cmd/compile: sort import strings for canonical obj files

This is not necessary for reproduceability but it removes
differences due to imported package order between compiles
using textual vs binary export format. The packages list
tends to be very short, so it's ok doing it always for now.

Guarded with a documented (const) flag so it's trivial to
disable and remove eventually.

Also, use the same flag now to enforce parameter numbering.

Change-Id: Ie05d2490df770239696ecbecc07532ed62ccd5c0
Reviewed-on: https://go-review.googlesource.com/22445
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -123,12 +123,11 @@ const posInfoFormat = false
 // TODO(gri) remove eventually
 const forceNewExport = false // force new export format - DO NOT SUBMIT with this flag set
 
-// forceNumberedParams keeps parameter numbering in exported parameter names
-// even where we don't really need it (because the parameter names are not used
-// elsewhere). Leave it enabled for now to remove this difference in generated
-// object files so we can more easily compare old and new format.
-// TODO(gri) remove once we switched to new format
-const forceNumberedParams = true
+// forceObjFileStability enforces additional constraints in export data
+// and other parts of the compiler to eliminate object file differences
+// only due to the choice of export format.
+// TODO(gri) disable and remove once there is only one export format again
+const forceObjFileStability = true
 
 const exportVersion = "v0"
 
@@ -882,7 +881,7 @@ func parName(f *Field, numbered bool) string {
 	// Functions that can be inlined use numbered parameters so we can distingish them
 	// from other names in their context after inlining (i.e., the parameter numbering
 	// is a form of parameter rewriting). See issue 4326 for an example and test case.
-	if forceNumberedParams || numbered {
+	if forceObjFileStability || numbered {
 		if !strings.Contains(name, "·") && f.Nname != nil && f.Nname.Name != nil && f.Nname.Name.Vargen > 0 {
 			name = fmt.Sprintf("%s·%d", name, f.Nname.Name.Vargen) // append Vargen
 		}

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

@@ -1391,6 +1391,11 @@ func dumptypestructs() {
 	}
 
 	// generate import strings for imported packages
+	if forceObjFileStability {
+		// Sorting the packages is not necessary but to compare binaries created
+		// using textual and binary format we sort by path to reduce differences.
+		sort.Sort(pkgByPath(pkgs))
+	}
 	for _, p := range pkgs {
 		if p.Direct {
 			dimportpath(p)
@@ -1429,6 +1434,12 @@ func dumptypestructs() {
 	}
 }
 
+type pkgByPath []*Pkg
+
+func (a pkgByPath) Len() int           { return len(a) }
+func (a pkgByPath) Less(i, j int) bool { return a[i].Path < a[j].Path }
+func (a pkgByPath) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
 func dalgsym(t *Type) *Sym {
 	var s *Sym
 	var hashfunc *Sym