cmd/compile: inline convT2{I,E} when result doesn't escape

No point in calling a function when we can build the interface
using a known type (or itab) and the address of a local.

Get rid of third arg (preallocated stack space) to convT2{I,E}.

Makes go binary smaller by 0.2%

benchmark                   old ns/op     new ns/op     delta
BenchmarkEfaceInteger-8     16.7          10.1          -39.52%

Update #17118
Update #15375

Change-Id: I9724a1f802bfa1e3957bf1856b55558278e198a2
Reviewed-on: https://go-review.googlesource.com/29373
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -0,0 +1,128 @@
+// Copyright 2016 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
+
+// Test to make sure we make copies of the values we
+// put in interfaces.
+
+import (
+	"testing"
+)
+
+var x int
+
+func TestEfaceConv1(t *testing.T) {
+	a := 5
+	i := interface{}(a)
+	a += 2
+	if got := i.(int); got != 5 {
+		t.Errorf("wanted 5, got %d\n", got)
+	}
+}
+
+func TestEfaceConv2(t *testing.T) {
+	a := 5
+	sink = &a
+	i := interface{}(a)
+	a += 2
+	if got := i.(int); got != 5 {
+		t.Errorf("wanted 5, got %d\n", got)
+	}
+}
+
+func TestEfaceConv3(t *testing.T) {
+	x = 5
+	if got := e2int3(x); got != 5 {
+		t.Errorf("wanted 5, got %d\n", got)
+	}
+}
+
+//go:noinline
+func e2int3(i interface{}) int {
+	x = 7
+	return i.(int)
+}
+
+func TestEfaceConv4(t *testing.T) {
+	a := 5
+	if got := e2int4(a, &a); got != 5 {
+		t.Errorf("wanted 5, got %d\n", got)
+	}
+}
+
+//go:noinline
+func e2int4(i interface{}, p *int) int {
+	*p = 7
+	return i.(int)
+}
+
+type Int int
+
+var y Int
+
+type I interface {
+	foo()
+}
+
+func (i Int) foo() {
+}
+
+func TestIfaceConv1(t *testing.T) {
+	a := Int(5)
+	i := interface{}(a)
+	a += 2
+	if got := i.(Int); got != 5 {
+		t.Errorf("wanted 5, got %d\n", int(got))
+	}
+}
+
+func TestIfaceConv2(t *testing.T) {
+	a := Int(5)
+	sink = &a
+	i := interface{}(a)
+	a += 2
+	if got := i.(Int); got != 5 {
+		t.Errorf("wanted 5, got %d\n", int(got))
+	}
+}
+
+func TestIfaceConv3(t *testing.T) {
+	y = 5
+	if got := i2Int3(y); got != 5 {
+		t.Errorf("wanted 5, got %d\n", int(got))
+	}
+}
+
+//go:noinline
+func i2Int3(i I) Int {
+	y = 7
+	return i.(Int)
+}
+
+func TestIfaceConv4(t *testing.T) {
+	a := Int(5)
+	if got := i2Int4(a, &a); got != 5 {
+		t.Errorf("wanted 5, got %d\n", int(got))
+	}
+}
+
+//go:noinline
+func i2Int4(i I, p *Int) Int {
+	*p = 7
+	return i.(Int)
+}
+
+func BenchmarkEfaceInteger(b *testing.B) {
+	sum := 0
+	for i := 0; i < b.N; i++ {
+		sum += i2int(i)
+	}
+	sink = sum
+}
+
+//go:noinline
+func i2int(i interface{}) int {
+	return i.(int)
+}