cmd/link: when pruning methods also prune funcType

Remove method type information for pruned methods from any program
that does not reflect on methods. This can be a significant saving:

	addr2line: -310KB (8.8%)

A future update might want to consider a more aggressive variant of
this: setting the Type and Func fields of reflect.Method to nil for
unexported methods. That would shrink cmd/go by 2% and jujud by 2.6%
but could be considered an API change. So this CL sticks to the
uncontroversial change.

For #6853.

Change-Id: I5d186d9f822dc118ee89dc572c4912a3b3c72577
Reviewed-on: https://go-review.googlesource.com/20701
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 53 insertions, 21 deletions

diff --git a/src/cmd/link/internal/ld/deadcode.go b/src/cmd/link/internal/ld/deadcode.go
index a2286eb872..9367375102 100644
--- a/src/cmd/link/internal/ld/deadcode.go
+++ b/src/cmd/link/internal/ld/deadcode.go
@@ -62,6 +62,12 @@ func deadcode(ctxt *Link) {
 	methSym := Linkrlookup(ctxt, "reflect.Value.Method", 0)
 	reflectSeen := false
 
+	if DynlinkingGo() {
+		// Exported methods may satisfy interfaces we don't know
+		// about yet when dynamically linking.
+		reflectSeen = true
+	}
+
 	for {
 		if !reflectSeen {
 			if d.reflectMethod || (callSym != nil && callSym.Attr.Reachable()) || (methSym != nil && methSym.Attr.Reachable()) {
@@ -80,6 +86,17 @@ func deadcode(ctxt *Link) {
 		for _, m := range d.markableMethods {
 			if (reflectSeen && m.isExported()) || d.ifaceMethod[m.m] {
 				d.markMethod(m)
+			} else if reflectSeen {
+				// This ensures the Type and Func fields of
+				// reflect.Method are filled as they were in
+				// Go 1.
+				//
+				// An argument could be made for changing this
+				// and setting those fields to nil. Doing so
+				// would reduce the binary size of typical
+				// programs like cmd/go by ~2%.
+				d.markMethodType(m)
+				rem = append(rem, m)
 			} else {
 				rem = append(rem, m)
 			}
@@ -95,8 +112,9 @@ func deadcode(ctxt *Link) {
 
 	// Remove all remaining unreached R_METHOD relocations.
 	for _, m := range d.markableMethods {
-		d.cleanupReloc(m.r0)
-		d.cleanupReloc(m.r1)
+		for _, r := range m.r {
+			d.cleanupReloc(r)
+		}
 	}
 
 	if Buildmode != BuildmodeShared {
@@ -153,15 +171,19 @@ var markextra = []string{
 }
 
 // methodref holds the relocations from a receiver type symbol to its
-// method. There are two relocations, one for the method type without
-// receiver, one with receiver
+// method. There are four relocations, one for each of the fields in
+// the reflect.method struct: mtyp, typ, ifn, and tfn.
 type methodref struct {
 	m   methodsig
-	src *LSym // receiver type symbol
-	r0  *Reloc
-	r1  *Reloc
+	src *LSym     // receiver type symbol
+	r   [4]*Reloc // R_METHOD relocations to fields of runtime.method
 }
 
+func (m methodref) mtyp() *LSym { return m.r[0].Sym }
+func (m methodref) typ() *LSym  { return m.r[1].Sym }
+func (m methodref) ifn() *LSym  { return m.r[2].Sym }
+func (m methodref) tfn() *LSym  { return m.r[3].Sym }
+
 func (m methodref) isExported() bool {
 	for _, r := range m.m {
 		return unicode.IsUpper(r)
@@ -203,12 +225,18 @@ func (d *deadcodepass) mark(s, parent *LSym) {
 	d.markQueue = append(d.markQueue, s)
 }
 
-// markMethod marks a method as reachable and preps its R_METHOD relocations.
+// markMethod marks a method as reachable.
 func (d *deadcodepass) markMethod(m methodref) {
-	d.mark(m.r0.Sym, m.src)
-	d.mark(m.r1.Sym, m.src)
-	m.r0.Type = obj.R_ADDR
-	m.r1.Type = obj.R_ADDR
+	for _, r := range m.r {
+		d.mark(r.Sym, m.src)
+		r.Type = obj.R_ADDR
+	}
+}
+
+// markMethodType marks just a method's types as reachable.
+func (d *deadcodepass) markMethodType(m methodref) {
+	d.mark(m.mtyp(), m.src)
+	d.mark(m.typ(), m.src)
 }
 
 // init marks all initial symbols as reachable.
@@ -278,6 +306,7 @@ func (d *deadcodepass) flood() {
 			}
 		}
 
+		mpos := 0 // 0-3, the R_METHOD relocs of runtime.uncommontype
 		var methods []methodref
 		for i := 0; i < len(s.R); i++ {
 			r := &s.R[i]
@@ -290,14 +319,17 @@ func (d *deadcodepass) flood() {
 			}
 			// Collect rtype pointers to methods for
 			// later processing in deadcode.
-			if len(methods) > 0 {
-				mref := &methods[len(methods)-1]
-				if mref.r1 == nil {
-					mref.r1 = r
-					continue
-				}
+			if mpos == 0 {
+				m := methodref{src: s}
+				m.r[0] = r
+				methods = append(methods, m)
+			} else {
+				methods[len(methods)-1].r[mpos] = r
+			}
+			mpos++
+			if mpos == len(methodref{}.r) {
+				mpos = 0
 			}
-			methods = append(methods, methodref{src: s, r0: r})
 		}
 		if len(methods) > 0 {
 			// Decode runtime type information for type methods
@@ -310,8 +342,8 @@ func (d *deadcodepass) flood() {
 			for i, m := range methodsigs {
 				name := string(m)
 				name = name[:strings.Index(name, "(")]
-				if !strings.HasSuffix(methods[i].r0.Sym.Name, name) {
-					panic(fmt.Sprintf("%q relocation for %q does not match method %q", s.Name, methods[i].r0.Sym.Name, name))
+				if !strings.HasSuffix(methods[i].ifn().Name, name) {
+					panic(fmt.Sprintf("%q relocation for %q does not match method %q", s.Name, methods[i].ifn().Name, name))
 				}
 				methods[i].m = m
 			}