cmd/compile: output DWARF lexical blocks for local variables

Change compiler and linker to emit DWARF lexical blocks in debug_info.
Version of debug_info is updated from DWARF v.2 to DWARF v.3 since version 2
does not allow lexical blocks with discontinuous ranges.

Second attempt at https://go-review.googlesource.com/#/c/29591/

Remaining open problems:
- scope information is removed from inlined functions
- variables in debug_info do not have DW_AT_start_scope attributes so a
variable will shadow other variables with the same name as soon as its
containing scope begins, before its declaration.

Updates golang/go#12899, golang/go#6913

Change-Id: I0e260a45b564d14a87b88974eb16c5387cb410a5
Reviewed-on: https://go-review.googlesource.com/36879
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

21 files changed, 888 insertions, 108 deletions

diff --git a/src/cmd/compile/internal/gc/alg.go b/src/cmd/compile/internal/gc/alg.go
index 8ee158c5f7..f094e8e934 100644
--- a/src/cmd/compile/internal/gc/alg.go
+++ b/src/cmd/compile/internal/gc/alg.go
@@ -6,6 +6,7 @@ package gc
 
 import (
 	"cmd/compile/internal/types"
+	"cmd/internal/src"
 	"fmt"
 )
 
@@ -191,7 +192,7 @@ func genhash(sym *types.Sym, t *types.Type) {
 
 	lineno = autogeneratedPos // less confusing than end of input
 	dclcontext = PEXTERN
-	markdcl()
+	markdcl(src.NoPos)
 
 	// func sym(p *T, h uintptr) uintptr
 	fn := nod(ODCLFUNC, nil, nil)
@@ -210,7 +211,7 @@ func genhash(sym *types.Sym, t *types.Type) {
 	n = anonfield(types.Types[TUINTPTR]) // return value
 	tfn.Rlist.Append(n)
 
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 	fn.Func.Nname.Name.Param.Ntype = typecheck(fn.Func.Nname.Name.Param.Ntype, Etype)
 
 	// genhash is only called for types that have equality but
@@ -299,13 +300,13 @@ func genhash(sym *types.Sym, t *types.Type) {
 		dumplist("genhash body", fn.Nbody)
 	}
 
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 	Curfn = fn
 	fn.Func.SetDupok(true)
 	fn = typecheck(fn, Etop)
 	typecheckslice(fn.Nbody.Slice(), Etop)
+	popdcl(src.NoPos)
 	Curfn = nil
-	popdcl()
 	if debug_dclstack != 0 {
 		testdclstack()
 	}
@@ -369,7 +370,7 @@ func geneq(sym *types.Sym, t *types.Type) {
 
 	lineno = autogeneratedPos // less confusing than end of input
 	dclcontext = PEXTERN
-	markdcl()
+	markdcl(src.NoPos)
 
 	// func sym(p, q *T) bool
 	fn := nod(ODCLFUNC, nil, nil)
@@ -388,7 +389,7 @@ func geneq(sym *types.Sym, t *types.Type) {
 	n = anonfield(types.Types[TBOOL])
 	tfn.Rlist.Append(n)
 
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 	fn.Func.Nname.Name.Param.Ntype = typecheck(fn.Func.Nname.Name.Param.Ntype, Etype)
 
 	// geneq is only called for types that have equality but
@@ -491,13 +492,13 @@ func geneq(sym *types.Sym, t *types.Type) {
 		dumplist("geneq body", fn.Nbody)
 	}
 
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 	Curfn = fn
 	fn.Func.SetDupok(true)
 	fn = typecheck(fn, Etop)
 	typecheckslice(fn.Nbody.Slice(), Etop)
+	popdcl(src.NoPos)
 	Curfn = nil
-	popdcl()
 	if debug_dclstack != 0 {
 		testdclstack()
 	}
diff --git a/src/cmd/compile/internal/gc/bimport.go b/src/cmd/compile/internal/gc/bimport.go
index 64f410fa80..5a33aec7b5 100644
--- a/src/cmd/compile/internal/gc/bimport.go
+++ b/src/cmd/compile/internal/gc/bimport.go
@@ -187,7 +187,7 @@ func Import(imp *types.Pkg, in *bufio.Reader) {
 
 		if f := p.funcList[i]; f != nil {
 			// function not yet imported - read body and set it
-			funchdr(f)
+			funchdr(f, src.NoPos)
 			body := p.stmtList()
 			if body == nil {
 				// Make sure empty body is not interpreted as
@@ -198,7 +198,7 @@ func Import(imp *types.Pkg, in *bufio.Reader) {
 				body = []*Node{nod(OEMPTY, nil, nil)}
 			}
 			f.Func.Inl.Set(body)
-			funcbody(f)
+			funcbody(f, src.NoPos)
 		} else {
 			// function already imported - read body but discard declarations
 			dclcontext = PDISCARD // throw away any declarations
@@ -1091,54 +1091,54 @@ func (p *importer) node() *Node {
 		return nodl(p.pos(), op, p.expr(), nil)
 
 	case OIF:
-		markdcl()
+		markdcl(src.NoPos)
 		n := nodl(p.pos(), OIF, nil, nil)
 		n.Ninit.Set(p.stmtList())
 		n.Left = p.expr()
 		n.Nbody.Set(p.stmtList())
 		n.Rlist.Set(p.stmtList())
-		popdcl()
+		popdcl(src.NoPos)
 		return n
 
 	case OFOR:
-		markdcl()
+		markdcl(src.NoPos)
 		n := nodl(p.pos(), OFOR, nil, nil)
 		n.Ninit.Set(p.stmtList())
 		n.Left, n.Right = p.exprsOrNil()
 		n.Nbody.Set(p.stmtList())
-		popdcl()
+		popdcl(src.NoPos)
 		return n
 
 	case ORANGE:
-		markdcl()
+		markdcl(src.NoPos)
 		n := nodl(p.pos(), ORANGE, nil, nil)
 		n.List.Set(p.stmtList())
 		n.Right = p.expr()
 		n.Nbody.Set(p.stmtList())
-		popdcl()
+		popdcl(src.NoPos)
 		return n
 
 	case OSELECT, OSWITCH:
-		markdcl()
+		markdcl(src.NoPos)
 		n := nodl(p.pos(), op, nil, nil)
 		n.Ninit.Set(p.stmtList())
 		n.Left, _ = p.exprsOrNil()
 		n.List.Set(p.stmtList())
-		popdcl()
+		popdcl(src.NoPos)
 		return n
 
 	// case OCASE, OXCASE:
 	// 	unreachable - mapped to OXCASE case below by exporter
 
 	case OXCASE:
-		markdcl()
+		markdcl(src.NoPos)
 		n := nodl(p.pos(), OXCASE, nil, nil)
 		n.Xoffset = int64(block)
 		n.List.Set(p.exprList())
 		// TODO(gri) eventually we must declare variables for type switch
 		// statements (type switch statements are not yet exported)
 		n.Nbody.Set(p.stmtList())
-		popdcl()
+		popdcl(src.NoPos)
 		return n
 
 	// case OFALL:
diff --git a/src/cmd/compile/internal/gc/closure.go b/src/cmd/compile/internal/gc/closure.go
index cc6f8eb6a6..283d771c9c 100644
--- a/src/cmd/compile/internal/gc/closure.go
+++ b/src/cmd/compile/internal/gc/closure.go
@@ -6,6 +6,7 @@ package gc
 
 import (
 	"cmd/compile/internal/types"
+	"cmd/internal/src"
 	"fmt"
 )
 
@@ -17,7 +18,7 @@ func closurehdr(ntype *Node) {
 	n.Func.Depth = funcdepth
 	n.Func.Outerfunc = Curfn
 
-	funchdr(n)
+	funchdr(n, src.NoPos)
 
 	// steal ntype's argument names and
 	// leave a fresh copy in their place.
@@ -58,7 +59,7 @@ func closurebody(body []*Node) *Node {
 	func_ := Curfn
 	func_.Nbody.Set(body)
 	func_.Func.Endlineno = lineno
-	funcbody(func_)
+	funcbody(func_, src.NoPos)
 
 	// closure-specific variables are hanging off the
 	// ordinary ones in the symbol table; see oldname.
@@ -226,8 +227,10 @@ func makeclosure(func_ *Node) *Node {
 	}
 
 	xfunc.Nbody.Set(func_.Nbody.Slice())
+	xfunc.Func.scopes = func_.Func.scopes
 	xfunc.Func.Dcl = append(func_.Func.Dcl, xfunc.Func.Dcl...)
 	func_.Func.Dcl = nil
+	func_.Func.scopes.Scopes = nil
 	if xfunc.Nbody.Len() == 0 {
 		Fatalf("empty body - won't generate any code")
 	}
@@ -609,6 +612,7 @@ func makepartialcall(fn *Node, t0 *types.Type, meth *types.Sym) *Node {
 	xfunc.Func.Nname.Sym.SetExported(true) // disable export
 	xfunc.Func.Nname.Name.Param.Ntype = xtype
 	xfunc.Func.Nname.Name.Defn = xfunc
+	xfunc.Func.scopes.Scopes = []src.Scope{src.Scope{}}
 	declare(xfunc.Func.Nname, PFUNC)
 
 	// Declare and initialize variable holding receiver.
diff --git a/src/cmd/compile/internal/gc/dcl.go b/src/cmd/compile/internal/gc/dcl.go
index 0704d6ef5d..ecd5e9d806 100644
--- a/src/cmd/compile/internal/gc/dcl.go
+++ b/src/cmd/compile/internal/gc/dcl.go
@@ -33,6 +33,9 @@ var block int32 // current block number
 // Sym definitions, and only a subset of its fields are actually used.
 var dclstack *types.Sym
 
+// lastPopdclPos is the last pos passed to popdcl
+var lastPopdclPos src.Pos
+
 func dcopy(a, b *types.Sym) {
 	a.Pkg = b.Pkg
 	a.Name = b.Name
@@ -59,7 +62,7 @@ func pushdcl(s *types.Sym) *types.Sym {
 
 // popdcl pops the innermost block scope and restores all symbol declarations
 // to their previous state.
-func popdcl() {
+func popdcl(pos src.Pos) {
 	d := dclstack
 	for ; d != nil && d.Name != ""; d = d.Link {
 		s := d.Pkg.Lookup(d.Name)
@@ -74,16 +77,29 @@ func popdcl() {
 
 	dclstack = d.Link // pop mark
 	block = d.Block
+
+	if Curfn != nil {
+		Curfn.Func.scopes.Close(&Ctxt.PosTable, pos)
+		lastPopdclPos = pos
+	}
+
+	return
 }
 
 // markdcl records the start of a new block scope for declarations.
-func markdcl() {
+func markdcl(pos src.Pos) {
 	d := push()
 	d.Name = "" // used as a mark in fifo
 	d.Block = block
 
 	blockgen++
 	block = blockgen
+
+	if Curfn != nil {
+		Curfn.Func.scopes.Open(&Ctxt.PosTable, pos)
+	}
+
+	return
 }
 
 // keep around for debugging
@@ -451,14 +467,14 @@ func ifacedcl(n *Node) {
 // and declare the arguments.
 // called in extern-declaration context
 // returns in auto-declaration context.
-func funchdr(n *Node) {
+func funchdr(n *Node, pos src.Pos) {
 	// change the declaration context from extern to auto
 	if funcdepth == 0 && dclcontext != PEXTERN {
 		Fatalf("funchdr: dclcontext = %d", dclcontext)
 	}
 
 	dclcontext = PAUTO
-	funcstart(n)
+	funcstart(n, pos)
 
 	if n.Func.Nname != nil {
 		funcargs(n.Func.Nname.Name.Param.Ntype)
@@ -596,22 +612,22 @@ var funcdepth int32   // len(funcstack) during parsing, but then forced to be th
 
 // start the function.
 // called before funcargs; undone at end of funcbody.
-func funcstart(n *Node) {
-	markdcl()
+func funcstart(n *Node, pos src.Pos) {
 	funcstack = append(funcstack, Curfn)
 	funcdepth++
 	Curfn = n
+	markdcl(pos)
 }
 
 // finish the body.
 // called in auto-declaration context.
 // returns in extern-declaration context.
-func funcbody(n *Node) {
+func funcbody(n *Node, pos src.Pos) {
 	// change the declaration context from auto to extern
 	if dclcontext != PAUTO {
 		Fatalf("funcbody: unexpected dclcontext %d", dclcontext)
 	}
-	popdcl()
+	popdcl(pos)
 	funcstack, Curfn = funcstack[:len(funcstack)-1], funcstack[len(funcstack)-1]
 	funcdepth--
 	if funcdepth == 0 {
diff --git a/src/cmd/compile/internal/gc/init.go b/src/cmd/compile/internal/gc/init.go
index e26718dfdc..9d11decc60 100644
--- a/src/cmd/compile/internal/gc/init.go
+++ b/src/cmd/compile/internal/gc/init.go
@@ -4,7 +4,10 @@
 
 package gc
 
-import "cmd/compile/internal/types"
+import (
+	"cmd/compile/internal/types"
+	"cmd/internal/src"
+)
 
 // a function named init is a special case.
 // it is called by the initialization before
@@ -92,7 +95,7 @@ func fninit(n []*Node) {
 	fn.Func.Nname.Name.Defn = fn
 	fn.Func.Nname.Name.Param.Ntype = nod(OTFUNC, nil, nil)
 	declare(fn.Func.Nname, PFUNC)
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 
 	// (3)
 	a := nod(OIF, nil, nil)
@@ -152,7 +155,7 @@ func fninit(n []*Node) {
 	exportsym(fn.Func.Nname)
 
 	fn.Nbody.Set(r)
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 
 	Curfn = fn
 	fn = typecheck(fn, Etop)
diff --git a/src/cmd/compile/internal/gc/noder.go b/src/cmd/compile/internal/gc/noder.go
index bd769b6126..61a338d214 100644
--- a/src/cmd/compile/internal/gc/noder.go
+++ b/src/cmd/compile/internal/gc/noder.go
@@ -326,7 +326,7 @@ func (p *noder) funcDecl(fun *syntax.FuncDecl) *Node {
 		declare(f.Func.Nname, PFUNC)
 	}
 
-	funchdr(f)
+	funchdr(f, fun.Pos())
 
 	if fun.Body != nil {
 		if f.Noescape() {
@@ -347,7 +347,7 @@ func (p *noder) funcDecl(fun *syntax.FuncDecl) *Node {
 		}
 	}
 
-	funcbody(f)
+	funcbody(f, fun.Pos())
 
 	return f
 }
@@ -777,14 +777,14 @@ func (p *noder) stmt(stmt syntax.Stmt) *Node {
 }
 
 func (p *noder) blockStmt(stmt *syntax.BlockStmt) []*Node {
-	markdcl()
+	markdcl(stmt.Pos())
 	nodes := p.stmts(stmt.List)
-	popdcl()
+	popdcl(stmt.Rbrace)
 	return nodes
 }
 
 func (p *noder) ifStmt(stmt *syntax.IfStmt) *Node {
-	markdcl()
+	markdcl(stmt.Pos())
 	n := p.nod(stmt, OIF, nil, nil)
 	if stmt.Init != nil {
 		n.Ninit.Set1(p.stmt(stmt.Init))
@@ -801,12 +801,12 @@ func (p *noder) ifStmt(stmt *syntax.IfStmt) *Node {
 			n.Rlist.Set1(e)
 		}
 	}
-	popdcl()
+	popdcl(lastPopdclPos)
 	return n
 }
 
 func (p *noder) forStmt(stmt *syntax.ForStmt) *Node {
-	markdcl()
+	markdcl(stmt.Pos())
 	var n *Node
 	if r, ok := stmt.Init.(*syntax.RangeClause); ok {
 		if stmt.Cond != nil || stmt.Post != nil {
@@ -835,12 +835,12 @@ func (p *noder) forStmt(stmt *syntax.ForStmt) *Node {
 		}
 	}
 	n.Nbody.Set(p.blockStmt(stmt.Body))
-	popdcl()
+	popdcl(stmt.Body.Rbrace)
 	return n
 }
 
 func (p *noder) switchStmt(stmt *syntax.SwitchStmt) *Node {
-	markdcl()
+	markdcl(stmt.Pos())
 	n := p.nod(stmt, OSWITCH, nil, nil)
 	if stmt.Init != nil {
 		n.Ninit.Set1(p.stmt(stmt.Init))
@@ -854,17 +854,17 @@ func (p *noder) switchStmt(stmt *syntax.SwitchStmt) *Node {
 		tswitch = nil
 	}
 
-	n.List.Set(p.caseClauses(stmt.Body, tswitch))
+	n.List.Set(p.caseClauses(stmt.Body, tswitch, stmt.Rbrace))
 
-	popdcl()
+	popdcl(stmt.Rbrace)
 	return n
 }
 
-func (p *noder) caseClauses(clauses []*syntax.CaseClause, tswitch *Node) []*Node {
+func (p *noder) caseClauses(clauses []*syntax.CaseClause, tswitch *Node, rbrace src.Pos) []*Node {
 	var nodes []*Node
-	for _, clause := range clauses {
+	for i, clause := range clauses {
 		p.lineno(clause)
-		markdcl()
+		markdcl(clause.Pos())
 		n := p.nod(clause, OXCASE, nil, nil)
 		if clause.Cases != nil {
 			n.List.Set(p.exprList(clause.Cases))
@@ -878,7 +878,11 @@ func (p *noder) caseClauses(clauses []*syntax.CaseClause, tswitch *Node) []*Node
 		}
 		n.Xoffset = int64(block)
 		n.Nbody.Set(p.stmts(clause.Body))
-		popdcl()
+		if i+1 < len(clauses) {
+			popdcl(clauses[i+1].Pos())
+		} else {
+			popdcl(rbrace)
+		}
 		nodes = append(nodes, n)
 	}
 	return nodes
@@ -886,22 +890,26 @@ func (p *noder) caseClauses(clauses []*syntax.CaseClause, tswitch *Node) []*Node
 
 func (p *noder) selectStmt(stmt *syntax.SelectStmt) *Node {
 	n := p.nod(stmt, OSELECT, nil, nil)
-	n.List.Set(p.commClauses(stmt.Body))
+	n.List.Set(p.commClauses(stmt.Body, stmt.Rbrace))
 	return n
 }
 
-func (p *noder) commClauses(clauses []*syntax.CommClause) []*Node {
+func (p *noder) commClauses(clauses []*syntax.CommClause, rbrace src.Pos) []*Node {
 	var nodes []*Node
-	for _, clause := range clauses {
+	for i, clause := range clauses {
 		p.lineno(clause)
-		markdcl()
+		markdcl(clause.Pos())
 		n := p.nod(clause, OXCASE, nil, nil)
 		if clause.Comm != nil {
 			n.List.Set1(p.stmt(clause.Comm))
 		}
 		n.Xoffset = int64(block)
 		n.Nbody.Set(p.stmts(clause.Body))
-		popdcl()
+		if i+1 < len(clauses) {
+			popdcl(clauses[i+1].Pos())
+		} else {
+			popdcl(rbrace)
+		}
 		nodes = append(nodes, n)
 	}
 	return nodes
diff --git a/src/cmd/compile/internal/gc/pgen.go b/src/cmd/compile/internal/gc/pgen.go
index ec6483fe8e..c2fa7dfcae 100644
--- a/src/cmd/compile/internal/gc/pgen.go
+++ b/src/cmd/compile/internal/gc/pgen.go
@@ -310,13 +310,18 @@ func compile(fn *Node) {
 	pp.Free()
 }
 
-func debuginfo(fnsym *obj.LSym, curfn interface{}) []*dwarf.Var {
+func debuginfo(fnsym *obj.LSym, curfn interface{}) []dwarf.Scope {
 	fn := curfn.(*Node)
 	if expect := Linksym(fn.Func.Nname.Sym); fnsym != expect {
 		Fatalf("unexpected fnsym: %v != %v", fnsym, expect)
 	}
 
-	var vars []*dwarf.Var
+	scopes := make([]dwarf.Scope, len(fn.Func.scopes.Scopes))
+
+	for i := range scopes {
+		scopes[i] = dwarf.Scope{Scope: fn.Func.scopes.Scopes[i]}
+	}
+
 	for _, n := range fn.Func.Dcl {
 		if n.Op != ONAME { // might be OTYPE or OLITERAL
 			continue
@@ -363,8 +368,14 @@ func debuginfo(fnsym *obj.LSym, curfn interface{}) []*dwarf.Var {
 			continue
 		}
 
+		var scope int32 = 0
+		// Only record scope information when inlining is disabled.
+		if Debug['l'] == 0 {
+			scope = findScope(n.Pos, scopes)
+		}
+
 		typename := dwarf.InfoPrefix + gotype.Name[len("type."):]
-		vars = append(vars, &dwarf.Var{
+		scopes[scope].Vars = append(scopes[scope].Vars, &dwarf.Var{
 			Name:   n.Sym.Name,
 			Abbrev: abbrev,
 			Offset: int32(offs),
@@ -373,9 +384,13 @@ func debuginfo(fnsym *obj.LSym, curfn interface{}) []*dwarf.Var {
 	}
 
 	// Stable sort so that ties are broken with declaration order.
-	sort.Stable(dwarf.VarsByOffset(vars))
+	for i := range scopes {
+		sort.Stable(dwarf.VarsByOffset(scopes[i].Vars))
+	}
+
+	scopeRanges(fnsym, scopes)
 
-	return vars
+	return scopes
 }
 
 // fieldtrack adds R_USEFIELD relocations to fnsym to record any
diff --git a/src/cmd/compile/internal/gc/scope.go b/src/cmd/compile/internal/gc/scope.go
new file mode 100644
index 0000000000..82cc371016
--- /dev/null
+++ b/src/cmd/compile/internal/gc/scope.go
@@ -0,0 +1,91 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gc
+
+import (
+	"cmd/internal/dwarf"
+	"cmd/internal/obj"
+	"cmd/internal/src"
+)
+
+// findScope returns the most specific scope containing pos.
+func findScope(pos src.XPos, scopes []dwarf.Scope) int32 {
+	if !pos.IsKnown() {
+		return 0
+	}
+	for i := len(scopes) - 1; i > 0; i-- {
+		if pos.After(scopes[i].Start) && pos.Before(scopes[i].End) {
+			return int32(i)
+		}
+	}
+	return 0
+}
+
+type scopedProg struct {
+	p     *obj.Prog
+	scope int32
+}
+
+// scopeRanges calculates scope ranges for symbol fnsym.
+func scopeRanges(fnsym *obj.LSym, scopes []dwarf.Scope) {
+	var sp []scopedProg
+
+	for p := fnsym.Text; p != nil; p = p.Link {
+		sp = append(sp, scopedProg{p, -1})
+	}
+
+	for scopeID := int32(len(scopes) - 1); scopeID >= 0; scopeID-- {
+		if scope := &scopes[scopeID]; scope.Start.IsKnown() && scope.End.IsKnown() {
+			scopeProgs(sp, scopeID, scope.Start, scope.End)
+		}
+	}
+
+	scopedProgsToRanges(sp, scopes, fnsym.Size)
+
+	// Propagate scope's pc ranges to parent
+	for i := len(scopes) - 1; i > 0; i-- {
+		cur := &scopes[i]
+		if scopes[i].Parent != 0 {
+			parent := &scopes[scopes[i].Parent]
+			parent.UnifyRanges(cur)
+		}
+	}
+}
+
+// scopeProgs marks all scopedProgs between start and end that don't already
+// belong to a scope as belonging to scopeId.
+func scopeProgs(sp []scopedProg, scopeId int32, start, end src.XPos) {
+	for i := range sp {
+		if sp[i].scope >= 0 {
+			continue
+		}
+		if pos := sp[i].p.Pos; pos.After(start) && pos.Before(end) {
+			sp[i].scope = scopeId
+		}
+	}
+}
+
+// scopedProgsToRanges scans sp and collects in the Ranges field of each
+// scope the start and end instruction of the scope.
+func scopedProgsToRanges(sp []scopedProg, scopes []dwarf.Scope, symSize int64) {
+	var curscope int32 = -1
+	for i := range sp {
+		if sp[i].scope == curscope {
+			continue
+		}
+		if curscope >= 0 {
+			curranges := scopes[curscope].Ranges
+			curranges[len(curranges)-1].End = sp[i].p.Pc
+		}
+		curscope = sp[i].scope
+		if curscope >= 0 {
+			scopes[curscope].Ranges = append(scopes[curscope].Ranges, dwarf.Range{Start: sp[i].p.Pc, End: -1})
+		}
+	}
+	if curscope >= 0 {
+		curranges := scopes[curscope].Ranges
+		curranges[len(curranges)-1].End = symSize
+	}
+}
diff --git a/src/cmd/compile/internal/gc/scope_test.go b/src/cmd/compile/internal/gc/scope_test.go
new file mode 100644
index 0000000000..0d01abad09
--- /dev/null
+++ b/src/cmd/compile/internal/gc/scope_test.go
@@ -0,0 +1,410 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gc_test
+
+import (
+	"cmd/internal/objfile"
+	"debug/dwarf"
+	"internal/testenv"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"testing"
+)
+
+type testline struct {
+	// line is one line of go source
+	line string
+
+	// scopes is a list of scope IDs of all the lexical scopes that this line
+	// of code belongs to.
+	// Scope IDs are assigned by traversing the tree of lexical blocks of a
+	// function in pre-order
+	// Scope IDs are function specific, i.e. scope 0 is always the root scope
+	// of the function that this line belongs to. Empty scopes are not assigned
+	// an ID (because they are not saved in debug_info).
+	// Scope 0 is always omitted from this list since all lines always belong
+	// to it.
+	scopes []int
+
+	// vars is the list of variables that belong in scopes[len(scopes)-1].
+	// Local variables are prefixed with "var ", formal parameters with "arg ".
+	// Must be ordered alphabetically.
+	// Set to nil to skip the check.
+	vars []string
+}
+
+var testfile = []testline{
+	{line: "package main"},
+	{line: "func f1(x int) { }"},
+	{line: "func f2(x int) { }"},
+	{line: "func f3(x int) { }"},
+	{line: "func f4(x int) { }"},
+	{line: "func f5(x int) { }"},
+	{line: "func f6(x int) { }"},
+	{line: "func gret1() int { return 2 }"},
+	{line: "func gretbool() bool { return true }"},
+	{line: "func gret3() (int, int, int) { return 0, 1, 2 }"},
+	{line: "var v = []int{ 0, 1, 2 }"},
+	{line: "var ch = make(chan int)"},
+	{line: "var floatch = make(chan float64)"},
+	{line: "var iface interface{}"},
+	{line: "func TestNestedFor() {", vars: []string{"var a int"}},
+	{line: "	a := 0"},
+	{line: "	f1(a)"},
+	{line: "	for i := 0; i < 5; i++ {", scopes: []int{1}, vars: []string{"var i int"}},
+	{line: "		f2(i)", scopes: []int{1}},
+	{line: "		for i := 0; i < 5; i++ {", scopes: []int{1, 2}, vars: []string{"var i int"}},
+	{line: "			f3(i)", scopes: []int{1, 2}},
+	{line: "		}"},
+	{line: "		f4(i)", scopes: []int{1}},
+	{line: "	}"},
+	{line: "	f5(a)"},
+	{line: "}"},
+	{line: "func TestOas2() {", vars: []string{}},
+	{line: "	if a, b, c := gret3(); a != 1 {", scopes: []int{1}, vars: []string{"var a int", "var b int", "var c int"}},
+	{line: "		f1(a)", scopes: []int{1}},
+	{line: "		f1(b)", scopes: []int{1}},
+	{line: "		f1(c)", scopes: []int{1}},
+	{line: "	}"},
+	{line: "	for i, x := range v {", scopes: []int{2}, vars: []string{"var i int", "var x int"}},
+	{line: "		f1(i)", scopes: []int{2}},
+	{line: "		f1(x)", scopes: []int{2}},
+	{line: "	}"},
+	{line: "	if a, ok := <- ch; ok {", scopes: []int{3}, vars: []string{"var a int", "var ok bool"}},
+	{line: "		f1(a)", scopes: []int{3}},
+	{line: "	}"},
+	{line: "	if a, ok := iface.(int); ok {", scopes: []int{4}, vars: []string{"var a int", "var ok bool"}},
+	{line: "		f1(a)", scopes: []int{4}},
+	{line: "	}"},
+	{line: "}"},
+	{line: "func TestIfElse() {"},
+	{line: "	if x := gret1(); x != 0 {", scopes: []int{1}, vars: []string{"var x int"}},
+	{line: "		a := 0", scopes: []int{1, 2}, vars: []string{"var a int"}},
+	{line: "		f1(a); f1(x)", scopes: []int{1, 2}},
+	{line: "	} else {"},
+	{line: "		b := 1", scopes: []int{1, 3}, vars: []string{"var b int"}},
+	{line: "		f1(b); f1(x+1)", scopes: []int{1, 3}},
+	{line: "	}"},
+	{line: "}"},
+	{line: "func TestSwitch() {", vars: []string{}},
+	{line: "	switch x := gret1(); x {", scopes: []int{1}, vars: []string{"var x int"}},
+	{line: "	case 0:", scopes: []int{1}},
+	{line: "		i := x + 5", scopes: []int{1, 2}, vars: []string{"var i int"}},
+	{line: "		f1(x); f1(i)", scopes: []int{1, 2}},
+	{line: "	case 1:", scopes: []int{1}},
+	{line: "		j := x + 10", scopes: []int{1, 3}, vars: []string{"var j int"}},
+	{line: "		f1(x); f1(j)", scopes: []int{1, 3}},
+	{line: "	case 2:", scopes: []int{1}},
+	{line: "		k := x + 2", scopes: []int{1, 4}, vars: []string{"var k int"}},
+	{line: "		f1(x); f1(k)", scopes: []int{1, 4}},
+	{line: "	}"},
+	{line: "}"},
+	{line: "func TestTypeSwitch() {", vars: []string{}},
+	{line: "	switch x := iface.(type) {"},
+	{line: "	case int:"},
+	{line: "		f1(x)", scopes: []int{1}, vars: []string{"var x int"}},
+	{line: "	case uint8:"},
+	{line: "		f1(int(x))", scopes: []int{2}, vars: []string{"var x uint8"}},
+	{line: "	case float64:"},
+	{line: "		f1(int(x)+1)", scopes: []int{3}, vars: []string{"var x float64"}},
+	{line: "	}"},
+	{line: "}"},
+	{line: "func TestSelectScope() {"},
+	{line: "	select {"},
+	{line: "	case i := <- ch:"},
+	{line: "		f1(i)", scopes: []int{1}, vars: []string{"var i int"}},
+	{line: "	case f := <- floatch:"},
+	{line: "		f1(int(f))", scopes: []int{2}, vars: []string{"var f float64"}},
+	{line: "	}"},
+	{line: "}"},
+	{line: "func TestBlock() {", vars: []string{"var a int"}},
+	{line: "	a := 1"},
+	{line: "	{"},
+	{line: "		b := 2", scopes: []int{1}, vars: []string{"var b int"}},
+	{line: "		f1(b)", scopes: []int{1}},
+	{line: "		f1(a)", scopes: []int{1}},
+	{line: "	}"},
+	{line: "}"},
+	{line: "func TestDiscontiguousRanges() {", vars: []string{"var a int"}},
+	{line: "	a := 0"},
+	{line: "	f1(a)"},
+	{line: "	{"},
+	{line: "		b := 0", scopes: []int{1}, vars: []string{"var b int"}},
+	{line: "		f2(b)", scopes: []int{1}},
+	{line: "		if gretbool() {", scopes: []int{1}},
+	{line: "			c := 0", scopes: []int{1, 2}, vars: []string{"var c int"}},
+	{line: "			f3(c)", scopes: []int{1, 2}},
+	{line: "		} else {"},
+	{line: "			c := 1.1", scopes: []int{1, 3}, vars: []string{"var c float64"}},
+	{line: "			f4(int(c))", scopes: []int{1, 3}},
+	{line: "		}"},
+	{line: "		f5(b)", scopes: []int{1}},
+	{line: "	}"},
+	{line: "	f6(a)"},
+	{line: "}"},
+	{line: "func TestClosureScope() {", vars: []string{"var a int", "var b int", "var f func(int)"}},
+	{line: "	a := 1; b := 1"},
+	{line: "	f := func(c int) {", scopes: []int{0}, vars: []string{"arg c int", "var a int", "var d int"}},
+	{line: "		d := 3"},
+	{line: "		f1(a); f1(c); f1(d)"},
+	{line: "		if e := 3; e != 0 {", scopes: []int{1}, vars: []string{"var e int"}},
+	{line: "			f1(e)", scopes: []int{1}},
+	{line: "		}"},
+	{line: "	}"},
+	{line: "	f(3); f1(b)"},
+	{line: "}"},
+	{line: "func main() {"},
+	{line: "	TestNestedFor()"},
+	{line: "	TestOas2()"},
+	{line: "	TestIfElse()"},
+	{line: "	TestSwitch()"},
+	{line: "	TestTypeSwitch()"},
+	{line: "	TestSelectScope()"},
+	{line: "	TestBlock()"},
+	{line: "	TestDiscontiguousRanges()"},
+	{line: "	TestClosureScope()"},
+	{line: "}"},
+}
+
+const detailOutput = false
+
+// Compiles testfile checks that the description of lexical blocks emitted
+// by the linker in debug_info, for each function in the main package,
+// corresponds to what we expect it to be.
+func TestScopeRanges(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	dir, err := ioutil.TempDir("", "TestScopeRanges")
+	if err != nil {
+		t.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	src, f := gobuild(t, dir, testfile)
+	defer f.Close()
+
+	// the compiler uses forward slashes for paths even on windows
+	src = strings.Replace(src, "\\", "/", -1)
+
+	pcln, err := f.PCLineTable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	dwarfData, err := f.DWARF()
+	if err != nil {
+		t.Fatal(err)
+	}
+	dwarfReader := dwarfData.Reader()
+
+	lines := make(map[line][]*lexblock)
+
+	for {
+		entry, err := dwarfReader.Next()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if entry == nil {
+			break
+		}
+
+		if entry.Tag != dwarf.TagSubprogram {
+			continue
+		}
+
+		name, ok := entry.Val(dwarf.AttrName).(string)
+		if !ok || !strings.HasPrefix(name, "main.Test") {
+			continue
+		}
+
+		var scope lexblock
+		ctxt := scopexplainContext{
+			dwarfData:   dwarfData,
+			dwarfReader: dwarfReader,
+			scopegen:    1,
+		}
+
+		readScope(&ctxt, &scope, entry)
+
+		scope.markLines(pcln, lines)
+	}
+
+	anyerror := false
+	for i := range testfile {
+		tgt := testfile[i].scopes
+		out := lines[line{src, i + 1}]
+
+		if detailOutput {
+			t.Logf("%s // %v", testfile[i].line, out)
+		}
+
+		scopesok := checkScopes(tgt, out)
+		if !scopesok {
+			t.Logf("mismatch at line %d %q: expected: %v got: %v\n", i, testfile[i].line, tgt, scopesToString(out))
+		}
+
+		varsok := true
+		if testfile[i].vars != nil {
+			if len(out) > 0 {
+				varsok = checkVars(testfile[i].vars, out[len(out)-1].vars)
+				if !varsok {
+					t.Logf("variable mismatch at line %d %q for scope %d: expected: %v got: %v\n", i, testfile[i].line, out[len(out)-1].id, testfile[i].vars, out[len(out)-1].vars)
+				}
+			}
+		}
+
+		anyerror = anyerror || !scopesok || !varsok
+	}
+
+	if anyerror {
+		t.Fatalf("mismatched output")
+	}
+}
+
+func scopesToString(v []*lexblock) string {
+	r := make([]string, len(v))
+	for i, s := range v {
+		r[i] = strconv.Itoa(s.id)
+	}
+	return "[ " + strings.Join(r, ", ") + " ]"
+}
+
+func checkScopes(tgt []int, out []*lexblock) bool {
+	if len(out) > 0 {
+		// omit scope 0
+		out = out[1:]
+	}
+	if len(tgt) != len(out) {
+		return false
+	}
+	for i := range tgt {
+		if tgt[i] != out[i].id {
+			return false
+		}
+	}
+	return true
+}
+
+func checkVars(tgt, out []string) bool {
+	if len(tgt) != len(out) {
+		return false
+	}
+	for i := range tgt {
+		if tgt[i] != out[i] {
+			return false
+		}
+	}
+	return true
+}
+
+type lexblock struct {
+	id     int
+	ranges [][2]uint64
+	vars   []string
+	scopes []lexblock
+}
+
+type line struct {
+	file   string
+	lineno int
+}
+
+type scopexplainContext struct {
+	dwarfData   *dwarf.Data
+	dwarfReader *dwarf.Reader
+	scopegen    int
+	lines       map[line][]int
+}
+
+// readScope reads the DW_TAG_lexical_block or the DW_TAG_subprogram in
+// entry and writes a description in scope.
+// Nested DW_TAG_lexical_block entries are read recursively.
+func readScope(ctxt *scopexplainContext, scope *lexblock, entry *dwarf.Entry) {
+	if lowpc, ok := entry.Val(dwarf.AttrLowpc).(uint64); ok {
+		highpc, _ := entry.Val(dwarf.AttrHighpc).(uint64)
+		scope.ranges = [][2]uint64{[2]uint64{lowpc, highpc}}
+	} else {
+		ranges, err := ctxt.dwarfData.Ranges(entry)
+		if err != nil {
+			panic(err)
+		}
+		scope.ranges = ranges
+	}
+	for {
+		e, err := ctxt.dwarfReader.Next()
+		if err != nil {
+			panic(err)
+		}
+		switch e.Tag {
+		case 0:
+			sort.Strings(scope.vars)
+			return
+		case dwarf.TagFormalParameter:
+			typ, err := ctxt.dwarfData.Type(e.Val(dwarf.AttrType).(dwarf.Offset))
+			if err != nil {
+				panic(err)
+			}
+			scope.vars = append(scope.vars, "arg "+e.Val(dwarf.AttrName).(string)+" "+typ.String())
+		case dwarf.TagVariable:
+			typ, err := ctxt.dwarfData.Type(e.Val(dwarf.AttrType).(dwarf.Offset))
+			if err != nil {
+				panic(err)
+			}
+			scope.vars = append(scope.vars, "var "+e.Val(dwarf.AttrName).(string)+" "+typ.String())
+		case dwarf.TagLexDwarfBlock:
+			scope.scopes = append(scope.scopes, lexblock{id: ctxt.scopegen})
+			ctxt.scopegen++
+			readScope(ctxt, &scope.scopes[len(scope.scopes)-1], e)
+		}
+	}
+}
+
+// markLines marks all lines that belong to this scope with this scope
+// Recursively calls markLines for all children scopes.
+func (scope *lexblock) markLines(pcln objfile.Liner, lines map[line][]*lexblock) {
+	for _, r := range scope.ranges {
+		for pc := r[0]; pc < r[1]; pc++ {
+			file, lineno, _ := pcln.PCToLine(pc)
+			l := line{file, lineno}
+			if len(lines[l]) == 0 || lines[l][len(lines[l])-1] != scope {
+				lines[l] = append(lines[l], scope)
+			}
+		}
+	}
+
+	for i := range scope.scopes {
+		scope.scopes[i].markLines(pcln, lines)
+	}
+}
+
+func gobuild(t *testing.T, dir string, testfile []testline) (string, *objfile.File) {
+	src := filepath.Join(dir, "test.go")
+	dst := filepath.Join(dir, "out.o")
+
+	f, err := os.Create(src)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := range testfile {
+		f.Write([]byte(testfile[i].line))
+		f.Write([]byte{'\n'})
+	}
+	f.Close()
+
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags=-N -l", "-o", dst, src)
+	if b, err := cmd.CombinedOutput(); err != nil {
+		t.Logf("build: %s\n", string(b))
+		t.Fatal(err)
+	}
+
+	pkg, err := objfile.Open(dst)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return src, pkg
+}
diff --git a/src/cmd/compile/internal/gc/sizeof_test.go b/src/cmd/compile/internal/gc/sizeof_test.go
index bea25dde2b..f2d6728863 100644
--- a/src/cmd/compile/internal/gc/sizeof_test.go
+++ b/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{}, 96, 160},
+		{Func{}, 112, 192},
 		{Name{}, 36, 56},
 		{Param{}, 28, 56},
 		{Node{}, 84, 136},
diff --git a/src/cmd/compile/internal/gc/subr.go b/src/cmd/compile/internal/gc/subr.go
index ddf6f77c07..a7d2adf4a8 100644
--- a/src/cmd/compile/internal/gc/subr.go
+++ b/src/cmd/compile/internal/gc/subr.go
@@ -1687,7 +1687,7 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym, iface
 	lineno = autogeneratedPos
 
 	dclcontext = PEXTERN
-	markdcl()
+	markdcl(src.NoPos)
 
 	this := namedfield(".this", rcvr)
 	this.Left.Name.Param.Ntype = this.Right
@@ -1717,7 +1717,7 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym, iface
 	fn.Func.Nname.Name.Param.Ntype = t
 	fn.Func.Nname.Sym.SetExported(true) // prevent export; see closure.go
 	declare(fn.Func.Nname, PFUNC)
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 
 	// arg list
 	var args []*Node
@@ -1782,9 +1782,9 @@ func genwrapper(rcvr *types.Type, method *types.Field, newnam *types.Sym, iface
 		dumplist("genwrapper body", fn.Nbody)
 	}
 
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 	Curfn = fn
-	popdcl()
+	popdcl(src.NoPos)
 	if debug_dclstack != 0 {
 		testdclstack()
 	}
diff --git a/src/cmd/compile/internal/gc/syntax.go b/src/cmd/compile/internal/gc/syntax.go
index ef70e4b1f7..92f2a29288 100644
--- a/src/cmd/compile/internal/gc/syntax.go
+++ b/src/cmd/compile/internal/gc/syntax.go
@@ -321,9 +321,10 @@ type Func struct {
 	Shortname  *types.Sym
 	Enter      Nodes // for example, allocate and initialize memory for escaping parameters
 	Exit       Nodes
-	Cvars      Nodes   // closure params
-	Dcl        []*Node // autodcl for this func/closure
-	Inldcl     Nodes   // copy of dcl for use in inlining
+	Cvars      Nodes      // closure params
+	Dcl        []*Node    // autodcl for this func/closure
+	Inldcl     Nodes      // copy of dcl for use in inlining
+	scopes     src.Scopes // lexical scopes of the function
 	Closgen    int
 	Outerfunc  *Node // outer function (for closure)
 	FieldTrack map[*types.Sym]struct{}
diff --git a/src/cmd/compile/internal/gc/walk.go b/src/cmd/compile/internal/gc/walk.go
index 7a8becb35a..95b72c873c 100644
--- a/src/cmd/compile/internal/gc/walk.go
+++ b/src/cmd/compile/internal/gc/walk.go
@@ -7,6 +7,7 @@ package gc
 import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
+	"cmd/internal/src"
 	"cmd/internal/sys"
 	"fmt"
 	"strings"
@@ -3661,7 +3662,7 @@ func walkprintfunc(n *Node, init *Nodes) *Node {
 
 	oldfn := Curfn
 	Curfn = nil
-	funchdr(fn)
+	funchdr(fn, src.NoPos)
 
 	a = nod(n.Op, nil, nil)
 	a.List.Set(printargs)
@@ -3670,7 +3671,7 @@ func walkprintfunc(n *Node, init *Nodes) *Node {
 
 	fn.Nbody.Set1(a)
 
-	funcbody(fn)
+	funcbody(fn, src.NoPos)
 
 	fn = typecheck(fn, Etop)
 	typecheckslice(fn.Nbody.Slice(), Etop)
diff --git a/src/cmd/internal/dwarf/dwarf.go b/src/cmd/internal/dwarf/dwarf.go
index 7625149914..da419d8574 100644
--- a/src/cmd/internal/dwarf/dwarf.go
+++ b/src/cmd/internal/dwarf/dwarf.go
@@ -8,14 +8,20 @@
 package dwarf
 
 import (
+	"cmd/internal/src"
+	"errors"
 	"fmt"
 )
 
 // 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 range lists.
+const RangePrefix = "go.range."
+
 // Sym represents a symbol.
 type Sym interface {
+	Len() int64
 }
 
 // A Var represents a local variable or a function parameter.
@@ -26,6 +32,63 @@ type Var struct {
 	Type   Sym
 }
 
+// A Scope represents a lexical scope, all variables contained in a scope
+// will only be visible to instructions covered by the scope.
+// Lexical scopes are contiguous in source files but can end up being
+// compiled to discontiguous blocks of instructions in the executable, the
+// Ranges field lists all the blocks of instructions that belong in this
+// scope.
+type Scope struct {
+	src.Scope
+	Ranges []Range
+	Vars   []*Var
+}
+
+// A Range represents a half-open interval [Start, End).
+type Range struct {
+	Start, End int64
+}
+
+// UnifyRanges merges the list of ranges of c into the list of ranges of s
+func (s *Scope) UnifyRanges(c *Scope) {
+	out := make([]Range, 0, len(s.Ranges)+len(c.Ranges))
+
+	i, j := 0, 0
+	for {
+		var cur Range
+		if i < len(s.Ranges) && j < len(c.Ranges) {
+			if s.Ranges[i].Start < c.Ranges[j].Start {
+				cur = s.Ranges[i]
+				i++
+			} else {
+				cur = c.Ranges[j]
+				j++
+			}
+		} else if i < len(s.Ranges) {
+			cur = s.Ranges[i]
+			i++
+		} else if j < len(c.Ranges) {
+			cur = c.Ranges[j]
+			j++
+		} else {
+			break
+		}
+
+		if len(out) == 0 {
+			out = append(out, cur)
+		} else {
+			last := &out[len(out)-1]
+			if cur.Start > last.End {
+				out = append(out, cur)
+			} else if cur.End > last.End {
+				last.End = cur.End
+			}
+		}
+	}
+
+	s.Ranges = out
+}
+
 // A Context specifies how to add data to a Sym.
 type Context interface {
 	PtrSize() int
@@ -153,6 +216,8 @@ const (
 	DW_ABRV_VARIABLE
 	DW_ABRV_AUTO
 	DW_ABRV_PARAM
+	DW_ABRV_LEXICAL_BLOCK_RANGES
+	DW_ABRV_LEXICAL_BLOCK_SIMPLE
 	DW_ABRV_STRUCTFIELD
 	DW_ABRV_FUNCTYPEPARAM
 	DW_ABRV_DOTDOTDOT
@@ -243,6 +308,25 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		},
 	},
 
+	/* LEXICAL_BLOCK_RANGES */
+	{
+		DW_TAG_lexical_block,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_ranges, DW_FORM_data4}, // replace with DW_FORM_sec_offset in DWARFv4.
+		},
+	},
+
+	/* LEXICAL_BLOCK_SIMPLE */
+	{
+		DW_TAG_lexical_block,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_low_pc, DW_FORM_addr},
+			{DW_AT_high_pc, DW_FORM_addr},
+		},
+	},
+
 	/* STRUCTFIELD */
 	{
 		DW_TAG_member,
@@ -520,8 +604,8 @@ func putattr(ctxt Context, s Sym, abbrev int, form int, cls int, value int64, da
 		ctxt.AddInt(s, 2, value)
 
 	case DW_FORM_data4: // constant, {line,loclist,mac,rangelist}ptr
-		if cls == DW_CLS_PTR { // DW_AT_stmt_list
-			ctxt.AddSectionOffset(s, 4, data, 0)
+		if cls == DW_CLS_PTR { // DW_AT_stmt_list and DW_AT_ranges
+			ctxt.AddSectionOffset(s, 4, data, value)
 			break
 		}
 		ctxt.AddInt(s, 4, value)
@@ -550,15 +634,13 @@ func putattr(ctxt Context, s Sym, abbrev int, form int, cls int, value int64, da
 			ctxt.AddInt(s, 1, 0)
 		}
 
-	// In DWARF 2 (which is what we claim to generate),
-	// the ref_addr is the same size as a normal address.
-	// In DWARF 3 it is always 32 bits, unless emitting a large
+	// In DWARF 3 the ref_addr is always 32 bits, unless emitting a large
 	// (> 4 GB of debug info aka "64-bit") unit, which we don't implement.
 	case DW_FORM_ref_addr: // reference to a DIE in the .info section
 		if data == nil {
 			return fmt.Errorf("dwarf: null reference in %d", abbrev)
 		} else {
-			ctxt.AddSectionOffset(s, ctxt.PtrSize(), data, 0)
+			ctxt.AddSectionOffset(s, 4, data, 0)
 		}
 
 	case DW_FORM_ref1, // reference within the compilation unit
@@ -601,7 +683,7 @@ 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 Sym, name string, external bool, startPC Sym, size int64, vars []*Var) {
+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)
@@ -611,29 +693,72 @@ func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size
 		ev = 1
 	}
 	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
-	names := make(map[string]bool)
-	var encbuf [20]byte
-	for _, v := range vars {
-		var n string
-		if names[v.Name] {
-			n = fmt.Sprintf("%s#%d", v.Name, len(names))
-		} else {
-			n = v.Name
+	if len(scopes) > 0 {
+		var encbuf [20]byte
+		if putscope(ctxt, s, ranges, startPC, 0, scopes, encbuf[:0]) < int32(len(scopes)) {
+			return errors.New("multiple toplevel scopes")
 		}
-		names[n] = true
+	}
+
+	Uleb128put(ctxt, s, 0)
+	return nil
+}
 
-		Uleb128put(ctxt, s, int64(v.Abbrev))
-		putattr(ctxt, s, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
-		loc := append(encbuf[:0], DW_OP_call_frame_cfa)
-		if v.Offset != 0 {
-			loc = append(loc, DW_OP_consts)
-			loc = AppendSleb128(loc, int64(v.Offset))
-			loc = append(loc, DW_OP_plus)
+func putvar(ctxt Context, s Sym, v *Var, 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)
+	loc := append(encbuf[:0], DW_OP_call_frame_cfa)
+	if v.Offset != 0 {
+		loc = append(loc, DW_OP_consts)
+		loc = AppendSleb128(loc, int64(v.Offset))
+		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)
+}
+
+func putscope(ctxt Context, s, ranges Sym, startPC Sym, curscope int32, scopes []Scope, encbuf []byte) int32 {
+	for _, v := range scopes[curscope].Vars {
+		putvar(ctxt, s, v, encbuf)
+	}
+	this := curscope
+	curscope++
+	for curscope < int32(len(scopes)) {
+		scope := scopes[curscope]
+		if scope.Parent != this {
+			return curscope
+		}
+
+		emptyscope := len(scope.Vars) == 0 || len(scope.Ranges) == 0
+
+		if !emptyscope {
+			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)
+			} else {
+				Uleb128put(ctxt, s, DW_ABRV_LEXICAL_BLOCK_RANGES)
+				putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_RANGES, DW_FORM_data4, DW_CLS_PTR, ranges.Len(), ranges)
+				ctxt.AddAddress(ranges, nil, -1)
+				ctxt.AddAddress(ranges, startPC, 0)
+				for _, pcrange := range scope.Ranges {
+					ctxt.AddAddress(ranges, nil, pcrange.Start)
+					ctxt.AddAddress(ranges, nil, pcrange.End)
+				}
+				ctxt.AddAddress(ranges, nil, 0)
+				ctxt.AddAddress(ranges, nil, 0)
+			}
+		}
+
+		curscope = putscope(ctxt, s, ranges, startPC, curscope, scopes, encbuf)
+
+		if !emptyscope {
+			Uleb128put(ctxt, s, 0)
 		}
-		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)
 	}
-	Uleb128put(ctxt, s, 0)
+	return curscope
 }
 
 // VarsByOffset attaches the methods of sort.Interface to []*Var,
diff --git a/src/cmd/internal/obj/data.go b/src/cmd/internal/obj/data.go
index 114841dedb..7a19e3a38c 100644
--- a/src/cmd/internal/obj/data.go
+++ b/src/cmd/internal/obj/data.go
@@ -114,7 +114,7 @@ func (s *LSym) WriteInt(ctxt *Link, off int64, siz int, i int64) {
 // WriteAddr writes an address of size siz into s at offset off.
 // rsym and roff specify the relocation for the address.
 func (s *LSym) WriteAddr(ctxt *Link, off int64, siz int, rsym *LSym, roff int64) {
-	if siz != ctxt.Arch.PtrSize {
+	if siz != ctxt.Arch.PtrSize && siz != 4 {
 		ctxt.Diag("WriteAddr: bad address size %d in %s", siz, s.Name)
 	}
 	s.prepwrite(ctxt, off, siz)
diff --git a/src/cmd/internal/obj/link.go b/src/cmd/internal/obj/link.go
index fd07851f3b..e08d7a0340 100644
--- a/src/cmd/internal/obj/link.go
+++ b/src/cmd/internal/obj/link.go
@@ -483,6 +483,7 @@ const (
 	SHOSTOBJ
 	SDWARFSECT
 	SDWARFINFO
+	SDWARFRANGE
 	SSUB       = SymKind(1 << 8)
 	SMASK      = SymKind(SSUB - 1)
 	SHIDDEN    = SymKind(1 << 9)
@@ -739,7 +740,7 @@ type Link struct {
 	Armsize       int32
 	Pc            int64
 	DiagFunc      func(string, ...interface{})
-	DebugInfo     func(fn *LSym, curfn interface{}) []*dwarf.Var // if non-nil, curfn is a *gc.Node
+	DebugInfo     func(fn *LSym, curfn interface{}) []dwarf.Scope // if non-nil, curfn is a *gc.Node
 	Cursym        *LSym
 	Version       int
 	Errors        int
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index 6858143674..314c7a9de6 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -542,10 +542,14 @@ func (c dwCtxt) SymValue(s dwarf.Sym) int64 {
 	return 0
 }
 func (c dwCtxt) AddAddress(s dwarf.Sym, data interface{}, value int64) {
-	rsym := data.(*LSym)
 	ls := s.(*LSym)
 	size := c.PtrSize()
-	ls.WriteAddr(c.Link, ls.Size, size, rsym, value)
+	if data != nil {
+		rsym := data.(*LSym)
+		ls.WriteAddr(c.Link, ls.Size, size, rsym, value)
+	} else {
+		ls.WriteInt(c.Link, ls.Size, size, value)
+	}
 }
 func (c dwCtxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64) {
 	ls := s.(*LSym)
@@ -555,6 +559,10 @@ func (c dwCtxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64
 	r.Type = R_DWARFREF
 }
 
+func (s *LSym) Len() int64 {
+	return s.Size
+}
+
 // makeFuncDebugEntry makes a DWARF Debugging Information Entry
 // for TEXT symbol s.
 func makeFuncDebugEntry(ctxt *Link, curfn interface{}, s *LSym) {
@@ -564,10 +572,21 @@ func makeFuncDebugEntry(ctxt *Link, curfn interface{}, s *LSym) {
 	}
 	dsym.Type = SDWARFINFO
 	dsym.Set(AttrDuplicateOK, s.DuplicateOK())
-	var vars []*dwarf.Var
+
+	drsym := ctxt.Lookup(dwarf.RangePrefix+s.Name, int(s.Version))
+	if drsym.Size != 0 {
+		return
+	}
+	drsym.Type = SDWARFRANGE
+	drsym.Set(AttrDuplicateOK, s.DuplicateOK())
+
+	var scopes []dwarf.Scope
 	if ctxt.DebugInfo != nil {
-		vars = ctxt.DebugInfo(s, curfn)
+		scopes = ctxt.DebugInfo(s, curfn)
+	}
+	err := dwarf.PutFunc(dwCtxt{ctxt}, dsym, drsym, s.Name, s.Version == 0, s, s.Size, scopes)
+	if err != nil {
+		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
 	}
-	dwarf.PutFunc(dwCtxt{ctxt}, dsym, s.Name, s.Version == 0, s, s.Size, vars)
-	ctxt.Data = append(ctxt.Data, dsym)
+	ctxt.Data = append(ctxt.Data, dsym, drsym)
 }
diff --git a/src/cmd/internal/src/scope.go b/src/cmd/internal/src/scope.go
new file mode 100644
index 0000000000..d9ca1f5bfa
--- /dev/null
+++ b/src/cmd/internal/src/scope.go
@@ -0,0 +1,36 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package src
+
+// Scope represents a lexical scope.
+type Scope struct {
+	Start, End XPos
+	Parent     int32
+}
+
+// Scopes represents a tree of lexical scopes
+type Scopes struct {
+	Scopes   []Scope // lexical scopes
+	curscope int32   // current scope index (during noding)
+}
+
+// Open starts a new scope, scopes.Curscope is the parent.
+func (scopes *Scopes) Open(posTable *PosTable, pos Pos) {
+	scope := Scope{Parent: scopes.curscope, Start: NoXPos, End: NoXPos}
+	if pos.IsKnown() {
+		scope.Start = posTable.XPos(pos)
+	}
+	scopes.Scopes = append(scopes.Scopes, scope)
+	scopes.curscope = int32(len(scopes.Scopes) - 1)
+}
+
+// Close ends the current scope.
+func (scopes *Scopes) Close(posTable *PosTable, pos Pos) {
+	scope := &scopes.Scopes[scopes.curscope]
+	if pos.IsKnown() {
+		scope.End = posTable.XPos(pos)
+	}
+	scopes.curscope = scope.Parent
+}
diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index 094df86dce..6f45b2fa06 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -570,9 +570,18 @@ func relocsym(ctxt *Link, s *Symbol) {
 			}
 
 		case obj.R_DWARFREF:
-			if r.Sym.Sect == nil {
+			sectName := ""
+			vaddr := int64(0)
+			if r.Sym.Sect != nil {
+				sectName = r.Sym.Sect.Name
+				vaddr = int64(r.Sym.Sect.Vaddr)
+			} else if r.Sym.Type == obj.SDWARFRANGE {
+				sectName = ".debug_ranges"
+				vaddr = 0
+			} else {
 				Errorf(s, "missing DWARF section for relocation target %s", r.Sym.Name)
 			}
+
 			if Linkmode == LinkExternal {
 				r.Done = 0
 				// PE code emits IMAGE_REL_I386_SECREL and IMAGE_REL_AMD64_SECREL
@@ -584,8 +593,9 @@ func relocsym(ctxt *Link, s *Symbol) {
 					r.Type = obj.R_ADDR
 				}
 
-				r.Xsym = ctxt.Syms.ROLookup(r.Sym.Sect.Name, 0)
-				r.Xadd = r.Add + Symaddr(r.Sym) - int64(r.Sym.Sect.Vaddr)
+				r.Xsym = ctxt.Syms.ROLookup(sectName, 0)
+				r.Xadd = r.Add + Symaddr(r.Sym) - vaddr
+
 				o = r.Xadd
 				rs = r.Xsym
 				if Iself && SysArch.Family == sys.AMD64 {
@@ -593,7 +603,7 @@ func relocsym(ctxt *Link, s *Symbol) {
 				}
 				break
 			}
-			o = Symaddr(r.Sym) + r.Add - int64(r.Sym.Sect.Vaddr)
+			o = Symaddr(r.Sym) + r.Add - vaddr
 
 		case obj.R_WEAKADDROFF:
 			if !r.Sym.Attr.Reachable() {
@@ -1822,6 +1832,7 @@ func (ctxt *Link) dodata() {
 		if s.Type != obj.SDWARFSECT {
 			break
 		}
+
 		sect = addsection(&Segdwarf, s.Name, 04)
 		sect.Align = 1
 		datsize = Rnd(datsize, int64(sect.Align))
diff --git a/src/cmd/link/internal/ld/dwarf.go b/src/cmd/link/internal/ld/dwarf.go
index 696a80bcdc..579e13e2ab 100644
--- a/src/cmd/link/internal/ld/dwarf.go
+++ b/src/cmd/link/internal/ld/dwarf.go
@@ -8,7 +8,6 @@
 //   - assign global variables and types to their packages
 //   - gdb uses c syntax, meaning clumsy quoting is needed for go identifiers. eg
 //     ptype struct '[]uint8' and qualifiers need to be quoted away
-//   - lexical scoping is lost, so gdb gets confused as to which 'main.i' you mean.
 //   - file:line info for variables
 //   - make strings a typedef so prettyprinters can see the underlying string type
 
@@ -76,6 +75,7 @@ var arangessec *Symbol
 var framesec *Symbol
 var infosec *Symbol
 var linesec *Symbol
+var rangesec *Symbol
 
 var gdbscript string
 
@@ -1288,6 +1288,34 @@ func writeframes(ctxt *Link, syms []*Symbol) []*Symbol {
 	return syms
 }
 
+func writeranges(ctxt *Link, syms []*Symbol) []*Symbol {
+	if rangesec == nil {
+		rangesec = ctxt.Syms.Lookup(".debug_ranges", 0)
+	}
+	rangesec.Type = obj.SDWARFSECT
+	rangesec.Attr |= AttrReachable
+	rangesec.R = rangesec.R[:0]
+
+	for _, s := range ctxt.Textp {
+		rangeSym := ctxt.Syms.Lookup(dwarf.RangePrefix+s.Name, int(s.Version))
+		rangeSym.Attr |= AttrHidden
+		rangeSym.Attr |= AttrReachable
+		rangeSym.Type = obj.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)
+		}
+		rangesec.Size += rangeSym.Size
+	}
+	if rangesec.Size > 0 {
+		// PE does not like empty sections
+		syms = append(syms, rangesec)
+	}
+	return syms
+}
+
 /*
  *  Walk DWarfDebugInfoEntries, and emit .debug_info
  */
@@ -1318,7 +1346,7 @@ func writeinfo(ctxt *Link, syms []*Symbol, funcs []*Symbol) []*Symbol {
 		// Fields marked with (*) must be changed for 64-bit dwarf
 		// This must match COMPUNITHEADERSIZE above.
 		Adduint32(ctxt, s, 0) // unit_length (*), will be filled in later.
-		Adduint16(ctxt, s, 2) // dwarf version (appendix F)
+		Adduint16(ctxt, s, 3) // dwarf version (appendix F)
 
 		// debug_abbrev_offset (*)
 		adddwarfref(ctxt, s, abbrevsym, 4)
@@ -1531,6 +1559,7 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 	syms := writeabbrev(ctxt, nil)
 	syms, funcs := writelines(ctxt, syms)
 	syms = writeframes(ctxt, syms)
+	syms = writeranges(ctxt, syms)
 
 	synthesizestringtypes(ctxt, dwtypes.Child)
 	synthesizeslicetypes(ctxt, dwtypes.Child)
@@ -1572,6 +1601,7 @@ func dwarfaddshstrings(ctxt *Link, shstrtab *Symbol) {
 	Addstring(shstrtab, ".debug_pubnames")
 	Addstring(shstrtab, ".debug_pubtypes")
 	Addstring(shstrtab, ".debug_gdb_scripts")
+	Addstring(shstrtab, ".debug_ranges")
 	if Linkmode == LinkExternal {
 		Addstring(shstrtab, elfRelType+".debug_info")
 		Addstring(shstrtab, elfRelType+".debug_aranges")
@@ -1599,6 +1629,10 @@ 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_ranges", 0)
+	if sym.Sect != nil {
+		putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
+	}
 }
 
 /*
diff --git a/src/cmd/link/internal/ld/link.go b/src/cmd/link/internal/ld/link.go
index 1c61835da6..5ed8e894b9 100644
--- a/src/cmd/link/internal/ld/link.go
+++ b/src/cmd/link/internal/ld/link.go
@@ -89,6 +89,10 @@ func (s *Symbol) ElfsymForReloc() int32 {
 	}
 }
 
+func (s *Symbol) Len() int64 {
+	return s.Size
+}
+
 // Attribute is a set of common symbol attributes.
 type Attribute int16