5 files changed, 99 insertions, 49 deletions
diff --git a/src/internal/pkgbits/decoder.go b/src/internal/pkgbits/decoder.go
index 0b5fd9705c..5e233b8770 100644
--- a/src/internal/pkgbits/decoder.go
+++ b/src/internal/pkgbits/decoder.go
@@ -18,6 +18,12 @@ import (
 // A PkgDecoder provides methods for decoding a package's Unified IR
 // export data.
 type PkgDecoder struct {
+	// version is the file format version.
+	version uint32
+
+	// sync indicates whether the file uses sync markers.
+	sync bool
+
 	// pkgPath is the package path for the package to be decoded.
 	//
 	// TODO(mdempsky): Remove; unneeded since CL 391014.
@@ -52,6 +58,9 @@ type PkgDecoder struct {
 // TODO(mdempsky): Remove; unneeded since CL 391014.
 func (pr *PkgDecoder) PkgPath() string { return pr.pkgPath }
 
+// SyncMarkers reports whether pr uses sync markers.
+func (pr *PkgDecoder) SyncMarkers() bool { return pr.sync }
+
 // NewPkgDecoder returns a PkgDecoder initialized to read the Unified
 // IR export data from input. pkgPath is the package path for the
 // compilation unit that produced the export data.
@@ -67,9 +76,18 @@ func NewPkgDecoder(pkgPath, input string) PkgDecoder {
 
 	r := strings.NewReader(input)
 
-	var version uint32
-	assert(binary.Read(r, binary.LittleEndian, &version) == nil)
-	assert(version == 0)
+	assert(binary.Read(r, binary.LittleEndian, &pr.version) == nil)
+
+	switch pr.version {
+	default:
+		panic(fmt.Errorf("unsupported version: %v", pr.version))
+	case 0:
+		// no flags
+	case 1:
+		var flags uint32
+		assert(binary.Read(r, binary.LittleEndian, &flags) == nil)
+		pr.sync = flags&flagSyncMarkers != 0
+	}
 
 	assert(binary.Read(r, binary.LittleEndian, pr.elemEndsEnds[:]) == nil)
 
@@ -215,7 +233,7 @@ func (r *Decoder) rawReloc(k RelocKind, idx int) Index {
 //
 // If EnableSync is false, then Sync is a no-op.
 func (r *Decoder) Sync(mWant SyncMarker) {
-	if !EnableSync {
+	if !r.common.sync {
 		return
 	}
 
diff --git a/src/internal/pkgbits/encoder.go b/src/internal/pkgbits/encoder.go
index 1326a135cf..ec47e352cb 100644
--- a/src/internal/pkgbits/encoder.go
+++ b/src/internal/pkgbits/encoder.go
@@ -14,6 +14,16 @@ import (
 	"runtime"
 )
 
+// currentVersion is the current version number.
+//
+//   - v0: initial prototype
+//
+//   - v1: adds the flags uint32 word
+//
+// TODO(mdempsky): For the next version bump:
+//   - remove the legacy "has init" bool from the public root
+const currentVersion uint32 = 1
+
 // A PkgEncoder provides methods for encoding a package's Unified IR
 // export data.
 type PkgEncoder struct {
@@ -25,15 +35,21 @@ type PkgEncoder struct {
 	// elems[RelocString][stringsIdx[s]] == s (if present).
 	stringsIdx map[string]Index
 
+	// syncFrames is the number of frames to write at each sync
+	// marker. A negative value means sync markers are omitted.
 	syncFrames int
 }
 
+// SyncMarkers reports whether pw uses sync markers.
+func (pw *PkgEncoder) SyncMarkers() bool { return pw.syncFrames >= 0 }
+
 // NewPkgEncoder returns an initialized PkgEncoder.
 //
 // syncFrames is the number of caller frames that should be serialized
 // at Sync points. Serializing additional frames results in larger
 // export data files, but can help diagnosing desync errors in
-// higher-level Unified IR reader/writer code.
+// higher-level Unified IR reader/writer code. If syncFrames is
+// negative, then sync markers are omitted entirely.
 func NewPkgEncoder(syncFrames int) PkgEncoder {
 	return PkgEncoder{
 		stringsIdx: make(map[string]Index),
@@ -51,7 +67,13 @@ func (pw *PkgEncoder) DumpTo(out0 io.Writer) (fingerprint [8]byte) {
 		assert(binary.Write(out, binary.LittleEndian, x) == nil)
 	}
 
-	writeUint32(0) // version
+	writeUint32(currentVersion)
+
+	var flags uint32
+	if pw.SyncMarkers() {
+		flags |= flagSyncMarkers
+	}
+	writeUint32(flags)
 
 	// Write elemEndsEnds.
 	var sum uint32
@@ -204,7 +226,7 @@ func (w *Encoder) rawReloc(r RelocKind, idx Index) int {
 }
 
 func (w *Encoder) Sync(m SyncMarker) {
-	if !EnableSync {
+	if !w.p.SyncMarkers() {
 		return
 	}
 
@@ -297,8 +319,14 @@ func (w *Encoder) Code(c Code) {
 // section (if not already present), and then writing a relocation
 // into the element bitstream.
 func (w *Encoder) String(s string) {
+	w.StringRef(w.p.StringIdx(s))
+}
+
+// StringRef writes a reference to the given index, which must be a
+// previously encoded string value.
+func (w *Encoder) StringRef(idx Index) {
 	w.Sync(SyncString)
-	w.Reloc(RelocString, w.p.StringIdx(s))
+	w.Reloc(RelocString, idx)
 }
 
 // Strings encodes and writes a variable-length slice of strings into
diff --git a/src/internal/pkgbits/flags.go b/src/internal/pkgbits/flags.go
new file mode 100644
index 0000000000..654222745f
--- /dev/null
+++ b/src/internal/pkgbits/flags.go
@@ -0,0 +1,9 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package pkgbits
+
+const (
+	flagSyncMarkers = 1 << iota // file format contains sync markers
+)
diff --git a/src/internal/pkgbits/sync.go b/src/internal/pkgbits/sync.go
index 72f776af05..1520b73afb 100644
--- a/src/internal/pkgbits/sync.go
+++ b/src/internal/pkgbits/sync.go
@@ -10,17 +10,6 @@ import (
 	"strings"
 )
 
-// EnableSync controls whether sync markers are written into unified
-// IR's export data format and also whether they're expected when
-// reading them back in. They're inessential to the correct
-// functioning of unified IR, but are helpful during development to
-// detect mistakes.
-//
-// When sync is enabled, writer stack frames will also be included in
-// the export data. Currently, a fixed number of frames are included,
-// controlled by -d=syncframes (default 0).
-const EnableSync = true
-
 // fmtFrames formats a backtrace for reporting reader/writer desyncs.
 func fmtFrames(pcs ...uintptr) []string {
 	res := make([]string, 0, len(pcs))
@@ -109,6 +98,7 @@ const (
 	SyncExprs
 	SyncExpr
 	SyncExprType
+	SyncAssign
 	SyncOp
 	SyncFuncLit
 	SyncCompLit
@@ -139,4 +129,8 @@ const (
 	SyncStmtsEnd
 	SyncLabel
 	SyncOptLabel
+
+	SyncMultiExpr
+	SyncRType
+	SyncConvRTTI
 )
diff --git a/src/internal/pkgbits/syncmarker_string.go b/src/internal/pkgbits/syncmarker_string.go
index 39db9eddad..4a5b0ca5f2 100644
--- a/src/internal/pkgbits/syncmarker_string.go
+++ b/src/internal/pkgbits/syncmarker_string.go
@@ -45,39 +45,40 @@ func _() {
 	_ = x[SyncExprs-35]
 	_ = x[SyncExpr-36]
 	_ = x[SyncExprType-37]
-	_ = x[SyncOp-38]
-	_ = x[SyncFuncLit-39]
-	_ = x[SyncCompLit-40]
-	_ = x[SyncDecl-41]
-	_ = x[SyncFuncBody-42]
-	_ = x[SyncOpenScope-43]
-	_ = x[SyncCloseScope-44]
-	_ = x[SyncCloseAnotherScope-45]
-	_ = x[SyncDeclNames-46]
-	_ = x[SyncDeclName-47]
-	_ = x[SyncStmts-48]
-	_ = x[SyncBlockStmt-49]
-	_ = x[SyncIfStmt-50]
-	_ = x[SyncForStmt-51]
-	_ = x[SyncSwitchStmt-52]
-	_ = x[SyncRangeStmt-53]
-	_ = x[SyncCaseClause-54]
-	_ = x[SyncCommClause-55]
-	_ = x[SyncSelectStmt-56]
-	_ = x[SyncDecls-57]
-	_ = x[SyncLabeledStmt-58]
-	_ = x[SyncUseObjLocal-59]
-	_ = x[SyncAddLocal-60]
-	_ = x[SyncLinkname-61]
-	_ = x[SyncStmt1-62]
-	_ = x[SyncStmtsEnd-63]
-	_ = x[SyncLabel-64]
-	_ = x[SyncOptLabel-65]
+	_ = x[SyncAssign-38]
+	_ = x[SyncOp-39]
+	_ = x[SyncFuncLit-40]
+	_ = x[SyncCompLit-41]
+	_ = x[SyncDecl-42]
+	_ = x[SyncFuncBody-43]
+	_ = x[SyncOpenScope-44]
+	_ = x[SyncCloseScope-45]
+	_ = x[SyncCloseAnotherScope-46]
+	_ = x[SyncDeclNames-47]
+	_ = x[SyncDeclName-48]
+	_ = x[SyncStmts-49]
+	_ = x[SyncBlockStmt-50]
+	_ = x[SyncIfStmt-51]
+	_ = x[SyncForStmt-52]
+	_ = x[SyncSwitchStmt-53]
+	_ = x[SyncRangeStmt-54]
+	_ = x[SyncCaseClause-55]
+	_ = x[SyncCommClause-56]
+	_ = x[SyncSelectStmt-57]
+	_ = x[SyncDecls-58]
+	_ = x[SyncLabeledStmt-59]
+	_ = x[SyncUseObjLocal-60]
+	_ = x[SyncAddLocal-61]
+	_ = x[SyncLinkname-62]
+	_ = x[SyncStmt1-63]
+	_ = x[SyncStmtsEnd-64]
+	_ = x[SyncLabel-65]
+	_ = x[SyncOptLabel-66]
 }
 
-const _SyncMarker_name = "EOFBoolInt64Uint64StringValueValRelocsRelocUseRelocPublicPosPosBaseObjectObject1PkgPkgDefMethodTypeTypeIdxTypeParamNamesSignatureParamsParamCodeObjSymLocalIdentSelectorPrivateFuncExtVarExtTypeExtPragmaExprListExprsExprAssertTypeOpFuncLitCompLitDeclFuncBodyOpenScopeCloseScopeCloseAnotherScopeDeclNamesDeclNameStmtsBlockStmtIfStmtForStmtSwitchStmtRangeStmtCaseClauseCommClauseSelectStmtDeclsLabeledStmtUseObjLocalAddLocalLinknameStmt1StmtsEndLabelOptLabel"
+const _SyncMarker_name = "EOFBoolInt64Uint64StringValueValRelocsRelocUseRelocPublicPosPosBaseObjectObject1PkgPkgDefMethodTypeTypeIdxTypeParamNamesSignatureParamsParamCodeObjSymLocalIdentSelectorPrivateFuncExtVarExtTypeExtPragmaExprListExprsExprExprTypeAssignOpFuncLitCompLitDeclFuncBodyOpenScopeCloseScopeCloseAnotherScopeDeclNamesDeclNameStmtsBlockStmtIfStmtForStmtSwitchStmtRangeStmtCaseClauseCommClauseSelectStmtDeclsLabeledStmtUseObjLocalAddLocalLinknameStmt1StmtsEndLabelOptLabel"
 
-var _SyncMarker_index = [...]uint16{0, 3, 7, 12, 18, 24, 29, 32, 38, 43, 51, 57, 60, 67, 73, 80, 83, 89, 95, 99, 106, 120, 129, 135, 140, 147, 150, 160, 168, 175, 182, 188, 195, 201, 209, 214, 218, 228, 230, 237, 244, 248, 256, 265, 275, 292, 301, 309, 314, 323, 329, 336, 346, 355, 365, 375, 385, 390, 401, 412, 420, 428, 433, 441, 446, 454}
+var _SyncMarker_index = [...]uint16{0, 3, 7, 12, 18, 24, 29, 32, 38, 43, 51, 57, 60, 67, 73, 80, 83, 89, 95, 99, 106, 120, 129, 135, 140, 147, 150, 160, 168, 175, 182, 188, 195, 201, 209, 214, 218, 226, 232, 234, 241, 248, 252, 260, 269, 279, 296, 305, 313, 318, 327, 333, 340, 350, 359, 369, 379, 389, 394, 405, 416, 424, 432, 437, 445, 450, 458}
 
 func (i SyncMarker) String() string {
 	i -= 1