[dev.link] cmd/internal/goobj2, cmd/link: experiment new aux symbol accessors

Following the previous CLs, do the same for aux symbols. This has
some small speedup:

(linking cmd/compile)
Dostkcheck    41.0ms ± 1%    38.6ms ± 1%    -6.00%  (p=0.008 n=5+5)

Change-Id: Id62b2fc9e4ef1be92e60e4c03faec0a953eee94e
Reviewed-on: https://go-review.googlesource.com/c/go/+/222303
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/goobj2/objfile.go

@@ -368,6 +368,15 @@ func (a *Aux) Size() int {
 	return 1 + a.Sym.Size()
 }
 
+const AuxSize = 9 // TODO: is it possible to not hard-code this?
+
+type Aux2 [AuxSize]byte
+
+func (a *Aux2) Type() uint8 { return a[0] }
+func (a *Aux2) Sym() SymRef {
+	return SymRef{binary.LittleEndian.Uint32(a[1:]), binary.LittleEndian.Uint32(a[5:])}
+}
+
 type Writer struct {
 	wr        *bio.Writer
 	stringMap map[string]uint32
@@ -616,6 +625,19 @@ func (r *Reader) AuxOff(i int, j int) uint32 {
 	return r.h.Offsets[BlkAux] + (auxIdx+uint32(j))*uint32(auxsiz)
 }
 
+// Aux2 returns a pointer to the j-th aux symbol of the i-th symbol.
+func (r *Reader) Aux2(i int, j int) *Aux2 {
+	off := r.AuxOff(i, j)
+	return (*Aux2)(unsafe.Pointer(&r.b[off]))
+}
+
+// Auxs2 returns the aux symbols of the i-th symbol.
+func (r *Reader) Auxs2(i int) []Aux2 {
+	off := r.AuxOff(i, 0)
+	n := r.NAux(i)
+	return (*[1 << 20]Aux2)(unsafe.Pointer(&r.b[off]))[:n:n]
+}
+
 // DataOff returns the offset of the i-th symbol's data.
 func (r *Reader) DataOff(i int) uint32 {
 	dataIdxOff := r.h.Offsets[BlkDataIdx] + uint32(i*4)

src/cmd/internal/goobj2/objfile_test.go

@@ -20,10 +20,20 @@ func dummyWriter() *Writer {
 func TestSize(t *testing.T) {
 	// This test checks that hard-coded sizes match the actual sizes
 	// in the object file format.
+	tests := []struct {
+		x    interface{ Write(*Writer) }
+		want uint32
+	}{
+		{&Reloc{}, RelocSize},
+		{&Aux{}, AuxSize},
+	}
 	w := dummyWriter()
-	(&Reloc{}).Write(w)
-	off := w.off
-	if sz := uint32(RelocSize); off != sz {
-		t.Errorf("size mismatch: %d bytes written, but size=%d", off, sz)
+	for _, test := range tests {
+		off0 := w.off
+		test.x.Write(w)
+		got := w.off - off0
+		if got != test.want {
+			t.Errorf("size(%T) mismatch: %d bytes written, but size=%d", test.x, got, test.want)
+		}
 	}
 }

src/cmd/link/internal/ld/deadcode2.go

@@ -118,7 +118,6 @@ func (d *deadcodePass2) init() {
 }
 
 func (d *deadcodePass2) flood() {
-	auxSyms := []loader.Sym{}
 	for !d.wq.empty() {
 		symIdx := d.wq.pop()
 
@@ -162,9 +161,9 @@ func (d *deadcodePass2) flood() {
 			}
 			d.mark(r.Sym(), symIdx)
 		}
-		auxSyms = d.ldr.ReadAuxSyms(symIdx, auxSyms)
-		for i := 0; i < len(auxSyms); i++ {
-			d.mark(auxSyms[i], symIdx)
+		naux := d.ldr.NAux(symIdx)
+		for i := 0; i < naux; i++ {
+			d.mark(d.ldr.Aux2(symIdx, i).Sym(), symIdx)
 		}
 		// Some host object symbols have an outer object, which acts like a
 		// "carrier" symbol, or it holds all the symbols for a particular

src/cmd/link/internal/loader/loader.go

@@ -69,6 +69,16 @@ type Reloc2 struct {
 func (rel Reloc2) Type() objabi.RelocType { return objabi.RelocType(rel.Reloc2.Type()) + rel.typ }
 func (rel Reloc2) Sym() Sym               { return rel.l.resolve(rel.r, rel.Reloc2.Sym()) }
 
+// Aux2 holds a "handle" to access an aux symbol record from an
+// object file.
+type Aux2 struct {
+	*goobj2.Aux2
+	r *oReader
+	l *Loader
+}
+
+func (a Aux2) Sym() Sym { return a.l.resolve(a.r, a.Aux2.Sym()) }
+
 // oReader is a wrapper type of obj.Reader, along with some
 // extra information.
 // TODO: rename to objReader once the old one is gone?
@@ -1164,13 +1174,12 @@ func (l *Loader) SymGoType(i Sym) Sym {
 		return pp.gotype
 	}
 	r, li := l.toLocal(i)
-	naux := r.NAux(li)
-	for j := 0; j < naux; j++ {
-		a := goobj2.Aux{}
-		a.Read(r.Reader, r.AuxOff(li, j))
-		switch a.Type {
+	auxs := r.Auxs2(li)
+	for j := range auxs {
+		a := &auxs[j]
+		switch a.Type() {
 		case goobj2.AuxGotype:
-			return l.resolve(r, a.Sym)
+			return l.resolve(r, a.Sym())
 		}
 	}
 	return 0
@@ -1266,6 +1275,18 @@ func (l *Loader) AuxSym(i Sym, j int) Sym {
 	return l.resolve(r, a.Sym)
 }
 
+// Returns the "handle" to the j-th aux symbol of the i-th symbol.
+func (l *Loader) Aux2(i Sym, j int) Aux2 {
+	if l.IsExternal(i) {
+		return Aux2{}
+	}
+	r, li := l.toLocal(i)
+	if j >= r.NAux(li) {
+		return Aux2{}
+	}
+	return Aux2{r.Aux2(li, j), r, l}
+}
+
 // GetFuncDwarfAuxSyms collects and returns the auxiliary DWARF
 // symbols associated with a given function symbol.  Prior to the
 // introduction of the loader, this was done purely using name
@@ -1593,12 +1614,11 @@ func (l *Loader) FuncInfo(i Sym) FuncInfo {
 		return FuncInfo{}
 	}
 	r, li := l.toLocal(i)
-	n := r.NAux(li)
-	for j := 0; j < n; j++ {
-		a := goobj2.Aux{}
-		a.Read(r.Reader, r.AuxOff(li, j))
-		if a.Type == goobj2.AuxFuncInfo {
-			b := r.Data(int(a.Sym.SymIdx))
+	auxs := r.Auxs2(li)
+	for j := range auxs {
+		a := &auxs[j]
+		if a.Type() == goobj2.AuxFuncInfo {
+			b := r.Data(int(a.Sym().SymIdx))
 			return FuncInfo{l, r, b}
 		}
 	}