cmd/compile,cmd/link: export int global consts to DWARF

Updates #14517

Change-Id: I23ef88e71c89da12dffcadf5562ea2d7557b62cf
Reviewed-on: https://go-review.googlesource.com/61019
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime-gdb_test.go

@@ -381,3 +381,61 @@ func TestGdbAutotmpTypes(t *testing.T) {
 		}
 	}
 }
+
+const constsSource = `
+package main
+
+const aConstant int = 42
+const largeConstant uint64 = ^uint64(0)
+const minusOne int64 = -1
+
+func main() {
+	println("hello world")
+}
+`
+
+func TestGdbConst(t *testing.T) {
+	t.Parallel()
+	checkGdbEnvironment(t)
+	checkGdbVersion(t)
+
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatalf("failed to create temp directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	// Build the source code.
+	src := filepath.Join(dir, "main.go")
+	err = ioutil.WriteFile(src, []byte(constsSource), 0644)
+	if err != nil {
+		t.Fatalf("failed to create file: %v", err)
+	}
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags=-N -l", "-o", "a.exe")
+	cmd.Dir = dir
+	out, err := testenv.CleanCmdEnv(cmd).CombinedOutput()
+	if err != nil {
+		t.Fatalf("building source %v\n%s", err, out)
+	}
+
+	// Execute gdb commands.
+	args := []string{"-nx", "-batch",
+		"-ex", "set startup-with-shell off",
+		"-ex", "break main.main",
+		"-ex", "run",
+		"-ex", "print main.aConstant",
+		"-ex", "print main.largeConstant",
+		"-ex", "print main.minusOne",
+		"-ex", "print 'runtime._MSpanInUse'",
+		filepath.Join(dir, "a.exe"),
+	}
+	got, _ := exec.Command("gdb", args...).CombinedOutput()
+
+	sgot := string(got)
+
+	t.Logf("output %q", sgot)
+
+	if strings.Index(sgot, "\n$1 = 42\n$2 = 18446744073709551615\n$3 = -1\n$4 = 1 '\\001'") < 0 {
+		t.Fatalf("output mismatch")
+	}
+}