cmd/compile: update object file format for DWARF file table

In CL 188317, we generate the debug_lines in the compiler, and created a
new symbol to hold the line table. Here we modify the object file format
to output the file table.

Change-Id: Ibee192e80b86ff6af36467a0b1c26ee747dfee37
Reviewed-on: https://go-review.googlesource.com/c/go/+/191167Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/goobj/read.go

@@ -136,6 +136,7 @@ type Package struct {
 	MaxVersion    int64           // maximum Version in any SymID in Syms
 	Arch          string          // architecture
 	Native        []*NativeReader // native object data (e.g. ELF)
+	DWARFFileList []string        // List of files for the DWARF .debug_lines section
 }
 
 type NativeReader struct {
@@ -502,7 +503,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 	// TODO: extract OS + build ID if/when we need it
 
 	r.readFull(r.tmp[:8])
-	if !bytes.Equal(r.tmp[:8], []byte("\x00go112ld")) {
+	if !bytes.Equal(r.tmp[:8], []byte("\x00go114ld")) {
 		return r.error(errCorruptObject)
 	}
 
@@ -520,6 +521,12 @@ func (r *objReader) parseObject(prefix []byte) error {
 		r.p.Imports = append(r.p.Imports, s)
 	}
 
+	// Read filenames for dwarf info.
+	count := r.readInt()
+	for i := int64(0); i < count; i++ {
+		r.p.DWARFFileList = append(r.p.DWARFFileList, r.readString())
+	}
+
 	r.p.SymRefs = []SymID{{"", 0}}
 	for {
 		if b := r.readByte(); b != 0xfe {
@@ -619,7 +626,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 	}
 
 	r.readFull(r.tmp[:7])
-	if !bytes.Equal(r.tmp[:7], []byte("go112ld")) {
+	if !bytes.Equal(r.tmp[:7], []byte("go114ld")) {
 		return r.error(errCorruptObject)
 	}
 

src/cmd/internal/obj/dwarf.go

@@ -235,15 +235,3 @@ func putpclcdelta(linkctxt *Link, dctxt dwCtxt, s *LSym, deltaPC uint64, deltaLC
 	// Output the special opcode.
 	dctxt.AddUint8(s, uint8(opcode))
 }
-
-// createDebugLinesFileTable creates a new symbol holding the list of files
-// in our package.
-func (ctxt *Link) createDebugLinesFileTable() {
-	dctxt := dwCtxt{ctxt}
-
-	fileLUT := ctxt.PosTable.DebugLinesFileTable()
-	s := ctxt.dwarfFileTableSymbol()
-	for _, file := range fileLUT {
-		dctxt.AddString(s, file)
-	}
-}

src/cmd/internal/obj/objfile.go

@@ -86,7 +86,7 @@ func WriteObjFile(ctxt *Link, b *bufio.Writer, pkgpath string) {
 	w := newObjWriter(ctxt, b, pkgpath)
 
 	// Magic header
-	w.wr.WriteString("\x00go112ld")
+	w.wr.WriteString("\x00go114ld")
 
 	// Version
 	w.wr.WriteByte(1)
@@ -97,6 +97,13 @@ func WriteObjFile(ctxt *Link, b *bufio.Writer, pkgpath string) {
 	}
 	w.writeString("")
 
+	// DWARF File Table
+	fileTable := ctxt.PosTable.DebugLinesFileTable()
+	w.writeInt(int64(len(fileTable)))
+	for _, str := range fileTable {
+		w.writeString(str)
+	}
+
 	// Symbol references
 	for _, s := range ctxt.Text {
 		w.writeRefs(s)
@@ -161,7 +168,7 @@ func WriteObjFile(ctxt *Link, b *bufio.Writer, pkgpath string) {
 	}
 
 	// Magic footer
-	w.wr.WriteString("\xffgo112ld")
+	w.wr.WriteString("\xffgo114ld")
 }
 
 // Symbols are prefixed so their content doesn't get confused with the magic footer.
@@ -641,23 +648,6 @@ func (ctxt *Link) DwarfIntConst(myimportpath, name, typename string, val int64)
 	dwarf.PutIntConst(dwCtxt{ctxt}, s, ctxt.Lookup(dwarf.InfoPrefix+typename), myimportpath+"."+name, val)
 }
 
-// dwarfFileTableSymbol creates (or finds) the symbol for holding the line table for this package.
-//
-// The symbol WILL NOT be unique at the per package/archive level. For example,
-// when writing a package archive, we'll write this symbol for the Go code, and
-// one for each assembly file in the package. As such, we can't treat this
-// symbol the same when we read in the object files in the linker. This symbol
-// won't make it to the symbol table, and compilation units will keep track of
-// it.
-// TODO: Actually save this to the object file, and read it back in the linker.
-func (ctxt *Link) dwarfFileTableSymbol() *LSym {
-	s := ctxt.LookupInit(dwarf.DebugLinesPrefix+".package", func(s *LSym) {
-		s.Type = objabi.SDWARFLINES
-		//ctxt.Data = append(ctxt.Data, s)
-	})
-	return s
-}
-
 func (ctxt *Link) DwarfAbstractFunc(curfn interface{}, s *LSym, myimportpath string) {
 	absfn := ctxt.DwFixups.AbsFuncDwarfSym(s)
 	if absfn.Size != 0 {

src/cmd/internal/objabi/doc.go

@@ -22,10 +22,12 @@
 //
 // The file format is:
 //
-//	- magic header: "\x00go112ld"
+//	- magic header: "\x00go114ld"
 //	- byte 1 - version number
 //	- sequence of strings giving dependencies (imported packages)
 //	- empty string (marks end of sequence)
+//	- number of entries in the following sequence
+//	- sequence of filename strings to generate debug information
 //	- sequence of symbol references used by the defined symbols
 //	- byte 0xff (marks end of sequence)
 //	- sequence of integer lengths:
@@ -38,7 +40,7 @@
 //	- data, the content of the defined symbols
 //	- sequence of defined symbols
 //	- byte 0xff (marks end of sequence)
-//	- magic footer: "\xffgo112ld"
+//	- magic footer: "\xffgo114ld"
 //
 // All integers are stored in a zigzag varint format.
 // See golang.org/s/go12symtab for a definition.

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

@@ -27,8 +27,8 @@ import (
 )
 
 const (
-	startmagic = "\x00go112ld"
-	endmagic   = "\xffgo112ld"
+	startmagic = "\x00go114ld"
+	endmagic   = "\xffgo114ld"
 )
 
 var emptyPkg = []byte(`"".`)
@@ -136,6 +136,14 @@ func (r *objReader) loadObjFile() {
 		r.lib.ImportStrings = append(r.lib.ImportStrings, lib)
 	}
 
+	// DWARF strings
+	count := r.readInt()
+	r.unit.DWARFFileTable = make([]string, count)
+	for i := 0; i < count; i++ {
+		// TODO: This should probably be a call to mkROString.
+		r.unit.DWARFFileTable[i] = r.readString()
+	}
+
 	// Symbol references
 	r.refs = []*sym.Symbol{nil} // zeroth ref is nil
 	for {

src/cmd/link/internal/sym/compilation_unit.go

@@ -19,4 +19,5 @@ type CompilationUnit struct {
 	AbsFnDIEs      []*Symbol     // Abstract function DIE subtrees
 	RangeSyms      []*Symbol     // Symbols for debug_range
 	Textp          []*Symbol     // Text symbols in this CU
+	DWARFFileTable []string      // The file table used to generate the .debug_lines
 }