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// 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 ld
import (
"bytes"
"cmd/internal/objabi"
"cmd/internal/sys"
"cmd/link/internal/loader"
"cmd/link/internal/sym"
"container/heap"
"fmt"
"unicode"
)
var _ = fmt.Print
type workQueue []loader.Sym
// Implement container/heap.Interface.
func (q *workQueue) Len() int { return len(*q) }
func (q *workQueue) Less(i, j int) bool { return (*q)[i] < (*q)[j] }
func (q *workQueue) Swap(i, j int) { (*q)[i], (*q)[j] = (*q)[j], (*q)[i] }
func (q *workQueue) Push(i interface{}) { *q = append(*q, i.(loader.Sym)) }
func (q *workQueue) Pop() interface{} { i := (*q)[len(*q)-1]; *q = (*q)[:len(*q)-1]; return i }
// Functions for deadcode pass to use.
// Deadcode pass should call push/pop, not Push/Pop.
func (q *workQueue) push(i loader.Sym) { heap.Push(q, i) }
func (q *workQueue) pop() loader.Sym { return heap.Pop(q).(loader.Sym) }
func (q *workQueue) empty() bool { return len(*q) == 0 }
type deadcodePass struct {
ctxt *Link
ldr *loader.Loader
wq workQueue
ifaceMethod map[methodsig]bool // methods declared in reached interfaces
markableMethods []methodref // methods of reached types
reflectSeen bool // whether we have seen a reflect method call
}
func (d *deadcodePass) init() {
d.ldr.InitReachable()
d.ifaceMethod = make(map[methodsig]bool)
if objabi.Fieldtrack_enabled != 0 {
d.ldr.Reachparent = make([]loader.Sym, d.ldr.NSym())
}
heap.Init(&d.wq)
if d.ctxt.BuildMode == BuildModeShared {
// Mark all symbols defined in this library as reachable when
// building a shared library.
n := d.ldr.NDef()
for i := 1; i < n; i++ {
s := loader.Sym(i)
d.mark(s, 0)
}
return
}
var names []string
// In a normal binary, start at main.main and the init
// functions and mark what is reachable from there.
if d.ctxt.linkShared && (d.ctxt.BuildMode == BuildModeExe || d.ctxt.BuildMode == BuildModePIE) {
names = append(names, "main.main", "main..inittask")
} else {
// The external linker refers main symbol directly.
if d.ctxt.LinkMode == LinkExternal && (d.ctxt.BuildMode == BuildModeExe || d.ctxt.BuildMode == BuildModePIE) {
if d.ctxt.HeadType == objabi.Hwindows && d.ctxt.Arch.Family == sys.I386 {
*flagEntrySymbol = "_main"
} else {
*flagEntrySymbol = "main"
}
}
names = append(names, *flagEntrySymbol)
if d.ctxt.BuildMode == BuildModePlugin {
names = append(names, objabi.PathToPrefix(*flagPluginPath)+"..inittask", objabi.PathToPrefix(*flagPluginPath)+".main", "go.plugin.tabs")
// We don't keep the go.plugin.exports symbol,
// but we do keep the symbols it refers to.
exportsIdx := d.ldr.Lookup("go.plugin.exports", 0)
if exportsIdx != 0 {
relocs := d.ldr.Relocs(exportsIdx)
for i := 0; i < relocs.Count(); i++ {
d.mark(relocs.At2(i).Sym(), 0)
}
}
}
}
dynexpMap := d.ctxt.cgo_export_dynamic
if d.ctxt.LinkMode == LinkExternal {
dynexpMap = d.ctxt.cgo_export_static
}
for exp := range dynexpMap {
names = append(names, exp)
}
for _, name := range names {
// Mark symbol as a data/ABI0 symbol.
d.mark(d.ldr.Lookup(name, 0), 0)
// Also mark any Go functions (internal ABI).
d.mark(d.ldr.Lookup(name, sym.SymVerABIInternal), 0)
}
}
func (d *deadcodePass) flood() {
for !d.wq.empty() {
symIdx := d.wq.pop()
d.reflectSeen = d.reflectSeen || d.ldr.IsReflectMethod(symIdx)
isgotype := d.ldr.IsGoType(symIdx)
relocs := d.ldr.Relocs(symIdx)
if isgotype {
p := d.ldr.Data(symIdx)
if len(p) != 0 && decodetypeKind(d.ctxt.Arch, p)&kindMask == kindInterface {
for _, sig := range d.decodeIfaceMethods(d.ldr, d.ctxt.Arch, symIdx, &relocs) {
if d.ctxt.Debugvlog > 1 {
d.ctxt.Logf("reached iface method: %s\n", sig)
}
d.ifaceMethod[sig] = true
}
}
}
var methods []methodref
for i := 0; i < relocs.Count(); i++ {
r := relocs.At2(i)
t := r.Type()
if t == objabi.R_WEAKADDROFF {
continue
}
if t == objabi.R_METHODOFF {
if i+2 >= relocs.Count() {
panic("expect three consecutive R_METHODOFF relocs")
}
methods = append(methods, methodref{src: symIdx, r: i})
i += 2
continue
}
if t == objabi.R_USETYPE {
// type symbol used for DWARF. we need to load the symbol but it may not
// be otherwise reachable in the program.
// do nothing for now as we still load all type symbols.
continue
}
d.mark(r.Sym(), symIdx)
}
naux := d.ldr.NAux(symIdx)
for i := 0; i < naux; i++ {
d.mark(d.ldr.Aux2(symIdx, i).Sym(), symIdx)
}
// Some host object symbols have an outer object, which acts like a
// "carrier" symbol, or it holds all the symbols for a particular
// section. We need to mark all "referenced" symbols from that carrier,
// so we make sure we're pulling in all outer symbols, and their sub
// symbols. This is not ideal, and these carrier/section symbols could
// be removed.
if d.ldr.IsExternal(symIdx) {
d.mark(d.ldr.OuterSym(symIdx), symIdx)
d.mark(d.ldr.SubSym(symIdx), symIdx)
}
if len(methods) != 0 {
if !isgotype {
panic("method found on non-type symbol")
}
// Decode runtime type information for type methods
// to help work out which methods can be called
// dynamically via interfaces.
methodsigs := d.decodetypeMethods(d.ldr, d.ctxt.Arch, symIdx, &relocs)
if len(methods) != len(methodsigs) {
panic(fmt.Sprintf("%q has %d method relocations for %d methods", d.ldr.SymName(symIdx), len(methods), len(methodsigs)))
}
for i, m := range methodsigs {
methods[i].m = m
}
d.markableMethods = append(d.markableMethods, methods...)
}
}
}
func (d *deadcodePass) mark(symIdx, parent loader.Sym) {
if symIdx != 0 && !d.ldr.AttrReachable(symIdx) {
d.wq.push(symIdx)
d.ldr.SetAttrReachable(symIdx, true)
if objabi.Fieldtrack_enabled != 0 {
d.ldr.Reachparent[symIdx] = parent
}
if *flagDumpDep {
to := d.ldr.SymName(symIdx)
if to != "" {
from := "_"
if parent != 0 {
from = d.ldr.SymName(parent)
}
fmt.Printf("%s -> %s\n", from, to)
}
}
}
}
func (d *deadcodePass) markMethod(m methodref) {
relocs := d.ldr.Relocs(m.src)
d.mark(relocs.At2(m.r).Sym(), m.src)
d.mark(relocs.At2(m.r+1).Sym(), m.src)
d.mark(relocs.At2(m.r+2).Sym(), m.src)
}
// deadcode marks all reachable symbols.
//
// The basis of the dead code elimination is a flood fill of symbols,
// following their relocations, beginning at *flagEntrySymbol.
//
// This flood fill is wrapped in logic for pruning unused methods.
// All methods are mentioned by relocations on their receiver's *rtype.
// These relocations are specially defined as R_METHODOFF by the compiler
// so we can detect and manipulated them here.
//
// There are three ways a method of a reachable type can be invoked:
//
// 1. direct call
// 2. through a reachable interface type
// 3. reflect.Value.Method (or MethodByName), or reflect.Type.Method
// (or MethodByName)
//
// The first case is handled by the flood fill, a directly called method
// is marked as reachable.
//
// The second case is handled by decomposing all reachable interface
// types into method signatures. Each encountered method is compared
// against the interface method signatures, if it matches it is marked
// as reachable. This is extremely conservative, but easy and correct.
//
// The third case is handled by looking to see if any of:
// - reflect.Value.Method or MethodByName is reachable
// - reflect.Type.Method or MethodByName is called (through the
// REFLECTMETHOD attribute marked by the compiler).
// If any of these happen, all bets are off and all exported methods
// of reachable types are marked reachable.
//
// Any unreached text symbols are removed from ctxt.Textp.
func deadcode(ctxt *Link) {
ldr := ctxt.loader
d := deadcodePass{ctxt: ctxt, ldr: ldr}
d.init()
d.flood()
methSym := ldr.Lookup("reflect.Value.Method", sym.SymVerABIInternal)
methByNameSym := ldr.Lookup("reflect.Value.MethodByName", sym.SymVerABIInternal)
if ctxt.DynlinkingGo() {
// Exported methods may satisfy interfaces we don't know
// about yet when dynamically linking.
d.reflectSeen = true
}
for {
// Methods might be called via reflection. Give up on
// static analysis, mark all exported methods of
// all reachable types as reachable.
d.reflectSeen = d.reflectSeen || (methSym != 0 && ldr.AttrReachable(methSym)) || (methByNameSym != 0 && ldr.AttrReachable(methByNameSym))
// Mark all methods that could satisfy a discovered
// interface as reachable. We recheck old marked interfaces
// as new types (with new methods) may have been discovered
// in the last pass.
rem := d.markableMethods[:0]
for _, m := range d.markableMethods {
if (d.reflectSeen && m.isExported()) || d.ifaceMethod[m.m] {
d.markMethod(m)
} else {
rem = append(rem, m)
}
}
d.markableMethods = rem
if d.wq.empty() {
// No new work was discovered. Done.
break
}
d.flood()
}
n := ldr.NSym()
if ctxt.BuildMode != BuildModeShared {
// Keep a itablink if the symbol it points at is being kept.
// (When BuildModeShared, always keep itablinks.)
for i := 1; i < n; i++ {
s := loader.Sym(i)
if ldr.IsItabLink(s) {
relocs := ldr.Relocs(s)
if relocs.Count() > 0 && ldr.AttrReachable(relocs.At2(0).Sym()) {
ldr.SetAttrReachable(s, true)
}
}
}
}
}
// methodref holds the relocations from a receiver type symbol to its
// method. There are three relocations, one for each of the fields in
// the reflect.method struct: mtyp, ifn, and tfn.
type methodref struct {
m methodsig
src loader.Sym // receiver type symbol
r int // the index of R_METHODOFF relocations
}
func (m methodref) isExported() bool {
for _, r := range m.m {
return unicode.IsUpper(r)
}
panic("methodref has no signature")
}
// decodeMethodSig2 decodes an array of method signature information.
// Each element of the array is size bytes. The first 4 bytes is a
// nameOff for the method name, and the next 4 bytes is a typeOff for
// the function type.
//
// Conveniently this is the layout of both runtime.method and runtime.imethod.
func (d *deadcodePass) decodeMethodSig(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs, off, size, count int) []methodsig {
var buf bytes.Buffer
var methods []methodsig
for i := 0; i < count; i++ {
buf.WriteString(decodetypeName(ldr, symIdx, relocs, off))
mtypSym := decodeRelocSym(ldr, symIdx, relocs, int32(off+4))
// FIXME: add some sort of caching here, since we may see some of the
// same symbols over time for param types.
mrelocs := ldr.Relocs(mtypSym)
mp := ldr.Data(mtypSym)
buf.WriteRune('(')
inCount := decodetypeFuncInCount(arch, mp)
for i := 0; i < inCount; i++ {
if i > 0 {
buf.WriteString(", ")
}
a := decodetypeFuncInType(ldr, arch, mtypSym, &mrelocs, i)
buf.WriteString(ldr.SymName(a))
}
buf.WriteString(") (")
outCount := decodetypeFuncOutCount(arch, mp)
for i := 0; i < outCount; i++ {
if i > 0 {
buf.WriteString(", ")
}
a := decodetypeFuncOutType(ldr, arch, mtypSym, &mrelocs, i)
buf.WriteString(ldr.SymName(a))
}
buf.WriteRune(')')
off += size
methods = append(methods, methodsig(buf.String()))
buf.Reset()
}
return methods
}
func (d *deadcodePass) decodeIfaceMethods(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs) []methodsig {
p := ldr.Data(symIdx)
if decodetypeKind(arch, p)&kindMask != kindInterface {
panic(fmt.Sprintf("symbol %q is not an interface", ldr.SymName(symIdx)))
}
rel := decodeReloc(ldr, symIdx, relocs, int32(commonsize(arch)+arch.PtrSize))
s := rel.Sym()
if s == 0 {
return nil
}
if s != symIdx {
panic(fmt.Sprintf("imethod slice pointer in %q leads to a different symbol", ldr.SymName(symIdx)))
}
off := int(rel.Add()) // array of reflect.imethod values
numMethods := int(decodetypeIfaceMethodCount(arch, p))
sizeofIMethod := 4 + 4
return d.decodeMethodSig(ldr, arch, symIdx, relocs, off, sizeofIMethod, numMethods)
}
func (d *deadcodePass) decodetypeMethods(ldr *loader.Loader, arch *sys.Arch, symIdx loader.Sym, relocs *loader.Relocs) []methodsig {
p := ldr.Data(symIdx)
if !decodetypeHasUncommon(arch, p) {
panic(fmt.Sprintf("no methods on %q", ldr.SymName(symIdx)))
}
off := commonsize(arch) // reflect.rtype
switch decodetypeKind(arch, p) & kindMask {
case kindStruct: // reflect.structType
off += 4 * arch.PtrSize
case kindPtr: // reflect.ptrType
off += arch.PtrSize
case kindFunc: // reflect.funcType
off += arch.PtrSize // 4 bytes, pointer aligned
case kindSlice: // reflect.sliceType
off += arch.PtrSize
case kindArray: // reflect.arrayType
off += 3 * arch.PtrSize
case kindChan: // reflect.chanType
off += 2 * arch.PtrSize
case kindMap: // reflect.mapType
off += 4*arch.PtrSize + 8
case kindInterface: // reflect.interfaceType
off += 3 * arch.PtrSize
default:
// just Sizeof(rtype)
}
mcount := int(decodeInuxi(arch, p[off+4:], 2))
moff := int(decodeInuxi(arch, p[off+4+2+2:], 4))
off += moff // offset to array of reflect.method values
const sizeofMethod = 4 * 4 // sizeof reflect.method in program
return d.decodeMethodSig(ldr, arch, symIdx, relocs, off, sizeofMethod, mcount)
}