runtime: get rid of concatstring's vararg C argument.

Pass as a slice of strings instead.  For 2-5 strings, implement
dedicated routines so no slices are needed.

static call counts in the go binary:
 2 strings: 342 occurrences
 3 strings:  98
 4 strings:  30
 5 strings:  13
6+ strings:  14

Why?  C varags, bad for stack scanning and copying.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/36380043

src/cmd/gc/builtin.c

@@ -23,7 +23,11 @@ char *runtimeimport =
 	"func @\"\".printnl ()\n"
 	"func @\"\".printsp ()\n"
 	"func @\"\".goprintf ()\n"
-	"func @\"\".concatstring ()\n"
+	"func @\"\".concatstring2 (? string, ? string) (? string)\n"
+	"func @\"\".concatstring3 (? string, ? string, ? string) (? string)\n"
+	"func @\"\".concatstring4 (? string, ? string, ? string, ? string) (? string)\n"
+	"func @\"\".concatstring5 (? string, ? string, ? string, ? string, ? string) (? string)\n"
+	"func @\"\".concatstrings (? []string) (? string)\n"
 	"func @\"\".cmpstring (? string, ? string) (? int)\n"
 	"func @\"\".eqstring (? string, ? string) (? bool)\n"
 	"func @\"\".intstring (? int64) (? string)\n"

src/cmd/gc/runtime.go

@@ -36,8 +36,11 @@ func printnl()
 func printsp()
 func goprintf()
 
-// filled in by compiler: int n, string, string, ...
-func concatstring()
+func concatstring2(string, string) string
+func concatstring3(string, string, string) string
+func concatstring4(string, string, string, string) string
+func concatstring5(string, string, string, string, string) string
+func concatstrings([]string) string
 
 func cmpstring(string, string) int
 func eqstring(string, string) bool

src/cmd/gc/walk.c

@@ -2558,33 +2558,39 @@ mapfndel(char *name, Type *t)
 static Node*
 addstr(Node *n, NodeList **init)
 {
-	Node *r, *cat, *typstr;
-	NodeList *in, *args;
-	int i, count;
+	Node *r, *cat, *slice;
+	NodeList *args;
+	int count;
+	Type *t;
 
 	count = 0;
 	for(r=n; r->op == OADDSTR; r=r->left)
 		count++;	// r->right
 	count++;	// r
+	if(count < 2)
+		yyerror("addstr count %d too small", count);
 
-	// prepare call of runtime.catstring of type int, string, string, string
-	// with as many strings as we have.
-	cat = syslook("concatstring", 1);
-	cat->type = T;
-	cat->ntype = nod(OTFUNC, N, N);
-	in = list1(nod(ODCLFIELD, N, typenod(types[TINT])));	// count
-	typstr = typenod(types[TSTRING]);
-	for(i=0; i<count; i++)
-		in = list(in, nod(ODCLFIELD, N, typstr));
-	cat->ntype->list = in;
-	cat->ntype->rlist = list1(nod(ODCLFIELD, N, typstr));
-
+	// build list of string arguments
 	args = nil;
 	for(r=n; r->op == OADDSTR; r=r->left)
 		args = concat(list1(conv(r->right, types[TSTRING])), args);
 	args = concat(list1(conv(r, types[TSTRING])), args);
-	args = concat(list1(nodintconst(count)), args);
 
+	if(count <= 5) {
+		// small numbers of strings use direct runtime helpers.
+		snprint(namebuf, sizeof(namebuf), "concatstring%d", count);
+	} else {
+		// large numbers of strings are passed to the runtime as a slice.
+		strcpy(namebuf, "concatstrings");
+		t = typ(TARRAY);
+		t->type = types[TSTRING];
+		t->bound = -1;
+		slice = nod(OCOMPLIT, N, typenod(t));
+		slice->list = args;
+		slice->esc = EscNone;
+		args = list1(slice);
+	}
+	cat = syslook(namebuf, 1);
 	r = nod(OCALL, cat, N);
 	r->list = args;
 	typecheck(&r, Erv);

src/pkg/runtime/string.goc

@@ -179,14 +179,35 @@ concatstring(intgo n, String *s)
 	return out;
 }
 
-// NOTE: Cannot use func syntax, because we need the ...,
-// to signal to the garbage collector that this function does
-// not have a fixed size argument count.
 #pragma textflag NOSPLIT
-void
-runtime·concatstring(intgo n, String s1, ...)
-{
-	(&s1)[n] = concatstring(n, &s1);
+func concatstring2(s1 String, s2 String) (res String) {
+	USED(&s2);
+	res = concatstring(2, &s1);
+}
+#pragma textflag NOSPLIT
+func concatstring3(s1 String, s2 String, s3 String) (res String) {
+	USED(&s2);
+	USED(&s3);
+	res = concatstring(3, &s1);
+}
+#pragma textflag NOSPLIT
+func concatstring4(s1 String, s2 String, s3 String, s4 String) (res String) {
+	USED(&s2);
+	USED(&s3);
+	USED(&s4);
+	res = concatstring(4, &s1);
+}
+#pragma textflag NOSPLIT
+func concatstring5(s1 String, s2 String, s3 String, s4 String, s5 String) (res String) {
+	USED(&s2);
+	USED(&s3);
+	USED(&s4);
+	USED(&s5);
+	res = concatstring(5, &s1);
+}
+#pragma textflag NOSPLIT
+func concatstrings(s Slice) (res String) {
+	res = concatstring(s.len, (String*)s.array);
 }
 
 func eqstring(s1 String, s2 String) (v bool) {