cmd/internal/obj: Fix generation of assembly with -S

We can't drop Prog entries when we want to print disassembly.

Added a test for -S.

Fixes #14515

Change-Id: I44c72f70f7a3919acc01c559d30335d26669e76f
Reviewed-on: https://go-review.googlesource.com/19930
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -12,6 +12,7 @@ import (
 	"os"
 	"os/exec"
 	"path"
+	"strings"
 	"testing"
 )
 
@@ -62,3 +63,53 @@ func main() {
 		log.Fatalf("scanf code not removed from helloworld")
 	}
 }
+
+// Make sure -S prints assembly code.  See issue 14515.
+func TestDashS(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// Make a directory to work in.
+	dir, err := ioutil.TempDir("", "issue14515-")
+	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()
+
+	// Compile source.
+	cmd := exec.Command("go", "build", "-gcflags", "-S", src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		log.Fatalf("could not build target: %v", err)
+	}
+
+	patterns := []string{
+		// It is hard to look for actual instructions in an
+		// arch-independent way.  So we'll just look for
+		// pseudo-ops that are arch-independent.
+		"\tTEXT\t",
+		"\tFUNCDATA\t",
+		"\tPCDATA\t",
+	}
+	outstr := string(out)
+	for _, p := range patterns {
+		if !strings.Contains(outstr, p) {
+			println(outstr)
+			panic("can't find pattern " + p)
+		}
+	}
+}