5 files changed, 29 insertions, 10 deletions
diff --git a/src/cmd/internal/obj/line.go b/src/cmd/internal/obj/line.go
index fecf90c491..79ecb0068f 100644
--- a/src/cmd/internal/obj/line.go
+++ b/src/cmd/internal/obj/line.go
@@ -5,12 +5,13 @@
 package obj
 
 import (
+	"cmd/internal/goobj2"
 	"cmd/internal/src"
 )
 
 // AddImport adds a package to the list of imported packages.
-func (ctxt *Link) AddImport(pkg string) {
-	ctxt.Imports = append(ctxt.Imports, pkg)
+func (ctxt *Link) AddImport(pkg string, fingerprint goobj2.FingerprintType) {
+	ctxt.Imports = append(ctxt.Imports, goobj2.ImportedPkg{Pkg: pkg, Fingerprint: fingerprint})
 }
 
 func linkgetlineFromPos(ctxt *Link, xpos src.XPos) (f string, l int32) {
diff --git a/src/cmd/internal/obj/link.go b/src/cmd/internal/obj/link.go
index 046ad53ac7..e6f917dedb 100644
--- a/src/cmd/internal/obj/link.go
+++ b/src/cmd/internal/obj/link.go
@@ -33,6 +33,7 @@ package obj
 import (
 	"bufio"
 	"cmd/internal/dwarf"
+	"cmd/internal/goobj2"
 	"cmd/internal/objabi"
 	"cmd/internal/src"
 	"cmd/internal/sys"
@@ -666,7 +667,7 @@ type Link struct {
 	PosTable           src.PosTable
 	InlTree            InlTree // global inlining tree used by gc/inl.go
 	DwFixups           *DwarfFixupTable
-	Imports            []string
+	Imports            []goobj2.ImportedPkg
 	DiagFunc           func(string, ...interface{})
 	DiagFlush          func()
 	DebugInfo          func(fn *LSym, info *LSym, curfn interface{}) ([]dwarf.Scope, dwarf.InlCalls) // if non-nil, curfn is a *gc.Node
@@ -698,6 +699,8 @@ type Link struct {
 	defs       []*LSym // list of defined symbols in the current package
 	nonpkgdefs []*LSym // list of defined non-package symbols
 	nonpkgrefs []*LSym // list of referenced non-package symbols
+
+	Fingerprint goobj2.FingerprintType // fingerprint of symbol indices, to catch index mismatch
 }
 
 func (ctxt *Link) Diag(format string, args ...interface{}) {
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 2b0c45d6b2..6d7f42ed0b 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -98,8 +98,9 @@ func WriteObjFile(ctxt *Link, bout *bio.Writer, pkgpath string) {
 	w.wr.WriteByte(1)
 
 	// Autolib
-	for _, pkg := range ctxt.Imports {
-		w.writeString(pkg)
+	for _, p := range ctxt.Imports {
+		w.writeString(p.Pkg)
+		// This object format ignores p.Fingerprint.
 	}
 	w.writeString("")
 
diff --git a/src/cmd/internal/obj/objfile2.go b/src/cmd/internal/obj/objfile2.go
index 9792ef0846..061e43c434 100644
--- a/src/cmd/internal/obj/objfile2.go
+++ b/src/cmd/internal/obj/objfile2.go
@@ -38,7 +38,11 @@ func WriteObjFile2(ctxt *Link, b *bio.Writer, pkgpath string) {
 	if ctxt.Flag_shared {
 		flags |= goobj2.ObjFlagShared
 	}
-	h := goobj2.Header{Magic: goobj2.Magic, Flags: flags}
+	h := goobj2.Header{
+		Magic:       goobj2.Magic,
+		Fingerprint: ctxt.Fingerprint,
+		Flags:       flags,
+	}
 	h.Write(w.Writer)
 
 	// String table
@@ -46,8 +50,8 @@ func WriteObjFile2(ctxt *Link, b *bio.Writer, pkgpath string) {
 
 	// Autolib
 	h.Offsets[goobj2.BlkAutolib] = w.Offset()
-	for _, pkg := range ctxt.Imports {
-		w.StringRef(pkg)
+	for i := range ctxt.Imports {
+		ctxt.Imports[i].Write(w.Writer)
 	}
 
 	// Package references
@@ -180,8 +184,8 @@ func (w *writer) init() {
 
 func (w *writer) StringTable() {
 	w.AddString("")
-	for _, pkg := range w.ctxt.Imports {
-		w.AddString(pkg)
+	for _, p := range w.ctxt.Imports {
+		w.AddString(p.Pkg)
 	}
 	for _, pkg := range w.pkglist {
 		w.AddString(pkg)
diff --git a/src/cmd/internal/obj/sym.go b/src/cmd/internal/obj/sym.go
index 03ce8ddc5a..4a8b0ebb6f 100644
--- a/src/cmd/internal/obj/sym.go
+++ b/src/cmd/internal/obj/sym.go
@@ -34,6 +34,7 @@ package obj
 import (
 	"cmd/internal/goobj2"
 	"cmd/internal/objabi"
+	"crypto/md5"
 	"fmt"
 	"log"
 	"math"
@@ -241,6 +242,15 @@ func (ctxt *Link) NumberSyms(asm bool) {
 		ctxt.pkgIdx[pkg] = ipkg
 		ipkg++
 	})
+
+	// Compute a fingerprint of the indices, for exporting.
+	if !asm {
+		h := md5.New()
+		for _, s := range ctxt.defs {
+			h.Write([]byte(s.Name))
+		}
+		copy(ctxt.Fingerprint[:], h.Sum(nil)[:])
+	}
 }
 
 // Returns whether s is a non-package symbol, which needs to be referenced