objdump: implement disassembly

There is some duplication here with cmd/nm.
There is a TODO to address that after 1.3 is out.

Update #7452

x86 disassembly works and is tested.

The arm disassembler does not exist yet
and is therefore not yet hooked up.

LGTM=crawshaw, iant
R=crawshaw, iant
CC=golang-codereviews
https://golang.org/cl/91360046

src/cmd/objdump/objdump_test.go

@@ -79,19 +79,10 @@ func TestObjDump(t *testing.T) {
 	}
 	syms := loadSyms(t)
 
-	tmpDir, err := ioutil.TempDir("", "TestObjDump")
-	if err != nil {
-		t.Fatal("TempDir failed: ", err)
-	}
-	defer os.RemoveAll(tmpDir)
+	tmp, exe := buildObjdump(t)
+	defer os.RemoveAll(tmp)
 
-	exepath := filepath.Join(tmpDir, "testobjdump.exe")
-	out, err := exec.Command("go", "build", "-o", exepath, "cmd/objdump").CombinedOutput()
-	if err != nil {
-		t.Fatalf("go build -o %v cmd/objdump: %v\n%s", exepath, err, string(out))
-	}
-
-	srcPath, srcLineNo := runObjDump(t, exepath, syms["cmd/objdump.TestObjDump"])
+	srcPath, srcLineNo := runObjDump(t, exe, syms["cmd/objdump.TestObjDump"])
 	fi1, err := os.Stat("objdump_test.go")
 	if err != nil {
 		t.Fatalf("Stat failed: %v", err)
@@ -107,3 +98,86 @@ func TestObjDump(t *testing.T) {
 		t.Fatalf("line number = %v; want 76", srcLineNo)
 	}
 }
+
+func buildObjdump(t *testing.T) (tmp, exe string) {
+	tmp, err := ioutil.TempDir("", "TestObjDump")
+	if err != nil {
+		t.Fatal("TempDir failed: ", err)
+	}
+
+	exe = filepath.Join(tmp, "testobjdump.exe")
+	out, err := exec.Command("go", "build", "-o", exe, "cmd/objdump").CombinedOutput()
+	if err != nil {
+		os.RemoveAll(tmp)
+		t.Fatalf("go build -o %v cmd/objdump: %v\n%s", exe, err, string(out))
+	}
+	return
+}
+
+var x86Need = []string{
+	"fmthello.go:6",
+	"TEXT main.main(SB)",
+	"JMP main.main(SB)",
+	"CALL fmt.Println(SB)",
+	"RET",
+}
+
+var armNeed = []string{
+	"fmthello.go:6",
+	"TEXT main.main(SB)",
+	"B main.main(SB)",
+	"BL fmt.Println(SB)",
+	"RET",
+}
+
+// objdump is fully cross platform: it can handle binaries
+// from any known operating system and architecture.
+// We could in principle add binaries to testdata and check
+// all the supported systems during this test. However, the
+// binaries would be about 1 MB each, and we don't want to
+// add that much junk to the hg repository. Instead, build a
+// binary for the current system (only) and test that objdump
+// can handle that one.
+
+func TestDisasm(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping test; see http://golang.org/issue/7947")
+	}
+
+	tmp, exe := buildObjdump(t)
+	defer os.RemoveAll(tmp)
+
+	hello := filepath.Join(tmp, "hello.exe")
+	out, err := exec.Command("go", "build", "-o", hello, "testdata/fmthello.go").CombinedOutput()
+	if err != nil {
+		t.Fatalf("go build fmthello.go: %v\n%s", err, out)
+	}
+	need := []string{
+		"fmthello.go:6",
+		"TEXT main.main(SB)",
+	}
+	switch runtime.GOARCH {
+	case "amd64", "386":
+		need = append(need, x86Need...)
+	case "arm":
+		need = append(need, armNeed...)
+		t.Skip("disassembler not ready on arm yet")
+	}
+
+	out, err = exec.Command(exe, "-s", "main.main", hello).CombinedOutput()
+	if err != nil {
+		t.Fatalf("objdump fmthello.exe: %v\n%s", err, out)
+	}
+
+	text := string(out)
+	ok := true
+	for _, s := range need {
+		if !strings.Contains(text, s) {
+			t.Errorf("disassembly missing '%s'", s)
+			ok = false
+		}
+	}
+	if !ok {
+		t.Logf("full disassembly:\n%s", text)
+	}
+}