cmd/link: allow enabling DWARF with -s

The -s flag is to documented to disable symbol table, not DWARF
(which is the -w flag). However, due to a bug (#15166), -s was
made to also disable DWARF. That bug can be fixed without
disabling DWARF. So do that, and make it possible to enable DWARF
with -s.

Since -s has been disabling DWARF for quite some time, and users
who use -s may want to suppress all symbol information, as DWARF
also contains symbol information, we keep the current behavior,
having -s continue to disable DWARF by default. But we allow
enabling DWARF by specifying -w=0 (or false).

In summary, this is the behavior now:
-s       no symbol table, no DWARF
-w       has symbol table, no DWARF
-s -w    no symbol table, no DWARF (same as -s)
-s -w=0	 no symbol table, has DWARF

Change-Id: I1883f0aa3618abccfd735d104d983f7f531813d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/492984
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>

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

@@ -58,8 +58,8 @@ func (mode *BuildMode) Set(s string) error {
  return nil
 }
 
-func (mode *BuildMode) String() string {
- switch *mode {
+func (mode BuildMode) String() string {
+ switch mode {
  case BuildModeUnset:
  return "" // avoid showing a default in usage message
  case BuildModeExe:
@@ -75,7 +75,7 @@ func (mode *BuildMode) String() string {
  case BuildModePlugin:
  return "plugin"
  }
- return fmt.Sprintf("BuildMode(%d)", uint8(*mode))
+ return fmt.Sprintf("BuildMode(%d)", uint8(mode))
 }
 
 // LinkMode indicates whether an external linker is used for the final link.

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

@@ -1633,9 +1633,6 @@ func dwarfEnabled(ctxt *Link) bool {
  if *FlagW { // disable dwarf
  return false
  }
- if *FlagS && ctxt.HeadType != objabi.Hdarwin {
- return false
- }
  if ctxt.HeadType == objabi.Hplan9 || ctxt.HeadType == objabi.Hjs || ctxt.HeadType == objabi.Hwasip1 {
  return false
  }

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

@@ -1491,6 +1491,8 @@ func (ctxt *Link) doelf() {
  if !*FlagS {
  shstrtabAddstring(".symtab")
  shstrtabAddstring(".strtab")
+ }
+ if !*FlagW {
  dwarfaddshstrings(ctxt, shstrtabAddstring)
  }
 

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

@@ -43,6 +43,7 @@ import (
  "os"
  "runtime"
  "runtime/pprof"
+ "strconv"
  "strings"
 )
 
@@ -56,6 +57,7 @@ func init() {
  flag.Var(&rpath, "r", "set the ELF dynamic linker search `path` to dir1:dir2:...")
  flag.Var(&flagExtld, "extld", "use `linker` when linking in external mode")
  flag.Var(&flagExtldflags, "extldflags", "pass `flags` to external linker")
+ flag.Var(&flagW, "w", "disable DWARF generation")
 }
 
 // Flags used by the linker. The exported flags are used by the architecture-specific packages.
@@ -90,7 +92,6 @@ var (
  flagH = flag.Bool("h", false, "halt on error")
  flagN = flag.Bool("n", false, "dump symbol table")
  FlagS = flag.Bool("s", false, "disable symbol table")
- FlagW = flag.Bool("w", false, "disable DWARF generation")
  flag8 bool // use 64-bit addresses in symbol table
  flagInterpreter = flag.String("I", "", "use `linker` as ELF dynamic linker")
  FlagDebugTramp = flag.Int("debugtramp", 0, "debug trampolines")
@@ -106,8 +107,48 @@ var (
  memprofilerate = flag.Int64("memprofilerate", 0, "set runtime.MemProfileRate to `rate`")
  benchmarkFlag = flag.String("benchmark", "", "set to 'mem' or 'cpu' to enable phase benchmarking")
  benchmarkFileFlag = flag.String("benchmarkprofile", "", "emit phase profiles to `base`_phase.{cpu,mem}prof")
+
+ flagW ternaryFlag
+ FlagW = new(bool) // the -w flag, computed in main from flagW
 )
 
+// ternaryFlag is like a boolean flag, but has a default value that is
+// neither true nor false, allowing it to be set from context (e.g. from another
+// flag).
+// *ternaryFlag implements flag.Value.
+type ternaryFlag int
+
+const (
+ ternaryFlagUnset ternaryFlag = iota
+ ternaryFlagFalse
+ ternaryFlagTrue
+)
+
+func (t *ternaryFlag) Set(s string) error {
+ v, err := strconv.ParseBool(s)
+ if err != nil {
+ return err
+ }
+ if v {
+ *t = ternaryFlagTrue
+ } else {
+ *t = ternaryFlagFalse
+ }
+ return nil
+}
+
+func (t *ternaryFlag) String() string {
+ switch *t {
+ case ternaryFlagFalse:
+ return "false"
+ case ternaryFlagTrue:
+ return "true"
+ }
+ return "unset"
+}
+
+func (t *ternaryFlag) IsBoolFlag() bool { return true } // parse like a boolean flag
+
 // Main is the main entry point for the linker code.
 func Main(arch *sys.Arch, theArch Arch) {
  log.SetPrefix("link: ")
@@ -197,6 +238,15 @@ func Main(arch *sys.Arch, theArch Arch) {
 
  checkStrictDups = *FlagStrictDups
 
+ switch flagW {
+ case ternaryFlagFalse:
+ *FlagW = false
+ case ternaryFlagTrue:
+ *FlagW = true
+ case ternaryFlagUnset:
+ *FlagW = *FlagS // -s implies -w if not explicitly set
+ }
+
  if !buildcfg.Experiment.RegabiWrappers {
  abiInternalVer = 0
  }
@@ -252,6 +302,13 @@ func Main(arch *sys.Arch, theArch Arch) {
  }
 
  if ctxt.Debugvlog != 0 {
+ onOff := func(b bool) string {
+ if b {
+ return "on"
+ }
+ return "off"
+ }
+ ctxt.Logf("build mode: %s, symbol table: %s, DWARF: %s\n", ctxt.BuildMode, onOff(!*FlagS), onOff(dwarfEnabled(ctxt)))
  ctxt.Logf("HEADER = -H%d -T0x%x -R0x%x\n", ctxt.HeadType, uint64(*FlagTextAddr), uint32(*FlagRound))
  }