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cmd/link, cmd/internal/dwarf: add DWARF5 line table support 

This patch rolls out the necessary changes to migrate the DWARF line
table support in the compiler and linker to DWARF version 5, gated by
the "dwarf5" GOEXPERIMENT.

DWARF version 5 includes a number of changes to the line table,
notably a revamped prolog section and a change in the indexing system
used to refer to files and directories within the line table
program. Specifically, prior to DWARF 4 a compilation's directory
table was considered to have an implicit zero entry containing the
compilation directory of the translation unit (package), and the file
table was considered to have an implicit zero entry storing the
"primary source file" (stored in the compilation unit DIE name).
DWARF 5 does away with these implicity entries meaning that files and
dirs are now effectively a 0-based index.

Updates #26379.

Change-Id: I9b4f1be5415aacec1ba57366d60bd48819c56ea5
Reviewed-on: https://go-review.googlesource.com/c/go/+/633879
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
This commit is contained in:
Than McIntosh 2024-11-12 12:02:17 -05:00
parent 41a30dd192
commit 767e7680ec
2 changed files with 128 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
src/cmd/internal/dwarf/dwarf.go
View File

@ -1207,6 +1207,12 @@ func PutAbstractFunc(ctxt Context, s *FnState) error {
return nil
}
// dwarfFileIndex returns the DWARF file index value for the file associated
// with pos.
func dwarfFileIndex(pos src.Pos) int64 {
return int64(1 + pos.FileIndex())
}
// Emit DWARF attributes and child DIEs for an inlined subroutine. The
// first attribute of an inlined subroutine DIE is a reference back to
// its corresponding 'abstract' DIE (containing location-independent
@ -1240,7 +1246,7 @@ func putInlinedFunc(ctxt Context, s *FnState, callIdx int) error {
}
// Emit call file, line attrs.
putattr(ctxt, s.Info, abbrev, DW_FORM_data4, DW_CLS_CONSTANT, int64(1+ic.CallPos.FileIndex()), nil) // 1-based file table
putattr(ctxt, s.Info, abbrev, DW_FORM_data4, DW_CLS_CONSTANT, dwarfFileIndex(ic.CallPos), nil)
form := int(expandPseudoForm(DW_FORM_udata_pseudo))
putattr(ctxt, s.Info, abbrev, form, DW_CLS_CONSTANT, int64(ic.CallPos.RelLine()), nil)
@ -1343,7 +1349,7 @@ func PutDefaultFunc(ctxt Context, s *FnState, isWrapper bool) error {
if isWrapper {
putattr(ctxt, s.Info, abbrev, DW_FORM_flag, DW_CLS_FLAG, int64(1), 0)
} else {
putattr(ctxt, s.Info, abbrev, DW_FORM_data4, DW_CLS_CONSTANT, int64(1+s.StartPos.FileIndex()), nil) // 1-based file index
putattr(ctxt, s.Info, abbrev, DW_FORM_data4, DW_CLS_CONSTANT, dwarfFileIndex(s.StartPos), nil)
putattr(ctxt, s.Info, abbrev, DW_FORM_udata, DW_CLS_CONSTANT, int64(s.StartPos.RelLine()), nil)
var ev int64

139
src/cmd/link/internal/ld/dwarf.go
View File

@ -1212,18 +1212,70 @@ func expandFile(fname string) string {
// writeDirFileTables emits the portion of the DWARF line table
// prologue containing the include directories and file names,
// described in section 6.2.4 of the DWARF 4 standard. It walks the
// described in section 6.2.4 of the DWARF standard. It walks the
// filepaths for the unit to discover any common directories, which
// are emitted to the directory table first, then the file table is
// emitted after that.
//
// Note that there are some differences betwen DWARF versions 4 and 5
// regarding how files and directories are handled; the chief item is
// that in version 4 we have an implicit directory index 0 that holds
// the compilation dir, and an implicit file index 0 that holds the
// "primary" source file being compiled (with the assumption that
// we'll get these two pieces of info from attributes on the
// compilation unit DIE). DWARF version 5 does away with these
// implicit entries; if you want to refer to a file or dir, you have
// to mention it explicitly.
//
// So, let's say we have a Go package with import path
// "github.com/fruit/bowl" with two source files, "apple.go" and
// "orange.go". The directory and file table in version 4 might look
// like:
//
// Dir Table:
// 1 github.com/fruit/bowl
//
// File Table:
// Entry Dir Time Size Name
// 1 0 0 0 <autogenerated>
// 2 1 0 0 apple.go
// 3 1 0 0 orange.go
//
// With DWARF version 5, the file and directory tables might look like
//
// Dir Table:
// 0 .
// 1 github.com/fruit/bowl
//
// File Table:
// Entry Dir Name
// 0 0 ? /* see remark below about this entry */
// 1 0 <autogenerated>
// 2 1 apple.go
// 3 1 orange.go
//
// Also worth noting that the line table prolog allows you to control
// what items or fields appear in file and die entries (as opposed
// to the fixed dir/time/size/name format for files in DWARF 4).
func (d *dwctxt) writeDirFileTables(unit *sym.CompilationUnit, lsu *loader.SymbolBuilder) {
type fileDir struct {
base string
dir int
}
dirNums := make(map[string]int)
dirs := []string{""}
dirs := []string{"."}
dirNums["."] = 0
files := []fileDir{}
if buildcfg.Experiment.Dwarf5 {
// Add a dummy zero entry (not used for anything) so as to
// ensure that the first useful file index is 1, since that's
// the default value of the line table file register). Note
// the "?"; this was originally an empty string, but doing
// this triggers crashes in some versions of GDB, see
// https://sourceware.org/bugzilla/show_bug.cgi?id=30357 for
// the details.
files = append(files, fileDir{base: "?", dir: 0})
}
// Preprocess files to collect directories. This assumes that the
// file table is already de-duped.
@ -1241,7 +1293,7 @@ func (d *dwctxt) writeDirFileTables(unit *sym.CompilationUnit, lsu *loader.Symbo
dir := path.Dir(name)
dirIdx, ok := dirNums[dir]
if !ok && dir != "." {
dirIdx = len(dirNums) + 1
dirIdx = len(dirNums)
dirNums[dir] = dirIdx
dirs = append(dirs, dir)
}
@ -1260,22 +1312,61 @@ func (d *dwctxt) writeDirFileTables(unit *sym.CompilationUnit, lsu *loader.Symbo
}
}
// Emit directory section. This is a series of nul terminated
// strings, followed by a single zero byte.
lsDwsym := dwSym(lsu.Sym())
for k := 1; k < len(dirs); k++ {
d.AddString(lsDwsym, dirs[k])
}
lsu.AddUint8(0) // terminator
if buildcfg.Experiment.Dwarf5 {
// Emit DWARF5 directory and file sections. The key added
// element here for version 5 is that we emit a small prolog
// describing the format of each file/dir entry, then a count
// of the entries, then the entries themselves.
// Emit file section.
for k := 0; k < len(files); k++ {
d.AddString(lsDwsym, files[k].base)
dwarf.Uleb128put(d, lsDwsym, int64(files[k].dir))
lsu.AddUint8(0) // mtime
lsu.AddUint8(0) // length
// Dir table first...
lsu.AddUint8(1) // directory_entry_format_count
// each directory entry is just a single path string.
dwarf.Uleb128put(d, lsDwsym, dwarf.DW_LNCT_path)
dwarf.Uleb128put(d, lsDwsym, dwarf.DW_FORM_string)
// emit count, then dirs
dwarf.Uleb128put(d, lsDwsym, int64(len(dirs)))
for k := 0; k < len(dirs); k++ {
d.AddString(lsDwsym, dirs[k])
}
// ... now file table. With DWARF version 5 we put out a
// prolog that describes the format of each file entry; our
// chosen format is a pair <F,D> where F is the file and D is
// the dir index (zero-based).
lsu.AddUint8(2) // file_entry_format_count
// each file entry is just a file path followed by dir index.
dwarf.Uleb128put(d, lsDwsym, dwarf.DW_LNCT_path)
dwarf.Uleb128put(d, lsDwsym, dwarf.DW_FORM_string)
dwarf.Uleb128put(d, lsDwsym, dwarf.DW_LNCT_directory_index)
dwarf.Uleb128put(d, lsDwsym, dwarf.DW_FORM_udata)
// emit count, then files
dwarf.Uleb128put(d, lsDwsym, int64(len(files)))
for k := 0; k < len(files); k++ {
d.AddString(lsDwsym, files[k].base)
dwarf.Uleb128put(d, lsDwsym, int64(files[k].dir))
}
} else {
// Emit DWARF4 directory and file sections.
// Dir table first. This just a series of nul terminated
// strings, followed by a single zero byte.
for k := 1; k < len(dirs); k++ {
d.AddString(lsDwsym, dirs[k])
}
lsu.AddUint8(0) // terminator needed in V4 case.
// File table next. This is a series of
// file/dirindex/mtime/length tuples (where dirindex is
// 1-based) followed by a single zero byte.
for k := 0; k < len(files); k++ {
d.AddString(lsDwsym, files[k].base)
dwarf.Uleb128put(d, lsDwsym, int64(files[k].dir))
lsu.AddUint8(0) // mtime
lsu.AddUint8(0) // length
}
lsu.AddUint8(0) // terminator
}
lsu.AddUint8(0) // terminator
}
// writelines collects up and chains together the symbols needed to
@ -1304,13 +1395,23 @@ func (d *dwctxt) writelines(unit *sym.CompilationUnit, lineProlog loader.Sym) []
unitLengthOffset := lsu.Size()
d.createUnitLength(lsu, 0) // unit_length (*), filled in at end
unitstart = lsu.Size()
lsu.AddUint16(d.arch, 2) // dwarf version (appendix F) -- version 3 is incompatible w/ XCode 9.0's dsymutil, latest supported on OSX 10.12 as of 2018-05
if buildcfg.Experiment.Dwarf5 {
lsu.AddUint16(d.arch, 5)
// DWARF5 requires address_size and segment selector sizes
// here, both ubyte format.
lsu.AddUint8(uint8(d.arch.PtrSize))
lsu.AddUint8(0)
} else {
lsu.AddUint16(d.arch, 2) // dwarf version (appendix F) -- version 3 is incompatible w/ XCode 9.0's dsymutil, latest supported on OSX 10.12 as of 2018-05
}
headerLengthOffset := lsu.Size()
d.addDwarfAddrField(lsu, 0) // header_length (*), filled in at end
headerstart = lsu.Size()
// cpos == unitstart + 4 + 2 + 4
lsu.AddUint8(1) // minimum_instruction_length
lsu.AddUint8(1) // minimum_instruction_length
if buildcfg.Experiment.Dwarf5 {
lsu.AddUint8(1) // maximum_operations_per_instruction
}
lsu.AddUint8(is_stmt) // default_is_stmt
lsu.AddUint8(LINE_BASE & 0xFF) // line_base
lsu.AddUint8(LINE_RANGE) // line_range