[dev.link] cmd/compile, cmd/link: remove dead methods if type is not used in interface

Currently, a method of a reachable type is live if it matches a
method of a reachable interface. In fact, we only need to retain
the method if the type is actually converted to an interface. If
the type is never converted to an interface, there is no way to
call the method through an interface method call (but the type
descriptor could still be used, e.g. in calling
runtime.newobject).

A type can be used in an interface in two ways:
- directly converted to interface. (Any interface counts, as it
  is possible to convert one interface to another.)
- obtained by reflection from a related type (e.g. obtaining an
  interface of T from []T).

For the former, we let the compiler emit a marker on the type
descriptor symbol when it is converted to an interface. In the
linker, we only need to check methods of marked types.

For the latter, when the linker visits a marked type, it needs to
visit all its "child" types as marked (i.e. potentially could be
converted to interface).

This reduces binary size:
cmd/compile	18792016	18706096 (-0.5%)
cmd/go		14120572	13398948 (-5.1%)

Change-Id: I4465c7eeabf575f4dc84017214c610fa05ae31fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/237298
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/compile/internal/gc/sinit.go

@@ -277,6 +277,8 @@ func (s *InitSchedule) staticassign(l *Node, r *Node) bool {
 			return Isconst(val, CTNIL)
 		}
 
+		markTypeUsedInInterface(val.Type)
+
 		var itab *Node
 		if l.Type.IsEmptyInterface() {
 			itab = typename(val.Type)

src/cmd/compile/internal/gc/walk.go

@@ -6,6 +6,7 @@ package gc
 
 import (
 	"cmd/compile/internal/types"
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"encoding/binary"
@@ -797,6 +798,10 @@ opswitch:
 		fromType := n.Left.Type
 		toType := n.Type
 
+		if !fromType.IsInterface() {
+			markTypeUsedInInterface(fromType)
+		}
+
 		// typeword generates the type word of the interface value.
 		typeword := func() *Node {
 			if toType.IsEmptyInterface() {
@@ -1605,6 +1610,12 @@ opswitch:
 	return n
 }
 
+// markTypeUsedInInterface marks that type t is converted to an interface.
+// This information is used in the linker in dead method elimination.
+func markTypeUsedInInterface(t *types.Type) {
+	typenamesym(t).Linksym().Set(obj.AttrUsedInIface, true)
+}
+
 // rtconvfn returns the parameter and result types that will be used by a
 // runtime function to convert from type src to type dst. The runtime function
 // name can be derived from the names of the returned types.

src/cmd/internal/goobj2/objfile.go

@@ -228,12 +228,13 @@ func (p *ImportedPkg) Write(w *Writer) {
 //    ABI   uint16
 //    Type  uint8
 //    Flag  uint8
+//    Flag2 uint8
 //    Siz   uint32
 //    Align uint32
 // }
 type Sym [SymSize]byte
 
-const SymSize = stringRefSize + 2 + 1 + 1 + 4 + 4
+const SymSize = stringRefSize + 2 + 1 + 1 + 1 + 4 + 4
 
 const SymABIstatic = ^uint16(0)
 
@@ -242,6 +243,7 @@ const (
 	ObjFlagNeedNameExpansion             // the linker needs to expand `"".` to package path in symbol names
 )
 
+// Sym.Flag
 const (
 	SymFlagDupok = 1 << iota
 	SymFlagLocal
@@ -253,6 +255,11 @@ const (
 	SymFlagTopFrame
 )
 
+// Sym.Flag2
+const (
+	SymFlagUsedInIface = 1 << iota
+)
+
 func (s *Sym) Name(r *Reader) string {
 	len := binary.LittleEndian.Uint32(s[:])
 	off := binary.LittleEndian.Uint32(s[4:])
@@ -262,8 +269,9 @@ func (s *Sym) Name(r *Reader) string {
 func (s *Sym) ABI() uint16   { return binary.LittleEndian.Uint16(s[8:]) }
 func (s *Sym) Type() uint8   { return s[10] }
 func (s *Sym) Flag() uint8   { return s[11] }
-func (s *Sym) Siz() uint32   { return binary.LittleEndian.Uint32(s[12:]) }
-func (s *Sym) Align() uint32 { return binary.LittleEndian.Uint32(s[16:]) }
+func (s *Sym) Flag2() uint8  { return s[12] }
+func (s *Sym) Siz() uint32   { return binary.LittleEndian.Uint32(s[13:]) }
+func (s *Sym) Align() uint32 { return binary.LittleEndian.Uint32(s[17:]) }
 
 func (s *Sym) Dupok() bool         { return s.Flag()&SymFlagDupok != 0 }
 func (s *Sym) Local() bool         { return s.Flag()&SymFlagLocal != 0 }
@@ -273,6 +281,7 @@ func (s *Sym) NoSplit() bool       { return s.Flag()&SymFlagNoSplit != 0 }
 func (s *Sym) ReflectMethod() bool { return s.Flag()&SymFlagReflectMethod != 0 }
 func (s *Sym) IsGoType() bool      { return s.Flag()&SymFlagGoType != 0 }
 func (s *Sym) TopFrame() bool      { return s.Flag()&SymFlagTopFrame != 0 }
+func (s *Sym) UsedInIface() bool   { return s.Flag2()&SymFlagUsedInIface != 0 }
 
 func (s *Sym) SetName(x string, w *Writer) {
 	binary.LittleEndian.PutUint32(s[:], uint32(len(x)))
@@ -282,8 +291,9 @@ func (s *Sym) SetName(x string, w *Writer) {
 func (s *Sym) SetABI(x uint16)   { binary.LittleEndian.PutUint16(s[8:], x) }
 func (s *Sym) SetType(x uint8)   { s[10] = x }
 func (s *Sym) SetFlag(x uint8)   { s[11] = x }
-func (s *Sym) SetSiz(x uint32)   { binary.LittleEndian.PutUint32(s[12:], x) }
-func (s *Sym) SetAlign(x uint32) { binary.LittleEndian.PutUint32(s[16:], x) }
+func (s *Sym) SetFlag2(x uint8)  { s[12] = x }
+func (s *Sym) SetSiz(x uint32)   { binary.LittleEndian.PutUint32(s[13:], x) }
+func (s *Sym) SetAlign(x uint32) { binary.LittleEndian.PutUint32(s[17:], x) }
 
 func (s *Sym) Write(w *Writer) { w.Bytes(s[:]) }
 

src/cmd/internal/obj/link.go

@@ -514,6 +514,12 @@ const (
 	// new object file format).
 	AttrIndexed
 
+	// Only applied on type descriptor symbols, UsedInIface indicates this type is
+	// converted to an interface.
+	//
+	// Used by the linker to determine what methods can be pruned.
+	AttrUsedInIface
+
 	// attrABIBase is the value at which the ABI is encoded in
 	// Attribute. This must be last; all bits after this are
 	// assumed to be an ABI value.
@@ -538,6 +544,7 @@ func (a Attribute) Static() bool        { return a&AttrStatic != 0 }
 func (a Attribute) WasInlined() bool    { return a&AttrWasInlined != 0 }
 func (a Attribute) TopFrame() bool      { return a&AttrTopFrame != 0 }
 func (a Attribute) Indexed() bool       { return a&AttrIndexed != 0 }
+func (a Attribute) UsedInIface() bool   { return a&AttrUsedInIface != 0 }
 
 func (a *Attribute) Set(flag Attribute, value bool) {
 	if value {

src/cmd/internal/obj/objfile2.go

@@ -258,6 +258,10 @@ func (w *writer) Sym(s *LSym) {
 	if strings.HasPrefix(s.Name, "type.") && s.Name[5] != '.' && s.Type == objabi.SRODATA {
 		flag |= goobj2.SymFlagGoType
 	}
+	flag2 := uint8(0)
+	if s.UsedInIface() {
+		flag2 |= goobj2.SymFlagUsedInIface
+	}
 	name := s.Name
 	if strings.HasPrefix(name, "gofile..") {
 		name = filepath.ToSlash(name)
@@ -271,6 +275,7 @@ func (w *writer) Sym(s *LSym) {
 	o.SetABI(abi)
 	o.SetType(uint8(s.Type))
 	o.SetFlag(flag)
+	o.SetFlag2(flag2)
 	o.SetSiz(uint32(s.Size))
 	o.SetAlign(align)
 	o.Write(w.Writer)

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

@@ -102,8 +102,10 @@ func (d *deadcodePass) flood() {
 
 		isgotype := d.ldr.IsGoType(symIdx)
 		relocs := d.ldr.Relocs(symIdx)
+		var usedInIface bool
 
 		if isgotype {
+			usedInIface = d.ldr.AttrUsedInIface(symIdx)
 			p := d.ldr.Data(symIdx)
 			if len(p) != 0 && decodetypeKind(d.ctxt.Arch, p)&kindMask == kindInterface {
 				for _, sig := range d.decodeIfaceMethods(d.ldr, d.ctxt.Arch, symIdx, &relocs) {
@@ -126,7 +128,9 @@ func (d *deadcodePass) flood() {
 				if i+2 >= relocs.Count() {
 					panic("expect three consecutive R_METHODOFF relocs")
 				}
-				methods = append(methods, methodref{src: symIdx, r: i})
+				if usedInIface {
+					methods = append(methods, methodref{src: symIdx, r: i})
+				}
 				i += 2
 				continue
 			}
@@ -136,7 +140,23 @@ func (d *deadcodePass) flood() {
 				// do nothing for now as we still load all type symbols.
 				continue
 			}
-			d.mark(r.Sym(), symIdx)
+			rs := r.Sym()
+			if isgotype && usedInIface && d.ldr.IsGoType(rs) && !d.ldr.AttrUsedInIface(rs) {
+				// If a type is converted to an interface, it is possible to obtain an
+				// interface with a "child" type of it using reflection (e.g. obtain an
+				// interface of T from []chan T). We need to traverse its "child" types
+				// with UsedInIface attribute set.
+				// When visiting the child type (chan T in the example above), it will
+				// have UsedInIface set, so it in turn will mark and (re)visit its children
+				// (e.g. T above).
+				// We unset the reachable bit here, so if the child type is already visited,
+				// it will be visited again.
+				// Note that a type symbol can be visited at most twice, one without
+				// UsedInIface and one with. So termination is still guaranteed.
+				d.ldr.SetAttrUsedInIface(rs, true)
+				d.ldr.SetAttrReachable(rs, false)
+			}
+			d.mark(rs, symIdx)
 		}
 		naux := d.ldr.NAux(symIdx)
 		for i := 0; i < naux; i++ {

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

@@ -25,11 +25,13 @@ func TestDeadcode(t *testing.T) {
 	defer os.RemoveAll(tmpdir)
 
 	tests := []struct {
-		src     string
-		pattern string
+		src      string
+		pos, neg string // positive and negative patterns
 	}{
-		{"reflectcall", "main.T.M"},
-		{"typedesc", "type.main.T"},
+		{"reflectcall", "", "main.T.M"},
+		{"typedesc", "", "type.main.T"},
+		{"ifacemethod", "", "main.T.M"},
+		{"ifacemethod2", "main.T.M", ""},
 	}
 	for _, test := range tests {
 		test := test
@@ -42,8 +44,11 @@ func TestDeadcode(t *testing.T) {
 			if err != nil {
 				t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)
 			}
-			if bytes.Contains(out, []byte(test.pattern)) {
-				t.Errorf("%s should not be reachable. Output:\n%s", test.pattern, out)
+			if test.pos != "" && !bytes.Contains(out, []byte(test.pos)) {
+				t.Errorf("%s should be reachable. Output:\n%s", test.pos, out)
+			}
+			if test.neg != "" && bytes.Contains(out, []byte(test.neg)) {
+				t.Errorf("%s should not be reachable. Output:\n%s", test.neg, out)
 			}
 		})
 	}

src/cmd/link/internal/ld/testdata/deadcode/ifacemethod.go

@@ -0,0 +1,23 @@
+// Copyright 2020 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.
+
+// Test that a method of a reachable type is not necessarily
+// live even if it matches an interface method, as long as
+// the type is never converted to an interface.
+
+package main
+
+type I interface{ M() }
+
+type T int
+
+func (T) M() { println("XXX") }
+
+var p *T
+var e interface{}
+
+func main() {
+	p = new(T) // used T, but never converted to interface
+	e.(I).M()  // used I and I.M
+}

src/cmd/link/internal/ld/testdata/deadcode/ifacemethod2.go

@@ -0,0 +1,22 @@
+// Copyright 2020 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.
+
+// Test that a method *is* live if it matches an interface
+// method and the type is "indirectly" converted to an
+// interface through reflection.
+
+package main
+
+import "reflect"
+
+type I interface{ M() }
+
+type T int
+
+func (T) M() { println("XXX") }
+
+func main() {
+	e := reflect.ValueOf([]T{1}).Index(0).Interface()
+	e.(I).M()
+}

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

@@ -244,7 +244,8 @@ type Loader struct {
 	attrReachable        Bitmap // reachable symbols, indexed by global index
 	attrOnList           Bitmap // "on list" symbols, indexed by global index
 	attrLocal            Bitmap // "local" symbols, indexed by global index
-	attrNotInSymbolTable Bitmap // "not in symtab" symbols, indexed by glob idx
+	attrNotInSymbolTable Bitmap // "not in symtab" symbols, indexed by global idx
+	attrUsedInIface      Bitmap // "used in interface" symbols, indexed by global idx
 	attrVisibilityHidden Bitmap // hidden symbols, indexed by ext sym index
 	attrDuplicateOK      Bitmap // dupOK symbols, indexed by ext sym index
 	attrShared           Bitmap // shared symbols, indexed by ext sym index
@@ -768,6 +769,20 @@ func (l *Loader) SetAttrLocal(i Sym, v bool) {
 	}
 }
 
+// AttrUsedInIface returns true for a type symbol that is used in
+// an interface.
+func (l *Loader) AttrUsedInIface(i Sym) bool {
+	return l.attrUsedInIface.Has(i)
+}
+
+func (l *Loader) SetAttrUsedInIface(i Sym, v bool) {
+	if v {
+		l.attrUsedInIface.Set(i)
+	} else {
+		l.attrUsedInIface.Unset(i)
+	}
+}
+
 // SymAddr checks that a symbol is reachable, and returns its value.
 func (l *Loader) SymAddr(i Sym) int64 {
 	if !l.AttrReachable(i) {
@@ -1665,6 +1680,7 @@ func (l *Loader) growAttrBitmaps(reqLen int) {
 		l.attrOnList = growBitmap(reqLen, l.attrOnList)
 		l.attrLocal = growBitmap(reqLen, l.attrLocal)
 		l.attrNotInSymbolTable = growBitmap(reqLen, l.attrNotInSymbolTable)
+		l.attrUsedInIface = growBitmap(reqLen, l.attrUsedInIface)
 	}
 	l.growExtAttrBitmaps()
 }
@@ -1983,6 +1999,9 @@ func (l *Loader) preloadSyms(r *oReader, kind int) {
 		if osym.Local() {
 			l.SetAttrLocal(gi, true)
 		}
+		if osym.UsedInIface() {
+			l.SetAttrUsedInIface(gi, true)
+		}
 		if strings.HasPrefix(name, "go.itablink.") {
 			l.itablink[gi] = struct{}{}
 		}
@@ -2025,6 +2044,9 @@ func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch) {
 		if osym.Local() {
 			l.SetAttrLocal(gi, true)
 		}
+		if osym.UsedInIface() {
+			l.SetAttrUsedInIface(gi, true)
+		}
 		l.preprocess(arch, gi, name)
 	}
 }

src/net/http/http_test.go

@@ -91,7 +91,7 @@ func TestCmdGoNoHTTPServer(t *testing.T) {
 	}
 	wantSym := map[string]bool{
 		// Verify these exist: (sanity checking this test)
-		"net/http.(*Client).Get":          true,
+		"net/http.(*Client).do":           true,
 		"net/http.(*Transport).RoundTrip": true,
 
 		// Verify these don't exist: