loader.go

// Copyright 2019 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 loader

import (
	"bytes"
	"cmd/internal/bio"
	"cmd/internal/dwarf"
	"cmd/internal/goobj2"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/sys"
	"cmd/link/internal/sym"
	"fmt"
	"log"
	"os"
	"sort"
	"strconv"
	"strings"
)

var _ = fmt.Print

// Sym encapsulates a global symbol index, used to identify a specific
// Go symbol. The 0-valued Sym is corresponds to an invalid symbol.
type Sym int

// Relocs encapsulates the set of relocations on a given symbol; an
// instance of this type is returned by the Loader Relocs() method.
type Relocs struct {
	Count int // number of relocs

	li int      // local index of symbol whose relocs we're examining
	r  *oReader // object reader for containing package
	l  *Loader  // loader

	extIdx Sym // index of external symbol we're examining or 0
}

// Reloc contains the payload for a specific relocation.
// TODO: replace this with sym.Reloc, once we change the
// relocation target from "*sym.Symbol" to "loader.Sym" in sym.Reloc.
type Reloc struct {
	Off  int32            // offset to rewrite
	Size uint8            // number of bytes to rewrite: 0, 1, 2, or 4
	Type objabi.RelocType // the relocation type
	Add  int64            // addend
	Sym  Sym              // global index of symbol the reloc addresses
}

// oReader is a wrapper type of obj.Reader, along with some
// extra information.
// TODO: rename to objReader once the old one is gone?
type oReader struct {
	*goobj2.Reader
	unit      *sym.CompilationUnit
	version   int    // version of static symbol
	flags     uint32 // read from object file
	pkgprefix string
	rcache    []Sym // cache mapping local PkgNone symbol to resolved Sym
}

type objIdx struct {
	r *oReader
	i Sym // start index
	e Sym // end index
}

type nameVer struct {
	name string
	v    int
}

type bitmap []uint32

// set the i-th bit.
func (bm bitmap) Set(i Sym) {
	n, r := uint(i)/32, uint(i)%32
	bm[n] |= 1 << r
}

// whether the i-th bit is set.
func (bm bitmap) Has(i Sym) bool {
	n, r := uint(i)/32, uint(i)%32
	return bm[n]&(1<<r) != 0
}

// return current length of bitmap in bits.
func (bm bitmap) len() int {
	return len(bm) * 32
}
func makeBitmap(n int) bitmap {
	return make(bitmap, (n+31)/32)
}

// growBitmap insures that the specified bitmap has enough capacity,
// reallocating (doubling the size) if needed.
func growBitmap(reqLen int, b bitmap) bitmap {
	curLen := b.len()
	if reqLen > curLen {
		b = append(b, makeBitmap(reqLen-curLen)...)
	}
	return b
}

// A Loader loads new object files and resolves indexed symbol references.
//
// Notes on the layout of global symbol index space:
//
// - Go object files are read before host object files; each Go object
//   read allocates a new chunk of global index space of size P + NP,
//   where P is the number of package defined symbols in the object and
//   NP is the number of non-package defined symbols.
//
// - In loader.LoadRefs(), the loader makes a sweep through all of the
//   non-package references in each object file and allocates sym indices
//   for any symbols that have not yet been defined (start of this space
//   is marked by loader.extStart).
//
// - Host object file loading happens; the host object loader does a
//   name/version lookup for each symbol it finds; this can wind up
//   extending the external symbol index space range. The host object
//   loader currently stores symbol payloads in sym.Symbol objects,
//   which get handed off to the loader.
//
// - A given external symbol (Sym) either has a sym.Symbol acting as
//   its backing store (this will continue to be the case until we
//   finish rewriting the host object loader to work entirely with
//   loader.Sym) or it has a "payload" backing store (represented by
//   extSymPayload). Newly created external symbols (created by
//   a call to AddExtSym or equivalent) start out in the "has payload"
//   state, and continue until installSym is called for the sym
//   index in question.
//
// - At some point (when the wayfront is pushed through all of the
//   linker), all external symbols will be payload-based, and we can
//   get rid of the loader.Syms array.
//
type Loader struct {
	start       map[*oReader]Sym // map from object file to its start index
	objs        []objIdx         // sorted by start index (i.e. objIdx.i)
	max         Sym              // current max index
	extStart    Sym              // from this index on, the symbols are externally defined
	builtinSyms []Sym            // global index of builtin symbols
	ocache      int              // index (into 'objs') of most recent lookup

	symsByName    [2]map[string]Sym // map symbol name to index, two maps are for ABI0 and ABIInternal
	extStaticSyms map[nameVer]Sym   // externally defined static symbols, keyed by name
	overwrite     map[Sym]Sym       // overwrite[i]=j if symbol j overwrites symbol i

	payloads []extSymPayload // contents of linker-materialized external syms

	itablink map[Sym]struct{} // itablink[j] defined if j is go.itablink.*

	objByPkg map[string]*oReader // map package path to its Go object reader

	Syms     []*sym.Symbol // indexed symbols. XXX we still make sym.Symbol for now.
	symBatch []sym.Symbol  // batch of symbols.

	anonVersion int // most recently assigned ext static sym pseudo-version

	Reachable bitmap // bitmap of reachable symbols, indexed by global index

	// Used to implement field tracking; created during deadcode if
	// field tracking is enabled. Reachparent[K] contains the index of
	// the symbol that triggered the marking of symbol K as live.
	Reachparent []Sym

	relocBatch []sym.Reloc // for bulk allocation of relocations

	flags uint32

	strictDupMsgs int // number of strict-dup warning/errors, when FlagStrictDups is enabled
}

// extSymPayload holds the payload (data + relocations) for linker-synthesized
// external symbols.
type extSymPayload struct {
	name   string // TODO: would this be better as offset into str table?
	size   int64
	value  int64
	ver    int
	kind   sym.SymKind
	relocs []Reloc
	data   []byte
}

const (
	// Loader.flags
	FlagStrictDups = 1 << iota
)

func NewLoader(flags uint32) *Loader {
	nbuiltin := goobj2.NBuiltin()
	return &Loader{
		start:         make(map[*oReader]Sym),
		objs:          []objIdx{{nil, 0, 0}},
		symsByName:    [2]map[string]Sym{make(map[string]Sym), make(map[string]Sym)},
		objByPkg:      make(map[string]*oReader),
		overwrite:     make(map[Sym]Sym),
		itablink:      make(map[Sym]struct{}),
		extStaticSyms: make(map[nameVer]Sym),
		builtinSyms:   make([]Sym, nbuiltin),
		flags:         flags,
	}
}

// Return the start index in the global index space for a given object file.
func (l *Loader) startIndex(r *oReader) Sym {
	return l.start[r]
}

// Add object file r, return the start index.
func (l *Loader) addObj(pkg string, r *oReader) Sym {
	if _, ok := l.start[r]; ok {
		panic("already added")
	}
	pkg = objabi.PathToPrefix(pkg) // the object file contains escaped package path
	if _, ok := l.objByPkg[pkg]; !ok {
		l.objByPkg[pkg] = r
	}
	n := r.NSym() + r.NNonpkgdef()
	i := l.max + 1
	l.start[r] = i
	l.objs = append(l.objs, objIdx{r, i, i + Sym(n) - 1})
	l.max += Sym(n)
	return i
}

// Add a symbol with a given index, return if it is added.
func (l *Loader) AddSym(name string, ver int, i Sym, r *oReader, dupok bool, typ sym.SymKind) bool {
	if l.extStart != 0 {
		panic("AddSym called after AddExtSym is called")
	}
	if ver == r.version {
		// Static symbol. Add its global index but don't
		// add to name lookup table, as it cannot be
		// referenced by name.
		return true
	}
	if oldi, ok := l.symsByName[ver][name]; ok {
		if dupok {
			if l.flags&FlagStrictDups != 0 {
				l.checkdup(name, i, r, oldi)
			}
			return false
		}
		oldr, li := l.toLocal(oldi)
		oldsym := goobj2.Sym{}
		oldsym.Read(oldr.Reader, oldr.SymOff(li))
		if oldsym.Dupok() {
			return false
		}
		overwrite := r.DataSize(int(i-l.startIndex(r))) != 0
		if overwrite {
			// new symbol overwrites old symbol.
			oldtyp := sym.AbiSymKindToSymKind[objabi.SymKind(oldsym.Type)]
			if !oldtyp.IsData() && r.DataSize(li) == 0 {
				log.Fatalf("duplicated definition of symbol " + name)
			}
			l.overwrite[oldi] = i
		} else {
			// old symbol overwrites new symbol.
			if typ != sym.SDATA && typ != sym.SNOPTRDATA && typ != sym.SBSS && typ != sym.SNOPTRBSS { // only allow overwriting data symbol
				log.Fatalf("duplicated definition of symbol " + name)
			}
			l.overwrite[i] = oldi
			return false
		}
	}
	l.symsByName[ver][name] = i
	return true
}

// newExtSym creates a new external sym with the specified
// name/version.
func (l *Loader) newExtSym(name string, ver int) Sym {
	l.max++
	i := l.max
	if l.extStart == 0 {
		l.extStart = i
	}
	l.growSyms(int(i))
	pi := i - l.extStart
	l.payloads[pi].name = name
	l.payloads[pi].ver = ver
	return i
}

// Add an external symbol (without index). Return the index of newly added
// symbol, or 0 if not added.
func (l *Loader) AddExtSym(name string, ver int) Sym {
	i := l.Lookup(name, ver)
	if i != 0 {
		return 0
	}
	i = l.newExtSym(name, ver)
	static := ver >= sym.SymVerStatic || ver < 0
	if static {
		l.extStaticSyms[nameVer{name, ver}] = i
	} else {
		l.symsByName[ver][name] = i
	}
	return i
}

// LookupOrCreateSym looks up the symbol with the specified name/version,
// returning its Sym index if found. If the lookup fails, a new external
// Sym will be created, entered into the lookup tables, and returned.
func (l *Loader) LookupOrCreateSym(name string, ver int) Sym {
	i := l.Lookup(name, ver)
	if i != 0 {
		return i
	}
	i = l.newExtSym(name, ver)
	static := ver >= sym.SymVerStatic || ver < 0
	if static {
		l.extStaticSyms[nameVer{name, ver}] = i
	} else {
		l.symsByName[ver][name] = i
	}
	return i
}

func (l *Loader) IsExternal(i Sym) bool {
	return l.extStart != 0 && i >= l.extStart
}

// getPayload returns a pointer to the extSymPayload struct for an
// external symbol if the symbol has a payload, or nil if the
// data for the sym is being stored in a sym.Symbol. Will panic if
// the symbol in question is bogus (zero or not an external sym).
func (l *Loader) getPayload(i Sym) *extSymPayload {
	if l.extStart == 0 || i < l.extStart {
		panic(fmt.Sprintf("bogus symbol index %d in getPayload", i))
	}
	if l.Syms[i] != nil {
		return nil
	}
	pi := i - l.extStart
	return &l.payloads[pi]
}

// Ensure Syms slice has enough space.
func (l *Loader) growSyms(i int) {
	n := len(l.Syms)
	if n > i {
		return
	}
	l.Syms = append(l.Syms, make([]*sym.Symbol, i+1-n)...)
	l.payloads = append(l.payloads, make([]extSymPayload, i+1-n)...)
	l.growReachable(int(i))
}

// Convert a local index to a global index.
func (l *Loader) toGlobal(r *oReader, i int) Sym {
	g := l.startIndex(r) + Sym(i)
	if ov, ok := l.overwrite[g]; ok {
		return ov
	}
	return g
}

// Convert a global index to a local index.
func (l *Loader) toLocal(i Sym) (*oReader, int) {
	if ov, ok := l.overwrite[i]; ok {
		i = ov
	}
	if l.IsExternal(i) {
		return nil, int(i - l.extStart)
	}
	oc := l.ocache
	if oc != 0 && i >= l.objs[oc].i && i <= l.objs[oc].e {
		return l.objs[oc].r, int(i - l.objs[oc].i)
	}
	// Search for the local object holding index i.
	// Below k is the first one that has its start index > i,
	// so k-1 is the one we want.
	k := sort.Search(len(l.objs), func(k int) bool {
		return l.objs[k].i > i
	})
	l.ocache = k - 1
	return l.objs[k-1].r, int(i - l.objs[k-1].i)
}

// rcacheGet checks for a valid entry for 's' in the readers cache,
// where 's' is a local PkgIdxNone ref or def, or zero if
// the cache is empty or doesn't contain a value for 's'.
func (or *oReader) rcacheGet(symIdx uint32) Sym {
	if len(or.rcache) > 0 {
		return or.rcache[symIdx]
	}
	return 0
}

// rcacheSet installs a new entry in the oReader's PkgNone
// resolver cache for the specified PkgIdxNone ref or def,
// allocating a new cache if needed.
func (or *oReader) rcacheSet(symIdx uint32, gsym Sym) {
	if len(or.rcache) == 0 {
		or.rcache = make([]Sym, or.NNonpkgdef()+or.NNonpkgref())
	}
	or.rcache[symIdx] = gsym
}

// Resolve a local symbol reference. Return global index.
func (l *Loader) resolve(r *oReader, s goobj2.SymRef) Sym {
	var rr *oReader
	switch p := s.PkgIdx; p {
	case goobj2.PkgIdxInvalid:
		if s.SymIdx != 0 {
			panic("bad sym ref")
		}
		return 0
	case goobj2.PkgIdxNone:
		// Check for cached version first
		if cached := r.rcacheGet(s.SymIdx); cached != 0 {
			return cached
		}
		// Resolve by name
		i := int(s.SymIdx) + r.NSym()
		osym := goobj2.Sym{}
		osym.Read(r.Reader, r.SymOff(i))
		name := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
		v := abiToVer(osym.ABI, r.version)
		gsym := l.Lookup(name, v)
		// Add to cache, then return.
		r.rcacheSet(s.SymIdx, gsym)
		return gsym
	case goobj2.PkgIdxBuiltin:
		return l.builtinSyms[s.SymIdx]
	case goobj2.PkgIdxSelf:
		rr = r
	default:
		pkg := r.Pkg(int(p))
		var ok bool
		rr, ok = l.objByPkg[pkg]
		if !ok {
			log.Fatalf("reference of nonexisted package %s, from %v", pkg, r.unit.Lib)
		}
	}
	return l.toGlobal(rr, int(s.SymIdx))
}

// Look up a symbol by name, return global index, or 0 if not found.
// This is more like Syms.ROLookup than Lookup -- it doesn't create
// new symbol.
func (l *Loader) Lookup(name string, ver int) Sym {
	if ver >= sym.SymVerStatic || ver < 0 {
		return l.extStaticSyms[nameVer{name, ver}]
	}
	return l.symsByName[ver][name]
}

// Returns whether i is a dup of another symbol, and i is not
// "primary", i.e. Lookup i by name will not return i.
func (l *Loader) IsDup(i Sym) bool {
	if _, ok := l.overwrite[i]; ok {
		return true
	}
	if l.IsExternal(i) {
		return false
	}
	r, li := l.toLocal(i)
	osym := goobj2.Sym{}
	osym.Read(r.Reader, r.SymOff(li))
	if !osym.Dupok() {
		return false
	}
	if osym.Name == "" {
		return false // Unnamed aux symbol cannot be dup.
	}
	if osym.ABI == goobj2.SymABIstatic {
		return false // Static symbol cannot be dup.
	}
	name := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
	ver := abiToVer(osym.ABI, r.version)
	return l.symsByName[ver][name] != i
}

// Check that duplicate symbols have same contents.
func (l *Loader) checkdup(name string, i Sym, r *oReader, dup Sym) {
	li := int(i - l.startIndex(r))
	p := r.Data(li)
	if strings.HasPrefix(name, "go.info.") {
		p, _ = patchDWARFName1(p, r)
	}
	rdup, ldup := l.toLocal(dup)
	pdup := rdup.Data(ldup)
	if strings.HasPrefix(name, "go.info.") {
		pdup, _ = patchDWARFName1(pdup, rdup)
	}
	if bytes.Equal(p, pdup) {
		return
	}
	reason := "same length but different contents"
	if len(p) != len(pdup) {
		reason = fmt.Sprintf("new length %d != old length %d", len(p), len(pdup))
	}
	fmt.Fprintf(os.Stderr, "cmd/link: while reading object for '%v': duplicate symbol '%s', previous def at '%v', with mismatched payload: %s\n", r.unit.Lib, name, rdup.unit.Lib, reason)

	// For the moment, whitelist DWARF subprogram DIEs for
	// auto-generated wrapper functions. What seems to happen
	// here is that we get different line numbers on formal
	// params; I am guessing that the pos is being inherited
	// from the spot where the wrapper is needed.
	whitelist := strings.HasPrefix(name, "go.info.go.interface") ||
		strings.HasPrefix(name, "go.info.go.builtin") ||
		strings.HasPrefix(name, "go.debuglines")
	if !whitelist {
		l.strictDupMsgs++
	}
}

func (l *Loader) NStrictDupMsgs() int { return l.strictDupMsgs }

// Number of total symbols.
func (l *Loader) NSym() int {
	return int(l.max + 1)
}

// Number of defined Go symbols.
func (l *Loader) NDef() int {
	return int(l.extStart)
}

// Returns the raw (unpatched) name of the i-th symbol.
func (l *Loader) RawSymName(i Sym) string {
	if l.IsExternal(i) {
		if s := l.Syms[i]; s != nil {
			return s.Name
		}
		pp := l.getPayload(i)
		return pp.name
	}
	r, li := l.toLocal(i)
	osym := goobj2.Sym{}
	osym.Read(r.Reader, r.SymOff(li))
	return osym.Name
}

// Returns the (patched) name of the i-th symbol.
func (l *Loader) SymName(i Sym) string {
	if l.IsExternal(i) {
		if s := l.Syms[i]; s != nil {
			return s.Name // external name should already be patched?
		}
		pp := l.getPayload(i)
		return pp.name
	}
	r, li := l.toLocal(i)
	osym := goobj2.Sym{}
	osym.Read(r.Reader, r.SymOff(li))
	return strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
}

// Returns the type of the i-th symbol.
func (l *Loader) SymType(i Sym) sym.SymKind {
	if l.IsExternal(i) {
		if s := l.Syms[i]; s != nil {
			return s.Type
		}
		pp := l.getPayload(i)
		if pp != nil {
			return pp.kind
		}
		return 0
	}
	r, li := l.toLocal(i)
	osym := goobj2.Sym{}
	osym.Read(r.Reader, r.SymOff(li))
	return sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)]
}

// Returns the attributes of the i-th symbol.
func (l *Loader) SymAttr(i Sym) uint8 {
	if l.IsExternal(i) {
		// TODO: do something? External symbols have different representation of attributes. For now, ReflectMethod is the only thing matters and it cannot be set by external symbol.
		return 0
	}
	r, li := l.toLocal(i)
	osym := goobj2.Sym{}
	osym.Read(r.Reader, r.SymOff(li))
	return osym.Flag
}

// Returns whether the i-th symbol has ReflectMethod attribute set.
func (l *Loader) IsReflectMethod(i Sym) bool {
	return l.SymAttr(i)&goobj2.SymFlagReflectMethod != 0
}

// Returns whether this is a Go type symbol.
func (l *Loader) IsGoType(i Sym) bool {
	return l.SymAttr(i)&goobj2.SymFlagGoType != 0
}

// Returns whether this is a "go.itablink.*" symbol.
func (l *Loader) IsItabLink(i Sym) bool {
	if _, ok := l.itablink[i]; ok {
		return true
	}
	return false
}

// Returns the symbol content of the i-th symbol. i is global index.
func (l *Loader) Data(i Sym) []byte {
	if l.IsExternal(i) {
		if s := l.Syms[i]; s != nil {
			return s.P
		}
		pp := l.getPayload(i)
		if pp != nil {
			return pp.data
		}
		return nil
	}
	r, li := l.toLocal(i)
	return r.Data(li)
}

// Returns the number of aux symbols given a global index.
func (l *Loader) NAux(i Sym) int {
	if l.IsExternal(i) {
		return 0
	}
	r, li := l.toLocal(i)
	return r.NAux(li)
}

// Returns the referred symbol of the j-th aux symbol of the i-th
// symbol.
func (l *Loader) AuxSym(i Sym, j int) Sym {
	if l.IsExternal(i) {
		return 0
	}
	r, li := l.toLocal(i)
	a := goobj2.Aux{}
	a.Read(r.Reader, r.AuxOff(li, j))
	return l.resolve(r, a.Sym)
}

// ReadAuxSyms reads the aux symbol ids for the specified symbol into the
// slice passed as a parameter. If the slice capacity is not large enough, a new
// larger slice will be allocated. Final slice is returned.
func (l *Loader) ReadAuxSyms(symIdx Sym, dst []Sym) []Sym {
	if l.IsExternal(symIdx) {
		return dst[:0]
	}
	naux := l.NAux(symIdx)
	if naux == 0 {
		return dst[:0]
	}

	if cap(dst) < naux {
		dst = make([]Sym, naux)
	}
	dst = dst[:0]

	r, li := l.toLocal(symIdx)
	for i := 0; i < naux; i++ {
		a := goobj2.Aux{}
		a.Read(r.Reader, r.AuxOff(li, i))
		dst = append(dst, l.resolve(r, a.Sym))
	}

	return dst
}

// OuterSym gets the outer symbol for host object loaded symbols.
func (l *Loader) OuterSym(i Sym) Sym {
	sym := l.Syms[i]
	if sym != nil && sym.Outer != nil {
		outer := sym.Outer
		return l.Lookup(outer.Name, int(outer.Version))
	}
	return 0
}

// SubSym gets the subsymbol for host object loaded symbols.
func (l *Loader) SubSym(i Sym) Sym {
	sym := l.Syms[i]
	if sym != nil && sym.Sub != nil {
		sub := sym.Sub
		return l.Lookup(sub.Name, int(sub.Version))
	}
	return 0
}

// Initialize Reachable bitmap for running deadcode pass.
func (l *Loader) InitReachable() {
	l.growReachable(l.NSym())
}

// Insure that reachable bitmap has enough size.
func (l *Loader) growReachable(reqLen int) {
	if reqLen > l.Reachable.len() {
		l.Reachable = growBitmap(reqLen, l.Reachable)
	}
}

// At method returns the j-th reloc for a global symbol.
func (relocs *Relocs) At(j int) Reloc {
	if s := relocs.l.Syms[relocs.extIdx]; s != nil {
		rel := s.R[j]
		return Reloc{
			Off:  rel.Off,
			Size: rel.Siz,
			Type: rel.Type,
			Add:  rel.Add,
			Sym:  relocs.l.Lookup(rel.Sym.Name, int(rel.Sym.Version)),
		}
	}
	if relocs.extIdx != 0 {
		pp := relocs.l.getPayload(relocs.extIdx)
		return pp.relocs[j]
	}
	rel := goobj2.Reloc{}
	rel.Read(relocs.r.Reader, relocs.r.RelocOff(relocs.li, j))
	target := relocs.l.resolve(relocs.r, rel.Sym)
	return Reloc{
		Off:  rel.Off,
		Size: rel.Siz,
		Type: objabi.RelocType(rel.Type),
		Add:  rel.Add,
		Sym:  target,
	}
}

// ReadAll method reads all relocations for a symbol into the
// specified slice. If the slice capacity is not large enough, a new
// larger slice will be allocated. Final slice is returned.
func (relocs *Relocs) ReadAll(dst []Reloc) []Reloc {
	if relocs.Count == 0 {
		return dst[:0]
	}

	if cap(dst) < relocs.Count {
		dst = make([]Reloc, relocs.Count)
	}
	dst = dst[:0]

	if s := relocs.l.Syms[relocs.extIdx]; s != nil {
		for i := 0; i < relocs.Count; i++ {
			erel := &s.R[i]
			rel := Reloc{
				Off:  erel.Off,
				Size: erel.Siz,
				Type: erel.Type,
				Add:  erel.Add,
				Sym:  relocs.l.Lookup(erel.Sym.Name, int(erel.Sym.Version)),
			}
			dst = append(dst, rel)
		}
		return dst
	}

	if relocs.extIdx != 0 {
		pp := relocs.l.getPayload(relocs.extIdx)
		dst = append(dst, pp.relocs...)
		return dst
	}

	off := relocs.r.RelocOff(relocs.li, 0)
	for i := 0; i < relocs.Count; i++ {
		rel := goobj2.Reloc{}
		rel.Read(relocs.r.Reader, off)
		off += uint32(rel.Size())
		target := relocs.l.resolve(relocs.r, rel.Sym)
		dst = append(dst, Reloc{
			Off:  rel.Off,
			Size: rel.Siz,
			Type: objabi.RelocType(rel.Type),
			Add:  rel.Add,
			Sym:  target,
		})
	}
	return dst
}

// Relocs returns a Relocs object for the given global sym.
func (l *Loader) Relocs(i Sym) Relocs {
	if l.IsExternal(i) {
		if s := l.Syms[i]; s != nil {
			return Relocs{Count: len(s.R), l: l, extIdx: i}
		}
		pp := l.getPayload(i)
		if pp != nil {
			return Relocs{Count: len(pp.relocs), l: l, extIdx: i}
		}
		return Relocs{}
	}
	r, li := l.toLocal(i)
	if r == nil {
		panic(fmt.Sprintf("trying to get oreader for invalid sym %d\n\n", i))
	}
	return l.relocs(r, li)
}

// Relocs returns a Relocs object given a local sym index and reader.
func (l *Loader) relocs(r *oReader, li int) Relocs {
	return Relocs{
		Count: r.NReloc(li),
		li:    li,
		r:     r,
		l:     l,
	}
}

// Preload a package: add autolibs, add symbols to the symbol table.
// Does not read symbol data yet.
func (l *Loader) Preload(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, lib *sym.Library, unit *sym.CompilationUnit, length int64, pn string, flags int) {
	roObject, readonly, err := f.Slice(uint64(length))
	if err != nil {
		log.Fatal("cannot read object file:", err)
	}
	r := goobj2.NewReaderFromBytes(roObject, readonly)
	if r == nil {
		panic("cannot read object file")
	}
	localSymVersion := syms.IncVersion()
	pkgprefix := objabi.PathToPrefix(lib.Pkg) + "."
	or := &oReader{r, unit, localSymVersion, r.Flags(), pkgprefix, nil}

	// Autolib
	lib.ImportStrings = append(lib.ImportStrings, r.Autolib()...)

	// DWARF file table
	nfile := r.NDwarfFile()
	unit.DWARFFileTable = make([]string, nfile)
	for i := range unit.DWARFFileTable {
		unit.DWARFFileTable[i] = r.DwarfFile(i)
	}

	istart := l.addObj(lib.Pkg, or)

	ndef := r.NSym()
	nnonpkgdef := r.NNonpkgdef()
	for i, n := 0, ndef+nnonpkgdef; i < n; i++ {
		osym := goobj2.Sym{}
		osym.Read(r, r.SymOff(i))
		name := strings.Replace(osym.Name, "\"\".", pkgprefix, -1)
		if name == "" {
			continue // don't add unnamed aux symbol
		}
		v := abiToVer(osym.ABI, localSymVersion)
		dupok := osym.Dupok()
		added := l.AddSym(name, v, istart+Sym(i), or, dupok, sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)])
		if added && strings.HasPrefix(name, "go.itablink.") {
			l.itablink[istart+Sym(i)] = struct{}{}
		}
		if added && strings.HasPrefix(name, "runtime.") {
			if bi := goobj2.BuiltinIdx(name, v); bi != -1 {
				// This is a definition of a builtin symbol. Record where it is.
				l.builtinSyms[bi] = istart + Sym(i)
			}
		}
	}

	// The caller expects us consuming all the data
	f.MustSeek(length, os.SEEK_CUR)
}

// Make sure referenced symbols are added. Most of them should already be added.
// This should only be needed for referenced external symbols.
func (l *Loader) LoadRefs(arch *sys.Arch, syms *sym.Symbols) {
	for _, o := range l.objs[1:] {
		loadObjRefs(l, o.r, arch, syms)
	}
}

func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch, syms *sym.Symbols) {
	ndef := r.NSym() + r.NNonpkgdef()
	for i, n := 0, r.NNonpkgref(); i < n; i++ {
		osym := goobj2.Sym{}
		osym.Read(r.Reader, r.SymOff(ndef+i))
		name := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
		v := abiToVer(osym.ABI, r.version)
		l.AddExtSym(name, v)
	}
}

func abiToVer(abi uint16, localSymVersion int) int {
	var v int
	if abi == goobj2.SymABIstatic {
		// Static
		v = localSymVersion
	} else if abiver := sym.ABIToVersion(obj.ABI(abi)); abiver != -1 {
		// Note that data symbols are "ABI0", which maps to version 0.
		v = abiver
	} else {
		log.Fatalf("invalid symbol ABI: %d", abi)
	}
	return v
}

func preprocess(arch *sys.Arch, s *sym.Symbol) {
	if s.Name != "" && s.Name[0] == '$' && len(s.Name) > 5 && s.Type == 0 && len(s.P) == 0 {
		x, err := strconv.ParseUint(s.Name[5:], 16, 64)
		if err != nil {
			log.Panicf("failed to parse $-symbol %s: %v", s.Name, err)
		}
		s.Type = sym.SRODATA
		s.Attr |= sym.AttrLocal
		switch s.Name[:5] {
		case "$f32.":
			if uint64(uint32(x)) != x {
				log.Panicf("$-symbol %s too large: %d", s.Name, x)
			}
			s.AddUint32(arch, uint32(x))
		case "$f64.", "$i64.":
			s.AddUint64(arch, x)
		default:
			log.Panicf("unrecognized $-symbol: %s", s.Name)
		}
	}
}

// Load full contents.
func (l *Loader) LoadFull(arch *sys.Arch, syms *sym.Symbols) {
	// create all Symbols first.
	l.growSyms(l.NSym())

	nr := 0 // total number of sym.Reloc's we'll need
	for _, o := range l.objs[1:] {
		nr += loadObjSyms(l, syms, o.r)
	}

	// allocate a single large slab of relocations for all live symbols
	l.relocBatch = make([]sym.Reloc, nr)

	// external symbols
	for i := l.extStart; i <= l.max; i++ {
		if s := l.Syms[i]; s != nil {
			s.Attr.Set(sym.AttrReachable, l.Reachable.Has(i))
			continue // already loaded from external object
		}
		sname := l.payloads[i-l.extStart].name
		sver := l.payloads[i-l.extStart].ver
		if l.Reachable.Has(i) || strings.HasPrefix(sname, "gofile..") { // XXX file symbols are used but not marked
			s := l.allocSym(sname, sver)
			pp := l.getPayload(i)
			if pp != nil {
				if pp.kind != sym.Sxxx || len(pp.relocs) != 0 || len(pp.data) != 0 {
					// Unpack payload into sym. Currently there is nothing
					// to do here, but eventually we'll need a real
					// implementation.
					panic("need to handle this")
				}
			}
			preprocess(arch, s)
			s.Attr.Set(sym.AttrReachable, l.Reachable.Has(i))
			l.installSym(i, s)
		}
	}

	// load contents of defined symbols
	for _, o := range l.objs[1:] {
		loadObjFull(l, o.r)
	}

	// Resolve ABI aliases for external symbols. This is only
	// needed for internal cgo linking.
	// (The old code does this in deadcode, but deadcode2 doesn't
	// do this.)
	for i := l.extStart; i <= l.max; i++ {
		if s := l.Syms[i]; s != nil && s.Attr.Reachable() {
			for ri := range s.R {
				r := &s.R[ri]
				if r.Sym != nil && r.Sym.Type == sym.SABIALIAS {
					r.Sym = r.Sym.R[0].Sym
				}
			}
		}
	}
}

// ExtractSymbols grabs the symbols out of the loader for work that hasn't been
// ported to the new symbol type.
func (l *Loader) ExtractSymbols(syms *sym.Symbols) {
	// Nil out overwritten symbols.
	// Overwritten Go symbols aren't a problem (as they're lazy loaded), but
	// symbols loaded from host object loaders are fully loaded, and we might
	// have multiple symbols with the same name. This loop nils them out.
	for oldI := range l.overwrite {
		l.Syms[oldI] = nil
	}

	// Add symbols to the ctxt.Syms lookup table. This explicitly skips things
	// created via loader.Create (marked with versions less than zero), since
	// if we tried to add these we'd wind up with collisions. We do, however,
	// add these symbols to the list of global symbols so that other future
	// steps (like pclntab generation) can find these symbols if neceassary.
	// Along the way, update the version from the negative anon version to
	// something larger than sym.SymVerStatic (needed so that
	// sym.symbol.IsFileLocal() works properly).
	anonVerReplacement := syms.IncVersion()
	for _, s := range l.Syms {
		if s == nil {