cmd/compile, cmd/link: record compiler flags in DW_AT_producer

This adds a whitelisted subset of compiler flags to the DW_AT_producer
DWARF attribute of each package compilation unit DIE. This is common
practice in DWARF and can help debuggers determine the quality of the
produced debugging information.

Fixes #22168.

Change-Id: I1b994ef2262aa9b88b68eb6e883695d1103acc58
Reviewed-on: https://go-review.googlesource.com/71430Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -11,6 +11,7 @@ import (
 	"bytes"
 	"cmd/compile/internal/ssa"
 	"cmd/compile/internal/types"
+	"cmd/internal/dwarf"
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/src"
@@ -239,6 +240,11 @@ func Main(archInit func(*Arch)) {
 	flag.StringVar(&benchfile, "bench", "", "append benchmark times to `file`")
 	objabi.Flagparse(usage)
 
+	// Record flags that affect the build result. (And don't
+	// record flags that don't, since that would cause spurious
+	// changes in the binary.)
+	recordFlags("B", "N", "l", "msan", "race", "shared", "dynlink", "dwarflocationlists")
+
 	Ctxt.Flag_shared = flag_dynlink || flag_shared
 	Ctxt.Flag_dynlink = flag_dynlink
 	Ctxt.Flag_optimize = Debug['N'] == 0
@@ -1195,3 +1201,55 @@ func concurrentBackendAllowed() bool {
 	}
 	return true
 }
+
+// recordFlags records the specified command-line flags to be placed
+// in the DWARF info.
+func recordFlags(flags ...string) {
+	if myimportpath == "" {
+		// We can't record the flags if we don't know what the
+		// package name is.
+		return
+	}
+
+	type BoolFlag interface {
+		IsBoolFlag() bool
+	}
+	type CountFlag interface {
+		IsCountFlag() bool
+	}
+	var cmd bytes.Buffer
+	for _, name := range flags {
+		f := flag.Lookup(name)
+		if f == nil {
+			continue
+		}
+		getter := f.Value.(flag.Getter)
+		if getter.String() == f.DefValue {
+			// Flag has default value, so omit it.
+			continue
+		}
+		if bf, ok := f.Value.(BoolFlag); ok && bf.IsBoolFlag() {
+			val, ok := getter.Get().(bool)
+			if ok && val {
+				fmt.Fprintf(&cmd, " -%s", f.Name)
+				continue
+			}
+		}
+		if cf, ok := f.Value.(CountFlag); ok && cf.IsCountFlag() {
+			val, ok := getter.Get().(int)
+			if ok && val == 1 {
+				fmt.Fprintf(&cmd, " -%s", f.Name)
+				continue
+			}
+		}
+		fmt.Fprintf(&cmd, " -%s=%v", f.Name, getter.Get())
+	}
+
+	if cmd.Len() == 0 {
+		return
+	}
+	s := Ctxt.Lookup(dwarf.CUInfoPrefix + "producer." + myimportpath)
+	s.Type = objabi.SDWARFINFO
+	Ctxt.Data = append(Ctxt.Data, s)
+	s.P = cmd.Bytes()[1:]
+}

src/cmd/internal/dwarf/dwarf.go

@@ -21,10 +21,14 @@ const LocPrefix = "go.loc."
 // RangePrefix is the prefix for all the symbols containing DWARF range lists.
 const RangePrefix = "go.range."
 
-// InfoConstPrefix is the prefix for all symbols containing DWARF info
+// ConstInfoPrefix is the prefix for all symbols containing DWARF info
 // entries that contain constants.
 const ConstInfoPrefix = "go.constinfo."
 
+// CUInfoPrefix is the prefix for symbols containing information to
+// populate the DWARF compilation unit info entries.
+const CUInfoPrefix = "go.cuinfo."
+
 // Sym represents a symbol.
 type Sym interface {
 	Len() int64

src/cmd/internal/objabi/flag.go

@@ -94,10 +94,18 @@ func (c *count) Set(s string) error {
 	return nil
 }
 
+func (c *count) Get() interface{} {
+	return int(*c)
+}
+
 func (c *count) IsBoolFlag() bool {
 	return true
 }
 
+func (c *count) IsCountFlag() bool {
+	return true
+}
+
 type fn0 func()
 
 func (f fn0) Set(s string) error {

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

@@ -1059,7 +1059,17 @@ func writelines(ctxt *Link, lib *sym.Library, textp []*sym.Symbol, ls *sym.Symbo
 	// the linker directory. If we move CU construction into the
 	// compiler, this should happen naturally.
 	newattr(dwinfo, dwarf.DW_AT_comp_dir, dwarf.DW_CLS_STRING, int64(len(compDir)), compDir)
+	producerExtra := ctxt.Syms.Lookup(dwarf.CUInfoPrefix+"producer."+lib.Pkg, 0)
 	producer := "Go cmd/compile " + objabi.Version
+	if len(producerExtra.P) > 0 {
+		// We put a semicolon before the flags to clearly
+		// separate them from the version, which can be long
+		// and have lots of weird things in it in development
+		// versions. We promise not to put a semicolon in the
+		// version, so it should be safe for readers to scan
+		// forward to the semicolon.
+		producer += "; " + string(producerExtra.P)
+	}
 	newattr(dwinfo, dwarf.DW_AT_producer, dwarf.DW_CLS_STRING, int64(len(producer)), producer)
 
 	// Write .debug_line Line Number Program Header (sec 6.2.4)