[dev.cc] cmd/go: run C and Go assemblers side by side and verify matching output

When an assembly file must be assembled, cmd/go now runs
both (say) 6a and new6a and checks that they write identical
output files.

This serves as a build-time test that the new assemblers are accurate
conversions of the old ones. As long as they are producing identical
bytes, there's no need for run-time testing.

Once the C conversion is done, we'll throw away the C code
and this checking.

Change-Id: I0216dad56b7e79011eecd27f1aff4fe79bfe720b
Reviewed-on: https://go-review.googlesource.com/3145Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/build.go

@@ -1672,11 +1672,40 @@ func (gcToolchain) gc(b *builder, p *Package, archive, obj string, asmhdr bool,
 	return ofile, output, err
 }
 
+// verifyAsm specifies whether to check the assemblers written in Go
+// against the assemblers written in C. If set, asm will run both (say) 6a and new6a
+// and fail if the two produce different output files.
+const verifyAsm = true
+
 func (gcToolchain) asm(b *builder, p *Package, obj, ofile, sfile string) error {
 	// Add -I pkg/GOOS_GOARCH so #include "textflag.h" works in .s files.
 	inc := filepath.Join(goroot, "pkg", fmt.Sprintf("%s_%s", goos, goarch))
 	sfile = mkAbs(p.Dir, sfile)
-	return b.run(p.Dir, p.ImportPath, nil, tool(archChar+"a"), "-trimpath", b.work, "-I", obj, "-I", inc, "-o", ofile, "-D", "GOOS_"+goos, "-D", "GOARCH_"+goarch, sfile)
+	args := []interface{}{tool(archChar + "a"), "-o", ofile, "-trimpath", b.work, "-I", obj, "-I", inc, "-D", "GOOS_" + goos, "-D", "GOARCH_" + goarch, sfile}
+	if err := b.run(p.Dir, p.ImportPath, nil, args...); err != nil {
+		return err
+	}
+	if verifyAsm {
+		newArgs := make([]interface{}, len(args))
+		copy(newArgs, args)
+		newArgs[0] = tool("new" + archChar + "a")
+		newArgs[2] = ofile + ".new" // x.6 becomes x.6.new
+		if err := b.run(p.Dir, p.ImportPath, nil, newArgs...); err != nil {
+			return err
+		}
+		data1, err := ioutil.ReadFile(ofile)
+		if err != nil {
+			return err
+		}
+		data2, err := ioutil.ReadFile(ofile + ".new")
+		if err != nil {
+			return err
+		}
+		if !bytes.Equal(data1, data2) {
+			return fmt.Errorf("%sa and n%sa produced different output files:\n%s\n%s", archChar, archChar, strings.Join(stringList(args...), " "), strings.Join(stringList(newArgs...), " "))
+		}
+	}
+	return nil
 }
 
 func (gcToolchain) pkgpath(basedir string, p *Package) string {

src/cmd/go/main.go

@@ -662,7 +662,7 @@ func stringList(args ...interface{}) []string {
 		case string:
 			x = append(x, arg)
 		default:
-			panic("stringList: invalid argument")
+			panic("stringList: invalid argument of type " + fmt.Sprintf("%T", arg))
 		}
 	}
 	return x