cmd/link: combine all string data into one symbol

This CL introduces a mergestrings pass after the reachability
analysis to combine all reachable go.string."..." character data
symbols into a single symbol.

Shrinks juju by 1.2mb (1.5%).
Shrinks cmd/go by 0.5% when building without DWARF.
No noticable effect on linker speed.

Change-Id: I2ba3e60bf418f65766bda257f6ca9eea26d895b6
Reviewed-on: https://go-review.googlesource.com/20165
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

+// Copyright 2016 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.
+
+package ld
+
+import (
+	"cmd/internal/obj"
+	"strings"
+)
+
+// mergestrings merges all go.string.* character data into a single symbol.
+//
+// Combining string data symbols reduces the total binary size and
+// makes deduplication possible.
+func mergestrings() {
+	if Buildmode == BuildmodeShared {
+		return
+	}
+
+	strs := make([]*LSym, 0, 256)
+	seenStr := make(map[string]bool, 256)         // symbol name -> in strs slice
+	relocsToStrs := make(map[*LSym][]*Reloc, 256) // string -> relocation to string
+	size := 0                                     // number of bytes in all strings
+
+	// Collect strings and relocations that point to strings.
+	for _, s := range Ctxt.Allsym {
+		if !s.Attr.Reachable() || s.Attr.Special() {
+			continue
+		}
+		for i := range s.R {
+			r := &s.R[i]
+			if r.Sym == nil {
+				continue
+			}
+			if !seenStr[r.Sym.Name] {
+				if !strings.HasPrefix(r.Sym.Name, "go.string.") {
+					continue
+				}
+				if strings.HasPrefix(r.Sym.Name, "go.string.hdr") {
+					continue
+				}
+				strs = append(strs, r.Sym)
+				seenStr[r.Sym.Name] = true
+				size += len(r.Sym.P)
+			}
+			relocsToStrs[r.Sym] = append(relocsToStrs[r.Sym], r)
+		}
+	}
+
+	// Put all string data into a single symbol and update the relocations.
+	alldata := Linklookup(Ctxt, "go.string.alldata", 0)
+	alldata.Type = obj.SGOSTRING
+	alldata.Attr |= AttrReachable
+	alldata.Size = int64(size)
+	alldata.P = make([]byte, 0, size)
+	for _, str := range strs {
+		off := len(alldata.P)
+		alldata.P = append(alldata.P, str.P...)
+		str.Attr.Set(AttrReachable, false)
+		for _, r := range relocsToStrs[str] {
+			r.Add += int64(off)
+			r.Sym = alldata
+		}
+	}
+}

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

@@ -197,6 +197,7 @@ func Ldmain() {
 	checkstrdata()
 	deadcode()
 	callgraph()
+	mergestrings()
 
 	doelf()
 	if HEADTYPE == obj.Hdarwin {