[dev.link] cmd/internal/goobj2: separate Autolib from Pkglist in new object file

In CL 196030 we decided to combine the imported package list
(Autolib) and referenced package list (PkgIdx, or Pkglist).
However, in some cases the Autolib list may contain file name,
instead of package path, e.g.
https://go.googlesource.com/go/+/refs/heads/dev.link/src/cmd/compile/internal/gc/main.go#1181
And the linker needs that to locate the file. This mostly happens
with direct invocation of the compiler and linker (i.e., not
through "go build").

Instead of letting the linker make guess of the file name based
on the package path, make Autolib a separate list.

Change-Id: If195a69462d04db515346ee67cdec925f5a69e2e
Reviewed-on: https://go-review.googlesource.com/c/go/+/200157
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/internal/goobj/readnew.go

@@ -21,8 +21,9 @@ func (r *objReader) readNew() {
 	}
 
 	// Imports
+	r.p.Imports = rr.Autolib()
+
 	pkglist := rr.Pkglist()
-	r.p.Imports = pkglist[1:] // index 0 is a dummy invalid package
 
 	abiToVer := func(abi uint16) int64 {
 		var vers int64

src/cmd/internal/goobj2/objfile.go

@@ -29,7 +29,8 @@ import (
 //       Data [...]byte
 //    }
 //
-//    PkgIndex [...]stringOff // TODO: add fingerprints
+//    Autolib  [...]stringOff // imported packages (for file loading) // TODO: add fingerprints
+//    PkgIndex [...]stringOff // referenced packages by index
 //
 //    DwarfFiles [...]stringOff // XXX as a separate block for now
 //
@@ -127,7 +128,8 @@ const (
 
 // Blocks
 const (
-	BlkPkgIdx = iota
+	BlkAutolib = iota
+	BlkPkgIdx
 	BlkDwarfFile
 	BlkSymdef
 	BlkNonpkgdef
@@ -469,6 +471,16 @@ func (r *Reader) StringRef(off uint32) string {
 	return r.StringAt(r.uint32At(off))
 }
 
+func (r *Reader) Autolib() []string {
+	n := (r.h.Offsets[BlkAutolib+1] - r.h.Offsets[BlkAutolib]) / 4
+	s := make([]string, n)
+	for i := range s {
+		off := r.h.Offsets[BlkAutolib] + uint32(i)*4
+		s[i] = r.StringRef(off)
+	}
+	return s
+}
+
 func (r *Reader) Pkglist() []string {
 	n := (r.h.Offsets[BlkPkgIdx+1] - r.h.Offsets[BlkPkgIdx]) / 4
 	s := make([]string, n)

src/cmd/internal/obj/objfile2.go

@@ -40,6 +40,12 @@ func WriteObjFile2(ctxt *Link, b *bio.Writer, pkgpath string) {
 	// String table
 	w.StringTable()
 
+	// Autolib
+	h.Offsets[goobj2.BlkAutolib] = w.Offset()
+	for _, pkg := range ctxt.Imports {
+		w.StringRef(pkg)
+	}
+
 	// Package references
 	h.Offsets[goobj2.BlkPkgIdx] = w.Offset()
 	for _, pkg := range w.pkglist {
@@ -172,13 +178,6 @@ func (w *writer) init() {
 	for pkg, i := range w.ctxt.pkgIdx {
 		w.pkglist[i] = pkg
 	}
-
-	// Also make sure imported packages appear in the list (even if no symbol is referenced).
-	for _, pkg := range w.ctxt.Imports {
-		if _, ok := w.ctxt.pkgIdx[pkg]; !ok {
-			w.pkglist = append(w.pkglist, pkg)
-		}
-	}
 }
 
 func (w *writer) StringTable() {

src/cmd/link/internal/objfile/objfile2.go

@@ -366,11 +366,7 @@ func LoadNew(l *Loader, arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, lib *s
 	or := &oReader{r, unit, localSymVersion, pkgprefix}
 
 	// Autolib
-	npkg := r.NPkg()
-	lib.ImportStrings = append(lib.ImportStrings, make([]string, npkg-1)...)[:len(lib.ImportStrings)]
-	for i := 1; i < npkg; i++ {
-		lib.ImportStrings = append(lib.ImportStrings, r.Pkg(i))
-	}
+	lib.ImportStrings = append(lib.ImportStrings, r.Autolib()...)
 
 	// DWARF file table
 	nfile := r.NDwarfFile()