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>

1 files changed, 62 insertions, 8 deletions

diff --git a/src/internal/pkgbits/decoder.go b/src/internal/pkgbits/decoder.go
index 357e328a3b..1a18da3301 100644
--- a/src/internal/pkgbits/decoder.go
+++ b/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)
-	path := r.String()
+	var path 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)
-	r.Sync(SyncSym)
-	r.Sync(SyncPkg)
-	path := pr.PeekPkgPath(r.Reloc(RelocPkg))
-	name := r.String()
+	var ridx Index
+	var name string
+	var rcode int
+	{
+		r := pr.TempDecoder(RelocName, idx, SyncObject1)
+		r.Sync(SyncSym)
+		r.Sync(SyncPkg)
+		ridx = r.Reloc(RelocPkg)
+		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
 }