cmd/compile/internal/noder: write V2 bitstream aliastypeparams=1

Enables V2 unified IR bitstreams when GOEXPERIMENT aliastypeparams
are enabled.

Allows pkgbits.NewPkgEncoder to set the output version.
Reenables support for writing V0 streams.

Updates #68778
Updates #68526

Change-Id: I590c494d81ab7db148232ceaba52229068d1e986
Reviewed-on: https://go-review.googlesource.com/c/go/+/608595
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

3 files changed, 29 insertions, 23 deletions

diff --git a/src/internal/pkgbits/decoder.go b/src/internal/pkgbits/decoder.go
index ca66446dba..09f26a84bd 100644
--- a/src/internal/pkgbits/decoder.go
+++ b/src/internal/pkgbits/decoder.go
@@ -82,8 +82,8 @@ func NewPkgDecoder(pkgPath, input string) PkgDecoder {
 	assert(binary.Read(r, binary.LittleEndian, &ver) == nil)
 	pr.version = Version(ver)
 
-	if pr.version >= V2 { // TODO(taking): Switch to numVersions.
-		panic(fmt.Errorf("cannot decode %q, export data version %d is too new", pkgPath, pr.version))
+	if pr.version >= numVersions {
+		panic(fmt.Errorf("cannot decode %q, export data version %d is greater than maximum supported version %d", pkgPath, pr.version, numVersions-1))
 	}
 
 	if pr.version.Has(Flags) {
@@ -101,7 +101,9 @@ func NewPkgDecoder(pkgPath, input string) PkgDecoder {
 	assert(err == nil)
 
 	pr.elemData = input[pos:]
-	assert(len(pr.elemData)-8 == int(pr.elemEnds[len(pr.elemEnds)-1]))
+
+	const fingerprintSize = 8
+	assert(len(pr.elemData)-fingerprintSize == int(pr.elemEnds[len(pr.elemEnds)-1]))
 
 	return pr
 }
diff --git a/src/internal/pkgbits/encoder.go b/src/internal/pkgbits/encoder.go
index b632b58ca0..c17a12399d 100644
--- a/src/internal/pkgbits/encoder.go
+++ b/src/internal/pkgbits/encoder.go
@@ -15,9 +15,6 @@ import (
 	"strings"
 )
 
-// currentVersion is the current version number written.
-const currentVersion = V1
-
 // A PkgEncoder provides methods for encoding a package's Unified IR
 // export data.
 type PkgEncoder struct {
@@ -47,10 +44,9 @@ func (pw *PkgEncoder) SyncMarkers() bool { return pw.syncFrames >= 0 }
 // export data files, but can help diagnosing desync errors in
 // higher-level Unified IR reader/writer code. If syncFrames is
 // negative, then sync markers are omitted entirely.
-func NewPkgEncoder(syncFrames int) PkgEncoder {
+func NewPkgEncoder(version Version, syncFrames int) PkgEncoder {
 	return PkgEncoder{
-		// TODO(taking): Change NewPkgEncoder to take a version as an argument, and remove currentVersion.
-		version:    currentVersion,
+		version:    version,
 		stringsIdx: make(map[string]Index),
 		syncFrames: syncFrames,
 	}
@@ -68,11 +64,13 @@ func (pw *PkgEncoder) DumpTo(out0 io.Writer) (fingerprint [8]byte) {
 
 	writeUint32(uint32(pw.version))
 
-	var flags uint32
-	if pw.SyncMarkers() {
-		flags |= flagSyncMarkers
+	if pw.version.Has(Flags) {
+		var flags uint32
+		if pw.SyncMarkers() {
+			flags |= flagSyncMarkers
+		}
+		writeUint32(flags)
 	}
-	writeUint32(flags)
 
 	// Write elemEndsEnds.
 	var sum uint32
diff --git a/src/internal/pkgbits/pkgbits_test.go b/src/internal/pkgbits/pkgbits_test.go
index f4c3719446..6f4004741b 100644
--- a/src/internal/pkgbits/pkgbits_test.go
+++ b/src/internal/pkgbits/pkgbits_test.go
@@ -11,19 +11,25 @@ import (
 )
 
 func TestRoundTrip(t *testing.T) {
-	pw := pkgbits.NewPkgEncoder(-1)
-	w := pw.NewEncoder(pkgbits.RelocMeta, pkgbits.SyncPublic)
-	w.Flush()
+	for _, version := range []pkgbits.Version{
+		pkgbits.V0,
+		pkgbits.V1,
+		pkgbits.V2,
+	} {
+		pw := pkgbits.NewPkgEncoder(version, -1)
+		w := pw.NewEncoder(pkgbits.RelocMeta, pkgbits.SyncPublic)
+		w.Flush()
 
-	var b strings.Builder
-	_ = pw.DumpTo(&b)
-	input := b.String()
+		var b strings.Builder
+		_ = pw.DumpTo(&b)
+		input := b.String()
 
-	pr := pkgbits.NewPkgDecoder("package_id", input)
-	r := pr.NewDecoder(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
+		pr := pkgbits.NewPkgDecoder("package_id", input)
+		r := pr.NewDecoder(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
 
-	if r.Version() != w.Version() {
-		t.Errorf("Expected reader version %q to be the writer version %q", r.Version(), w.Version())
+		if r.Version() != w.Version() {
+			t.Errorf("Expected reader version %q to be the writer version %q", r.Version(), w.Version())
+		}
 	}
 }