code review 51470043: cmd/link: implement dead code removal

src/cmd/link/dead.go

 // Copyright 2014 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.
 
 // Removal of dead code and data.
 
 package main
 
 import "debug/goobj"
 
 // dead removes unreachable code and data from the program.
 // It is basically a mark-sweep garbage collection: traverse all the
 // symbols reachable from the entry (startSymID) and then delete
 // the rest.
 func (p *Prog) dead() {
+        p.Dead = make(map[goobj.SymID]bool)
         reachable := make(map[goobj.SymID]bool)
         p.walkDead(p.startSym, reachable)
 
         for sym := range p.Syms {
                 if !reachable[sym] {
                         delete(p.Syms, sym)
+                        p.Dead[sym] = true
                 }
         }
 
         for sym := range p.Missing {
                 if !reachable[sym] {
                         delete(p.Missing, sym)
+                        p.Dead[sym] = true
                 }
         }
 
         p.SymOrder = removeDead(p.SymOrder, reachable)
 
         for _, pkg := range p.Packages {
                 pkg.Syms = removeDead(pkg.Syms, reachable)
         }
 }
 
@@ skipping 24 matching lines @@
 // returning a shortened slice using the same underlying array.
 func removeDead(syms []*Sym, reachable map[goobj.SymID]bool) []*Sym {
         keep := syms[:0]
         for _, sym := range syms {
                 if reachable[sym.SymID] {
                         keep = append(keep, sym)
                 }
         }
         return keep
 }