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objfile.go
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objfile.go
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// Copyright 2013 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.
// Writing of Go object files.
//
// Originally, Go object files were Plan 9 object files, but no longer.
// Now they are more like standard object files, in that each symbol is defined
// by an associated memory image (bytes) and a list of relocations to apply
// during linking. We do not (yet?) use a standard file format, however.
// For now, the format is chosen to be as simple as possible to read and write.
// It may change for reasons of efficiency, or we may even switch to a
// standard file format if there are compelling benefits to doing so.
// See golang.org/s/go13linker for more background.
//
// The file format is:
//
// - magic header: "\x00\x00go17ld"
// - byte 1 - version number
// - sequence of strings giving dependencies (imported packages)
// - empty string (marks end of sequence)
// - sequence of symbol references used by the defined symbols
// - byte 0xff (marks end of sequence)
// - sequence of integer lengths:
// - total data length
// - total number of relocations
// - total number of pcdata
// - total number of automatics
// - total number of funcdata
// - total number of files
// - data, the content of the defined symbols
// - sequence of defined symbols
// - byte 0xff (marks end of sequence)
// - magic footer: "\xff\xffgo17ld"
//
// All integers are stored in a zigzag varint format.
// See golang.org/s/go12symtab for a definition.
//
// Data blocks and strings are both stored as an integer
// followed by that many bytes.
//
// A symbol reference is a string name followed by a version.
//
// A symbol points to other symbols using an index into the symbol
// reference sequence. Index 0 corresponds to a nil LSym* pointer.
// In the symbol layout described below "symref index" stands for this
// index.
//
// Each symbol is laid out as the following fields (taken from LSym*):
//
// - byte 0xfe (sanity check for synchronization)
// - type [int]
// - name & version [symref index]
// - flags [int]
// 1 dupok
// - size [int]
// - gotype [symref index]
// - p [data block]
// - nr [int]
// - r [nr relocations, sorted by off]
//
// If type == STEXT, there are a few more fields:
//
// - args [int]
// - locals [int]
// - nosplit [int]
// - flags [int]
// 1<<0 leaf
// 1<<1 C function
// 1<<2 function may call reflect.Type.Method
// - nlocal [int]
// - local [nlocal automatics]
// - pcln [pcln table]
//
// Each relocation has the encoding:
//
// - off [int]
// - siz [int]
// - type [int]
// - add [int]
// - sym [symref index]
//
// Each local has the encoding:
//
// - asym [symref index]
// - offset [int]
// - type [int]
// - gotype [symref index]
//
// The pcln table has the encoding:
//
// - pcsp [data block]
// - pcfile [data block]
// - pcline [data block]
// - npcdata [int]
// - pcdata [npcdata data blocks]
// - nfuncdata [int]
// - funcdata [nfuncdata symref index]
// - funcdatasym [nfuncdata ints]
// - nfile [int]
// - file [nfile symref index]
//
// The file layout and meaning of type integers are architecture-independent.
//
// TODO(rsc): The file format is good for a first pass but needs work.
// - There are SymID in the object file that should really just be strings.
package obj
import (
"bufio"
"cmd/internal/sys"
"fmt"
"log"
"path/filepath"
"sort"
)
// The Go and C compilers, and the assembler, call writeobj to write
// out a Go object file. The linker does not call this; the linker
// does not write out object files.
func Writeobjdirect(ctxt *Link, b *bufio.Writer) {
Flushplist(ctxt)
WriteObjFile(ctxt, b)
}
// objWriter writes Go object files.
type objWriter struct {
wr *bufio.Writer
ctxt *Link
// Temporary buffer for zigzag int writing.
varintbuf [10]uint8
// Provide the the index of a symbol reference by symbol name.
// One map for versioned symbols and one for unversioned symbols.
// Used for deduplicating the symbol reference list.
refIdx map[string]int
vrefIdx map[string]int
// Number of objects written of each type.
nRefs int
nData int
nReloc int
nPcdata int
nAutom int
nFuncdata int
nFile int
}
func (w *objWriter) addLengths(s *LSym) {
w.nData += len(s.P)
w.nReloc += len(s.R)
if s.Type != STEXT {
return
}
pc := s.Pcln
data := 0
data += len(pc.Pcsp.P)
data += len(pc.Pcfile.P)
data += len(pc.Pcline.P)
for i := 0; i < len(pc.Pcdata); i++ {
data += len(pc.Pcdata[i].P)
}
w.nData += data
w.nPcdata += len(pc.Pcdata)
autom := 0
for a := s.Autom; a != nil; a = a.Link {
autom++
}
w.nAutom += autom
w.nFuncdata += len(pc.Funcdataoff)
w.nFile += len(pc.File)
}
func (w *objWriter) writeLengths() {
w.writeInt(int64(w.nData))
w.writeInt(int64(w.nReloc))
w.writeInt(int64(w.nPcdata))
w.writeInt(int64(w.nAutom))
w.writeInt(int64(w.nFuncdata))
w.writeInt(int64(w.nFile))
}
func newObjWriter(ctxt *Link, b *bufio.Writer) *objWriter {
return &objWriter{
ctxt: ctxt,
wr: b,
vrefIdx: make(map[string]int),
refIdx: make(map[string]int),
}
}
func WriteObjFile(ctxt *Link, b *bufio.Writer) {
w := newObjWriter(ctxt, b)
// Magic header
w.wr.WriteString("\x00\x00go17ld")
// Version
w.wr.WriteByte(1)
// Autolib
for _, pkg := range ctxt.Imports {
w.writeString(pkg)
}
w.writeString("")
// Symbol references
for _, s := range ctxt.Text {
w.writeRefs(s)
w.addLengths(s)
}
for _, s := range ctxt.Data {
w.writeRefs(s)
w.addLengths(s)
}
// End symbol references
w.wr.WriteByte(0xff)
// Lengths
w.writeLengths()
// Data block
for _, s := range ctxt.Text {
w.wr.Write(s.P)
pc := s.Pcln
w.wr.Write(pc.Pcsp.P)
w.wr.Write(pc.Pcfile.P)
w.wr.Write(pc.Pcline.P)
for i := 0; i < len(pc.Pcdata); i++ {
w.wr.Write(pc.Pcdata[i].P)
}
}
for _, s := range ctxt.Data {
w.wr.Write(s.P)
}
// Symbols
for _, s := range ctxt.Text {
w.writeSym(s)
}
for _, s := range ctxt.Data {
w.writeSym(s)
}
// Magic footer
w.wr.WriteString("\xff\xffgo17ld")
}
// Symbols are prefixed so their content doesn't get confused with the magic footer.
const symPrefix = 0xfe
func (w *objWriter) writeRef(s *LSym, isPath bool) {
if s == nil || s.RefIdx != 0 {
return
}
var m map[string]int
switch s.Version {
case 0:
m = w.refIdx
case 1:
m = w.vrefIdx
default:
log.Fatalf("%s: invalid version number %d", s.Name, s.Version)
}
idx := m[s.Name]
if idx != 0 {
s.RefIdx = idx
return
}
w.wr.WriteByte(symPrefix)
if isPath {
w.writeString(filepath.ToSlash(s.Name))
} else {
w.writeString(s.Name)
}
w.writeInt(int64(s.Version))
w.nRefs++
s.RefIdx = w.nRefs
m[s.Name] = w.nRefs
}
func (w *objWriter) writeRefs(s *LSym) {
w.writeRef(s, false)
w.writeRef(s.Gotype, false)
for i := range s.R {
w.writeRef(s.R[i].Sym, false)
}
if s.Type == STEXT {
for a := s.Autom; a != nil; a = a.Link {
w.writeRef(a.Asym, false)
w.writeRef(a.Gotype, false)
}
pc := s.Pcln
for _, d := range pc.Funcdata {
w.writeRef(d, false)
}
for _, f := range pc.File {
w.writeRef(f, true)
}
}
}
func (w *objWriter) writeSymDebug(s *LSym) {
ctxt := w.ctxt
fmt.Fprintf(ctxt.Bso, "%s ", s.Name)
if s.Version != 0 {
fmt.Fprintf(ctxt.Bso, "v=%d ", s.Version)
}
if s.Type != 0 {
fmt.Fprintf(ctxt.Bso, "t=%d ", s.Type)
}
if s.Dupok {
fmt.Fprintf(ctxt.Bso, "dupok ")
}
if s.Cfunc {
fmt.Fprintf(ctxt.Bso, "cfunc ")
}
if s.Nosplit {
fmt.Fprintf(ctxt.Bso, "nosplit ")
}
fmt.Fprintf(ctxt.Bso, "size=%d", s.Size)
if s.Type == STEXT {
fmt.Fprintf(ctxt.Bso, " args=%#x locals=%#x", uint64(s.Args), uint64(s.Locals))
if s.Leaf {
fmt.Fprintf(ctxt.Bso, " leaf")
}
}
fmt.Fprintf(ctxt.Bso, "\n")
for p := s.Text; p != nil; p = p.Link {
fmt.Fprintf(ctxt.Bso, "\t%#04x %v\n", uint(int(p.Pc)), p)
}
var c int
var j int
for i := 0; i < len(s.P); {
fmt.Fprintf(ctxt.Bso, "\t%#04x", uint(i))
for j = i; j < i+16 && j < len(s.P); j++ {
fmt.Fprintf(ctxt.Bso, " %02x", s.P[j])
}
for ; j < i+16; j++ {
fmt.Fprintf(ctxt.Bso, " ")
}
fmt.Fprintf(ctxt.Bso, " ")
for j = i; j < i+16 && j < len(s.P); j++ {
c = int(s.P[j])
if ' ' <= c && c <= 0x7e {
fmt.Fprintf(ctxt.Bso, "%c", c)
} else {
fmt.Fprintf(ctxt.Bso, ".")
}
}
fmt.Fprintf(ctxt.Bso, "\n")
i += 16
}
sort.Sort(relocByOff(s.R)) // generate stable output
for _, r := range s.R {
name := ""
if r.Sym != nil {
name = r.Sym.Name
} else if r.Type == R_TLS_LE {
name = "TLS"
}
if ctxt.Arch.InFamily(sys.ARM, sys.PPC64) {
fmt.Fprintf(ctxt.Bso, "\trel %d+%d t=%d %s+%x\n", int(r.Off), r.Siz, r.Type, name, uint64(r.Add))
} else {
fmt.Fprintf(ctxt.Bso, "\trel %d+%d t=%d %s+%d\n", int(r.Off), r.Siz, r.Type, name, r.Add)
}
}
}
func (w *objWriter) writeSym(s *LSym) {
ctxt := w.ctxt
if ctxt.Debugasm != 0 {
w.writeSymDebug(s)
}
w.wr.WriteByte(symPrefix)
w.writeInt(int64(s.Type))
w.writeRefIndex(s)
flags := int64(0)
if s.Dupok {
flags |= 1
}
if s.Local {
flags |= 1 << 1
}
w.writeInt(flags)
w.writeInt(s.Size)
w.writeRefIndex(s.Gotype)
w.writeInt(int64(len(s.P)))
w.writeInt(int64(len(s.R)))
var r *Reloc
for i := 0; i < len(s.R); i++ {
r = &s.R[i]
w.writeInt(int64(r.Off))
w.writeInt(int64(r.Siz))
w.writeInt(int64(r.Type))
w.writeInt(r.Add)
w.writeRefIndex(r.Sym)
}
if s.Type != STEXT {
return
}
w.writeInt(int64(s.Args))
w.writeInt(int64(s.Locals))
if s.Nosplit {
w.writeInt(1)
} else {
w.writeInt(0)
}
flags = int64(0)
if s.Leaf {
flags |= 1
}
if s.Cfunc {
flags |= 1 << 1
}
if s.ReflectMethod {
flags |= 1 << 2
}
w.writeInt(flags)
n := 0
for a := s.Autom; a != nil; a = a.Link {
n++
}
w.writeInt(int64(n))
for a := s.Autom; a != nil; a = a.Link {
w.writeRefIndex(a.Asym)
w.writeInt(int64(a.Aoffset))
if a.Name == NAME_AUTO {
w.writeInt(A_AUTO)
} else if a.Name == NAME_PARAM {
w.writeInt(A_PARAM)
} else {
log.Fatalf("%s: invalid local variable type %d", s.Name, a.Name)
}
w.writeRefIndex(a.Gotype)
}
pc := s.Pcln
w.writeInt(int64(len(pc.Pcsp.P)))
w.writeInt(int64(len(pc.Pcfile.P)))
w.writeInt(int64(len(pc.Pcline.P)))
w.writeInt(int64(len(pc.Pcdata)))
for i := 0; i < len(pc.Pcdata); i++ {
w.writeInt(int64(len(pc.Pcdata[i].P)))
}
w.writeInt(int64(len(pc.Funcdataoff)))
for i := 0; i < len(pc.Funcdataoff); i++ {
w.writeRefIndex(pc.Funcdata[i])
}
for i := 0; i < len(pc.Funcdataoff); i++ {
w.writeInt(pc.Funcdataoff[i])
}
w.writeInt(int64(len(pc.File)))
for _, f := range pc.File {
w.writeRefIndex(f)
}
}
func (w *objWriter) writeInt(sval int64) {
var v uint64
uv := (uint64(sval) << 1) ^ uint64(sval>>63)
p := w.varintbuf[:]
for v = uv; v >= 0x80; v >>= 7 {
p[0] = uint8(v | 0x80)
p = p[1:]
}
p[0] = uint8(v)
p = p[1:]
w.wr.Write(w.varintbuf[:len(w.varintbuf)-len(p)])
}
func (w *objWriter) writeString(s string) {
w.writeInt(int64(len(s)))
w.wr.WriteString(s)
}
func (w *objWriter) writeRefIndex(s *LSym) {
if s == nil {
w.writeInt(0)
return
}
if s.RefIdx == 0 {
log.Fatalln("writing an unreferenced symbol", s.Name)
}
w.writeInt(int64(s.RefIdx))
}
// relocByOff sorts relocations by their offsets.
type relocByOff []Reloc
func (x relocByOff) Len() int { return len(x) }
func (x relocByOff) Less(i, j int) bool { return x[i].Off < x[j].Off }
func (x relocByOff) Swap(i, j int) { x[i], x[j] = x[j], x[i] }