cmd/compile: use innermost line number for -S

When functions are inlined, for instructions in the inlined body, does
-S print the location of the call, or the location of the body? Right
now, we do the former. I'd like to do the latter by default, it makes
much more sense when reading disassembly. With mid-stack inlining
enabled in more cases, this quandry will come up more often.

The original behavior is still available with -S=2. Some tests
use this mode (so they can find assembly generated by a particular
source line).

This helped me with understanding what the compiler was doing
while fixing #29007.

Change-Id: Id14a3a41e1b18901e7c5e460aa4caf6d940ed064
Reviewed-on: https://go-review.googlesource.com/c/153241Reviewed-by: David Chase <drchase@google.com>

src/cmd/asm/main.go

@@ -36,7 +36,7 @@ func main() {
 
 	ctxt := obj.Linknew(architecture.LinkArch)
 	if *flags.PrintOut {
-		ctxt.Debugasm = true
+		ctxt.Debugasm = 1
 	}
 	ctxt.Flag_dynlink = *flags.Dynlink
 	ctxt.Flag_shared = *flags.Shared || *flags.Dynlink

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

@@ -8,7 +8,6 @@ import (
 	"bytes"
 	"internal/testenv"
 	"io/ioutil"
-	"log"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -24,7 +23,7 @@ func TestScanfRemoval(t *testing.T) {
 	// Make a directory to work in.
 	dir, err := ioutil.TempDir("", "issue6853a-")
 	if err != nil {
-		log.Fatalf("could not create directory: %v", err)
+		t.Fatalf("could not create directory: %v", err)
 	}
 	defer os.RemoveAll(dir)
 
@@ -32,7 +31,7 @@ func TestScanfRemoval(t *testing.T) {
 	src := filepath.Join(dir, "test.go")
 	f, err := os.Create(src)
 	if err != nil {
-		log.Fatalf("could not create source file: %v", err)
+		t.Fatalf("could not create source file: %v", err)
 	}
 	f.Write([]byte(`
 package main
@@ -50,17 +49,17 @@ func main() {
 	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", dst, src)
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		log.Fatalf("could not build target: %v", err)
+		t.Fatalf("could not build target: %v", err)
 	}
 
 	// Check destination to see if scanf code was included.
 	cmd = exec.Command(testenv.GoToolPath(t), "tool", "nm", dst)
 	out, err = cmd.CombinedOutput()
 	if err != nil {
-		log.Fatalf("could not read target: %v", err)
+		t.Fatalf("could not read target: %v", err)
 	}
 	if bytes.Contains(out, []byte("scanInt")) {
-		log.Fatalf("scanf code not removed from helloworld")
+		t.Fatalf("scanf code not removed from helloworld")
 	}
 }
 
@@ -71,7 +70,7 @@ func TestDashS(t *testing.T) {
 	// Make a directory to work in.
 	dir, err := ioutil.TempDir("", "issue14515-")
 	if err != nil {
-		log.Fatalf("could not create directory: %v", err)
+		t.Fatalf("could not create directory: %v", err)
 	}
 	defer os.RemoveAll(dir)
 
@@ -79,7 +78,7 @@ func TestDashS(t *testing.T) {
 	src := filepath.Join(dir, "test.go")
 	f, err := os.Create(src)
 	if err != nil {
-		log.Fatalf("could not create source file: %v", err)
+		t.Fatalf("could not create source file: %v", err)
 	}
 	f.Write([]byte(`
 package main
@@ -94,7 +93,7 @@ func main() {
 	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags", "-S", "-o", filepath.Join(dir, "test"), src)
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		log.Fatalf("could not build target: %v", err)
+		t.Fatalf("could not build target: %v", err)
 	}
 
 	patterns := []string{

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

@@ -39,7 +39,6 @@ var (
 
 var (
 	Debug_append       int
-	Debug_asm          bool
 	Debug_closure      int
 	Debug_compilelater int
 	debug_dclstack     int
@@ -195,7 +194,7 @@ func Main(archInit func(*Arch)) {
 	objabi.Flagcount("K", "debug missing line numbers", &Debug['K'])
 	objabi.Flagcount("L", "show full file names in error messages", &Debug['L'])
 	objabi.Flagcount("N", "disable optimizations", &Debug['N'])
-	flag.BoolVar(&Debug_asm, "S", false, "print assembly listing")
+	objabi.Flagcount("S", "print assembly listing", &Debug['S'])
 	objabi.AddVersionFlag() // -V
 	objabi.Flagcount("W", "debug parse tree after type checking", &Debug['W'])
 	flag.StringVar(&asmhdr, "asmhdr", "", "write assembly header to `file`")
@@ -265,7 +264,7 @@ func Main(archInit func(*Arch)) {
 	Ctxt.Flag_dynlink = flag_dynlink
 	Ctxt.Flag_optimize = Debug['N'] == 0
 
-	Ctxt.Debugasm = Debug_asm
+	Ctxt.Debugasm = Debug['S']
 	Ctxt.Debugvlog = Debug_vlog
 	if flagDWARF {
 		Ctxt.DebugInfo = debuginfo
@@ -1330,6 +1329,7 @@ var concurrentFlagOK = [256]bool{
 	'l': true, // disable inlining
 	'w': true, // all printing happens before compilation
 	'W': true, // all printing happens before compilation
+	'S': true, // printing disassembly happens at the end (but see concurrentBackendAllowed below)
 }
 
 func concurrentBackendAllowed() bool {
@@ -1338,9 +1338,9 @@ func concurrentBackendAllowed() bool {
 			return false
 		}
 	}
-	// Debug_asm by itself is ok, because all printing occurs
+	// Debug['S'] by itself is ok, because all printing occurs
 	// while writing the object file, and that is non-concurrent.
-	// Adding Debug_vlog, however, causes Debug_asm to also print
+	// Adding Debug_vlog, however, causes Debug['S'] to also print
 	// while flushing the plist, which happens concurrently.
 	if Debug_vlog || debugstr != "" || debuglive > 0 {
 		return false

src/cmd/internal/obj/link.go

@@ -599,7 +599,7 @@ type Pcdata struct {
 type Link struct {
 	Headtype           objabi.HeadType
 	Arch               *LinkArch
-	Debugasm           bool
+	Debugasm           int
 	Debugvlog          bool
 	Debugpcln          string
 	Flag_shared        bool

src/cmd/internal/obj/objfile.go

@@ -240,7 +240,13 @@ func (w *objWriter) writeSymDebug(s *LSym) {
 	fmt.Fprintf(ctxt.Bso, "\n")
 	if s.Type == objabi.STEXT {
 		for p := s.Func.Text; p != nil; p = p.Link {
-			fmt.Fprintf(ctxt.Bso, "\t%#04x %v\n", uint(int(p.Pc)), p)
+			var s string
+			if ctxt.Debugasm > 1 {
+				s = p.String()
+			} else {
+				s = p.InnermostString()
+			}
+			fmt.Fprintf(ctxt.Bso, "\t%#04x %s\n", uint(int(p.Pc)), s)
 		}
 	}
 	for i := 0; i < len(s.P); i += 16 {
@@ -283,7 +289,7 @@ func (w *objWriter) writeSymDebug(s *LSym) {
 
 func (w *objWriter) writeSym(s *LSym) {
 	ctxt := w.ctxt
-	if ctxt.Debugasm {
+	if ctxt.Debugasm > 0 {
 		w.writeSymDebug(s)
 	}
 

src/cmd/internal/obj/plist.go

@@ -27,7 +27,7 @@ func Flushplist(ctxt *Link, plist *Plist, newprog ProgAlloc, myimportpath string
 
 	var plink *Prog
 	for p := plist.Firstpc; p != nil; p = plink {
-		if ctxt.Debugasm && ctxt.Debugvlog {
+		if ctxt.Debugasm > 0 && ctxt.Debugvlog {
 			fmt.Printf("obj: %v\n", p)
 		}
 		plink = p.Link

src/cmd/internal/obj/util.go

@@ -17,6 +17,9 @@ const REG_NONE = 0
 func (p *Prog) Line() string {
 	return p.Ctxt.OutermostPos(p.Pos).Format(false, true)
 }
+func (p *Prog) InnermostLine() string {
+	return p.Ctxt.InnermostPos(p.Pos).Format(false, true)
+}
 
 // InnermostLineNumber returns a string containing the line number for the
 // innermost inlined function (if any inlining) at p's position
@@ -118,6 +121,16 @@ func (p *Prog) String() string {
 	return fmt.Sprintf("%.5d (%v)\t%s", p.Pc, p.Line(), p.InstructionString())
 }
 
+func (p *Prog) InnermostString() string {
+	if p == nil {
+		return "<nil Prog>"
+	}
+	if p.Ctxt == nil {
+		return "<Prog without ctxt>"
+	}
+	return fmt.Sprintf("%.5d (%v)\t%s", p.Pc, p.InnermostLine(), p.InstructionString())
+}
+
 // InstructionString returns a string representation of the instruction without preceding
 // program counter or file and line number.
 func (p *Prog) InstructionString() string {
@@ -177,7 +190,7 @@ func (ctxt *Link) NewProg() *Prog {
 }
 
 func (ctxt *Link) CanReuseProgs() bool {
-	return !ctxt.Debugasm
+	return ctxt.Debugasm == 0
 }
 
 func Dconv(p *Prog, a *Addr) string {

test/run.go

@@ -642,7 +642,8 @@ func (t *test) run() {
 		// against a set of regexps in comments.
 		ops := t.wantedAsmOpcodes(long)
 		for _, env := range ops.Envs() {
-			cmdline := []string{"build", "-gcflags", "-S"}
+			// -S=2 forces outermost line numbers when disassembling inlined code.
+			cmdline := []string{"build", "-gcflags", "-S=2"}
 			cmdline = append(cmdline, flags...)
 			cmdline = append(cmdline, long)
 			cmd := exec.Command(goTool(), cmdline...)