cmd/link: eliminate .debug_aranges

The .debug_aranges section is an odd vestige of DWARF, since its
contents are easy and efficient for a debugger to reconstruct from the
attributes of the top-level compilation unit DIEs. Neither GCC nor
clang emit it by default these days. GDB and Delve ignore it entirely.
LLDB will use it if present, but is happy to construct the index from
the compilation unit attributes (and, indeed, a remarkable variety of
other ways if those aren't available either).

We're about to split up the compilation units by package, which means
they'll have discontiguous PC ranges, which is going to make
.debug_aranges harder to construct (and larger).

Rather than try to maintain this essentially unused code, let's
simplify things and remove it.

Change-Id: I8e0ccc033b583b5b8908cbb2c879b2f2d5f9a50b
Reviewed-on: https://go-review.googlesource.com/69972
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/link/internal/ld/dwarf.go

@@ -1308,9 +1308,6 @@ func writeinfo(ctxt *Link, syms []*sym.Symbol, funcs, consts []*sym.Symbol, abbr
 	infosec.Attr |= sym.AttrReachable
 	syms = append(syms, infosec)
 
-	arangessec := ctxt.Syms.Lookup(".dwarfaranges", 0)
-	arangessec.R = arangessec.R[:0]
-
 	var dwarfctxt dwarf.Context = dwctxt{ctxt}
 
 	for compunit := dwroot.Child; compunit != nil; compunit = compunit.Link {
@@ -1406,52 +1403,6 @@ func writepub(ctxt *Link, sname string, ispub func(*dwarf.DWDie) bool, syms []*s
 	return syms
 }
 
-/*
- *  emit .debug_aranges.  _info must have been written before,
- *  because we need die->offs of dwarf.DW_globals.
- */
-func writearanges(ctxt *Link, syms []*sym.Symbol) []*sym.Symbol {
-	s := ctxt.Syms.Lookup(".debug_aranges", 0)
-	s.Type = sym.SDWARFSECT
-	// The first tuple is aligned to a multiple of the size of a single tuple
-	// (twice the size of an address)
-	headersize := int(Rnd(4+2+4+1+1, int64(ctxt.Arch.PtrSize*2))) // don't count unit_length field itself
-
-	for compunit := dwroot.Child; compunit != nil; compunit = compunit.Link {
-		b := getattr(compunit, dwarf.DW_AT_low_pc)
-		if b == nil {
-			continue
-		}
-		e := getattr(compunit, dwarf.DW_AT_high_pc)
-		if e == nil {
-			continue
-		}
-
-		// Write .debug_aranges	 Header + entry	 (sec 6.1.2)
-		unitlength := uint32(headersize) + 4*uint32(ctxt.Arch.PtrSize) - 4
-		s.AddUint32(ctxt.Arch, unitlength) // unit_length (*)
-		s.AddUint16(ctxt.Arch, 2)          // dwarf version (appendix F)
-
-		adddwarfref(ctxt, s, dtolsym(compunit.Sym), 4)
-
-		s.AddUint8(uint8(ctxt.Arch.PtrSize)) // address_size
-		s.AddUint8(0)                        // segment_size
-		padding := headersize - (4 + 2 + 4 + 1 + 1)
-		for i := 0; i < padding; i++ {
-			s.AddUint8(0)
-		}
-
-		s.AddAddrPlus(ctxt.Arch, b.Data.(*sym.Symbol), b.Value-(b.Data.(*sym.Symbol)).Value)
-		s.AddUintXX(ctxt.Arch, uint64(e.Value-b.Value), ctxt.Arch.PtrSize)
-		s.AddUintXX(ctxt.Arch, 0, ctxt.Arch.PtrSize)
-		s.AddUintXX(ctxt.Arch, 0, ctxt.Arch.PtrSize)
-	}
-	if s.Size > 0 {
-		syms = append(syms, s)
-	}
-	return syms
-}
-
 func writegdbscript(ctxt *Link, syms []*sym.Symbol) []*sym.Symbol {
 	if ctxt.LinkMode == LinkExternal && Headtype == objabi.Hwindows && ctxt.BuildMode == BuildModeCArchive {
 		// gcc on Windows places .debug_gdb_scripts in the wrong location, which
@@ -1584,7 +1535,6 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 
 	syms = writepub(ctxt, ".debug_pubnames", ispubname, syms)
 	syms = writepub(ctxt, ".debug_pubtypes", ispubtype, syms)
-	syms = writearanges(ctxt, syms)
 	syms = writegdbscript(ctxt, syms)
 	syms = append(syms, infosyms...)
 	syms = collectlocs(ctxt, syms, funcs)
@@ -1625,7 +1575,6 @@ func dwarfaddshstrings(ctxt *Link, shstrtab *sym.Symbol) {
 	}
 
 	Addstring(shstrtab, ".debug_abbrev")
-	Addstring(shstrtab, ".debug_aranges")
 	Addstring(shstrtab, ".debug_frame")
 	Addstring(shstrtab, ".debug_info")
 	Addstring(shstrtab, ".debug_loc")
@@ -1637,7 +1586,6 @@ func dwarfaddshstrings(ctxt *Link, shstrtab *sym.Symbol) {
 	if ctxt.LinkMode == LinkExternal {
 		Addstring(shstrtab, elfRelType+".debug_info")
 		Addstring(shstrtab, elfRelType+".debug_loc")
-		Addstring(shstrtab, elfRelType+".debug_aranges")
 		Addstring(shstrtab, elfRelType+".debug_line")
 		Addstring(shstrtab, elfRelType+".debug_frame")
 		Addstring(shstrtab, elfRelType+".debug_pubnames")

src/runtime/runtime-lldb_test.go

@@ -5,11 +5,7 @@
 package runtime_test
 
 import (
-	"debug/elf"
-	"debug/macho"
-	"encoding/binary"
 	"internal/testenv"
-	"io"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -182,81 +178,3 @@ func TestLldbPython(t *testing.T) {
 		t.Fatalf("Unexpected lldb output:\n%s", got)
 	}
 }
-
-// Check that aranges are valid even when lldb isn't installed.
-func TestDwarfAranges(t *testing.T) {
-	testenv.MustHaveGoBuild(t)
-	dir, err := ioutil.TempDir("", "go-build")
-	if err != nil {
-		t.Fatalf("failed to create temp directory: %v", err)
-	}
-	defer os.RemoveAll(dir)
-
-	src := filepath.Join(dir, "main.go")
-	err = ioutil.WriteFile(src, []byte(lldbHelloSource), 0644)
-	if err != nil {
-		t.Fatalf("failed to create file: %v", err)
-	}
-
-	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", "a.exe")
-	cmd.Dir = dir
-	out, err := cmd.CombinedOutput()
-	if err != nil {
-		t.Fatalf("building source %v\n%s", err, out)
-	}
-
-	filename := filepath.Join(dir, "a.exe")
-	if f, err := elf.Open(filename); err == nil {
-		sect := f.Section(".debug_aranges")
-		if sect == nil {
-			t.Fatal("Missing aranges section")
-		}
-		verifyAranges(t, f.ByteOrder, sect.Open())
-	} else if f, err := macho.Open(filename); err == nil {
-		sect := f.Section("__debug_aranges")
-		if sect == nil {
-			t.Fatal("Missing aranges section")
-		}
-		verifyAranges(t, f.ByteOrder, sect.Open())
-	} else {
-		t.Skip("Not an elf or macho binary.")
-	}
-}
-
-func verifyAranges(t *testing.T, byteorder binary.ByteOrder, data io.ReadSeeker) {
-	var header struct {
-		UnitLength  uint32 // does not include the UnitLength field
-		Version     uint16
-		Offset      uint32
-		AddressSize uint8
-		SegmentSize uint8
-	}
-	for {
-		offset, err := data.Seek(0, io.SeekCurrent)
-		if err != nil {
-			t.Fatalf("Seek error: %v", err)
-		}
-		if err = binary.Read(data, byteorder, &header); err == io.EOF {
-			return
-		} else if err != nil {
-			t.Fatalf("Error reading arange header: %v", err)
-		}
-		tupleSize := int64(header.SegmentSize) + 2*int64(header.AddressSize)
-		lastTupleOffset := offset + int64(header.UnitLength) + 4 - tupleSize
-		if lastTupleOffset%tupleSize != 0 {
-			t.Fatalf("Invalid arange length %d, (addr %d, seg %d)", header.UnitLength, header.AddressSize, header.SegmentSize)
-		}
-		if _, err = data.Seek(lastTupleOffset, io.SeekStart); err != nil {
-			t.Fatalf("Seek error: %v", err)
-		}
-		buf := make([]byte, tupleSize)
-		if n, err := data.Read(buf); err != nil || int64(n) < tupleSize {
-			t.Fatalf("Read error: %v", err)
-		}
-		for _, val := range buf {
-			if val != 0 {
-				t.Fatalf("Invalid terminator")
-			}
-		}
-	}
-}