cmd/compile: keep JMPs around with -N

When -N, make sure we don't drop every instruction from
a block, even ones which would otherwise be empty.
Helps keep line numbers around for debugging, particularly
for break and continue statements (which often compile
down to nothing).

Fixes #14379

Change-Id: I33722c4f0dcd502f146fa48af262ba3a477c959a
Reviewed-on: https://go-review.googlesource.com/19854
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

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

@@ -236,6 +236,7 @@ func Main() {
 	}
 	Ctxt.Flag_shared = int32(flag_shared)
 	Ctxt.Flag_dynlink = flag_dynlink
+	Ctxt.Flag_optimize = Debug['N'] == 0
 
 	Ctxt.Debugasm = int32(Debug['S'])
 	Ctxt.Debugvlog = int32(Debug['v'])

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

@@ -138,15 +138,16 @@ func fixjmp(firstp *obj.Prog) {
 			fmt.Printf("%v\n", p)
 		}
 		if p.As != obj.ACALL && p.To.Type == obj.TYPE_BRANCH && p.To.Val.(*obj.Prog) != nil && p.To.Val.(*obj.Prog).As == obj.AJMP {
-			p.To.Val = chasejmp(p.To.Val.(*obj.Prog), &jmploop)
-			if Debug['R'] != 0 && Debug['v'] != 0 {
-				fmt.Printf("->%v\n", p)
+			if Debug['N'] == 0 {
+				p.To.Val = chasejmp(p.To.Val.(*obj.Prog), &jmploop)
+				if Debug['R'] != 0 && Debug['v'] != 0 {
+					fmt.Printf("->%v\n", p)
+				}
 			}
 		}
 
 		p.Opt = dead
 	}
-
 	if Debug['R'] != 0 && Debug['v'] != 0 {
 		fmt.Printf("\n")
 	}
@@ -186,7 +187,7 @@ func fixjmp(firstp *obj.Prog) {
 
 	// pass 4: elide JMP to next instruction.
 	// only safe if there are no jumps to JMPs anymore.
-	if jmploop == 0 {
+	if jmploop == 0 && Debug['N'] == 0 {
 		var last *obj.Prog
 		for p := firstp; p != nil; p = p.Link {
 			if p.As == obj.AJMP && p.To.Type == obj.TYPE_BRANCH && p.To.Val == p.Link {

src/cmd/internal/obj/link.go

@@ -572,6 +572,7 @@ type Link struct {
 	Debugpcln          int32
 	Flag_shared        int32
 	Flag_dynlink       bool
+	Flag_optimize      bool
 	Bso                *Biobuf
 	Pathname           string
 	Windows            int32

src/cmd/internal/obj/objfile.go

@@ -295,7 +295,9 @@ func Flushplist(ctxt *Link) {
 	for s := text; s != nil; s = s.Next {
 		mkfwd(s)
 		linkpatch(ctxt, s)
-		ctxt.Arch.Follow(ctxt, s)
+		if ctxt.Flag_optimize {
+			ctxt.Arch.Follow(ctxt, s)
+		}
 		ctxt.Arch.Preprocess(ctxt, s)
 		ctxt.Arch.Assemble(ctxt, s)
 		fieldtrack(ctxt, s)

src/cmd/internal/obj/pass.go

@@ -202,13 +202,15 @@ func linkpatch(ctxt *Link, sym *LSym) {
 		p.Pcond = q
 	}
 
-	for p := sym.Text; p != nil; p = p.Link {
-		p.Mark = 0 /* initialization for follow */
-		if p.Pcond != nil {
-			p.Pcond = brloop(ctxt, p.Pcond)
+	if ctxt.Flag_optimize {
+		for p := sym.Text; p != nil; p = p.Link {
+			p.Mark = 0 /* initialization for follow */
 			if p.Pcond != nil {
-				if p.To.Type == TYPE_BRANCH {
-					p.To.Offset = p.Pcond.Pc
+				p.Pcond = brloop(ctxt, p.Pcond)
+				if p.Pcond != nil {
+					if p.To.Type == TYPE_BRANCH {
+						p.To.Offset = p.Pcond.Pc
+					}
 				}
 			}
 		}

src/cmd/internal/obj/sym.go

@@ -110,6 +110,7 @@ func Linknew(arch *LinkArch) *Link {
 		ctxt.Goarm = Getgoarm()
 	}
 
+	ctxt.Flag_optimize = true
 	return ctxt
 }