types[TSTRING]),
+				conv(n->right->left, types[TINT]));
+		}
 		goto ret;
 
 	case OINDEXSTR:

src/pkg/runtime/string.cgo

@@ -142,6 +142,24 @@ func slicestring(si String, lindex int32, hindex int32) (so String) {
 //	mcpy(so.str, si.str+lindex, l);
 }
 
+func slicestring1(si String, lindex int32) (so String) {
+	int32 l;
+
+	if(lindex < 0 || lindex > si.len) {
+		runtime·printpc(&si);
+		prints(" ");
+		prbounds("slice", lindex, si.len, si.len);
+	}
+
+	l = si.len-lindex;
+	so.str = si.str + lindex;
+	so.len = l;
+
+//	alternate to create a new string
+//	so = gostringsize(l);
+//	mcpy(so.str, si.str+lindex, l);
+}
+
 func indexstring(s String, i int32) (b byte) {
 	if(i < 0 || i >= s.len) {
 		runtime·printpc(&s);

test/ken/string.go

@@ -64,7 +64,7 @@ main()
 	}
 
 	/* slice strings */
-	print(c[0:3], c[3:6]);
+	print(c[0:3], c[3:]);
 
 	print("\n");