cmd/internal/gc: add internConcat for alloc-free string concatenation

This is a follow-up to review comments on CL 7696.

I believe that this includes the first regular Go test in the compiler.

No functional changes. Passes toolstash -cmp.

Change-Id: Id45f51aa664c5d52ece2a61cd7d8417159ce3cf0
Reviewed-on: https://go-review.googlesource.com/7820Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/gc/lex.go

@@ -1511,6 +1511,15 @@ func internString(b []byte) string {
 	return s
 }
 
+func internConcat(ss ...string) string {
+	const bufsiz = 128 // big enough for most compiler uses; must be constant to avoid heap alloc
+	b := make([]byte, 0, bufsiz)
+	for _, s := range ss {
+		b = append(b, s...)
+	}
+	return internString(b)
+}
+
 func more(pp *string) bool {
 	p := *pp
 	for p != "" && yy_isspace(int(p[0])) {

src/cmd/internal/gc/lex_test.go

+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gc
+
+import "testing"
+
+func TestInternConcat(t *testing.T) {
+	fromKind := "T"
+	toKind := "E"
+	var s string
+	n := testing.AllocsPerRun(100, func() {
+		s = internConcat("conv", fromKind, "2", toKind)
+	})
+	if s != "convT2E" {
+		t.Fatalf("internConcat(\"conv\", \"T\", \"2\", \"E\")=%q want %q", s, "convT2E")
+	}
+	if n > 0 {
+		t.Errorf("internConcat allocs per run=%f", n)
+	}
+}

src/cmd/internal/gc/subr.go

@@ -3574,10 +3574,10 @@ func isdirectiface(t *Type) bool {
 	return false
 }
 
-// type2IET returns "T" if t is a concrete type,
-// "I" if t is an interface type, and "E" if t is an empty interface type.
+// IET returns "T" if t is a concrete type, "I" if t is an interface type, and
+// "E" if t is an empty interface type.
 // It is used to build calls to the conv* and assert* runtime routines.
-func type2IET(t *Type) string {
+func (t *Type) IET() string {
 	if isnilinter(t) {
 		return "E"
 	}

src/cmd/internal/gc/walk.go

@@ -678,7 +678,7 @@ func walkexpr(np **Node, init **NodeList) {
 			n1 := Nod(OADDR, n.Left, nil)
 			r := n.Right // i.(T)
 
-			buf := "assert" + type2IET(r.Left.Type) + "2" + type2IET(r.Type)
+			buf := internConcat("assert", r.Left.Type.IET(), "2", r.Type.IET())
 			fn := syslook(buf, 1)
 			substArgTypes(fn, r.Left.Type, r.Type)
 
@@ -869,8 +869,8 @@ func walkexpr(np **Node, init **NodeList) {
 			oktype = ok.Type
 		}
 
-		fromKind := type2IET(from.Type)
-		toKind := type2IET(t)
+		fromKind := from.Type.IET()
+		toKind := t.IET()
 
 		// Avoid runtime calls in a few cases of the form _, ok := i.(T).
 		// This is faster and shorter and allows the corresponding assertX2X2
@@ -903,7 +903,7 @@ func walkexpr(np **Node, init **NodeList) {
 		}
 		resptr.Etype = 1 // addr does not escape
 
-		buf := "assert" + fromKind + "2" + toKind + "2"
+		buf := internConcat("assert", fromKind, "2", toKind, "2")
 		fn := syslook(buf, 1)
 		substArgTypes(fn, from.Type, t)
 		call := mkcall1(fn, oktype, init, typename(t), from, resptr)
@@ -927,11 +927,7 @@ func walkexpr(np **Node, init **NodeList) {
 			goto ret
 		}
 
-		// Build name of function: convI2E etc.
-		// Not all names are possible
-		// (e.g., we'll never generate convE2E or convE2I).
-		buf := "conv" + type2IET(n.Left.Type) + "2" + type2IET(n.Type)
-		fn := syslook(buf, 1)
+		// Handle fast paths and special cases.
 		var ll *NodeList
 		if !Isinter(n.Left.Type) {
 			ll = list(ll, typename(n.Left.Type))
@@ -1010,6 +1006,11 @@ func walkexpr(np **Node, init **NodeList) {
 			}
 		}
 
+		// Build name of function: convI2E etc.
+		// Not all names are possible
+		// (e.g., we'll never generate convE2E or convE2I).
+		buf := internConcat("conv", n.Left.Type.IET(), "2", n.Type.IET())
+		fn := syslook(buf, 1)
 		substArgTypes(fn, n.Left.Type, n.Type)
 		dowidth(fn.Type)
 		n = Nod(OCALL, fn, nil)