cmd/compile: introduce "temporary" readers for more storage reuse

Change-Id: Id05d6099624284a9c1583b066d1a703e806b1e22
Reviewed-on: https://go-review.googlesource.com/c/go/+/433037
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

src/cmd/compile/internal/importer/ureader.go

@@ -83,6 +83,17 @@ func (pr *pkgReader) newReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pk
 	}
 }
 
+func (pr *pkgReader) tempReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pkgbits.SyncMarker) *reader {
+	return &reader{
+		Decoder: pr.TempDecoder(k, idx, marker),
+		p:       pr,
+	}
+}
+
+func (pr *pkgReader) retireReader(r *reader) {
+	pr.RetireDecoder(&r.Decoder)
+}
+
 // @@@ Positions
 
 func (r *reader) pos() syntax.Pos {
@@ -106,9 +117,9 @@ func (pr *pkgReader) posBaseIdx(idx pkgbits.Index) *syntax.PosBase {
 	if b := pr.posBases[idx]; b != nil {
 		return b
 	}
-
-	r := pr.newReader(pkgbits.RelocPosBase, idx, pkgbits.SyncPosBase)
 	var b *syntax.PosBase
+	{
+		r := pr.tempReader(pkgbits.RelocPosBase, idx, pkgbits.SyncPosBase)
 
 		filename := r.String()
 
@@ -120,6 +131,8 @@ func (pr *pkgReader) posBaseIdx(idx pkgbits.Index) *syntax.PosBase {
 			col := r.Uint()
 			b = syntax.NewLineBase(pos, filename, true, line, col)
 		}
+		pr.retireReader(r)
+	}
 
 	pr.posBases[idx] = b
 	return b
@@ -202,11 +215,15 @@ func (pr *pkgReader) typIdx(info typeInfo, dict *readerDict) types2.Type {
 		return typ
 	}
 
-	r := pr.newReader(pkgbits.RelocType, idx, pkgbits.SyncTypeIdx)
+	var typ types2.Type
+	{
+		r := pr.tempReader(pkgbits.RelocType, idx, pkgbits.SyncTypeIdx)
 		r.dict = dict
 
-	typ := r.doTyp()
+		typ = r.doTyp()
 		assert(typ != nil)
+		pr.retireReader(r)
+	}
 
 	// See comment in pkgReader.typIdx explaining how this happens.
 	if prev := *where; prev != nil {
@@ -361,12 +378,18 @@ func (r *reader) obj() (types2.Object, []types2.Type) {
 }
 
 func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types2.Package, string) {
-	rname := pr.newReader(pkgbits.RelocName, idx, pkgbits.SyncObject1)
+	var objPkg *types2.Package
+	var objName string
+	var tag pkgbits.CodeObj
+	{
+		rname := pr.tempReader(pkgbits.RelocName, idx, pkgbits.SyncObject1)
 
-	objPkg, objName := rname.qualifiedIdent()
+		objPkg, objName = rname.qualifiedIdent()
 		assert(objName != "")
 
-	tag := pkgbits.CodeObj(rname.Code(pkgbits.SyncCodeObj))
+		tag = pkgbits.CodeObj(rname.Code(pkgbits.SyncCodeObj))
+		pr.retireReader(rname)
+	}
 
 	if tag == pkgbits.ObjStub {
 		base.Assertf(objPkg == nil || objPkg == types2.Unsafe, "unexpected stub package: %v", objPkg)
@@ -431,9 +454,9 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types2.Package, string) {
 }
 
 func (pr *pkgReader) objDictIdx(idx pkgbits.Index) *readerDict {
-	r := pr.newReader(pkgbits.RelocObjDict, idx, pkgbits.SyncObject1)
-
 	var dict readerDict
+	{
+		r := pr.tempReader(pkgbits.RelocObjDict, idx, pkgbits.SyncObject1)
 
 		if implicits := r.Len(); implicits != 0 {
 			base.Fatalf("unexpected object with %v implicit type parameter(s)", implicits)
@@ -450,6 +473,8 @@ func (pr *pkgReader) objDictIdx(idx pkgbits.Index) *readerDict {
 			dict.derived[i] = derivedInfo{r.Reloc(pkgbits.RelocType), r.Bool()}
 		}
 
+		pr.retireReader(r)
+	}
 	// function references follow, but reader doesn't need those
 
 	return &dict

src/go/internal/gcimporter/ureader.go

@@ -131,6 +131,17 @@ func (pr *pkgReader) newReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pk
 	}
 }
 
+func (pr *pkgReader) tempReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pkgbits.SyncMarker) *reader {
+	return &reader{
+		Decoder: pr.TempDecoder(k, idx, marker),
+		p:       pr,
+	}
+}
+
+func (pr *pkgReader) retireReader(r *reader) {
+	pr.RetireDecoder(&r.Decoder)
+}
+
 // @@@ Positions
 
 func (r *reader) pos() token.Pos {
@@ -155,14 +166,16 @@ func (pr *pkgReader) posBaseIdx(idx pkgbits.Index) string {
 		return b
 	}
 
-	r := pr.newReader(pkgbits.RelocPosBase, idx, pkgbits.SyncPosBase)
+	var filename string
+	{
+		r := pr.tempReader(pkgbits.RelocPosBase, idx, pkgbits.SyncPosBase)
 
 		// Within types2, position bases have a lot more details (e.g.,
 		// keeping track of where //line directives appeared exactly).
 		//
 		// For go/types, we just track the file name.
 
-	filename := r.String()
+		filename = r.String()
 
 		if r.Bool() { // file base
 			// Was: "b = token.NewTrimmedFileBase(filename, true)"
@@ -174,7 +187,8 @@ func (pr *pkgReader) posBaseIdx(idx pkgbits.Index) string {
 			// Was: "b = token.NewLineBase(pos, filename, true, line, col)"
 			_, _, _ = pos, line, col
 		}
-
+		pr.retireReader(r)
+	}
 	b := filename
 	pr.posBases[idx] = b
 	return b
@@ -286,12 +300,15 @@ func (pr *pkgReader) typIdx(info typeInfo, dict *readerDict) types.Type {
 		return typ
 	}
 
-	r := pr.newReader(pkgbits.RelocType, idx, pkgbits.SyncTypeIdx)
+	var typ types.Type
+	{
+		r := pr.tempReader(pkgbits.RelocType, idx, pkgbits.SyncTypeIdx)
 		r.dict = dict
 
-	typ := r.doTyp()
+		typ = r.doTyp()
 		assert(typ != nil)
-
+		pr.retireReader(r)
+	}
 	// See comment in pkgReader.typIdx explaining how this happens.
 	if prev := *where; prev != nil {
 		return prev
@@ -457,12 +474,19 @@ func (r *reader) obj() (types.Object, []types.Type) {
 }
 
 func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
-	rname := pr.newReader(pkgbits.RelocName, idx, pkgbits.SyncObject1)
 
-	objPkg, objName := rname.qualifiedIdent()
+	var objPkg *types.Package
+	var objName string
+	var tag pkgbits.CodeObj
+	{
+		rname := pr.tempReader(pkgbits.RelocName, idx, pkgbits.SyncObject1)
+
+		objPkg, objName = rname.qualifiedIdent()
 		assert(objName != "")
 
-	tag := pkgbits.CodeObj(rname.Code(pkgbits.SyncCodeObj))
+		tag = pkgbits.CodeObj(rname.Code(pkgbits.SyncCodeObj))
+		pr.retireReader(rname)
+	}
 
 	if tag == pkgbits.ObjStub {
 		assert(objPkg == nil || objPkg == types.Unsafe)
@@ -567,10 +591,11 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
 }
 
 func (pr *pkgReader) objDictIdx(idx pkgbits.Index) *readerDict {
-	r := pr.newReader(pkgbits.RelocObjDict, idx, pkgbits.SyncObject1)
 
 	var dict readerDict
 
+	{
+		r := pr.tempReader(pkgbits.RelocObjDict, idx, pkgbits.SyncObject1)
 		if implicits := r.Len(); implicits != 0 {
 			errorf("unexpected object with %v implicit type parameter(s)", implicits)
 		}
@@ -586,6 +611,8 @@ func (pr *pkgReader) objDictIdx(idx pkgbits.Index) *readerDict {
 			dict.derived[i] = derivedInfo{r.Reloc(pkgbits.RelocType), r.Bool()}
 		}
 
+		pr.retireReader(r)
+	}
 	// function references follow, but reader doesn't need those
 
 	return &dict

src/internal/pkgbits/decoder.go

@@ -52,6 +52,8 @@ type PkgDecoder struct {
 	// For example, section K's end positions start at elemEndsEnds[K-1]
 	// (or 0, if K==0) and end at elemEndsEnds[K].
 	elemEndsEnds [numRelocs]uint32
+
+	scratchRelocEnt []RelocEnt
 }
 
 // PkgPath returns the package path for the package
@@ -165,6 +167,21 @@ func (pr *PkgDecoder) NewDecoder(k RelocKind, idx Index, marker SyncMarker) Deco
 	return r
 }
 
+// TempDecoder returns a Decoder for the given (section, index) pair,
+// and decodes the given SyncMarker from the element bitstream.
+// If possible the Decoder should be RetireDecoder'd when it is no longer
+// needed, this will avoid heap allocations.
+func (pr *PkgDecoder) TempDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
+	r := pr.TempDecoderRaw(k, idx)
+	r.Sync(marker)
+	return r
+}
+
+func (pr *PkgDecoder) RetireDecoder(d *Decoder) {
+	pr.scratchRelocEnt = d.Relocs
+	d.Relocs = nil
+}
+
 // NewDecoderRaw returns a Decoder for the given (section, index) pair.
 //
 // Most callers should use NewDecoder instead.
@@ -186,6 +203,30 @@ func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx Index) Decoder {
 	return r
 }
 
+func (pr *PkgDecoder) TempDecoderRaw(k RelocKind, idx Index) Decoder {
+	r := Decoder{
+		common: pr,
+		k:      k,
+		Idx:    idx,
+	}
+
+	r.Data.Reset(pr.DataIdx(k, idx))
+	r.Sync(SyncRelocs)
+	l := r.Len()
+	if cap(pr.scratchRelocEnt) >= l {
+		r.Relocs = pr.scratchRelocEnt[:l]
+		pr.scratchRelocEnt = nil
+	} else {
+		r.Relocs = make([]RelocEnt, l)
+	}
+	for i := range r.Relocs {
+		r.Sync(SyncReloc)
+		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
+	}
+
+	return r
+}
+
 // A Decoder provides methods for decoding an individual element's
 // bitstream data.
 type Decoder struct {
@@ -408,8 +449,12 @@ func (r *Decoder) bigFloat() *big.Float {
 // PeekPkgPath returns the package path for the specified package
 // index.
 func (pr *PkgDecoder) PeekPkgPath(idx Index) string {
-	r := pr.NewDecoder(RelocPkg, idx, SyncPkgDef)
+	var path string
-	path := r.String()
+	{
+		r := pr.TempDecoder(RelocPkg, idx, SyncPkgDef)
+		path = r.String()
+		pr.RetireDecoder(&r)
+	}
 	if path == "" {
 		path = pr.pkgPath
 	}
@@ -419,14 +464,23 @@ func (pr *PkgDecoder) PeekPkgPath(idx Index) string {
 // PeekObj returns the package path, object name, and CodeObj for the
 // specified object index.
 func (pr *PkgDecoder) PeekObj(idx Index) (string, string, CodeObj) {
-	r := pr.NewDecoder(RelocName, idx, SyncObject1)
+	var ridx Index
+	var name string
+	var rcode int
+	{
+		r := pr.TempDecoder(RelocName, idx, SyncObject1)
 		r.Sync(SyncSym)
 		r.Sync(SyncPkg)
-	path := pr.PeekPkgPath(r.Reloc(RelocPkg))
+		ridx = r.Reloc(RelocPkg)
-	name := r.String()
+		name = r.String()
+		rcode = r.Code(SyncCodeObj)
+		pr.RetireDecoder(&r)
+	}
+
+	path := pr.PeekPkgPath(ridx)
 	assert(name != "")
 
-	tag := CodeObj(r.Code(SyncCodeObj))
+	tag := CodeObj(rcode)
 
 	return path, name, tag
 }