From ad92148058ee060ddbc4ff789f537d8d94f57c5d Mon Sep 17 00:00:00 2001
From: Than McIntosh <thanm@google.com>
Date: Wed, 11 Mar 2020 11:16:19 -0400
Subject: [PATCH] [dev.link] cmd/link: demote DWARF line symbols to anonymous
 aux

Convert DWARF .debug_line symbols to anonymous aux syms, so as
to save space in object files and reduce the number of symbols
that have to be added to the linker's lookup tables.

Change-Id: I5b350f036e21a7a7128cb08148ab7c243aaf0d0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/223018
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/cmd/internal/dwarf/dwarf.go  | 3 ---
 src/cmd/internal/obj/objfile.go  | 4 +++-
 src/cmd/internal/obj/objfile2.go | 6 +++---
 src/cmd/internal/obj/plist.go    | 5 +----
 src/cmd/internal/obj/sym.go      | 2 +-
 5 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/src/cmd/internal/dwarf/dwarf.go b/src/cmd/internal/dwarf/dwarf.go
index 3ba0623c8c..62036aee2a 100644
--- a/src/cmd/internal/dwarf/dwarf.go
+++ b/src/cmd/internal/dwarf/dwarf.go
@@ -21,9 +21,6 @@ import (
 // InfoPrefix is the prefix for all the symbols containing DWARF info entries.
 const InfoPrefix = "go.info."
 
-// DebugLinesPrefix is the prefix for all the symbols containing DWARF debug_line information from the compiler.
-const DebugLinesPrefix = "go.debuglines."
-
 // ConstInfoPrefix is the prefix for all symbols containing DWARF info
 // entries that contain constants.
 const ConstInfoPrefix = "go.constinfo."
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 0f73e48b05..717d471569 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -219,7 +219,9 @@ func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym,
 		if s.WasInlined() {
 			s.Func.dwarfAbsFnSym = ctxt.DwFixups.AbsFuncDwarfSym(s)
 		}
-		s.Func.dwarfDebugLinesSym = ctxt.LookupDerived(s, dwarf.DebugLinesPrefix+s.Name)
+		s.Func.dwarfDebugLinesSym = &LSym{
+			Type: objabi.SDWARFLINES,
+		}
 	}
 	return s.Func.dwarfInfoSym, s.Func.dwarfLocSym, s.Func.dwarfRangesSym, s.Func.dwarfAbsFnSym, s.Func.dwarfDebugLinesSym
 }
diff --git a/src/cmd/internal/obj/objfile2.go b/src/cmd/internal/obj/objfile2.go
index 9ccbcd5352..359e82e138 100644
--- a/src/cmd/internal/obj/objfile2.go
+++ b/src/cmd/internal/obj/objfile2.go
@@ -320,7 +320,7 @@ func (w *writer) Aux(s *LSym) {
 			}
 			o.Write(w.Writer)
 		}
-		if s.Func.dwarfDebugLinesSym != nil {
+		if s.Func.dwarfDebugLinesSym != nil && s.Func.dwarfDebugLinesSym.Size != 0 {
 			o := goobj2.Aux{
 				Type: goobj2.AuxDwarfLines,
 				Sym:  makeSymRef(s.Func.dwarfDebugLinesSym),
@@ -348,7 +348,7 @@ func nAuxSym(s *LSym) int {
 		if s.Func.dwarfRangesSym != nil && s.Func.dwarfRangesSym.Size != 0 {
 			n++
 		}
-		if s.Func.dwarfDebugLinesSym != nil {
+		if s.Func.dwarfDebugLinesSym != nil && s.Func.dwarfDebugLinesSym.Size != 0 {
 			n++
 		}
 	}
@@ -419,7 +419,7 @@ func genFuncInfoSyms(ctxt *Link) {
 		s.Func.FuncInfoSym = isym
 		b.Reset()
 
-		dwsyms := []*LSym{s.Func.dwarfRangesSym, s.Func.dwarfLocSym}
+		dwsyms := []*LSym{s.Func.dwarfRangesSym, s.Func.dwarfLocSym, s.Func.dwarfDebugLinesSym}
 		for _, s := range dwsyms {
 			if s == nil || s.Size == 0 {
 				continue
diff --git a/src/cmd/internal/obj/plist.go b/src/cmd/internal/obj/plist.go
index fdb5490255..917343ea57 100644
--- a/src/cmd/internal/obj/plist.go
+++ b/src/cmd/internal/obj/plist.go
@@ -139,13 +139,10 @@ func (ctxt *Link) InitTextSym(s *LSym, flag int) {
 	ctxt.Text = append(ctxt.Text, s)
 
 	// Set up DWARF entries for s.
-	info, _, _, _, lines := ctxt.dwarfSym(s)
+	info, _, _, _, _ := ctxt.dwarfSym(s)
 	info.Type = objabi.SDWARFINFO
 	info.Set(AttrDuplicateOK, s.DuplicateOK())
 	ctxt.Data = append(ctxt.Data, info)
-	lines.Type = objabi.SDWARFLINES
-	lines.Set(AttrDuplicateOK, s.DuplicateOK())
-	ctxt.Data = append(ctxt.Data, lines)
 }
 
 func (ctxt *Link) Globl(s *LSym, size int64, flag int) {
diff --git a/src/cmd/internal/obj/sym.go b/src/cmd/internal/obj/sym.go
index 818d464c91..9cd1786f67 100644
--- a/src/cmd/internal/obj/sym.go
+++ b/src/cmd/internal/obj/sym.go
@@ -325,7 +325,7 @@ func (ctxt *Link) traverseFuncAux(fsym *LSym, fn func(parent *LSym, aux *LSym))
 			fn(fsym, filesym)
 		}
 	}
-	dwsyms := []*LSym{fsym.Func.dwarfRangesSym, fsym.Func.dwarfLocSym}
+	dwsyms := []*LSym{fsym.Func.dwarfRangesSym, fsym.Func.dwarfLocSym, fsym.Func.dwarfDebugLinesSym}
 	for _, dws := range dwsyms {
 		if dws == nil || dws.Size == 0 {
 			continue