cmd/link: implement and test automatic symbols

Related changes included in this CL:

 - Add explicit start symbol to Prog.
 - Add omitRuntime bool to Prog.
 - Introduce p.Packages[""] to hold automatic symbols
 - Add SymOrder to Prog to preserve symbol order.
 - Add layout test (and fix bug that was putting everything in text section).

R=iant
CC=golang-codereviews
https://golang.org/cl/51260045

src/cmd/link/auto.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.
+
+// Automatic symbol generation.
+
+// TODO(rsc): Handle go.typelink, go.track symbols.
+// TODO(rsc): Do not handle $f64. and $f32. symbols. Instead, generate those
+// from the compiler and assemblers as dupok data, and then remove autoData below.
+package main
+
+import (
+	"debug/goobj"
+	"strconv"
+	"strings"
+)
+
+// linkerDefined lists the symbols supplied by other parts of the linker
+// (runtime.go and layout.go).
+var linkerDefined = map[string]bool{
+	"bss":        true,
+	"data":       true,
+	"ebss":       true,
+	"edata":      true,
+	"efunctab":   true,
+	"end":        true,
+	"enoptrbss":  true,
+	"enoptrdata": true,
+	"erodata":    true,
+	"etext":      true,
+	"etypelink":  true,
+	"functab":    true,
+	"gcbss":      true,
+	"gcdata":     true,
+	"noptrbss":   true,
+	"noptrdata":  true,
+	"pclntab":    true,
+	"rodata":     true,
+	"text":       true,
+	"typelink":   true,
+}
+
+// isAuto reports whether sym is an automatically-generated data or constant symbol.
+func (p *Prog) isAuto(sym goobj.SymID) bool {
+	return strings.HasPrefix(sym.Name, "go.weak.") ||
+		strings.HasPrefix(sym.Name, "$f64.") ||
+		strings.HasPrefix(sym.Name, "$f32.") ||
+		linkerDefined[sym.Name]
+}
+
+// autoData defines the automatically generated data symbols needed by p.
+func (p *Prog) autoData() {
+	for sym := range p.Missing {
+		switch {
+		// Floating-point constants that need to be loaded from memory are
+		// written as $f64.{16 hex digits} or $f32.{8 hex digits}; the hex digits
+		// give the IEEE bit pattern of the constant. As far as the layout into
+		// memory is concerned, we interpret these as uint64 or uint32 constants.
+		case strings.HasPrefix(sym.Name, "$f64."), strings.HasPrefix(sym.Name, "$f32."):
+			size := 64
+			if sym.Name[2:4] == "32" {
+				size = 32
+			}
+			delete(p.Missing, sym)
+			fbits, err := strconv.ParseUint(sym.Name[len("$f64."):], 16, size)
+			if err != nil {
+				p.errorf("unexpected floating point symbol %s", sym)
+				continue
+			}
+			data := make([]byte, size/8)
+			if size == 64 {
+				p.byteorder.PutUint64(data, fbits)
+			} else {
+				p.byteorder.PutUint32(data, uint32(fbits))
+			}
+			p.addSym(&Sym{
+				Sym: &goobj.Sym{
+					SymID: sym,
+					Kind:  goobj.SRODATA,
+					Size:  size / 8,
+				},
+				Bytes: data,
+			})
+		}
+	}
+}
+
+// autoConst defines the automatically generated constant symbols needed by p.
+func (p *Prog) autoConst() {
+	for sym := range p.Missing {
+		switch {
+		case strings.HasPrefix(sym.Name, "go.weak."):
+			// weak symbol resolves to actual symbol if present, or else nil.
+			delete(p.Missing, sym)
+			targ := sym
+			targ.Name = sym.Name[len("go.weak."):]
+			var addr Addr
+			if s := p.Syms[targ]; s != nil {
+				addr = s.Addr
+			}
+			p.defineConst(sym.Name, addr)
+		}
+	}
+}
+
+// defineConst defines a new symbol with the given name and constant address.
+func (p *Prog) defineConst(name string, addr Addr) {
+	sym := goobj.SymID{Name: name}
+	p.addSym(&Sym{
+		Sym: &goobj.Sym{
+			SymID: sym,
+			Kind:  goobj.SCONST,
+		},
+		Package: nil,
+		Addr:    addr,
+	})
+}

src/cmd/link/auto_test.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.
+
+// Test for auto-generated symbols.
+
+// There is no test for $f64. and $f32. symbols, because those are
+// not possible to write in the assembler syntax. Instead of changing
+// the assembler to allow that, we plan to change the compilers
+// not to generate such symbols (plain dupok data is sufficient).
+
+package main
+
+import (
+	"bytes"
+	"debug/goobj"
+	"testing"
+)
+
+// Each test case is an object file, generated from a corresponding .s file.
+// The image of the autotab symbol should be a sequence of pairs of
+// identical 8-byte sequences.
+var autoTests = []string{
+	"testdata/autosection.6",
+	"testdata/autoweak.6",
+}
+
+func TestAuto(t *testing.T) {
+	for _, obj := range autoTests {
+		p := Prog{GOOS: "darwin", GOARCH: "amd64", StartSym: "start"}
+		p.omitRuntime = true
+		p.Error = func(s string) { t.Error(s) }
+		var buf bytes.Buffer
+		p.link(&buf, obj)
+		if p.NumError > 0 {
+			continue // already reported
+		}
+
+		const name = "autotab"
+		sym := p.Syms[goobj.SymID{Name: name}]
+		if sym == nil {
+			t.Errorf("%s is missing %s symbol", obj, name)
+			return
+		}
+		if sym.Size == 0 {
+			return
+		}
+
+		seg := sym.Section.Segment
+		off := sym.Addr - seg.VirtAddr
+		data := seg.Data[off : off+Addr(sym.Size)]
+		if len(data)%16 != 0 {
+			t.Errorf("%s: %s.Size = %d, want multiple of 16", obj, name, len(data))
+			return
+		}
+	Data:
+		for i := 0; i < len(data); i += 16 {
+			have := p.byteorder.Uint64(data[i : i+8])
+			want := p.byteorder.Uint64(data[i+8 : i+16])
+			if have != want {
+				// Look for relocation so we can explain what went wrong.
+				for _, r := range sym.Reloc {
+					if r.Offset == i {
+						t.Errorf("%s: %s+%#x: %s: have %#x want %#x", obj, name, i, r.Sym, have, want)
+						continue Data
+					}
+				}
+				t.Errorf("%s: %s+%#x: have %#x want %#x", obj, name, i, have, want)
+			}
+		}
+	}
+}

src/cmd/link/layout.go

@@ -25,21 +25,19 @@ type layoutSection struct {
 // Entries with the same Segment name must be contiguous.
 var layout = []layoutSection{
 	{Segment: "text", Section: "text", Kind: goobj.STEXT},
+	{Segment: "rodata", Section: "rodata", Kind: goobj.SRODATA},
+	{Segment: "rodata", Section: "functab", Kind: goobj.SPCLNTAB},
+	{Segment: "rodata", Section: "typelink", Kind: goobj.STYPELINK},
+	{Segment: "data", Section: "noptrdata", Kind: goobj.SNOPTRDATA},
 	{Segment: "data", Section: "data", Kind: goobj.SDATA},
+	{Segment: "data", Section: "bss", Kind: goobj.SBSS},
+	{Segment: "data", Section: "noptrbss", Kind: goobj.SNOPTRBSS},
 
 	// Later:
 	//	{"rodata", "type", goobj.STYPE},
 	//	{"rodata", "string", goobj.SSTRING},
 	//	{"rodata", "gostring", goobj.SGOSTRING},
 	//	{"rodata", "gofunc", goobj.SGOFUNC},
-	//	{"rodata", "rodata", goobj.SRODATA},
-	//	{"rodata", "functab", goobj.SFUNCTAB},
-	//	{"rodata", "typelink", goobj.STYPELINK},
-	//	{"rodata", "symtab", goobj.SSYMTAB},
-	//	{"rodata", "pclntab", goobj.SPCLNTAB},
-	//	{"data", "noptrdata", goobj.SNOPTRDATA},
-	//	{"data", "bss", goobj.SBSS},
-	//	{"data", "noptrbss", goobj.SNOPTRBSS},
 }
 
 // layoutByKind maps from SymKind to an entry in layout.
@@ -54,8 +52,9 @@ func init() {
 		}
 	}
 	layoutByKind = make([]*layoutSection, max)
-	for i, sect := range layout {
-		layoutByKind[sect.Kind] = &layout[i]
+	for i := range layout {
+		sect := &layout[i]
+		layoutByKind[sect.Kind] = sect
 		sect.Index = i
 	}
 }
@@ -67,7 +66,7 @@ func (p *Prog) layout() {
 
 	// Assign symbols to sections using index, creating sections as needed.
 	// Could keep sections separated by type during input instead.
-	for _, sym := range p.Syms {
+	for _, sym := range p.SymOrder {
 		kind := sym.Kind
 		if kind < 0 || int(kind) >= len(layoutByKind) || layoutByKind[kind] == nil {
 			p.errorf("%s: unexpected symbol kind %v", sym.SymID, kind)
@@ -82,7 +81,7 @@ func (p *Prog) layout() {
 			}
 			sections[lsect.Index] = sect
 		}
-		if sym.Data.Size > 0 {
+		if sym.Data.Size > 0 || len(sym.Bytes) > 0 {
 			sect.InFile = true
 		}
 		sym.Section = sect
@@ -102,9 +101,17 @@ func (p *Prog) layout() {
 		if sect == nil {
 			continue
 		}
-		if seg == nil || seg.Name != layout[i].Segment {
+		segName := layout[i].Segment
+
+		// Special case: Mach-O does not support "rodata" segment,
+		// so store read-only data in text segment.
+		if p.GOOS == "darwin" && segName == "rodata" {
+			segName = "text"
+		}
+
+		if seg == nil || seg.Name != segName {
 			seg = &Segment{
-				Name: layout[i].Segment,
+				Name: segName,
 			}
 			p.Segments = append(p.Segments, seg)
 		}
@@ -153,6 +160,21 @@ func (p *Prog) layout() {
 				seg.FileSize = addr - seg.VirtAddr
 			}
 		}
-		seg.VirtSize = addr
+		seg.VirtSize = addr - seg.VirtAddr
+	}
+
+	// Define symbols for section names.
+	var progEnd Addr
+	for i, sect := range sections {
+		name := layout[i].Section
+		var start, end Addr
+		if sect != nil {
+			start = sect.VirtAddr
+			end = sect.VirtAddr + sect.Size
+		}
+		p.defineConst(name, start)
+		p.defineConst("e"+name, end)
+		progEnd = end
 	}
+	p.defineConst("end", progEnd)
 }

src/cmd/link/layout_test.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.
+
+package main
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestLayout(t *testing.T) {
+	p := Prog{GOOS: "darwin", GOARCH: "amd64", StartSym: "text_start"}
+	p.omitRuntime = true
+	p.Error = func(s string) { t.Error(s) }
+	var buf bytes.Buffer
+	const obj = "testdata/layout.6"
+	p.link(&buf, obj)
+	if p.NumError > 0 {
+		return // already reported
+	}
+	if len(p.Dead) > 0 {
+		t.Errorf("%s: unexpected dead symbols %v", obj, p.Dead)
+		return
+	}
+
+	for _, sym := range p.SymOrder {
+		if p.isAuto(sym.SymID) {
+			continue
+		}
+		if sym.Section == nil {
+			t.Errorf("%s: symbol %s is missing section", obj, sym)
+			continue
+		}
+		i := strings.Index(sym.Name, "_")
+		if i < 0 {
+			t.Errorf("%s: unexpected symbol %s", obj, sym)
+			continue
+		}
+		if sym.Section.Name != sym.Name[:i] {
+			t.Errorf("%s: symbol %s in section %s, want %s", obj, sym, sym.Section.Name, sym.Name[:i])
+		}
+	}
+}

src/cmd/link/link_test.go

@@ -7,26 +7,29 @@ package main
 import (
 	"bytes"
 	"debug/goobj"
+	"io/ioutil"
 	"testing"
 )
 
 func TestLinkHello(t *testing.T) {
 	p := &Prog{
-		GOOS:   "darwin",
-		GOARCH: "amd64",
-		Error:  func(s string) { t.Error(s) },
+		GOOS:     "darwin",
+		GOARCH:   "amd64",
+		Error:    func(s string) { t.Error(s) },
+		StartSym: "_rt0_go",
 	}
 	var buf bytes.Buffer
 	p.link(&buf, "testdata/hello.6")
 	if p.NumError > 0 {
 		return
 	}
-	if len(p.Syms) != 2 || p.Syms[goobj.SymID{"_rt0_go", 0}] == nil || p.Syms[goobj.SymID{"hello", 1}] == nil {
-		t.Errorf("Syms = %v, want [_rt0_go hello<1>]", p.Syms)
+	if p.Syms[goobj.SymID{"_rt0_go", 0}] == nil || p.Syms[goobj.SymID{"hello", 1}] == nil {
+		t.Errorf("Syms = %v, want at least [_rt0_go hello<1>]", p.Syms)
 	}
 
-	checkGolden(t, buf.Bytes(), "testdata/link.hello.darwin.amd64")
-
 	// uncomment to leave file behind for execution:
-	// ioutil.WriteFile("a.out", buf.Bytes(), 0777)
+	if false {
+		ioutil.WriteFile("a.out", buf.Bytes(), 0777)
+	}
+	checkGolden(t, buf.Bytes(), "testdata/link.hello.darwin.amd64")
 }

src/cmd/link/load.go

@@ -6,10 +6,7 @@
 
 package main
 
-import (
-	"encoding/binary"
-	"os"
-)
+import "os"
 
 // load allocates segment images, populates them with data
 // read from package files, and applies relocations to the data.
@@ -27,6 +24,24 @@ func (p *Prog) load() {
 // loadPackage loads and relocates data for all the
 // symbols needed in the given package.
 func (p *Prog) loadPackage(pkg *Package) {
+	if pkg.File == "" {
+		// This "package" contains internally generated symbols only.
+		// All such symbols have a sym.Bytes field holding the actual data
+		// (if any), plus relocations.
+		for _, sym := range pkg.Syms {
+			if sym.Bytes == nil {
+				continue
+			}
+			seg := sym.Section.Segment
+			off := sym.Addr - seg.VirtAddr
+			data := seg.Data[off : off+Addr(sym.Size)]
+			copy(data, sym.Bytes)
+			p.relocateSym(sym, data)
+		}
+		return
+	}
+
+	// Package stored in file.
 	f, err := os.Open(pkg.File)
 	if err != nil {
 		p.errorf("%v", err)
@@ -41,8 +56,14 @@ func (p *Prog) loadPackage(pkg *Package) {
 			continue
 		}
 		// TODO(rsc): If not using mmap, at least coalesce nearby reads.
+		if sym.Section == nil {
+			p.errorf("internal error: missing section for %s", sym.Name)
+		}
 		seg := sym.Section.Segment
 		off := sym.Addr - seg.VirtAddr
+		if off >= Addr(len(seg.Data)) || off+Addr(sym.Data.Size) > Addr(len(seg.Data)) {
+			p.errorf("internal error: allocated space for %s too small: %d bytes for %d+%d (%d)", sym, len(seg.Data), off, sym.Data.Size, sym.Size)
+		}
 		data := seg.Data[off : off+Addr(sym.Data.Size)]
 		_, err := f.ReadAt(data, sym.Data.Offset)
 		if err != nil {
@@ -89,10 +110,9 @@ func (p *Prog) relocateSym(sym *Sym, data []byte) {
 			p.errorf("%v: unknown relocation size %d", sym, r.Size)
 		case 4:
 			// TODO(rsc): Check for overflow?
-			// TODO(rsc): Handle big-endian systems.
-			binary.LittleEndian.PutUint32(frag, uint32(val))
+			p.byteorder.PutUint32(frag, uint32(val))
 		case 8:
-			binary.LittleEndian.PutUint64(frag, uint64(val))
+			p.byteorder.PutUint64(frag, uint64(val))
 		}
 	}
 }

src/cmd/link/macho.go

@@ -315,12 +315,7 @@ func (h *machoHeader) size() int {
 func (h *machoHeader) encode() []byte {
 	w := &machoWriter{p: h.p}
 	w.is64 = h.CPU&macho64Bit != 0
-	switch h.SubCPU {
-	default:
-		h.p.errorf("mach-o error: unknown CPU")
-	case machoSubCPU386:
-		w.order = binary.LittleEndian
-	}
+	w.order = w.p.byteorder
 
 	loadSize := 0
 	for _, seg := range h.Segments {

src/cmd/link/macho_test.go

@@ -28,6 +28,7 @@ var machoWriteTests = []struct {
 		golden: true,
 		prog: &Prog{
 			GOARCH:       "amd64",
+			GOOS:         "darwin",
 			UnmappedSize: 0x1000,
 			Entry:        0x1000,
 			Segments: []*Segment{
@@ -62,6 +63,7 @@ var machoWriteTests = []struct {
 		golden: true,
 		prog: &Prog{
 			GOARCH:       "amd64",
+			GOOS:         "darwin",
 			UnmappedSize: 0x1000,
 			Entry:        0x1000,
 			Segments: []*Segment{
@@ -117,6 +119,7 @@ var machoWriteTests = []struct {
 		golden: true,
 		prog: &Prog{
 			GOARCH:       "amd64",
+			GOOS:         "darwin",
 			UnmappedSize: 0x1000,
 			Entry:        0x1000,
 			Segments: []*Segment{
@@ -173,6 +176,7 @@ func TestMachoWrite(t *testing.T) {
 	for _, tt := range machoWriteTests {
 		name := tt.prog.GOARCH + "." + tt.name
 		prog := cloneProg(tt.prog)
+		prog.init()
 		var f machoFormat
 		vsize, fsize := f.headerSize(prog)
 		shiftProg(prog, vsize, fsize)

src/cmd/link/prog.go

@@ -6,10 +6,12 @@ package main
 
 import (
 	"debug/goobj"
+	"encoding/binary"
 	"fmt"
 	"go/build"
 	"io"
 	"os"
+	"runtime"
 )
 
 // A Prog holds state for constructing an executable (program) image.
@@ -31,17 +33,26 @@ import (
 //
 type Prog struct {
 	// Context
-	GOOS      string // target operating system
-	GOARCH    string // target architecture
-	Format    string // desired file format ("elf", "macho", ...)
-	formatter formatter
-	Error     func(string) // called to report an error (if set)
-	NumError  int          // number of errors printed
+	GOOS     string       // target operating system
+	GOARCH   string       // target architecture
+	Format   string       // desired file format ("elf", "macho", ...)
+	Error    func(string) // called to report an error (if set)
+	NumError int          // number of errors printed
+	StartSym string
+
+	// Derived context
+	arch
+	formatter   formatter
+	startSym    goobj.SymID
+	pkgdir      string
+	omitRuntime bool // do not load runtime package
 
 	// Input
 	Packages   map[string]*Package  // loaded packages, by import path
 	Syms       map[goobj.SymID]*Sym // defined symbols, by symbol ID
-	Missing    map[goobj.SymID]bool // missing symbols, by symbol ID
+	Missing    map[goobj.SymID]bool // missing symbols
+	Dead       map[goobj.SymID]bool // symbols removed as dead
+	SymOrder   []*Sym               // order syms were scanned
 	MaxVersion int                  // max SymID.Version, for generating fresh symbol IDs
 
 	// Output
@@ -51,8 +62,11 @@ type Prog struct {
 	Segments     []*Segment // loaded memory segments
 }
 
-// startSymID is the symbol where program execution begins.
-var startSymID = goobj.SymID{Name: "_rt0_go"}
+// An arch describes architecture-dependent settings.
+type arch struct {
+	byteorder binary.ByteOrder
+	ptrsize   int
+}
 
 // A formatter takes care of the details of generating a particular
 // kind of executable file.
@@ -86,6 +100,7 @@ type Sym struct {
 	Package    *Package // package defining symbol
 	Section    *Section // section where symbol is placed in output program
 	Addr       Addr     // virtual address of symbol in output program
+	Bytes      []byte   // symbol data, for internally defined symbols
 }
 
 // A Segment is a loaded memory segment.
@@ -131,7 +146,9 @@ func (p *Prog) link(w io.Writer, mainFile string) {
 	}
 	p.dead()
 	p.runtime()
+	p.autoData()
 	p.layout()
+	p.autoConst()
 	if p.NumError > 0 {
 		return
 	}
@@ -162,6 +179,9 @@ func (p *Prog) init() {
 			return
 		}
 	}
+	if p.StartSym == "" {
+		p.StartSym = fmt.Sprintf("_rt0_%s_%s", p.GOARCH, p.GOOS)
+	}
 
 	// Derive internal context.
 	p.formatter = formatters[p.Format]
@@ -169,6 +189,15 @@ func (p *Prog) init() {
 		p.errorf("unknown output file format %q", p.Format)
 		return
 	}
+	p.startSym = goobj.SymID{Name: p.StartSym}
+	arch, ok := arches[p.GOARCH]
+	if !ok {
+		p.errorf("unknown GOOS %q", p.GOOS)
+		return
+	}
+	p.arch = arch
+
+	p.pkgdir = fmt.Sprintf("%s/pkg/%s_%s", runtime.GOROOT(), p.GOOS, p.GOARCH)
 }
 
 // goosFormat records the default format for each known GOOS value.
@@ -180,3 +209,10 @@ var goosFormat = map[string]string{
 var formatters = map[string]formatter{
 	"darwin": machoFormat{},
 }
+
+var arches = map[string]arch{
+	"amd64": {
+		byteorder: binary.LittleEndian,
+		ptrsize:   8,
+	},
+}

src/cmd/link/scan.go

@@ -13,6 +13,7 @@ package main
 import (
 	"debug/goobj"
 	"os"
+	"sort"
 	"strings"
 )
 
@@ -20,9 +21,20 @@ import (
 func (p *Prog) scan(mainfile string) {
 	p.initScan()
 	p.scanFile("main", mainfile)
-	if len(p.Missing) != 0 {
-		// TODO(rsc): iterate in deterministic order
-		for sym := range p.Missing {
+	if len(p.Missing) > 0 && !p.omitRuntime {
+		p.scanImport("runtime")
+	}
+
+	var missing []string
+	for sym := range p.Missing {
+		if !p.isAuto(sym) {
+			missing = append(missing, sym.String())
+		}
+	}
+
+	if missing != nil {
+		sort.Strings(missing)
+		for _, sym := range missing {
 			p.errorf("undefined: %s", sym)
 		}
 	}
@@ -35,7 +47,7 @@ func (p *Prog) initScan() {
 	p.Packages = make(map[string]*Package)
 	p.Syms = make(map[goobj.SymID]*Sym)
 	p.Missing = make(map[goobj.SymID]bool)
-	p.Missing[startSymID] = true
+	p.Missing[p.startSym] = true
 }
 
 // scanFile reads file to learn about the package with the given import path.
@@ -81,21 +93,62 @@ func (p *Prog) scanFile(pkgpath string, file string) {
 			if r.Sym.Version != 0 {
 				r.Sym.Version += p.MaxVersion
 			}
-			if p.Syms[r.Sym] != nil {
+			if p.Syms[r.Sym] == nil {
 				p.Missing[r.Sym] = true
 			}
 		}
+		if gs.Func != nil {
+			for i := range gs.Func.FuncData {
+				fdata := &gs.Func.FuncData[i]
+				if fdata.Sym.Name != "" {
+					if fdata.Sym.Version != 0 {
+						fdata.Sym.Version += p.MaxVersion
+					}
+					if p.Syms[fdata.Sym] == nil {
+						p.Missing[fdata.Sym] = true
+					}
+				}
+			}
+		}
 		if old := p.Syms[gs.SymID]; old != nil {
-			p.errorf("symbol %s defined in both %s and %s", old.Package.File, file)
-			continue
+			// Duplicate definition of symbol. Is it okay?
+			// TODO(rsc): Write test for this code.
+			switch {
+			// If both symbols are BSS (no data), take max of sizes
+			// but otherwise ignore second symbol.
+			case old.Data.Size == 0 && gs.Data.Size == 0:
+				if old.Size < gs.Size {
+					old.Size = gs.Size
+				}
+				continue
+
+			// If one is in BSS and one is not, use the one that is not.
+			case old.Data.Size > 0 && gs.Data.Size == 0:
+				continue
+			case gs.Data.Size > 0 && old.Data.Size == 0:
+				break // install gs as new symbol below
+
+			// If either is marked as DupOK, we can keep either one.
+			// Keep the one that we saw first.
+			case old.DupOK || gs.DupOK:
+				continue
+
+			// Otherwise, there's an actual conflict:
+			default:
+				p.errorf("symbol %s defined in both %s and %s %v %v", gs.SymID, old.Package.File, file, old.Data, gs.Data)
+				continue
+			}
 		}
 		s := &Sym{
 			Sym:     gs,
 			Package: pkg,
 		}
-		pkg.Syms = append(pkg.Syms, s)
-		p.Syms[gs.SymID] = s
+		p.addSym(s)
 		delete(p.Missing, gs.SymID)
+
+		if s.Data.Size > int64(s.Size) {
+			p.errorf("%s: initialized data larger than symbol (%d > %d)", s, s.Data.Size, s.Size)
+		}
 	}
 	p.MaxVersion += pkg.MaxVersion
 
@@ -108,6 +161,21 @@ func (p *Prog) scanFile(pkgpath string, file string) {
 	}
 }
 
+func (p *Prog) addSym(s *Sym) {
+	pkg := s.Package
+	if pkg == nil {
+		pkg = p.Packages[""]
+		if pkg == nil {
+			pkg = &Package{}
+			p.Packages[""] = pkg
+		}
+		s.Package = pkg
+	}
+	pkg.Syms = append(pkg.Syms, s)
+	p.Syms[s.SymID] = s
+	p.SymOrder = append(p.SymOrder, s)
+}
+
 // scanImport finds the object file for the given import path and then scans it.
 func (p *Prog) scanImport(pkgpath string) {
 	if p.Packages[pkgpath] != nil {
@@ -115,5 +183,5 @@ func (p *Prog) scanImport(pkgpath string) {
 	}
 
 	// TODO(rsc): Implement correct search to find file.
-	p.scanFile(pkgpath, "/Users/rsc/rscgo/pkg/darwin_amd64/"+pkgpath+".a")
+	p.scanFile(pkgpath, p.pkgdir+"/"+pkgpath+".a")
 }

src/cmd/link/testdata/autosection.s

+// 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.
+
+// Test of section-named symbols.
+
+#include "../../ld/textflag.h"
+
+TEXT start(SB),7,$0
+	MOVQ $autotab(SB),AX
+	MOVQ $autoptr(SB),AX
+	RET
+
+GLOBL zero(SB), $8
+
+GLOBL zeronoptr(SB), NOPTR, $16
+
+// text
+DATA autotab+0x00(SB)/8, $text(SB)
+DATA autotab+0x08(SB)/8, $start(SB)
+DATA autotab+0x10(SB)/8, $etext(SB)
+DATA autotab+0x18(SB)/8, $start+16(SB)
+
+// data
+DATA autotab+0x20(SB)/8, $data(SB)
+DATA autotab+0x28(SB)/8, $autotab(SB)
+DATA autotab+0x30(SB)/8, $edata(SB)
+DATA autotab+0x38(SB)/8, $nonzero+4(SB)
+
+// bss
+DATA autotab+0x40(SB)/8, $bss(SB)
+DATA autotab+0x48(SB)/8, $zero(SB)
+DATA autotab+0x50(SB)/8, $ebss(SB)
+DATA autotab+0x58(SB)/8, $zero+8(SB)
+
+// noptrdata
+DATA autotab+0x60(SB)/8, $noptrdata(SB)
+DATA autotab+0x68(SB)/8, $nonzeronoptr(SB)
+DATA autotab+0x70(SB)/8, $enoptrdata(SB)
+DATA autotab+0x78(SB)/8, $nonzeronoptr+8(SB)
+
+// noptrbss
+DATA autotab+0x80(SB)/8, $noptrbss(SB)
+DATA autotab+0x88(SB)/8, $zeronoptr(SB)
+DATA autotab+0x90(SB)/8, $enoptrbss(SB)
+DATA autotab+0x98(SB)/8, $zeronoptr+16(SB)
+
+// end
+DATA autotab+0xa0(SB)/8, $end(SB)
+DATA autotab+0xa8(SB)/8, $zeronoptr+16(SB)
+
+GLOBL autotab(SB), $0xb0
+
+DATA nonzero(SB)/4, $1
+GLOBL nonzero(SB), $4
+
+DATA nonzeronoptr(SB)/8, $2
+GLOBL nonzeronoptr(SB), NOPTR, $8
+
+GLOBL autoptr(SB), $0

src/cmd/link/testdata/autoweak.s

+// 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.
+
+// Test of go.weak symbols.
+
+TEXT start(SB),7,$0
+	MOVQ $autotab(SB),AX
+	MOVQ $autoptr(SB),AX
+	RET
+
+// go.weak.sym should resolve to sym, because sym is in the binary.
+DATA autotab+0(SB)/8, $go·weak·sym(SB)
+DATA autotab+8(SB)/8, $sym(SB)
+
+// go.weak.missingsym should resolve to 0, because missingsym is not in the binary.
+DATA autotab+16(SB)/8, $go·weak·missingsym(SB)
+DATA autotab+24(SB)/8, $0
+
+// go.weak.deadsym should resolve to 0, because deadsym is discarded during dead code removal
+DATA autotab+32(SB)/8, $go·weak·deadsym(SB)
+DATA autotab+40(SB)/8, $0
+
+GLOBL autotab(SB), $48
+
+GLOBL sym(SB), $1
+
+GLOBL deadsym(SB), $1
+
+GLOBL autoptr(SB), $0

src/cmd/link/testdata/layout.s

+// 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.
+
+// Test of section assignment in layout.go.
+// Each symbol should end up in the section named by the symbol name prefix (up to the underscore).
+
+#include "../../ld/textflag.h"
+
+TEXT text_start(SB),7,$0
+	MOVQ $rodata_sym(SB), AX
+	MOVQ $noptrdata_sym(SB), AX
+	MOVQ $data_sym(SB), AX
+	MOVQ $bss_sym(SB), AX
+	MOVQ $noptrbss_sym(SB), AX
+	RET
+
+DATA rodata_sym(SB)/4, $1
+GLOBL rodata_sym(SB), RODATA, $4
+
+DATA noptrdata_sym(SB)/4, $1
+GLOBL noptrdata_sym(SB), NOPTR, $4
+
+DATA data_sym(SB)/4, $1
+GLOBL data_sym(SB), $4
+
+GLOBL bss_sym(SB), $4
+
+GLOBL noptrbss_sym(SB), NOPTR, $4

src/cmd/link/write.go

@@ -9,6 +9,6 @@ package main
 import "io"
 
 func (p *Prog) write(w io.Writer) {
-	p.Entry = p.Syms[startSymID].Addr
+	p.Entry = p.Syms[p.startSym].Addr
 	p.formatter.write(w, p)
 }