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/153241
Reviewed-by: David Chase <drchase@google.com>

4 files changed, 24 insertions, 5 deletions

diff --git a/src/cmd/internal/obj/link.go b/src/cmd/internal/obj/link.go
index 6cff335ddf..dfecdfbb37 100644
--- a/src/cmd/internal/obj/link.go
+++ b/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
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 4fbaafe347..49301f04d5 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/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)
 	}
 
diff --git a/src/cmd/internal/obj/plist.go b/src/cmd/internal/obj/plist.go
index 6710b375f1..9d376f739f 100644
--- a/src/cmd/internal/obj/plist.go
+++ b/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
diff --git a/src/cmd/internal/obj/util.go b/src/cmd/internal/obj/util.go
index 46c3d7b17b..f1517d3d5d 100644
--- a/src/cmd/internal/obj/util.go
+++ b/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 {