cmd/compile: captureless closures are constants

In particular, we can initialize globals with them at link time instead
of generating code for them in an init() function.  Less code, less
startup cost.

But the real reason for this change is binary size.  This change reduces
the binary size of hello world by ~4%.

The culprit is fmt.ssFree, a global variable which is a sync.Pool of
scratch scan states.  It is initalized with a captureless closure as the
pool's New action.  That action in turn references all the scanf code.

If you never call any of the fmt.Scanf* routines, ssFree is never used.
But before this change, ssFree is still referenced by fmt's init
function.  That keeps ssFree and all the code it references in the
binary.  With this change, ssFree is initialized at link time.  As a
result, fmt.init never mentions ssFree.  If you don't call fmt.Scanf*,
ssFree is unreferenced and it and the scanf code are not included.

This change is an easy fix for what is generally a much harder problem,
the unnecessary initializing of unused globals (and retention of code
that they reference).  Ideally we should have separate init code for
each global and only include that code if the corresponding global is
live.  (We'd need to make sure that the initializing code has no side
effects, except on the global being initialized.)  That is a much harder
change.

Update #6853

Change-Id: I19d1e33992287882c83efea6ce113b7cfc504b67
Reviewed-on: https://go-review.googlesource.com/17398Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/global_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 (
+	"bytes"
+	"internal/testenv"
+	"io/ioutil"
+	"log"
+	"os"
+	"os/exec"
+	"path"
+	"testing"
+)
+
+// Make sure "hello world" does not link in all the
+// fmt.scanf routines.  See issue 6853.
+func TestScanfRemoval(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// Make a directory to work in.
+	dir, err := ioutil.TempDir("", "issue6853a-")
+	if err != nil {
+		log.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	// Create source.
+	src := path.Join(dir, "test.go")
+	f, err := os.Create(src)
+	if err != nil {
+		log.Fatalf("could not create source file: %v", err)
+	}
+	f.Write([]byte(`
+package main
+import "fmt"
+func main() {
+	fmt.Println("hello world")
+}
+`))
+	f.Close()
+
+	// Name of destination.
+	dst := path.Join(dir, "test")
+
+	// Compile source.
+	cmd := exec.Command("go", "build", "-o", dst, src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Fatalf("could not build target: %v", err)
+	}
+
+	// Check destination to see if scanf code was included.
+	cmd = exec.Command("go", "tool", "nm", dst)
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		log.Fatalf("could not read target: %v", err)
+	}
+	if bytes.Index(out, []byte("scanInt")) != -1 {
+		log.Fatalf("scanf code not removed from helloworld")
+	}
+}

src/cmd/compile/internal/gc/sinit.go

@@ -378,10 +378,6 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
 	}
 
 	switch r.Op {
-	//dump("not static", r);
-	default:
-		break
-
 	case ONAME:
 		return staticcopy(l, r, out)
 
@@ -404,12 +400,8 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
 
 	case OPTRLIT:
 		switch r.Left.Op {
-		//dump("not static ptrlit", r);
-		default:
-			break
-
-			// Init pointer.
 		case OARRAYLIT, OMAPLIT, OSTRUCTLIT:
+			// Init pointer.
 			a := staticname(r.Left.Type, 1)
 
 			inittemps[r] = a
@@ -421,6 +413,7 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
 			}
 			return true
 		}
+		//dump("not static ptrlit", r);
 
 	case OSTRARRAYBYTE:
 		if l.Class == PEXTERN && r.Left.Op == OLITERAL {
@@ -452,7 +445,6 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
 		}
 		fallthrough
 
-		// fall through
 	case OSTRUCTLIT:
 		initplan(r)
 
@@ -477,11 +469,21 @@ func staticassign(l *Node, r *Node, out **NodeList) bool {
 
 		return true
 
-		// TODO: Table-driven map insert.
 	case OMAPLIT:
+		// TODO: Table-driven map insert.
 		break
+
+	case OCLOSURE:
+		if r.Func.Cvars == nil {
+			// Closures with no captured variables are globals,
+			// so the assignment can be done at link time.
+			n := *l
+			gdata(&n, r.Func.Closure.Func.Nname, Widthptr)
+			return true
+		}
 	}
 
+	//dump("not static", r);
 	return false
 }