runtime: add TestIntendedInlining

The intent is to allow more aggressive refactoring
in the runtime without silent performance changes.

The test would be useful for many functions.
I've seeded it with the runtime functions tophash and add;
it will grow organically (or wither!) from here.

Updates #21536 and #17566

Change-Id: Ib26d9cfd395e7a8844150224da0856add7bedc42
Reviewed-on: https://go-review.googlesource.com/57410Reviewed-by: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/runtime_test.go

@@ -5,7 +5,10 @@
 package runtime_test
 
 import (
+	"bytes"
+	"internal/testenv"
 	"io"
+	"os/exec"
 	. "runtime"
 	"runtime/debug"
 	"strings"
@@ -354,3 +357,42 @@ func TestVersion(t *testing.T) {
 		t.Fatalf("cr/nl in version: %q", vers)
 	}
 }
+
+// TestIntendedInlining tests that specific runtime functions are inlined.
+// This allows refactoring for code clarity and re-use without fear that
+// changes to the compiler will cause silent performance regressions.
+func TestIntendedInlining(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	testenv.MustHaveGoRun(t)
+	t.Parallel()
+
+	// want is the list of function names that should be inlined.
+	want := []string{"tophash", "add"}
+
+	m := make(map[string]bool, len(want))
+	for _, s := range want {
+		m[s] = true
+	}
+
+	cmd := testEnv(exec.Command(testenv.GoToolPath(t), "build", "-gcflags=-m", "runtime"))
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatal(err)
+	}
+	lines := bytes.Split(out, []byte{'\n'})
+	for _, x := range lines {
+		f := bytes.Split(x, []byte(": can inline "))
+		if len(f) < 2 {
+			continue
+		}
+		fn := bytes.TrimSpace(f[1])
+		delete(m, string(fn))
+	}
+
+	for s := range m {
+		t.Errorf("function %s not inlined", s)
+	}
+}