debug/dwarf: speed up SkipChildren for compilation units

For a common pattern of iterating only over top-level compilation units (CU)
Reader.SkipChildren has decode and meterialize all CU subentries just
to skip them, because DW_TAG_compile_unit does not have DW_AT_sibling.
However, CUs have total size encoded before the unit and we already parse them
and know all unit sizes.
Optimize Reader.SkipChildren to use that size when skipping CUs children.

This speeds up iteration over a 1.3GB object file from 7.5s to 0.73s.

Change-Id: I2a8f00955159b4bd13571409f4817805f934cb69
Reviewed-on: https://go-review.googlesource.com/c/go/+/256217
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Than McIntosh <thanm@google.com>

src/debug/dwarf/entry.go

@@ -717,6 +717,7 @@ type Reader struct {
 	d            *Data
 	err          error
 	unit         int
+	lastUnit     bool   // set if last entry returned by Next is TagCompileUnit/TagPartialUnit
 	lastChildren bool   // .Children of last entry returned by Next
 	lastSibling  Offset // .Val(AttrSibling) of last entry returned by Next
 	cu           *Entry // current compilation unit
@@ -774,13 +775,18 @@ func (r *Reader) Seek(off Offset) {
 // maybeNextUnit advances to the next unit if this one is finished.
 func (r *Reader) maybeNextUnit() {
 	for len(r.b.data) == 0 && r.unit+1 < len(r.d.unit) {
-		r.unit++
-		u := &r.d.unit[r.unit]
-		r.b = makeBuf(r.d, u, "info", u.off, u.data)
-		r.cu = nil
+		r.nextUnit()
 	}
 }
 
+// nextUnit advances to the next unit.
+func (r *Reader) nextUnit() {
+	r.unit++
+	u := &r.d.unit[r.unit]
+	r.b = makeBuf(r.d, u, "info", u.off, u.data)
+	r.cu = nil
+}
+
 // Next reads the next entry from the encoded entry stream.
 // It returns nil, nil when it reaches the end of the section.
 // It returns an error if the current offset is invalid or the data at the
@@ -799,12 +805,14 @@ func (r *Reader) Next() (*Entry, error) {
 		r.err = r.b.err
 		return nil, r.err
 	}
+	r.lastUnit = false
 	if e != nil {
 		r.lastChildren = e.Children
 		if r.lastChildren {
 			r.lastSibling, _ = e.Val(AttrSibling).(Offset)
 		}
 		if e.Tag == TagCompileUnit || e.Tag == TagPartialUnit {
+			r.lastUnit = true
 			r.cu = e
 		}
 	} else {
@@ -830,6 +838,11 @@ func (r *Reader) SkipChildren() {
 		return
 	}
 
+	if r.lastUnit && r.unit+1 < len(r.d.unit) {
+		r.nextUnit()
+		return
+	}
+
 	for {
 		e, err := r.Next()
 		if err != nil || e == nil || e.Tag == 0 {

src/debug/dwarf/entry_test.go

@@ -7,6 +7,7 @@ package dwarf_test
 import (
 	. "debug/dwarf"
 	"encoding/binary"
+	"path/filepath"
 	"reflect"
 	"testing"
 )
@@ -209,3 +210,44 @@ func Test64Bit(t *testing.T) {
 		}
 	}
 }
+
+func TestUnitIteration(t *testing.T) {
+	// Iterate over all ELF test files we have and ensure that
+	// we get the same set of compilation units skipping (method 0)
+	// and not skipping (method 1) CU children.
+	files, err := filepath.Glob(filepath.Join("testdata", "*.elf"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, file := range files {
+		t.Run(file, func(t *testing.T) {
+			d := elfData(t, file)
+			var units [2][]interface{}
+			for method := range units {
+				for r := d.Reader(); ; {
+					ent, err := r.Next()
+					if err != nil {
+						t.Fatal(err)
+					}
+					if ent == nil {
+						break
+					}
+					if ent.Tag == TagCompileUnit {
+						units[method] = append(units[method], ent.Val(AttrName))
+					}
+					if method == 0 {
+						if ent.Tag != TagCompileUnit {
+							t.Fatalf("found unexpected tag %v on top level", ent.Tag)
+						}
+						r.SkipChildren()
+					}
+				}
+			}
+			t.Logf("skipping CUs:     %v", units[0])
+			t.Logf("not-skipping CUs: %v", units[1])
+			if !reflect.DeepEqual(units[0], units[1]) {
+				t.Fatal("set of CUs differ")
+			}
+		})
+	}
+}