Merge "Merge remote-tracking branch 'origin/dev.debug' into master"

src/cmd/compile/fmt_test.go

@@ -571,9 +571,16 @@ var knownFormats = map[string]string{
 	"*cmd/compile/internal/ssa.Block %s":              "",
 	"*cmd/compile/internal/ssa.Block %v":              "",
 	"*cmd/compile/internal/ssa.Func %s":               "",
+	"*cmd/compile/internal/ssa.Func %v":               "",
+	"*cmd/compile/internal/ssa.FuncDebug %v":          "",
+	"*cmd/compile/internal/ssa.LocalSlot %+v":         "",
+	"*cmd/compile/internal/ssa.LocalSlot %v":          "",
+	"*cmd/compile/internal/ssa.Register %v":           "",
 	"*cmd/compile/internal/ssa.SparseTreeNode %v":     "",
 	"*cmd/compile/internal/ssa.Value %s":              "",
 	"*cmd/compile/internal/ssa.Value %v":              "",
+	"*cmd/compile/internal/ssa.VarLoc %+v":            "",
+	"*cmd/compile/internal/ssa.VarLoc %v":             "",
 	"*cmd/compile/internal/ssa.sparseTreeMapEntry %v": "",
 	"*cmd/compile/internal/types.Field %p":            "",
 	"*cmd/compile/internal/types.Field %v":            "",
@@ -592,6 +599,7 @@ var knownFormats = map[string]string{
 	"*cmd/compile/internal/types.Type %p":             "",
 	"*cmd/compile/internal/types.Type %s":             "",
 	"*cmd/compile/internal/types.Type %v":             "",
+	"*cmd/internal/dwarf.Location %#v":                "",
 	"*cmd/internal/obj.Addr %v":                       "",
 	"*cmd/internal/obj.LSym %v":                       "",
 	"*cmd/internal/obj.Prog %s":                       "",
@@ -600,17 +608,21 @@ var knownFormats = map[string]string{
 	"[16]byte %x":                                     "",
 	"[]*cmd/compile/internal/gc.Node %v":              "",
 	"[]*cmd/compile/internal/gc.Sig %#v":              "",
+	"[]*cmd/compile/internal/ssa.Block %+v":           "",
 	"[]*cmd/compile/internal/ssa.Value %v":            "",
+	"[][]cmd/compile/internal/ssa.SlotID %v":          "",
 	"[]byte %s":                                       "",
 	"[]byte %x":                                       "",
 	"[]cmd/compile/internal/ssa.Edge %v":              "",
 	"[]cmd/compile/internal/ssa.ID %v":                "",
+	"[]cmd/compile/internal/ssa.VarLocList %v":        "",
 	"[]string %v":                                     "",
 	"bool %v":                                         "",
 	"byte %08b":                                       "",
 	"byte %c":                                         "",
 	"cmd/compile/internal/arm.shift %d":               "",
 	"cmd/compile/internal/gc.Class %d":                "",
+	"cmd/compile/internal/gc.Class %v":                "",
 	"cmd/compile/internal/gc.Ctype %d":                "",
 	"cmd/compile/internal/gc.Ctype %v":                "",
 	"cmd/compile/internal/gc.Level %d":                "",
@@ -630,11 +642,13 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/ssa.Edge %v":                "",
 	"cmd/compile/internal/ssa.GCNode %v":              "",
 	"cmd/compile/internal/ssa.ID %d":                  "",
+	"cmd/compile/internal/ssa.ID %v":                  "",
 	"cmd/compile/internal/ssa.LocalSlot %v":           "",
 	"cmd/compile/internal/ssa.Location %v":            "",
 	"cmd/compile/internal/ssa.Op %s":                  "",
 	"cmd/compile/internal/ssa.Op %v":                  "",
 	"cmd/compile/internal/ssa.ValAndOff %s":           "",
+	"cmd/compile/internal/ssa.VarLocList %v":          "",
 	"cmd/compile/internal/ssa.rbrank %d":              "",
 	"cmd/compile/internal/ssa.regMask %d":             "",
 	"cmd/compile/internal/ssa.register %d":            "",
@@ -648,6 +662,7 @@ var knownFormats = map[string]string{
 	"cmd/compile/internal/types.EType %d":             "",
 	"cmd/compile/internal/types.EType %s":             "",
 	"cmd/compile/internal/types.EType %v":             "",
+	"cmd/internal/dwarf.Location %#v":                 "",
 	"cmd/internal/src.Pos %s":                         "",
 	"cmd/internal/src.Pos %v":                         "",
 	"error %v":                                        "",

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

@@ -44,6 +44,7 @@ var (
 	Debug_vlog         bool
 	Debug_wb           int
 	Debug_pctab        string
+	Debug_locationlist int
 )
 
 // Debug arguments.
@@ -69,6 +70,7 @@ var debugtab = []struct {
 	{"wb", "print information about write barriers", &Debug_wb},
 	{"export", "print export data", &Debug_export},
 	{"pctab", "print named pc-value table", &Debug_pctab},
+	{"locationlists", "print information about DWARF location list creation", &Debug_locationlist},
 }
 
 const debugHelpHeader = `usage: -d arg[,arg]* and arg is <key>[=<value>]
@@ -192,6 +194,7 @@ func Main(archInit func(*Arch)) {
 	flag.BoolVar(&pure_go, "complete", false, "compiling complete package (no C or assembly)")
 	flag.StringVar(&debugstr, "d", "", "print debug information about items in `list`; try -d help")
 	flag.BoolVar(&flagDWARF, "dwarf", true, "generate DWARF symbols")
+	flag.BoolVar(&Ctxt.Flag_locationlists, "dwarflocationlists", false, "add location lists to DWARF in optimized mode")
 	objabi.Flagcount("e", "no limit on number of errors reported", &Debug['e'])
 	objabi.Flagcount("f", "debug stack frames", &Debug['f'])
 	objabi.Flagcount("h", "halt on error", &Debug['h'])
@@ -298,6 +301,9 @@ func Main(archInit func(*Arch)) {
 	if nBackendWorkers > 1 && !concurrentBackendAllowed() {
 		log.Fatalf("cannot use concurrent backend compilation with provided flags; invoked as %v", os.Args)
 	}
+	if Ctxt.Flag_locationlists && len(Ctxt.Arch.DWARFRegisters) == 0 {
+		log.Fatalf("location lists requested but register mapping not available on %v", Ctxt.Arch.Name)
+	}
 
 	// parse -d argument
 	if debugstr != "" {
@@ -383,7 +389,7 @@ func Main(archInit func(*Arch)) {
 		Debug['l'] = 1 - Debug['l']
 	}
 
-	trackScopes = flagDWARF && Debug['l'] == 0 && Debug['N'] != 0
+	trackScopes = flagDWARF && ((Debug['l'] == 0 && Debug['N'] != 0) || Ctxt.Flag_locationlists)
 
 	Widthptr = thearch.LinkArch.PtrSize
 	Widthreg = thearch.LinkArch.RegSize

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

@@ -168,7 +168,7 @@ func (v varsByScopeAndOffset) Less(i, j int) bool {
 	if v.scopes[i] != v.scopes[j] {
 		return v.scopes[i] < v.scopes[j]
 	}
-	return v.vars[i].Offset < v.vars[j].Offset
+	return v.vars[i].StackOffset < v.vars[j].StackOffset
 }
 
 func (v varsByScopeAndOffset) Swap(i, j int) {

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

@@ -22,7 +22,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Func{}, 124, 216},
+		{Func{}, 128, 224},
 		{Name{}, 36, 56},
 		{Param{}, 28, 56},
 		{Node{}, 76, 128},

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

@@ -4401,14 +4401,15 @@ func genssa(f *ssa.Func, pp *Progs) {
 	// Remember where each block starts.
 	s.bstart = make([]*obj.Prog, f.NumBlocks())
 	s.pp = pp
-	var valueProgs map[*obj.Prog]*ssa.Value
-	var blockProgs map[*obj.Prog]*ssa.Block
+	var progToValue map[*obj.Prog]*ssa.Value
+	var progToBlock map[*obj.Prog]*ssa.Block
+	var valueToProg []*obj.Prog
 	var logProgs = e.log
 	if logProgs {
-		valueProgs = make(map[*obj.Prog]*ssa.Value, f.NumValues())
-		blockProgs = make(map[*obj.Prog]*ssa.Block, f.NumBlocks())
+		progToValue = make(map[*obj.Prog]*ssa.Value, f.NumValues())
+		progToBlock = make(map[*obj.Prog]*ssa.Block, f.NumBlocks())
 		f.Logf("genssa %s\n", f.Name)
-		blockProgs[s.pp.next] = f.Blocks[0]
+		progToBlock[s.pp.next] = f.Blocks[0]
 	}
 
 	if thearch.Use387 {
@@ -4417,6 +4418,11 @@ func genssa(f *ssa.Func, pp *Progs) {
 
 	s.ScratchFpMem = e.scratchFpMem
 
+	logLocationLists := Debug_locationlist != 0
+	if Ctxt.Flag_locationlists {
+		e.curfn.Func.DebugInfo = ssa.BuildFuncDebug(f, logLocationLists)
+		valueToProg = make([]*obj.Prog, f.NumValues())
+	}
 	// Emit basic blocks
 	for i, b := range f.Blocks {
 		s.bstart[b.ID] = s.pp.next
@@ -4457,15 +4463,19 @@ func genssa(f *ssa.Func, pp *Progs) {
 				}
 			case ssa.OpPhi:
 				CheckLoweredPhi(v)
-
+			case ssa.OpRegKill:
+				// nothing to do
 			default:
 				// let the backend handle it
 				thearch.SSAGenValue(&s, v)
 			}
 
+			if Ctxt.Flag_locationlists {
+				valueToProg[v.ID] = x
+			}
 			if logProgs {
 				for ; x != s.pp.next; x = x.Link {
-					valueProgs[x] = v
+					progToValue[x] = v
 				}
 			}
 		}
@@ -4483,7 +4493,23 @@ func genssa(f *ssa.Func, pp *Progs) {
 		thearch.SSAGenBlock(&s, b, next)
 		if logProgs {
 			for ; x != s.pp.next; x = x.Link {
-				blockProgs[x] = b
+				progToBlock[x] = b
+			}
+		}
+	}
+
+	if Ctxt.Flag_locationlists {
+		for _, locList := range e.curfn.Func.DebugInfo.Variables {
+			for _, loc := range locList.Locations {
+				loc.StartProg = valueToProg[loc.Start.ID]
+				if loc.End == nil {
+					Fatalf("empty loc %v compiling %v", loc, f.Name)
+				}
+				loc.EndProg = valueToProg[loc.End.ID]
+				if !logLocationLists {
+					loc.Start = nil
+					loc.End = nil
+				}
 			}
 		}
 	}
@@ -4496,9 +4522,9 @@ func genssa(f *ssa.Func, pp *Progs) {
 	if logProgs {
 		for p := pp.Text; p != nil; p = p.Link {
 			var s string
-			if v, ok := valueProgs[p]; ok {
+			if v, ok := progToValue[p]; ok {
 				s = v.String()
-			} else if b, ok := blockProgs[p]; ok {
+			} else if b, ok := progToBlock[p]; ok {
 				s = b.String()
 			} else {
 				s = "   " // most value and branch strings are 2-3 characters long
@@ -4516,9 +4542,9 @@ func genssa(f *ssa.Func, pp *Progs) {
 			buf.WriteString("<dl class=\"ssa-gen\">")
 			for p := pp.Text; p != nil; p = p.Link {
 				buf.WriteString("<dt class=\"ssa-prog-src\">")
-				if v, ok := valueProgs[p]; ok {
+				if v, ok := progToValue[p]; ok {
 					buf.WriteString(v.HTML())
-				} else if b, ok := blockProgs[p]; ok {
+				} else if b, ok := progToBlock[p]; ok {
 					buf.WriteString(b.HTML())
 				}
 				buf.WriteString("</dt>")
@@ -4895,9 +4921,9 @@ func (e *ssafn) SplitString(name ssa.LocalSlot) (ssa.LocalSlot, ssa.LocalSlot) {
 	lenType := types.Types[TINT]
 	if n.Class() == PAUTO && !n.Addrtaken() {
 		// Split this string up into two separate variables.
-		p := e.namedAuto(n.Sym.Name+".ptr", ptrType, n.Pos)
-		l := e.namedAuto(n.Sym.Name+".len", lenType, n.Pos)
-		return ssa.LocalSlot{N: p, Type: ptrType, Off: 0}, ssa.LocalSlot{N: l, Type: lenType, Off: 0}
+		p := e.splitSlot(&name, ".ptr", 0, ptrType)
+		l := e.splitSlot(&name, ".len", ptrType.Size(), lenType)
+		return p, l
 	}
 	// Return the two parts of the larger variable.
 	return ssa.LocalSlot{N: n, Type: ptrType, Off: name.Off}, ssa.LocalSlot{N: n, Type: lenType, Off: name.Off + int64(Widthptr)}
@@ -4912,9 +4938,9 @@ func (e *ssafn) SplitInterface(name ssa.LocalSlot) (ssa.LocalSlot, ssa.LocalSlot
 		if n.Type.IsEmptyInterface() {
 			f = ".type"
 		}
-		c := e.namedAuto(n.Sym.Name+f, t, n.Pos)
-		d := e.namedAuto(n.Sym.Name+".data", t, n.Pos)
-		return ssa.LocalSlot{N: c, Type: t, Off: 0}, ssa.LocalSlot{N: d, Type: t, Off: 0}
+		c := e.splitSlot(&name, f, 0, t)
+		d := e.splitSlot(&name, ".data", t.Size(), t)
+		return c, d
 	}
 	// Return the two parts of the larger variable.
 	return ssa.LocalSlot{N: n, Type: t, Off: name.Off}, ssa.LocalSlot{N: n, Type: t, Off: name.Off + int64(Widthptr)}
@@ -4926,10 +4952,10 @@ func (e *ssafn) SplitSlice(name ssa.LocalSlot) (ssa.LocalSlot, ssa.LocalSlot, ss
 	lenType := types.Types[TINT]
 	if n.Class() == PAUTO && !n.Addrtaken() {
 		// Split this slice up into three separate variables.
-		p := e.namedAuto(n.Sym.Name+".ptr", ptrType, n.Pos)
-		l := e.namedAuto(n.Sym.Name+".len", lenType, n.Pos)
-		c := e.namedAuto(n.Sym.Name+".cap", lenType, n.Pos)
-		return ssa.LocalSlot{N: p, Type: ptrType, Off: 0}, ssa.LocalSlot{N: l, Type: lenType, Off: 0}, ssa.LocalSlot{N: c, Type: lenType, Off: 0}
+		p := e.splitSlot(&name, ".ptr", 0, ptrType)
+		l := e.splitSlot(&name, ".len", ptrType.Size(), lenType)
+		c := e.splitSlot(&name, ".cap", ptrType.Size()+lenType.Size(), lenType)
+		return p, l, c
 	}
 	// Return the three parts of the larger variable.
 	return ssa.LocalSlot{N: n, Type: ptrType, Off: name.Off},
@@ -4948,9 +4974,9 @@ func (e *ssafn) SplitComplex(name ssa.LocalSlot) (ssa.LocalSlot, ssa.LocalSlot)
 	}
 	if n.Class() == PAUTO && !n.Addrtaken() {
 		// Split this complex up into two separate variables.
-		c := e.namedAuto(n.Sym.Name+".real", t, n.Pos)
-		d := e.namedAuto(n.Sym.Name+".imag", t, n.Pos)
-		return ssa.LocalSlot{N: c, Type: t, Off: 0}, ssa.LocalSlot{N: d, Type: t, Off: 0}
+		r := e.splitSlot(&name, ".real", 0, t)
+		i := e.splitSlot(&name, ".imag", t.Size(), t)
+		return r, i
 	}
 	// Return the two parts of the larger variable.
 	return ssa.LocalSlot{N: n, Type: t, Off: name.Off}, ssa.LocalSlot{N: n, Type: t, Off: name.Off + s}
@@ -4966,9 +4992,10 @@ func (e *ssafn) SplitInt64(name ssa.LocalSlot) (ssa.LocalSlot, ssa.LocalSlot) {
 	}
 	if n.Class() == PAUTO && !n.Addrtaken() {
 		// Split this int64 up into two separate variables.
-		h := e.namedAuto(n.Sym.Name+".hi", t, n.Pos)
-		l := e.namedAuto(n.Sym.Name+".lo", types.Types[TUINT32], n.Pos)
-		return ssa.LocalSlot{N: h, Type: t, Off: 0}, ssa.LocalSlot{N: l, Type: types.Types[TUINT32], Off: 0}
+		if thearch.LinkArch.ByteOrder == binary.BigEndian {
+			return e.splitSlot(&name, ".hi", 0, t), e.splitSlot(&name, ".lo", t.Size(), types.Types[TUINT32])
+		}
+		return e.splitSlot(&name, ".hi", t.Size(), t), e.splitSlot(&name, ".lo", 0, types.Types[TUINT32])
 	}
 	// Return the two parts of the larger variable.
 	if thearch.LinkArch.ByteOrder == binary.BigEndian {
@@ -4981,12 +5008,15 @@ func (e *ssafn) SplitStruct(name ssa.LocalSlot, i int) ssa.LocalSlot {
 	n := name.N.(*Node)
 	st := name.Type
 	ft := st.FieldType(i)
+	var offset int64
+	for f := 0; f < i; f++ {
+		offset += st.FieldType(f).Size()
+	}
 	if n.Class() == PAUTO && !n.Addrtaken() {
 		// Note: the _ field may appear several times.  But
 		// have no fear, identically-named but distinct Autos are
 		// ok, albeit maybe confusing for a debugger.
-		x := e.namedAuto(n.Sym.Name+"."+st.FieldName(i), ft, n.Pos)
-		return ssa.LocalSlot{N: x, Type: ft, Off: 0}
+		return e.splitSlot(&name, "."+st.FieldName(i), offset, ft)
 	}
 	return ssa.LocalSlot{N: n, Type: ft, Off: name.Off + st.FieldOff(i)}
 }
@@ -4999,8 +5029,7 @@ func (e *ssafn) SplitArray(name ssa.LocalSlot) ssa.LocalSlot {
 	}
 	et := at.ElemType()
 	if n.Class() == PAUTO && !n.Addrtaken() {
-		x := e.namedAuto(n.Sym.Name+"[0]", et, n.Pos)
-		return ssa.LocalSlot{N: x, Type: et, Off: 0}
+		return e.splitSlot(&name, "[0]", 0, et)
 	}
 	return ssa.LocalSlot{N: n, Type: et, Off: name.Off}
 }
@@ -5009,16 +5038,14 @@ func (e *ssafn) DerefItab(it *obj.LSym, offset int64) *obj.LSym {
 	return itabsym(it, offset)
 }
 
-// namedAuto returns a new AUTO variable with the given name and type.
-// These are exposed to the debugger.
-func (e *ssafn) namedAuto(name string, typ *types.Type, pos src.XPos) ssa.GCNode {
-	t := typ
-	s := &types.Sym{Name: name, Pkg: localpkg}
+// splitSlot returns a slot representing the data of parent starting at offset.
+func (e *ssafn) splitSlot(parent *ssa.LocalSlot, suffix string, offset int64, t *types.Type) ssa.LocalSlot {
+	s := &types.Sym{Name: parent.N.(*Node).Sym.Name + suffix, Pkg: localpkg}
 
 	n := new(Node)
 	n.Name = new(Name)
 	n.Op = ONAME
-	n.Pos = pos
+	n.Pos = parent.N.(*Node).Pos
 	n.Orig = n
 
 	s.Def = asTypesNode(n)
@@ -5031,7 +5058,7 @@ func (e *ssafn) namedAuto(name string, typ *types.Type, pos src.XPos) ssa.GCNode
 	n.Name.Curfn = e.curfn
 	e.curfn.Func.Dcl = append(e.curfn.Func.Dcl, n)
 	dowidth(t)
-	return n
+	return ssa.LocalSlot{N: n, Type: t, Off: 0, SplitOf: parent, SplitOffset: offset}
 }
 
 func (e *ssafn) CanSSA(t *types.Type) bool {

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

@@ -7,6 +7,7 @@
 package gc
 
 import (
+	"cmd/compile/internal/ssa"
 	"cmd/compile/internal/syntax"
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
@@ -369,6 +370,7 @@ type Func struct {
 	Closgen    int
 	Outerfunc  *Node // outer function (for closure)
 	FieldTrack map[*types.Sym]struct{}
+	DebugInfo  *ssa.FuncDebug
 	Ntype      *Node // signature
 	Top        int   // top context (Ecall, Eproc, etc)
 	Closure    *Node // OCLOSURE <-> ODCLFUNC

src/cmd/compile/internal/ssa/cache.go

@@ -14,6 +14,11 @@ type Cache struct {
 	blocks [200]Block
 	locs   [2000]Location
 
+	// Storage for DWARF variable locations. Lazily allocated
+	// since location lists are off by default.
+	varLocs   []VarLoc
+	curVarLoc int
+
 	// Reusable stackAllocState.
 	// See stackalloc.go's {new,put}StackAllocState.
 	stackAllocState *stackAllocState
@@ -38,4 +43,21 @@ func (c *Cache) Reset() {
 	for i := range xl {
 		xl[i] = nil
 	}
+	xvl := c.varLocs[:c.curVarLoc]
+	for i := range xvl {
+		xvl[i] = VarLoc{}
+	}
+	c.curVarLoc = 0
+}
+
+func (c *Cache) NewVarLoc() *VarLoc {
+	if c.varLocs == nil {
+		c.varLocs = make([]VarLoc, 4000)
+	}
+	if c.curVarLoc == len(c.varLocs) {
+		return &VarLoc{}
+	}
+	vl := &c.varLocs[c.curVarLoc]
+	c.curVarLoc++
+	return vl
 }

src/cmd/compile/internal/ssa/decompose.go

@@ -98,6 +98,7 @@ func decomposeBuiltIn(f *Func) {
 			delete(f.NamedValues, name)
 		case t.IsFloat():
 			// floats are never decomposed, even ones bigger than RegSize
+			newNames = append(newNames, name)
 		case t.Size() > f.Config.RegSize:
 			f.Fatalf("undecomposed named type %v %v", name, t)
 		default:

src/cmd/compile/internal/ssa/export_test.go

@@ -82,33 +82,33 @@ func (DummyFrontend) Auto(pos src.XPos, t *types.Type) GCNode {
 	return &DummyAuto{t: t, s: "aDummyAuto"}
 }
 func (d DummyFrontend) SplitString(s LocalSlot) (LocalSlot, LocalSlot) {
-	return LocalSlot{s.N, dummyTypes.BytePtr, s.Off}, LocalSlot{s.N, dummyTypes.Int, s.Off + 8}
+	return LocalSlot{N: s.N, Type: dummyTypes.BytePtr, Off: s.Off}, LocalSlot{N: s.N, Type: dummyTypes.Int, Off: s.Off + 8}
 }
 func (d DummyFrontend) SplitInterface(s LocalSlot) (LocalSlot, LocalSlot) {
-	return LocalSlot{s.N, dummyTypes.BytePtr, s.Off}, LocalSlot{s.N, dummyTypes.BytePtr, s.Off + 8}
+	return LocalSlot{N: s.N, Type: dummyTypes.BytePtr, Off: s.Off}, LocalSlot{N: s.N, Type: dummyTypes.BytePtr, Off: s.Off + 8}
 }
 func (d DummyFrontend) SplitSlice(s LocalSlot) (LocalSlot, LocalSlot, LocalSlot) {
-	return LocalSlot{s.N, s.Type.ElemType().PtrTo(), s.Off},
-		LocalSlot{s.N, dummyTypes.Int, s.Off + 8},
-		LocalSlot{s.N, dummyTypes.Int, s.Off + 16}
+	return LocalSlot{N: s.N, Type: s.Type.ElemType().PtrTo(), Off: s.Off},
+		LocalSlot{N: s.N, Type: dummyTypes.Int, Off: s.Off + 8},
+		LocalSlot{N: s.N, Type: dummyTypes.Int, Off: s.Off + 16}
 }
 func (d DummyFrontend) SplitComplex(s LocalSlot) (LocalSlot, LocalSlot) {
 	if s.Type.Size() == 16 {
-		return LocalSlot{s.N, dummyTypes.Float64, s.Off}, LocalSlot{s.N, dummyTypes.Float64, s.Off + 8}
+		return LocalSlot{N: s.N, Type: dummyTypes.Float64, Off: s.Off}, LocalSlot{N: s.N, Type: dummyTypes.Float64, Off: s.Off + 8}
 	}
-	return LocalSlot{s.N, dummyTypes.Float32, s.Off}, LocalSlot{s.N, dummyTypes.Float32, s.Off + 4}
+	return LocalSlot{N: s.N, Type: dummyTypes.Float32, Off: s.Off}, LocalSlot{N: s.N, Type: dummyTypes.Float32, Off: s.Off + 4}
 }
 func (d DummyFrontend) SplitInt64(s LocalSlot) (LocalSlot, LocalSlot) {
 	if s.Type.IsSigned() {
-		return LocalSlot{s.N, dummyTypes.Int32, s.Off + 4}, LocalSlot{s.N, dummyTypes.UInt32, s.Off}
+		return LocalSlot{N: s.N, Type: dummyTypes.Int32, Off: s.Off + 4}, LocalSlot{N: s.N, Type: dummyTypes.UInt32, Off: s.Off}
 	}
-	return LocalSlot{s.N, dummyTypes.UInt32, s.Off + 4}, LocalSlot{s.N, dummyTypes.UInt32, s.Off}
+	return LocalSlot{N: s.N, Type: dummyTypes.UInt32, Off: s.Off + 4}, LocalSlot{N: s.N, Type: dummyTypes.UInt32, Off: s.Off}
 }
 func (d DummyFrontend) SplitStruct(s LocalSlot, i int) LocalSlot {
-	return LocalSlot{s.N, s.Type.FieldType(i), s.Off + s.Type.FieldOff(i)}
+	return LocalSlot{N: s.N, Type: s.Type.FieldType(i), Off: s.Off + s.Type.FieldOff(i)}
 }
 func (d DummyFrontend) SplitArray(s LocalSlot) LocalSlot {
-	return LocalSlot{s.N, s.Type.ElemType(), s.Off}
+	return LocalSlot{N: s.N, Type: s.Type.ElemType(), Off: s.Off}
 }
 func (DummyFrontend) Line(_ src.XPos) string {
 	return "unknown.go:0"

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -420,6 +420,7 @@ var genericOps = []opData{
 	{name: "VarKill", argLength: 1, aux: "Sym", symEffect: "None"},            // aux is a *gc.Node of a variable that is known to be dead.  arg0=mem, returns mem
 	{name: "VarLive", argLength: 1, aux: "Sym", symEffect: "None"},            // aux is a *gc.Node of a variable that must be kept live.  arg0=mem, returns mem
 	{name: "KeepAlive", argLength: 2, typ: "Mem"},                             // arg[0] is a value that must be kept alive until this mark.  arg[1]=mem, returns mem
+	{name: "RegKill"},                                                         // regalloc has determined that the value in this register is dead
 
 	// Ops for breaking 64-bit operations on 32-bit architectures
 	{name: "Int64Make", argLength: 2, typ: "UInt64"}, // arg0=hi, arg1=lo

src/cmd/compile/internal/ssa/html.go

@@ -11,6 +11,7 @@ import (
 	"html"
 	"io"
 	"os"
+	"strings"
 )
 
 type HTMLWriter struct {
@@ -362,6 +363,18 @@ func (v *Value) LongHTML() string {
 	if int(v.ID) < len(r) && r[v.ID] != nil {
 		s += " : " + html.EscapeString(r[v.ID].Name())
 	}
+	var names []string
+	for name, values := range v.Block.Func.NamedValues {
+		for _, value := range values {
+			if value == v {
+				names = append(names, name.Name())
+				break // drop duplicates.
+			}
+		}
+	}
+	if len(names) != 0 {
+		s += " (" + strings.Join(names, ", ") + ")"
+	}
 	s += "</span>"
 	return s
 }

src/cmd/compile/internal/ssa/location.go

@@ -26,12 +26,38 @@ func (r *Register) Name() string {
 	return r.name
 }
 
-// A LocalSlot is a location in the stack frame.
-// It is (possibly a subpiece of) a PPARAM, PPARAMOUT, or PAUTO ONAME node.
+// ObjNum returns the register number from cmd/internal/obj/$ARCH that
+// corresponds to this register.
+func (r *Register) ObjNum() int16 {
+	return r.objNum
+}
+
+// A LocalSlot is a location in the stack frame, which identifies and stores
+// part or all of a PPARAM, PPARAMOUT, or PAUTO ONAME node.
+// It can represent a whole variable, part of a larger stack slot, or part of a
+// variable that has been decomposed into multiple stack slots.
+// As an example, a string could have the following configurations:
+//
+//           stack layout              LocalSlots
+//
+// Optimizations are disabled. s is on the stack and represented in its entirety.
+// [ ------- s string ---- ] { N: s, Type: string, Off: 0 }
+//
+// s was not decomposed, but the SSA operates on its parts individually, so
+// there is a LocalSlot for each of its fields that points into the single stack slot.
+// [ ------- s string ---- ] { N: s, Type: *uint8, Off: 0 }, {N: s, Type: int, Off: 8}
+//
+// s was decomposed. Each of its fields is in its own stack slot and has its own LocalSLot.
+// [ ptr *uint8 ] [ len int] { N: ptr, Type: *uint8, Off: 0, SplitOf: parent, SplitOffset: 0},
+//                           { N: len, Type: int, Off: 0, SplitOf: parent, SplitOffset: 8}
+//                           parent = &{N: s, Type: string}
 type LocalSlot struct {
-	N    GCNode      // an ONAME *gc.Node representing a variable on the stack
+	N    GCNode      // an ONAME *gc.Node representing a stack location.
 	Type *types.Type // type of slot
 	Off  int64       // offset of slot in N
+
+	SplitOf     *LocalSlot // slot is a decomposition of SplitOf
+	SplitOffset int64      // .. at this offset.
 }
 
 func (s LocalSlot) Name() string {

src/cmd/compile/internal/ssa/opGen.go

@@ -1903,6 +1903,7 @@ const (
 	OpVarKill
 	OpVarLive
 	OpKeepAlive
+	OpRegKill
 	OpInt64Make
 	OpInt64Hi
 	OpInt64Lo
@@ -22557,6 +22558,11 @@ var opcodeTable = [...]opInfo{
 		argLen:  2,
 		generic: true,
 	},
+	{
+		name:    "RegKill",
+		argLen:  0,
+		generic: true,
+	},
 	{
 		name:    "Int64Make",
 		argLen:  2,

src/cmd/compile/internal/ssa/regalloc.go

@@ -242,6 +242,9 @@ type regAllocState struct {
 	// current state of each (preregalloc) Value
 	values []valState
 
+	// names associated with each Value
+	valueNames [][]LocalSlot
+
 	// ID of SP, SB values
 	sp, sb ID
 
@@ -300,6 +303,13 @@ type startReg struct {
 
 // freeReg frees up register r. Any current user of r is kicked out.
 func (s *regAllocState) freeReg(r register) {
+	s.freeOrResetReg(r, false)
+}
+
+// freeOrResetReg frees up register r. Any current user of r is kicked out.
+// resetting indicates that the operation is only for bookkeeping,
+// e.g. when clearing out state upon entry to a new block.
+func (s *regAllocState) freeOrResetReg(r register, resetting bool) {
 	v := s.regs[r].v
 	if v == nil {
 		s.f.Fatalf("tried to free an already free register %d\n", r)
@@ -309,6 +319,16 @@ func (s *regAllocState) freeReg(r register) {
 	if s.f.pass.debug > regDebug {
 		fmt.Printf("freeReg %s (dump %s/%s)\n", s.registers[r].Name(), v, s.regs[r].c)
 	}
+	if !resetting && s.f.Config.ctxt.Flag_locationlists && len(s.valueNames[v.ID]) != 0 {
+		kill := s.curBlock.NewValue0(src.NoXPos, OpRegKill, types.TypeVoid)
+		for int(kill.ID) >= len(s.orig) {
+			s.orig = append(s.orig, nil)
+		}
+		for _, name := range s.valueNames[v.ID] {
+			s.f.NamedValues[name] = append(s.f.NamedValues[name], kill)
+		}
+		s.f.setHome(kill, &s.registers[r])
+	}
 	s.regs[r] = regState{}
 	s.values[v.ID].regs &^= regMask(1) << r
 	s.used &^= regMask(1) << r
@@ -599,6 +619,17 @@ func (s *regAllocState) init(f *Func) {
 	s.values = make([]valState, f.NumValues())
 	s.orig = make([]*Value, f.NumValues())
 	s.copies = make(map[*Value]bool)
+	if s.f.Config.ctxt.Flag_locationlists {
+		s.valueNames = make([][]LocalSlot, f.NumValues())
+		for slot, values := range f.NamedValues {
+			if isSynthetic(&slot) {
+				continue
+			}
+			for _, value := range values {
+				s.valueNames[value.ID] = append(s.valueNames[value.ID], slot)
+			}
+		}
+	}
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if !v.Type.IsMemory() && !v.Type.IsVoid() && !v.Type.IsFlags() && !v.Type.IsTuple() {
@@ -692,7 +723,9 @@ func (s *regAllocState) liveAfterCurrentInstruction(v *Value) bool {
 
 // Sets the state of the registers to that encoded in regs.
 func (s *regAllocState) setState(regs []endReg) {
-	s.freeRegs(s.used)
+	for s.used != 0 {
+		s.freeOrResetReg(pickReg(s.used), true)
+	}
 	for _, x := range regs {
 		s.assignReg(x.r, x.v, x.c)
 	}
@@ -735,6 +768,9 @@ func (s *regAllocState) regalloc(f *Func) {
 	}
 
 	for _, b := range f.Blocks {
+		if s.f.pass.debug > regDebug {
+			fmt.Printf("Begin processing block %v\n", b)
+		}
 		s.curBlock = b
 
 		// Initialize regValLiveSet and uses fields for this block.
@@ -830,9 +866,6 @@ func (s *regAllocState) regalloc(f *Func) {
 			// This is the complicated case. We have more than one predecessor,
 			// which means we may have Phi ops.
 
-			// Copy phi ops into new schedule.
-			b.Values = append(b.Values, phis...)
-
 			// Start with the final register state of the primary predecessor
 			idx := s.primary[b.ID]
 			if idx < 0 {
@@ -910,6 +943,9 @@ func (s *regAllocState) regalloc(f *Func) {
 				}
 			}
 
+			// Copy phi ops into new schedule.
+			b.Values = append(b.Values, phis...)
+
 			// Third pass - pick registers for phis whose inputs
 			// were not in a register.
 			for i, v := range phis {
@@ -1005,7 +1041,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			pidx := e.i
 			for _, v := range succ.Values {
 				if v.Op != OpPhi {
-					break
+					continue
 				}
 				if !s.values[v.ID].needReg {
 					continue
@@ -1565,6 +1601,9 @@ func (s *regAllocState) placeSpills() {
 	for _, b := range f.Blocks {
 		var m regMask
 		for _, v := range b.Values {
+			if v.Op == OpRegKill {
+				continue
+			}
 			if v.Op != OpPhi {
 				break
 			}
@@ -1675,7 +1714,7 @@ func (s *regAllocState) placeSpills() {
 	for _, b := range f.Blocks {
 		nphi := 0
 		for _, v := range b.Values {
-			if v.Op != OpPhi {
+			if v.Op != OpRegKill && v.Op != OpPhi {
 				break
 			}
 			nphi++
@@ -1800,6 +1839,9 @@ func (e *edgeState) setup(idx int, srcReg []endReg, dstReg []startReg, stacklive
 	}
 	// Phis need their args to end up in a specific location.
 	for _, v := range e.b.Values {
+		if v.Op == OpRegKill {
+			continue
+		}
 		if v.Op != OpPhi {
 			break
 		}
@@ -1878,6 +1920,7 @@ func (e *edgeState) process() {
 		if e.s.f.pass.debug > regDebug {
 			fmt.Printf("breaking cycle with v%d in %s:%s\n", vid, loc.Name(), c)
 		}
+		e.erase(r)
 		if _, isReg := loc.(*Register); isReg {
 			c = e.p.NewValue1(d.pos, OpCopy, c.Type, c)
 		} else {
@@ -1943,6 +1986,18 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 		}
 	}
 	_, dstReg := loc.(*Register)
+
+	// Pre-clobber destination. This avoids the
+	// following situation:
+	//   - v is currently held in R0 and stacktmp0.
+	//   - We want to copy stacktmp1 to stacktmp0.
+	//   - We choose R0 as the temporary register.
+	// During the copy, both R0 and stacktmp0 are
+	// clobbered, losing both copies of v. Oops!
+	// Erasing the destination early means R0 will not
+	// be chosen as the temp register, as it will then
+	// be the last copy of v.
+	e.erase(loc)
 	var x *Value
 	if c == nil {
 		if !e.s.values[vid].rematerializeable {
@@ -1953,8 +2008,8 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 		} else {
 			// Rematerialize into stack slot. Need a free
 			// register to accomplish this.
-			e.erase(loc) // see pre-clobber comment below
 			r := e.findRegFor(v.Type)
+			e.erase(r)
 			x = v.copyIntoNoXPos(e.p)
 			e.set(r, vid, x, false, pos)
 			// Make sure we spill with the size of the slot, not the
@@ -1976,20 +2031,8 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 				x = e.p.NewValue1(pos, OpLoadReg, c.Type, c)
 			} else {
 				// mem->mem. Use temp register.
-
-				// Pre-clobber destination. This avoids the
-				// following situation:
-				//   - v is currently held in R0 and stacktmp0.
-				//   - We want to copy stacktmp1 to stacktmp0.
-				//   - We choose R0 as the temporary register.
-				// During the copy, both R0 and stacktmp0 are
-				// clobbered, losing both copies of v. Oops!
-				// Erasing the destination early means R0 will not
-				// be chosen as the temp register, as it will then
-				// be the last copy of v.
-				e.erase(loc)
-
 				r := e.findRegFor(c.Type)
+				e.erase(r)
 				t := e.p.NewValue1(pos, OpLoadReg, c.Type, c)
 				e.set(r, vid, t, false, pos)
 				x = e.p.NewValue1(pos, OpStoreReg, loc.(LocalSlot).Type, t)
@@ -2008,7 +2051,6 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 // set changes the contents of location loc to hold the given value and its cached representative.
 func (e *edgeState) set(loc Location, vid ID, c *Value, final bool, pos src.XPos) {
 	e.s.f.setHome(c, loc)
-	e.erase(loc)
 	e.contents[loc] = contentRecord{vid, c, final, pos}
 	a := e.cache[vid]
 	if len(a) == 0 {
@@ -2059,6 +2101,16 @@ func (e *edgeState) erase(loc Location) {
 				fmt.Printf("v%d no longer available in %s:%s\n", vid, loc.Name(), c)
 			}
 			a[i], a = a[len(a)-1], a[:len(a)-1]
+			if e.s.f.Config.ctxt.Flag_locationlists {
+				if _, isReg := loc.(*Register); isReg && int(c.ID) < len(e.s.valueNames) && len(e.s.valueNames[c.ID]) != 0 {
+					kill := e.p.NewValue0(src.NoXPos, OpRegKill, types.TypeVoid)
+					e.s.f.setHome(kill, loc)
+					for _, name := range e.s.valueNames[c.ID] {
+						e.s.f.NamedValues[name] = append(e.s.f.NamedValues[name], kill)
+					}
+				}
+			}
+
 			break
 		}
 	}
@@ -2118,8 +2170,8 @@ func (e *edgeState) findRegFor(typ *types.Type) Location {
 					// Allocate a temp location to spill a register to.
 					// The type of the slot is immaterial - it will not be live across
 					// any safepoint. Just use a type big enough to hold any register.
-					t := LocalSlot{e.s.f.fe.Auto(c.Pos, types.Int64), types.Int64, 0}
-					// TODO: reuse these slots.
+					t := LocalSlot{N: e.s.f.fe.Auto(c.Pos, types.Int64), Type: types.Int64}
+					// TODO: reuse these slots. They'll need to be erased first.
 					e.set(t, vid, x, false, c.Pos)
 					if e.s.f.pass.debug > regDebug {
 						fmt.Printf("  SPILL %s->%s %s\n", r.Name(), t.Name(), x.LongString())

src/cmd/compile/internal/ssa/sizeof_test.go

@@ -24,6 +24,7 @@ func TestSizeof(t *testing.T) {
 	}{
 		{Value{}, 68, 112},
 		{Block{}, 152, 288},
+		{LocalSlot{}, 32, 48},
 		{valState{}, 28, 40},
 	}
 

src/cmd/compile/internal/ssa/stackalloc.go

@@ -151,7 +151,7 @@ func (s *stackAllocState) stackalloc() {
 		if v.Op != OpArg {
 			continue
 		}
-		loc := LocalSlot{v.Aux.(GCNode), v.Type, v.AuxInt}
+		loc := LocalSlot{N: v.Aux.(GCNode), Type: v.Type, Off: v.AuxInt}
 		if f.pass.debug > stackDebug {
 			fmt.Printf("stackalloc %s to %s\n", v, loc.Name())
 		}

src/cmd/compile/internal/ssa/value.go

@@ -10,6 +10,7 @@ import (
 	"cmd/internal/src"
 	"fmt"
 	"math"
+	"strings"
 )
 
 // A Value represents a value in the SSA representation of the program.
@@ -98,7 +99,7 @@ func (v *Value) AuxValAndOff() ValAndOff {
 	return ValAndOff(v.AuxInt)
 }
 
-// long form print.  v# = opcode <type> [aux] args [: reg]
+// long form print.  v# = opcode <type> [aux] args [: reg] (names)
 func (v *Value) LongString() string {
 	s := fmt.Sprintf("v%d = %s", v.ID, v.Op)
 	s += " <" + v.Type.String() + ">"
@@ -110,6 +111,18 @@ func (v *Value) LongString() string {
 	if int(v.ID) < len(r) && r[v.ID] != nil {
 		s += " : " + r[v.ID].Name()
 	}
+	var names []string
+	for name, values := range v.Block.Func.NamedValues {
+		for _, value := range values {
+			if value == v {
+				names = append(names, name.Name())
+				break // drop duplicates.
+			}
+		}
+	}
+	if len(names) != 0 {
+		s += " (" + strings.Join(names, ", ") + ")"
+	}
 	return s
 }
 

src/cmd/internal/dwarf/dwarf.go

@@ -15,6 +15,9 @@ import (
 // InfoPrefix is the prefix for all the symbols containing DWARF info entries.
 const InfoPrefix = "go.info."
 
+// RangePrefix is the prefix for all the symbols containing DWARF location lists.
+const LocPrefix = "go.loc."
+
 // RangePrefix is the prefix for all the symbols containing DWARF range lists.
 const RangePrefix = "go.range."
 
@@ -23,11 +26,29 @@ type Sym interface {
 	Len() int64
 }
 
+// A Location represents a variable's location at a particular PC range.
+// It becomes a location list entry in the DWARF.
+type Location struct {
+	StartPC, EndPC int64
+	Pieces         []Piece
+}
+
+// A Piece represents the location of a particular part of a variable.
+// It becomes part of a location list entry (a DW_OP_piece) in the DWARF.
+type Piece struct {
+	Length      int64
+	StackOffset int32
+	RegNum      int16
+	Missing     bool
+	OnStack     bool // if true, RegNum is unset.
+}
+
 // A Var represents a local variable or a function parameter.
 type Var struct {
 	Name         string
 	Abbrev       int // Either DW_ABRV_AUTO or DW_ABRV_PARAM
-	Offset int32
+	StackOffset  int32
+	LocationList []Location
 	Scope        int32
 	Type         Sym
 }
@@ -205,7 +226,7 @@ const (
 )
 
 // Index into the abbrevs table below.
-// Keep in sync with ispubname() and ispubtype() below.
+// Keep in sync with ispubname() and ispubtype() in ld/dwarf.go.
 // ispubtype considers >= NULLTYPE public
 const (
 	DW_ABRV_NULL = iota
@@ -213,7 +234,9 @@ const (
 	DW_ABRV_FUNCTION
 	DW_ABRV_VARIABLE
 	DW_ABRV_AUTO
+	DW_ABRV_AUTO_LOCLIST
 	DW_ABRV_PARAM
+	DW_ABRV_PARAM_LOCLIST
 	DW_ABRV_LEXICAL_BLOCK_RANGES
 	DW_ABRV_LEXICAL_BLOCK_SIMPLE
 	DW_ABRV_STRUCTFIELD
@@ -297,6 +320,17 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		},
 	},
 
+	/* AUTO_LOCLIST */
+	{
+		DW_TAG_variable,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_location, DW_FORM_sec_offset},
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+
 	/* PARAM */
 	{
 		DW_TAG_formal_parameter,
@@ -307,6 +341,18 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 			{DW_AT_type, DW_FORM_ref_addr},
 		},
 	},
+
+	/* PARAM_LOCLIST */
+	{
+		DW_TAG_formal_parameter,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_location, DW_FORM_sec_offset},
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+
 	/* LEXICAL_BLOCK_RANGES */
 	{
 		DW_TAG_lexical_block,
@@ -684,31 +730,30 @@ func HasChildren(die *DWDie) bool {
 
 // PutFunc writes a DIE for a function to s.
 // It also writes child DIEs for each variable in vars.
-func PutFunc(ctxt Context, s, ranges Sym, name string, external bool, startPC Sym, size int64, scopes []Scope) error {
-	Uleb128put(ctxt, s, DW_ABRV_FUNCTION)
-	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_string, DW_CLS_STRING, int64(len(name)), name)
-	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, 0, startPC)
-	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, size, startPC)
-	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_block1, DW_CLS_BLOCK, 1, []byte{DW_OP_call_frame_cfa})
+func PutFunc(ctxt Context, info, loc, ranges Sym, name string, external bool, startPC Sym, size int64, scopes []Scope) error {
+	Uleb128put(ctxt, info, DW_ABRV_FUNCTION)
+	putattr(ctxt, info, DW_ABRV_FUNCTION, DW_FORM_string, DW_CLS_STRING, int64(len(name)), name)
+	putattr(ctxt, info, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, 0, startPC)
+	putattr(ctxt, info, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, size, startPC)
+	putattr(ctxt, info, DW_ABRV_FUNCTION, DW_FORM_block1, DW_CLS_BLOCK, 1, []byte{DW_OP_call_frame_cfa})
 	var ev int64
 	if external {
 		ev = 1
 	}
-	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
+	putattr(ctxt, info, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
 	if len(scopes) > 0 {
 		var encbuf [20]byte
-		if putscope(ctxt, s, ranges, startPC, 0, scopes, encbuf[:0]) < int32(len(scopes)) {
+		if putscope(ctxt, info, loc, ranges, startPC, 0, scopes, encbuf[:0]) < int32(len(scopes)) {
 			return errors.New("multiple toplevel scopes")
 		}
 	}
-
-	Uleb128put(ctxt, s, 0)
+	Uleb128put(ctxt, info, 0)
 	return nil
 }
 
-func putscope(ctxt Context, s, ranges Sym, startPC Sym, curscope int32, scopes []Scope, encbuf []byte) int32 {
+func putscope(ctxt Context, info, loc, ranges, startPC Sym, curscope int32, scopes []Scope, encbuf []byte) int32 {
 	for _, v := range scopes[curscope].Vars {
-		putvar(ctxt, s, v, encbuf)
+		putvar(ctxt, info, loc, v, startPC, encbuf)
 	}
 	this := curscope
 	curscope++
@@ -719,12 +764,12 @@ func putscope(ctxt Context, s, ranges Sym, startPC Sym, curscope int32, scopes [
 		}
 
 		if len(scope.Ranges) == 1 {
-			Uleb128put(ctxt, s, DW_ABRV_LEXICAL_BLOCK_SIMPLE)
-			putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_SIMPLE, DW_FORM_addr, DW_CLS_ADDRESS, scope.Ranges[0].Start, startPC)
-			putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_SIMPLE, DW_FORM_addr, DW_CLS_ADDRESS, scope.Ranges[0].End, startPC)
+			Uleb128put(ctxt, info, DW_ABRV_LEXICAL_BLOCK_SIMPLE)
+			putattr(ctxt, info, DW_ABRV_LEXICAL_BLOCK_SIMPLE, DW_FORM_addr, DW_CLS_ADDRESS, scope.Ranges[0].Start, startPC)
+			putattr(ctxt, info, DW_ABRV_LEXICAL_BLOCK_SIMPLE, DW_FORM_addr, DW_CLS_ADDRESS, scope.Ranges[0].End, startPC)
 		} else {
-			Uleb128put(ctxt, s, DW_ABRV_LEXICAL_BLOCK_RANGES)
-			putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_RANGES, DW_FORM_sec_offset, DW_CLS_PTR, ranges.Len(), ranges)
+			Uleb128put(ctxt, info, DW_ABRV_LEXICAL_BLOCK_RANGES)
+			putattr(ctxt, info, DW_ABRV_LEXICAL_BLOCK_RANGES, DW_FORM_sec_offset, DW_CLS_PTR, ranges.Len(), ranges)
 
 			ctxt.AddAddress(ranges, nil, -1)
 			ctxt.AddAddress(ranges, startPC, 0)
@@ -736,32 +781,72 @@ func putscope(ctxt Context, s, ranges Sym, startPC Sym, curscope int32, scopes [
 			ctxt.AddAddress(ranges, nil, 0)
 		}
 
-		curscope = putscope(ctxt, s, ranges, startPC, curscope, scopes, encbuf)
+		curscope = putscope(ctxt, info, loc, ranges, startPC, curscope, scopes, encbuf)
 
-		Uleb128put(ctxt, s, 0)
+		Uleb128put(ctxt, info, 0)
 	}
 	return curscope
 }
 
-func putvar(ctxt Context, s Sym, v *Var, encbuf []byte) {
+func putvar(ctxt Context, info, loc Sym, v *Var, startPC Sym, encbuf []byte) {
 	n := v.Name
 
-	Uleb128put(ctxt, s, int64(v.Abbrev))
-	putattr(ctxt, s, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
+	Uleb128put(ctxt, info, int64(v.Abbrev))
+	putattr(ctxt, info, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
+	if v.Abbrev == DW_ABRV_AUTO_LOCLIST || v.Abbrev == DW_ABRV_PARAM_LOCLIST {
+		putattr(ctxt, info, v.Abbrev, DW_FORM_sec_offset, DW_CLS_PTR, int64(loc.Len()), loc)
+		addLocList(ctxt, loc, startPC, v, encbuf)
+	} else {
 		loc := append(encbuf[:0], DW_OP_call_frame_cfa)
-	if v.Offset != 0 {
+		if v.StackOffset != 0 {
 			loc = append(loc, DW_OP_consts)
-		loc = AppendSleb128(loc, int64(v.Offset))
+			loc = AppendSleb128(loc, int64(v.StackOffset))
 			loc = append(loc, DW_OP_plus)
 		}
-	putattr(ctxt, s, v.Abbrev, DW_FORM_block1, DW_CLS_BLOCK, int64(len(loc)), loc)
-	putattr(ctxt, s, v.Abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
+		putattr(ctxt, info, v.Abbrev, DW_FORM_block1, DW_CLS_BLOCK, int64(len(loc)), loc)
+	}
+	putattr(ctxt, info, v.Abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
+}
+
+func addLocList(ctxt Context, listSym, startPC Sym, v *Var, encbuf []byte) {
+	// Base address entry: max ptr followed by the base address.
+	ctxt.AddInt(listSym, ctxt.PtrSize(), ^0)
+	ctxt.AddAddress(listSym, startPC, 0)
+	for _, entry := range v.LocationList {
+		ctxt.AddInt(listSym, ctxt.PtrSize(), entry.StartPC)
+		ctxt.AddInt(listSym, ctxt.PtrSize(), entry.EndPC)
+		locBuf := encbuf[:0]
+		for _, piece := range entry.Pieces {
+			if !piece.Missing {
+				if piece.OnStack {
+					locBuf = append(locBuf, DW_OP_fbreg)
+					locBuf = AppendSleb128(locBuf, int64(piece.StackOffset))
+				} else {
+					if piece.RegNum < 32 {
+						locBuf = append(locBuf, DW_OP_reg0+byte(piece.RegNum))
+					} else {
+						locBuf = append(locBuf, DW_OP_regx)
+						locBuf = AppendUleb128(locBuf, uint64(piece.RegNum))
+					}
+				}
+			}
+			if len(entry.Pieces) > 1 {
+				locBuf = append(locBuf, DW_OP_piece)
+				locBuf = AppendUleb128(locBuf, uint64(piece.Length))
+			}
+		}
+		ctxt.AddInt(listSym, 2, int64(len(locBuf)))
+		ctxt.AddBytes(listSym, locBuf)
+	}
+	// End list
+	ctxt.AddInt(listSym, ctxt.PtrSize(), 0)
+	ctxt.AddInt(listSym, ctxt.PtrSize(), 0)
 }
 
 // VarsByOffset attaches the methods of sort.Interface to []*Var,
-// sorting in increasing Offset.
+// sorting in increasing StackOffset.
 type VarsByOffset []*Var
 
 func (s VarsByOffset) Len() int           { return len(s) }
-func (s VarsByOffset) Less(i, j int) bool { return s[i].Offset < s[j].Offset }
+func (s VarsByOffset) Less(i, j int) bool { return s[i].StackOffset < s[j].StackOffset }
 func (s VarsByOffset) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }

src/cmd/internal/obj/link.go

@@ -330,7 +330,8 @@ type FuncInfo struct {
 	Autom  []*Auto
 	Pcln   Pcln
 
-	dwarfSym       *LSym
+	dwarfInfoSym   *LSym
+	dwarfLocSym    *LSym
 	dwarfRangesSym *LSym
 
 	GCArgs   LSym
@@ -484,6 +485,7 @@ type Link struct {
 	Flag_shared        bool
 	Flag_dynlink       bool
 	Flag_optimize      bool
+	Flag_locationlists bool
 	Bso                *bufio.Writer
 	Pathname           string
 	hashmu             sync.Mutex       // protects hash
@@ -538,4 +540,5 @@ type LinkArch struct {
 	Assemble       func(*Link, *LSym, ProgAlloc)
 	Progedit       func(*Link, *Prog, ProgAlloc)
 	UnaryDst       map[As]bool // Instruction takes one operand, a destination.
+	DWARFRegisters map[int16]int16
 }

src/cmd/internal/obj/objfile.go

@@ -465,15 +465,18 @@ func (c dwCtxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64
 }
 
 // dwarfSym returns the DWARF symbols for TEXT symbol.
-func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfRangesSym *LSym) {
+func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym *LSym) {
 	if s.Type != objabi.STEXT {
 		ctxt.Diag("dwarfSym of non-TEXT %v", s)
 	}
-	if s.Func.dwarfSym == nil {
-		s.Func.dwarfSym = ctxt.LookupDerived(s, dwarf.InfoPrefix+s.Name)
+	if s.Func.dwarfInfoSym == nil {
+		s.Func.dwarfInfoSym = ctxt.LookupDerived(s, dwarf.InfoPrefix+s.Name)
+		if ctxt.Flag_locationlists {
+			s.Func.dwarfLocSym = ctxt.LookupDerived(s, dwarf.LocPrefix+s.Name)
+		}
 		s.Func.dwarfRangesSym = ctxt.LookupDerived(s, dwarf.RangePrefix+s.Name)
 	}
-	return s.Func.dwarfSym, s.Func.dwarfRangesSym
+	return s.Func.dwarfInfoSym, s.Func.dwarfLocSym, s.Func.dwarfRangesSym
 }
 
 func (s *LSym) Len() int64 {
@@ -483,15 +486,15 @@ func (s *LSym) Len() int64 {
 // populateDWARF fills in the DWARF Debugging Information Entries for TEXT symbol s.
 // The DWARFs symbol must already have been initialized in InitTextSym.
 func (ctxt *Link) populateDWARF(curfn interface{}, s *LSym) {
-	dsym, drsym := ctxt.dwarfSym(s)
-	if dsym.Size != 0 {
+	info, loc, ranges := ctxt.dwarfSym(s)
+	if info.Size != 0 {
 		ctxt.Diag("makeFuncDebugEntry double process %v", s)
 	}
 	var scopes []dwarf.Scope
 	if ctxt.DebugInfo != nil {
 		scopes = ctxt.DebugInfo(s, curfn)
 	}
-	err := dwarf.PutFunc(dwCtxt{ctxt}, dsym, drsym, s.Name, !s.Static(), s, s.Size, scopes)
+	err := dwarf.PutFunc(dwCtxt{ctxt}, info, loc, ranges, s.Name, !s.Static(), s, s.Size, scopes)
 	if err != nil {
 		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
 	}

src/cmd/internal/obj/plist.go

@@ -136,13 +136,17 @@ func (ctxt *Link) InitTextSym(s *LSym, flag int) {
 	ctxt.Text = append(ctxt.Text, s)
 
 	// Set up DWARF entries for s.
-	dsym, drsym := ctxt.dwarfSym(s)
-	dsym.Type = objabi.SDWARFINFO
-	dsym.Set(AttrDuplicateOK, s.DuplicateOK())
-	drsym.Type = objabi.SDWARFRANGE
-	drsym.Set(AttrDuplicateOK, s.DuplicateOK())
-	ctxt.Data = append(ctxt.Data, dsym)
-	ctxt.Data = append(ctxt.Data, drsym)
+	info, loc, ranges := ctxt.dwarfSym(s)
+	info.Type = objabi.SDWARFINFO
+	info.Set(AttrDuplicateOK, s.DuplicateOK())
+	if loc != nil {
+		loc.Type = objabi.SDWARFLOC
+		loc.Set(AttrDuplicateOK, s.DuplicateOK())
+		ctxt.Data = append(ctxt.Data, loc)
+	}
+	ranges.Type = objabi.SDWARFRANGE
+	ranges.Set(AttrDuplicateOK, s.DuplicateOK())
+	ctxt.Data = append(ctxt.Data, info, ranges)
 
 	// Set up the function's gcargs and gclocals.
 	// They will be filled in later if needed.

src/cmd/internal/obj/x86/a.out.go

@@ -1006,3 +1006,120 @@ const (
 	T_64     = 1 << 6
 	T_GOTYPE = 1 << 7
 )
+
+// https://www.uclibc.org/docs/psABI-x86_64.pdf, figure 3.36
+var AMD64DWARFRegisters = map[int16]int16{
+	REG_AX:  0,
+	REG_DX:  1,
+	REG_CX:  2,
+	REG_BX:  3,
+	REG_SI:  4,
+	REG_DI:  5,
+	REG_BP:  6,
+	REG_SP:  7,
+	REG_R8:  8,
+	REG_R9:  9,
+	REG_R10: 10,
+	REG_R11: 11,
+	REG_R12: 12,
+	REG_R13: 13,
+	REG_R14: 14,
+	REG_R15: 15,
+	// 16 is "Return Address RA", whatever that is.
+	// XMM registers. %xmmN => XN.
+	REG_X0:  17,
+	REG_X1:  18,
+	REG_X2:  19,
+	REG_X3:  20,
+	REG_X4:  21,
+	REG_X5:  22,
+	REG_X6:  23,
+	REG_X7:  24,
+	REG_X8:  25,
+	REG_X9:  26,
+	REG_X10: 27,
+	REG_X11: 28,
+	REG_X12: 29,
+	REG_X13: 30,
+	REG_X14: 31,
+	REG_X15: 32,
+	// ST registers. %stN => FN.
+	REG_F0: 33,
+	REG_F1: 34,
+	REG_F2: 35,
+	REG_F3: 36,
+	REG_F4: 37,
+	REG_F5: 38,
+	REG_F6: 39,
+	REG_F7: 40,
+	// MMX registers. %mmN => MN.
+	REG_M0: 41,
+	REG_M1: 42,
+	REG_M2: 43,
+	REG_M3: 44,
+	REG_M4: 45,
+	REG_M5: 46,
+	REG_M6: 47,
+	REG_M7: 48,
+	// 48 is flags, which doesn't have a name.
+	REG_ES: 50,
+	REG_CS: 51,
+	REG_SS: 52,
+	REG_DS: 53,
+	REG_FS: 54,
+	REG_GS: 55,
+	// 58 and 59 are {fs,gs}base, which don't have names.
+	REG_TR:   62,
+	REG_LDTR: 63,
+	// 64-66 are mxcsr, fcw, fsw, which don't have names.
+}
+
+// https://www.uclibc.org/docs/psABI-i386.pdf, table 2.14
+var X86DWARFRegisters = map[int16]int16{
+	REG_AX: 0,
+	REG_CX: 1,
+	REG_DX: 2,
+	REG_BX: 3,
+	REG_SP: 4,
+	REG_BP: 5,
+	REG_SI: 6,
+	REG_DI: 7,
+	// 8 is "Return Address RA", whatever that is.
+	// 9 is flags, which doesn't have a name.
+	// ST registers. %stN => FN.
+	REG_F0: 11,
+	REG_F1: 12,
+	REG_F2: 13,
+	REG_F3: 14,
+	REG_F4: 15,
+	REG_F5: 16,
+	REG_F6: 17,
+	REG_F7: 18,
+	// XMM registers. %xmmN => XN.
+	REG_X0: 21,
+	REG_X1: 22,
+	REG_X2: 23,
+	REG_X3: 24,
+	REG_X4: 25,
+	REG_X5: 26,
+	REG_X6: 27,
+	REG_X7: 28,
+	// MMX registers. %mmN => MN.
+	REG_M0: 29,
+	REG_M1: 30,
+	REG_M2: 31,
+	REG_M3: 32,
+	REG_M4: 33,
+	REG_M5: 34,
+	REG_M6: 35,
+	REG_M7: 36,
+	// 39 is mxcsr, which doesn't have a name.
+	REG_ES:   40,
+	REG_CS:   41,
+	REG_SS:   42,
+	REG_DS:   43,
+	REG_FS:   44,
+	REG_GS:   45,
+	REG_TR:   48,
+	REG_LDTR: 49,
+}

src/cmd/internal/obj/x86/obj6.go

@@ -1237,6 +1237,7 @@ var Linkamd64 = obj.LinkArch{
 	Assemble:       span6,
 	Progedit:       progedit,
 	UnaryDst:       unaryDst,
+	DWARFRegisters: AMD64DWARFRegisters,
 }
 
 var Linkamd64p32 = obj.LinkArch{
@@ -1246,6 +1247,7 @@ var Linkamd64p32 = obj.LinkArch{
 	Assemble:       span6,
 	Progedit:       progedit,
 	UnaryDst:       unaryDst,
+	DWARFRegisters: AMD64DWARFRegisters,
 }
 
 var Link386 = obj.LinkArch{
@@ -1255,4 +1257,5 @@ var Link386 = obj.LinkArch{
 	Assemble:       span6,
 	Progedit:       progedit,
 	UnaryDst:       unaryDst,
+	DWARFRegisters: AMD64DWARFRegisters,
 }

src/cmd/internal/objabi/symkind.go

@@ -57,4 +57,5 @@ const (
 	// Debugging data
 	SDWARFINFO
 	SDWARFRANGE
+	SDWARFLOC
 )

src/cmd/internal/objabi/symkind_string.go

@@ -4,9 +4,9 @@ package objabi
 
 import "fmt"
 
-const _SymKind_name = "SxxxSTEXTSRODATASNOPTRDATASDATASBSSSNOPTRBSSSTLSBSSSDWARFINFOSDWARFRANGE"
+const _SymKind_name = "SxxxSTEXTSRODATASNOPTRDATASDATASBSSSNOPTRBSSSTLSBSSSDWARFINFOSDWARFRANGESDWARFLOC"
 
-var _SymKind_index = [...]uint8{0, 4, 9, 16, 26, 31, 35, 44, 51, 61, 72}
+var _SymKind_index = [...]uint8{0, 4, 9, 16, 26, 31, 35, 44, 51, 61, 72, 81}
 
 func (i SymKind) String() string {
 	if i >= SymKind(len(_SymKind_index)-1) {

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

@@ -592,15 +592,7 @@ func relocsym(ctxt *Link, s *Symbol) {
 			}
 
 		case objabi.R_DWARFREF:
-			var sectName string
-			var vaddr int64
-			switch {
-			case r.Sym.Sect != nil:
-				sectName = r.Sym.Sect.Name
-				vaddr = int64(r.Sym.Sect.Vaddr)
-			case r.Sym.Type == SDWARFRANGE:
-				sectName = ".debug_ranges"
-			default:
+			if r.Sym.Sect == nil {
 				Errorf(s, "missing DWARF section for relocation target %s", r.Sym.Name)
 			}
 
@@ -615,8 +607,8 @@ func relocsym(ctxt *Link, s *Symbol) {
 					r.Type = objabi.R_ADDR
 				}
 
-				r.Xsym = ctxt.Syms.ROLookup(sectName, 0)
-				r.Xadd = r.Add + Symaddr(r.Sym) - vaddr
+				r.Xsym = ctxt.Syms.ROLookup(r.Sym.Sect.Name, 0)
+				r.Xadd = r.Add + Symaddr(r.Sym) - int64(r.Sym.Sect.Vaddr)
 
 				o = r.Xadd
 				rs = r.Xsym
@@ -625,7 +617,7 @@ func relocsym(ctxt *Link, s *Symbol) {
 				}
 				break
 			}
-			o = Symaddr(r.Sym) + r.Add - vaddr
+			o = Symaddr(r.Sym) + r.Add - int64(r.Sym.Sect.Vaddr)
 
 		case objabi.R_WEAKADDROFF:
 			if !r.Sym.Attr.Reachable() {
@@ -1843,9 +1835,9 @@ func (ctxt *Link) dodata() {
 
 	dwarfgeneratedebugsyms(ctxt)
 
-	var s *Symbol
 	var i int
-	for i, s = range dwarfp {
+	for ; i < len(dwarfp); i++ {
+		s := dwarfp[i]
 		if s.Type != SDWARFSECT {
 			break
 		}
@@ -1862,13 +1854,26 @@ func (ctxt *Link) dodata() {
 	}
 	checkdatsize(ctxt, datsize, SDWARFSECT)
 
-	if i < len(dwarfp) {
+	for i < len(dwarfp) {
+		curType := dwarfp[i].Type
+		var sect *Section
+		switch curType {
+		case SDWARFINFO:
 			sect = addsection(&Segdwarf, ".debug_info", 04)
+		case SDWARFRANGE:
+			sect = addsection(&Segdwarf, ".debug_ranges", 04)
+		case SDWARFLOC:
+			sect = addsection(&Segdwarf, ".debug_loc", 04)
+		default:
+			Errorf(dwarfp[i], "unknown DWARF section %v", curType)
+		}
+
 		sect.Align = 1
 		datsize = Rnd(datsize, int64(sect.Align))
 		sect.Vaddr = uint64(datsize)
-		for _, s := range dwarfp[i:] {
-			if s.Type != SDWARFINFO {
+		for ; i < len(dwarfp); i++ {
+			s := dwarfp[i]
+			if s.Type != curType {
 				break
 			}
 			s.Sect = sect
@@ -1878,7 +1883,7 @@ func (ctxt *Link) dodata() {
 			datsize += s.Size
 		}
 		sect.Length = uint64(datsize) - sect.Vaddr
-		checkdatsize(ctxt, datsize, SDWARFINFO)
+		checkdatsize(ctxt, datsize, curType)
 	}
 
 	/* number the sections */

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

@@ -67,26 +67,15 @@ func (c dwctxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64
 	r.Add = ofs
 }
 
-/*
- * Offsets and sizes of the debug_* sections in the cout file.
- */
-var abbrevsym *Symbol
-var arangessec *Symbol
-var framesec *Symbol
-var infosec *Symbol
-var linesec *Symbol
-var rangesec *Symbol
-
 var gdbscript string
 
 var dwarfp []*Symbol
 
-func writeabbrev(ctxt *Link, syms []*Symbol) []*Symbol {
+func writeabbrev(ctxt *Link) *Symbol {
 	s := ctxt.Syms.Lookup(".debug_abbrev", 0)
 	s.Type = SDWARFSECT
-	abbrevsym = s
 	Addbytes(s, dwarf.GetAbbrev())
-	return append(syms, s)
+	return s
 }
 
 /*
@@ -993,13 +982,10 @@ func getCompilationDir() string {
 
 func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
 	var dwarfctxt dwarf.Context = dwctxt{ctxt}
-	if linesec == nil {
-		linesec = ctxt.Syms.Lookup(".debug_line", 0)
-	}
-	linesec.Type = SDWARFSECT
-	linesec.R = linesec.R[:0]
+	ls := ctxt.Syms.Lookup(".debug_line", 0)
+	ls.Type = SDWARFSECT
+	ls.R = ls.R[:0]
 
-	ls := linesec
 	syms = append(syms, ls)
 	var funcs []*Symbol
 
@@ -1019,7 +1005,7 @@ func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
 
 	dwinfo = newdie(ctxt, &dwroot, dwarf.DW_ABRV_COMPUNIT, "go", 0)
 	newattr(dwinfo, dwarf.DW_AT_language, dwarf.DW_CLS_CONSTANT, int64(lang), 0)
-	newattr(dwinfo, dwarf.DW_AT_stmt_list, dwarf.DW_CLS_PTR, 0, linesec)
+	newattr(dwinfo, dwarf.DW_AT_stmt_list, dwarf.DW_CLS_PTR, 0, ls)
 	newattr(dwinfo, dwarf.DW_AT_low_pc, dwarf.DW_CLS_ADDRESS, s.Value, s)
 	// OS X linker requires compilation dir or absolute path in comp unit name to output debug info.
 	compDir := getCompilationDir()
@@ -1178,12 +1164,9 @@ func appendPCDeltaCFA(b []byte, deltapc, cfa int64) []byte {
 
 func writeframes(ctxt *Link, syms []*Symbol) []*Symbol {
 	var dwarfctxt dwarf.Context = dwctxt{ctxt}
-	if framesec == nil {
-		framesec = ctxt.Syms.Lookup(".debug_frame", 0)
-	}
-	framesec.Type = SDWARFSECT
-	framesec.R = framesec.R[:0]
-	fs := framesec
+	fs := ctxt.Syms.Lookup(".debug_frame", 0)
+	fs.Type = SDWARFSECT
+	fs.R = fs.R[:0]
 	syms = append(syms, fs)
 
 	// Emit the CIE, Section 6.4.1
@@ -1280,7 +1263,7 @@ func writeframes(ctxt *Link, syms []*Symbol) []*Symbol {
 		//	ptrsize: address range
 		Adduint32(ctxt, fs, uint32(4+2*SysArch.PtrSize+len(deltaBuf))) // length (excludes itself)
 		if Linkmode == LinkExternal {
-			adddwarfref(ctxt, fs, framesec, 4)
+			adddwarfref(ctxt, fs, fs, 4)
 		} else {
 			Adduint32(ctxt, fs, 0) // CIE offset
 		}
@@ -1292,27 +1275,24 @@ func writeframes(ctxt *Link, syms []*Symbol) []*Symbol {
 }
 
 func writeranges(ctxt *Link, syms []*Symbol) []*Symbol {
-	if rangesec == nil {
-		rangesec = ctxt.Syms.Lookup(".debug_ranges", 0)
-	}
-	rangesec.Type = SDWARFSECT
-	rangesec.Attr |= AttrReachable
-	rangesec.R = rangesec.R[:0]
-
+	empty := true
 	for _, s := range ctxt.Textp {
 		rangeSym := ctxt.Syms.Lookup(dwarf.RangePrefix+s.Name, int(s.Version))
-		rangeSym.Attr |= AttrReachable
-		rangeSym.Type = SDWARFRANGE
-		rangeSym.Value = rangesec.Size
-		rangesec.P = append(rangesec.P, rangeSym.P...)
-		for _, r := range rangeSym.R {
-			r.Off += int32(rangesec.Size)
-			rangesec.R = append(rangesec.R, r)
+		if rangeSym.Size == 0 {
+			continue
 		}
-		rangesec.Size += rangeSym.Size
+		rangeSym.Attr |= AttrReachable | AttrNotInSymbolTable
+		rangeSym.Type = SDWARFRANGE
+		syms = append(syms, rangeSym)
+		empty = false
 	}
-	if rangesec.Size > 0 {
+	if !empty {
 		// PE does not like empty sections
+		rangesec := ctxt.Syms.Lookup(".debug_ranges", 0)
+		rangesec.Type = SDWARFRANGE
+		rangesec.Attr |= AttrReachable
+		rangesec.R = rangesec.R[:0]
+
 		syms = append(syms, rangesec)
 	}
 	return syms
@@ -1325,18 +1305,14 @@ const (
 	COMPUNITHEADERSIZE = 4 + 2 + 4 + 1
 )
 
-func writeinfo(ctxt *Link, syms []*Symbol, funcs []*Symbol) []*Symbol {
-	if infosec == nil {
-		infosec = ctxt.Syms.Lookup(".debug_info", 0)
-	}
+func writeinfo(ctxt *Link, syms []*Symbol, funcs []*Symbol, abbrevsym *Symbol) []*Symbol {
+	infosec := ctxt.Syms.Lookup(".debug_info", 0)
 	infosec.R = infosec.R[:0]
 	infosec.Type = SDWARFINFO
 	infosec.Attr |= AttrReachable
 	syms = append(syms, infosec)
 
-	if arangessec == nil {
-		arangessec = ctxt.Syms.Lookup(".dwarfaranges", 0)
-	}
+	arangessec := ctxt.Syms.Lookup(".dwarfaranges", 0)
 	arangessec.R = arangessec.R[:0]
 
 	var dwarfctxt dwarf.Context = dwctxt{ctxt}
@@ -1577,10 +1553,10 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 
 	genasmsym(ctxt, defdwsymb)
 
-	syms := writeabbrev(ctxt, nil)
+	abbrev := writeabbrev(ctxt)
+	syms := []*Symbol{abbrev}
 	syms, funcs := writelines(ctxt, syms)
 	syms = writeframes(ctxt, syms)
-	syms = writeranges(ctxt, syms)
 
 	synthesizestringtypes(ctxt, dwtypes.Child)
 	synthesizeslicetypes(ctxt, dwtypes.Child)
@@ -1596,16 +1572,42 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 
 	// Need to reorder symbols so SDWARFINFO is after all SDWARFSECT
 	// (but we need to generate dies before writepub)
-	infosyms := writeinfo(ctxt, nil, funcs)
+	infosyms := writeinfo(ctxt, nil, funcs, abbrev)
 
 	syms = writepub(ctxt, ".debug_pubnames", ispubname, syms)
 	syms = writepub(ctxt, ".debug_pubtypes", ispubtype, syms)
 	syms = writearanges(ctxt, syms)
 	syms = writegdbscript(ctxt, syms)
 	syms = append(syms, infosyms...)
+	syms = collectlocs(ctxt, syms, funcs)
+	syms = writeranges(ctxt, syms)
 	dwarfp = syms
 }
 
+func collectlocs(ctxt *Link, syms []*Symbol, funcs []*Symbol) []*Symbol {
+	empty := true
+	for _, fn := range funcs {
+		for _, reloc := range fn.R {
+			if reloc.Type == objabi.R_DWARFREF && strings.HasPrefix(reloc.Sym.Name, dwarf.LocPrefix) {
+				reloc.Sym.Attr |= AttrReachable | AttrNotInSymbolTable
+				syms = append(syms, reloc.Sym)
+				empty = false
+				// One location list entry per function, but many relocations to it. Don't duplicate.
+				break
+			}
+		}
+	}
+	// Don't emit .debug_loc if it's empty -- it makes the ARM linker mad.
+	if !empty {
+		locsym := ctxt.Syms.Lookup(".debug_loc", 0)
+		locsym.R = locsym.R[:0]
+		locsym.Type = SDWARFLOC
+		locsym.Attr |= AttrReachable
+		syms = append(syms, locsym)
+	}
+	return syms
+}
+
 /*
  *  Elf.
  */
@@ -1618,6 +1620,7 @@ func dwarfaddshstrings(ctxt *Link, shstrtab *Symbol) {
 	Addstring(shstrtab, ".debug_aranges")
 	Addstring(shstrtab, ".debug_frame")
 	Addstring(shstrtab, ".debug_info")
+	Addstring(shstrtab, ".debug_loc")
 	Addstring(shstrtab, ".debug_line")
 	Addstring(shstrtab, ".debug_pubnames")
 	Addstring(shstrtab, ".debug_pubtypes")
@@ -1625,6 +1628,7 @@ func dwarfaddshstrings(ctxt *Link, shstrtab *Symbol) {
 	Addstring(shstrtab, ".debug_ranges")
 	if Linkmode == LinkExternal {
 		Addstring(shstrtab, elfRelType+".debug_info")
+		Addstring(shstrtab, elfRelType+".debug_loc")
 		Addstring(shstrtab, elfRelType+".debug_aranges")
 		Addstring(shstrtab, elfRelType+".debug_line")
 		Addstring(shstrtab, elfRelType+".debug_frame")
@@ -1651,6 +1655,10 @@ func dwarfaddelfsectionsyms(ctxt *Link) {
 	putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
 	sym = ctxt.Syms.Lookup(".debug_frame", 0)
 	putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
+	sym = ctxt.Syms.Lookup(".debug_loc", 0)
+	if sym.Sect != nil {
+		putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
+	}
 	sym = ctxt.Syms.Lookup(".debug_ranges", 0)
 	if sym.Sect != nil {
 		putelfsectionsym(sym, sym.Sect.Elfsect.shnum)

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

@@ -1808,7 +1808,7 @@ func elfrelocsect(ctxt *Link, sect *Section, syms []*Symbol) {
 				continue
 			}
 			if r.Xsym == nil {
-				Errorf(sym, "missing xsym in relocation")
+				Errorf(sym, "missing xsym in relocation %#v %#v", r.Sym.Name, sym)
 				continue
 			}
 			if r.Xsym.ElfsymForReloc() == 0 {
@@ -2596,12 +2596,9 @@ elfobj:
 			elfshreloc(sect)
 		}
 		for _, s := range dwarfp {
-			if len(s.R) > 0 || s.Type == SDWARFINFO {
+			if len(s.R) > 0 || s.Type == SDWARFINFO || s.Type == SDWARFLOC {
 				elfshreloc(s.Sect)
 			}
-			if s.Type == SDWARFINFO {
-				break
-			}
 		}
 		// add a .note.GNU-stack section to mark the stack as non-executable
 		sh := elfshname(".note.GNU-stack")

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

@@ -105,6 +105,7 @@ const (
 	SDWARFSECT
 	SDWARFINFO
 	SDWARFRANGE
+	SDWARFLOC
 	SSUB       = SymKind(1 << 8)
 	SMASK      = SymKind(SSUB - 1)
 	SHIDDEN    = SymKind(1 << 9)
@@ -124,6 +125,7 @@ var abiSymKindToSymKind = [...]SymKind{
 	STLSBSS,
 	SDWARFINFO,
 	SDWARFRANGE,
+	SDWARFLOC,
 }
 
 // readOnly are the symbol kinds that form read-only sections. In some

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

@@ -4,9 +4,9 @@ package ld
 
 import "fmt"
 
-const _SymKind_name = "SxxxSTEXTSELFRXSECTSTYPESSTRINGSGOSTRINGSGOFUNCSGCBITSSRODATASFUNCTABSELFROSECTSMACHOPLTSTYPERELROSSTRINGRELROSGOSTRINGRELROSGOFUNCRELROSGCBITSRELROSRODATARELROSFUNCTABRELROSTYPELINKSITABLINKSSYMTABSPCLNTABSELFSECTSMACHOSMACHOGOTSWINDOWSSELFGOTSNOPTRDATASINITARRSDATASBSSSNOPTRBSSSTLSBSSSXREFSMACHOSYMSTRSMACHOSYMTABSMACHOINDIRECTPLTSMACHOINDIRECTGOTSFILESFILEPATHSCONSTSDYNIMPORTSHOSTOBJSDWARFSECTSDWARFINFOSDWARFRANGE"
+const _SymKind_name = "SxxxSTEXTSELFRXSECTSTYPESSTRINGSGOSTRINGSGOFUNCSGCBITSSRODATASFUNCTABSELFROSECTSMACHOPLTSTYPERELROSSTRINGRELROSGOSTRINGRELROSGOFUNCRELROSGCBITSRELROSRODATARELROSFUNCTABRELROSTYPELINKSITABLINKSSYMTABSPCLNTABSELFSECTSMACHOSMACHOGOTSWINDOWSSELFGOTSNOPTRDATASINITARRSDATASBSSSNOPTRBSSSTLSBSSSXREFSMACHOSYMSTRSMACHOSYMTABSMACHOINDIRECTPLTSMACHOINDIRECTGOTSFILESFILEPATHSCONSTSDYNIMPORTSHOSTOBJSDWARFSECTSDWARFINFOSDWARFRANGESDWARFLOC"
 
-var _SymKind_index = [...]uint16{0, 4, 9, 19, 24, 31, 40, 47, 54, 61, 69, 79, 88, 98, 110, 124, 136, 148, 160, 173, 182, 191, 198, 206, 214, 220, 229, 237, 244, 254, 262, 267, 271, 280, 287, 292, 304, 316, 333, 350, 355, 364, 370, 380, 388, 398, 408, 419}
+var _SymKind_index = [...]uint16{0, 4, 9, 19, 24, 31, 40, 47, 54, 61, 69, 79, 88, 98, 110, 124, 136, 148, 160, 173, 182, 191, 198, 206, 214, 220, 229, 237, 244, 254, 262, 267, 271, 280, 287, 292, 304, 316, 333, 350, 355, 364, 370, 380, 388, 398, 408, 419, 428}
 
 func (i SymKind) String() string {
 	if i < 0 || i >= SymKind(len(_SymKind_index)-1) {