cmd: generate DWARF for functions in compile instead of link.

This is a copy of golang.org/cl/22092 by Ryan Brown.

Here's his original comment:
On my machine this increases the average time for 'go build cmd/go' from
2.25s to 2.36s. I tried to measure compile and link separately but saw
no significant change.

Change-Id: If0d2b756d52a0d30d4eda526929c82794d89dd7b
Reviewed-on: https://go-review.googlesource.com/25311
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/buildtool.go

@@ -41,6 +41,7 @@ var bootstrapDirs = []string{
 	"compile/internal/s390x",
 	"internal/bio",
 	"internal/gcprog",
+	"internal/dwarf",
 	"internal/obj",
 	"internal/obj/arm",
 	"internal/obj/arm64",

src/cmd/internal/dwarf/dwarf.go

@@ -0,0 +1,609 @@
+// 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 dwarf generates DWARF debugging information.
+// DWARF generation is split between the compiler and the linker,
+// this package contains the shared code.
+package dwarf
+
+import (
+	"fmt"
+	"strings"
+)
+
+// InfoPrefix is the prefix for all the symbols containing DWARF info entries.
+const InfoPrefix = "go.info."
+
+// Sym represents a symbol.
+type Sym interface {
+}
+
+// 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
+	Type   Sym
+	Link   *Var
+}
+
+// A Context specifies how to add data to a Sym.
+type Context interface {
+	PtrSize() int
+	AddInt(s Sym, size int, i int64)
+	AddBytes(s Sym, b []byte)
+	AddAddress(s Sym, t interface{}, ofs int64)
+	AddSectionOffset(s Sym, size int, t interface{}, ofs int64)
+	AddString(s Sym, v string)
+	SymValue(s Sym) int64
+}
+
+// AppendUleb128 appends v to b using DWARF's unsigned LEB128 encoding.
+func AppendUleb128(b []byte, v uint64) []byte {
+	for {
+		c := uint8(v & 0x7f)
+		v >>= 7
+		if v != 0 {
+			c |= 0x80
+		}
+		b = append(b, c)
+		if c&0x80 == 0 {
+			break
+		}
+	}
+	return b
+}
+
+// AppendSleb128 appends v to b using DWARF's signed LEB128 encoding.
+func AppendSleb128(b []byte, v int64) []byte {
+	for {
+		c := uint8(v & 0x7f)
+		s := uint8(v & 0x40)
+		v >>= 7
+		if (v != -1 || s == 0) && (v != 0 || s != 0) {
+			c |= 0x80
+		}
+		b = append(b, c)
+		if c&0x80 == 0 {
+			break
+		}
+	}
+	return b
+}
+
+var encbuf [20]byte
+
+// AppendUleb128 appends v to s using DWARF's unsigned LEB128 encoding.
+func Uleb128put(ctxt Context, s Sym, v int64) {
+	b := AppendUleb128(encbuf[:0], uint64(v))
+	ctxt.AddBytes(s, b)
+}
+
+// AppendUleb128 appends v to s using DWARF's signed LEB128 encoding.
+func Sleb128put(ctxt Context, s Sym, v int64) {
+	b := AppendSleb128(encbuf[:0], v)
+	ctxt.AddBytes(s, b)
+}
+
+/*
+ * Defining Abbrevs.  This is hardcoded, and there will be
+ * only a handful of them.  The DWARF spec places no restriction on
+ * the ordering of attributes in the Abbrevs and DIEs, and we will
+ * always write them out in the order of declaration in the abbrev.
+ */
+type dwAttrForm struct {
+	attr uint16
+	form uint8
+}
+
+// Go-specific type attributes.
+const (
+	DW_AT_go_kind = 0x2900
+	DW_AT_go_key  = 0x2901
+	DW_AT_go_elem = 0x2902
+
+	DW_AT_internal_location = 253 // params and locals; not emitted
+)
+
+// Index into the abbrevs table below.
+// Keep in sync with ispubname() and ispubtype() below.
+// ispubtype considers >= NULLTYPE public
+const (
+	DW_ABRV_NULL = iota
+	DW_ABRV_COMPUNIT
+	DW_ABRV_FUNCTION
+	DW_ABRV_VARIABLE
+	DW_ABRV_AUTO
+	DW_ABRV_PARAM
+	DW_ABRV_STRUCTFIELD
+	DW_ABRV_FUNCTYPEPARAM
+	DW_ABRV_DOTDOTDOT
+	DW_ABRV_ARRAYRANGE
+	DW_ABRV_NULLTYPE
+	DW_ABRV_BASETYPE
+	DW_ABRV_ARRAYTYPE
+	DW_ABRV_CHANTYPE
+	DW_ABRV_FUNCTYPE
+	DW_ABRV_IFACETYPE
+	DW_ABRV_MAPTYPE
+	DW_ABRV_PTRTYPE
+	DW_ABRV_BARE_PTRTYPE // only for void*, no DW_AT_type attr to please gdb 6.
+	DW_ABRV_SLICETYPE
+	DW_ABRV_STRINGTYPE
+	DW_ABRV_STRUCTTYPE
+	DW_ABRV_TYPEDECL
+	DW_NABRV
+)
+
+type dwAbbrev struct {
+	tag      uint8
+	children uint8
+	attr     []dwAttrForm
+}
+
+var abbrevs = [DW_NABRV]dwAbbrev{
+	/* The mandatory DW_ABRV_NULL entry. */
+	{0, 0, []dwAttrForm{}},
+
+	/* COMPUNIT */
+	{
+		DW_TAG_compile_unit,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_language, DW_FORM_data1},
+			{DW_AT_low_pc, DW_FORM_addr},
+			{DW_AT_high_pc, DW_FORM_addr},
+			{DW_AT_stmt_list, DW_FORM_data4},
+			{DW_AT_comp_dir, DW_FORM_string},
+		},
+	},
+
+	/* FUNCTION */
+	{
+		DW_TAG_subprogram,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_low_pc, DW_FORM_addr},
+			{DW_AT_high_pc, DW_FORM_addr},
+			{DW_AT_external, DW_FORM_flag},
+		},
+	},
+
+	/* VARIABLE */
+	{
+		DW_TAG_variable,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_location, DW_FORM_block1},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_external, DW_FORM_flag},
+		},
+	},
+
+	/* AUTO */
+	{
+		DW_TAG_variable,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_location, DW_FORM_block1},
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+
+	/* PARAM */
+	{
+		DW_TAG_formal_parameter,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_location, DW_FORM_block1},
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+
+	/* STRUCTFIELD */
+	{
+		DW_TAG_member,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_data_member_location, DW_FORM_block1},
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+
+	/* FUNCTYPEPARAM */
+	{
+		DW_TAG_formal_parameter,
+		DW_CHILDREN_no,
+
+		// No name!
+		[]dwAttrForm{
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+
+	/* DOTDOTDOT */
+	{
+		DW_TAG_unspecified_parameters,
+		DW_CHILDREN_no,
+		[]dwAttrForm{},
+	},
+
+	/* ARRAYRANGE */
+	{
+		DW_TAG_subrange_type,
+		DW_CHILDREN_no,
+
+		// No name!
+		[]dwAttrForm{
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_count, DW_FORM_udata},
+		},
+	},
+
+	// Below here are the types considered public by ispubtype
+	/* NULLTYPE */
+	{
+		DW_TAG_unspecified_type,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+		},
+	},
+
+	/* BASETYPE */
+	{
+		DW_TAG_base_type,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_encoding, DW_FORM_data1},
+			{DW_AT_byte_size, DW_FORM_data1},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* ARRAYTYPE */
+	// child is subrange with upper bound
+	{
+		DW_TAG_array_type,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_byte_size, DW_FORM_udata},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* CHANTYPE */
+	{
+		DW_TAG_typedef,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_go_kind, DW_FORM_data1},
+			{DW_AT_go_elem, DW_FORM_ref_addr},
+		},
+	},
+
+	/* FUNCTYPE */
+	{
+		DW_TAG_subroutine_type,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			// {DW_AT_type,	DW_FORM_ref_addr},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* IFACETYPE */
+	{
+		DW_TAG_typedef,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* MAPTYPE */
+	{
+		DW_TAG_typedef,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_go_kind, DW_FORM_data1},
+			{DW_AT_go_key, DW_FORM_ref_addr},
+			{DW_AT_go_elem, DW_FORM_ref_addr},
+		},
+	},
+
+	/* PTRTYPE */
+	{
+		DW_TAG_pointer_type,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* BARE_PTRTYPE */
+	{
+		DW_TAG_pointer_type,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+		},
+	},
+
+	/* SLICETYPE */
+	{
+		DW_TAG_structure_type,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_byte_size, DW_FORM_udata},
+			{DW_AT_go_kind, DW_FORM_data1},
+			{DW_AT_go_elem, DW_FORM_ref_addr},
+		},
+	},
+
+	/* STRINGTYPE */
+	{
+		DW_TAG_structure_type,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_byte_size, DW_FORM_udata},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* STRUCTTYPE */
+	{
+		DW_TAG_structure_type,
+		DW_CHILDREN_yes,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_byte_size, DW_FORM_udata},
+			{DW_AT_go_kind, DW_FORM_data1},
+		},
+	},
+
+	/* TYPEDECL */
+	{
+		DW_TAG_typedef,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+		},
+	},
+}
+
+// GetAbbrev returns the contents of the .debug_abbrev section.
+func GetAbbrev() []byte {
+	var buf []byte
+	for i := 1; i < DW_NABRV; i++ {
+		// See section 7.5.3
+		buf = AppendUleb128(buf, uint64(i))
+
+		buf = AppendUleb128(buf, uint64(abbrevs[i].tag))
+		buf = append(buf, byte(abbrevs[i].children))
+		for _, f := range abbrevs[i].attr {
+			buf = AppendUleb128(buf, uint64(f.attr))
+			buf = AppendUleb128(buf, uint64(f.form))
+		}
+		buf = append(buf, 0, 0)
+	}
+	return append(buf, 0)
+}
+
+/*
+ * Debugging Information Entries and their attributes.
+ */
+
+// DWAttr represents an attribute of a DWDie.
+//
+// For DW_CLS_string and _block, value should contain the length, and
+// data the data, for _reference, value is 0 and data is a DWDie* to
+// the referenced instance, for all others, value is the whole thing
+// and data is null.
+type DWAttr struct {
+	Link  *DWAttr
+	Atr   uint16 // DW_AT_
+	Cls   uint8  // DW_CLS_
+	Value int64
+	Data  interface{}
+}
+
+// DWDie represents a DWARF debug info entry.
+type DWDie struct {
+	Abbrev int
+	Link   *DWDie
+	Child  *DWDie
+	Attr   *DWAttr
+	Sym    Sym
+}
+
+func putattr(ctxt Context, s Sym, abbrev int, form int, cls int, value int64, data interface{}) error {
+	switch form {
+	case DW_FORM_addr: // address
+		ctxt.AddAddress(s, data, value)
+
+	case DW_FORM_block1: // block
+		if cls == DW_CLS_ADDRESS {
+			ctxt.AddInt(s, 1, int64(1+ctxt.PtrSize()))
+			ctxt.AddInt(s, 1, DW_OP_addr)
+			ctxt.AddAddress(s, data, 0)
+			break
+		}
+
+		value &= 0xff
+		ctxt.AddInt(s, 1, value)
+		p := data.([]byte)[:value]
+		ctxt.AddBytes(s, p)
+
+	case DW_FORM_block2: // block
+		value &= 0xffff
+
+		ctxt.AddInt(s, 2, value)
+		p := data.([]byte)[:value]
+		ctxt.AddBytes(s, p)
+
+	case DW_FORM_block4: // block
+		value &= 0xffffffff
+
+		ctxt.AddInt(s, 4, value)
+		p := data.([]byte)[:value]
+		ctxt.AddBytes(s, p)
+
+	case DW_FORM_block: // block
+		Uleb128put(ctxt, s, value)
+
+		p := data.([]byte)[:value]
+		ctxt.AddBytes(s, p)
+
+	case DW_FORM_data1: // constant
+		ctxt.AddInt(s, 1, value)
+
+	case DW_FORM_data2: // constant
+		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)
+			break
+		}
+		ctxt.AddInt(s, 4, value)
+
+	case DW_FORM_data8: // constant, {line,loclist,mac,rangelist}ptr
+		ctxt.AddInt(s, 8, value)
+
+	case DW_FORM_sdata: // constant
+		Sleb128put(ctxt, s, value)
+
+	case DW_FORM_udata: // constant
+		Uleb128put(ctxt, s, value)
+
+	case DW_FORM_string: // string
+		str := data.(string)
+		ctxt.AddString(s, str)
+		// TODO(ribrdb): verify padded strings are never used and remove this
+		for i := int64(len(str)); i < value; i++ {
+			ctxt.AddInt(s, 1, 0)
+		}
+
+	case DW_FORM_flag: // flag
+		if value != 0 {
+			ctxt.AddInt(s, 1, 1)
+		} else {
+			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
+	// (> 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)
+		}
+
+	case DW_FORM_ref1, // reference within the compilation unit
+		DW_FORM_ref2,      // reference
+		DW_FORM_ref4,      // reference
+		DW_FORM_ref8,      // reference
+		DW_FORM_ref_udata, // reference
+
+		DW_FORM_strp,     // string
+		DW_FORM_indirect: // (see Section 7.5.3)
+		fallthrough
+	default:
+		return fmt.Errorf("dwarf: unsupported attribute form %d / class %d", form, cls)
+	}
+	return nil
+}
+
+// PutAttrs writes the attributes for a DIE to symbol 's'.
+//
+// Note that we can (and do) add arbitrary attributes to a DIE, but
+// only the ones actually listed in the Abbrev will be written out.
+func PutAttrs(ctxt Context, s Sym, abbrev int, attr *DWAttr) {
+Outer:
+	for _, f := range abbrevs[abbrev].attr {
+		for ap := attr; ap != nil; ap = ap.Link {
+			if ap.Atr == f.attr {
+				putattr(ctxt, s, abbrev, int(f.form), int(ap.Cls), ap.Value, ap.Data)
+				continue Outer
+			}
+		}
+
+		putattr(ctxt, s, abbrev, int(f.form), 0, 0, nil)
+	}
+}
+
+// HasChildren returns true if 'die' uses an abbrev that supports children.
+func HasChildren(die *DWDie) bool {
+	return abbrevs[die.Abbrev].children != 0
+}
+
+// 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) {
+	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+ctxt.SymValue(startPC), startPC)
+	var ev int64
+	if external {
+		ev = 1
+	}
+	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
+	names := make(map[string]bool)
+	for v := vars; v != nil; v = v.Link {
+		if strings.Contains(v.Name, ".autotmp_") {
+			continue
+		}
+		var n string
+		if names[v.Name] {
+			n = fmt.Sprintf("%s#%d", v.Name, len(names))
+		} else {
+			n = v.Name
+		}
+		names[n] = true
+
+		// Drop the package prefix from locals and arguments.
+		if i := strings.LastIndex(n, "."); i >= 0 {
+			n = n[i+1:]
+		}
+
+		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)
+
+	}
+	Uleb128put(ctxt, s, 0)
+}

src/cmd/internal/dwarf/dwarf_defs.go

@@ -2,40 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package ld
-
-// TODO/NICETOHAVE:
-//   - eliminate DW_CLS_ if not used
-//   - package info in compilation units
-//   - 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 'obj.i' you mean.
-//   - file:line info for variables
-//   - make strings a typedef so prettyprinters can see the underlying string type
-//
-// Copyright 2010 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.
-
-/*
- * Emit debug_abbrevs, debug_info and debug_line sections to current
- * offset in cout.
- */
-
-/*
- * Add the dwarf section names to the ELF
- * s[ection]h[eader]str[ing]tab.  Prerequisite for
- * dwarfaddelfheaders().
- */
-
-/*
- * Add section headers pointing to the sections emitted in
- * dwarfemitdebugsections.
- */
-// Copyright 2010 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 dwarf
 
 // Cut, pasted, tr-and-awk'ed from tables in
 // http://dwarfstd.org/doc/Dwarf3.pdf

src/cmd/internal/obj/objfile.go

@@ -109,6 +109,7 @@ package obj
 
 import (
 	"bufio"
+	"cmd/internal/dwarf"
 	"cmd/internal/sys"
 	"fmt"
 	"log"
@@ -506,3 +507,94 @@ type relocByOff []Reloc
 func (x relocByOff) Len() int           { return len(x) }
 func (x relocByOff) Less(i, j int) bool { return x[i].Off < x[j].Off }
 func (x relocByOff) Swap(i, j int)      { x[i], x[j] = x[j], x[i] }
+
+// implement dwarf.Context
+type dwCtxt struct{ *Link }
+
+func (c dwCtxt) PtrSize() int {
+	return c.Arch.PtrSize
+}
+func (c dwCtxt) AddInt(s dwarf.Sym, size int, i int64) {
+	ls := s.(*LSym)
+	ls.WriteInt(c.Link, ls.Size, size, i)
+}
+func (c dwCtxt) AddBytes(s dwarf.Sym, b []byte) {
+	ls := s.(*LSym)
+	ls.WriteBytes(c.Link, ls.Size, b)
+}
+func (c dwCtxt) AddString(s dwarf.Sym, v string) {
+	ls := s.(*LSym)
+	ls.WriteString(c.Link, ls.Size, len(v), v)
+	ls.WriteInt(c.Link, ls.Size, 1, 0)
+}
+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)
+}
+func (c dwCtxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64) {
+	ls := s.(*LSym)
+	rsym := t.(*LSym)
+	ls.WriteAddr(c.Link, ls.Size, size, rsym, ofs)
+	r := &ls.R[len(ls.R)-1]
+	r.Type = R_DWARFREF
+}
+
+func gendwarf(ctxt *Link, text []*LSym) []*LSym {
+	dctxt := dwCtxt{ctxt}
+	var dw []*LSym
+
+	for _, s := range text {
+		dsym := Linklookup(ctxt, dwarf.InfoPrefix+s.Name, int(s.Version))
+		if dsym.Size != 0 {
+			continue
+		}
+		dw = append(dw, dsym)
+		dsym.Type = SDWARFINFO
+		dsym.Dupok = s.Dupok
+		var vars dwarf.Var
+		var abbrev int
+		var offs int32
+		for a := s.Autom; a != nil; a = a.Link {
+			switch a.Name {
+			case NAME_AUTO:
+				abbrev = dwarf.DW_ABRV_AUTO
+				offs = a.Aoffset
+				if ctxt.FixedFrameSize() == 0 {
+					offs -= int32(ctxt.Arch.PtrSize)
+				}
+				if Framepointer_enabled(Getgoos(), Getgoarch()) {
+					offs -= int32(ctxt.Arch.PtrSize)
+				}
+
+			case NAME_PARAM:
+				abbrev = dwarf.DW_ABRV_PARAM
+				offs = a.Aoffset + int32(ctxt.FixedFrameSize())
+
+			default:
+				continue
+			}
+			typename := dwarf.InfoPrefix + a.Gotype.Name[len("type."):]
+			dwvar := &dwarf.Var{
+				Name:   a.Asym.Name,
+				Abbrev: abbrev,
+				Offset: int32(offs),
+				Type:   Linklookup(ctxt, typename, 0),
+			}
+			dws := &vars.Link
+			for ; *dws != nil; dws = &(*dws).Link {
+				if offs <= (*dws).Offset {
+					break
+				}
+			}
+			dwvar.Link = *dws
+			*dws = dwvar
+		}
+		dwarf.PutFunc(dctxt, dsym, s.Name, s.Version == 0, s, s.Size, vars.Link)
+	}
+	return dw
+}

src/cmd/internal/obj/plist.go

@@ -209,6 +209,7 @@ func flushplist(ctxt *Link, freeProgs bool) {
 
 	// Add to running list in ctxt.
 	ctxt.Text = append(ctxt.Text, text...)
+	ctxt.Data = append(ctxt.Data, gendwarf(ctxt, text)...)
 	ctxt.Plist = nil
 	ctxt.Plast = nil
 	ctxt.Curp = nil

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

@@ -342,8 +342,14 @@ func relocsym(s *LSym) {
 		if r.Sym != nil && (r.Sym.Type&(obj.SMASK|obj.SHIDDEN) == 0 || r.Sym.Type&obj.SMASK == obj.SXREF) {
 			// When putting the runtime but not main into a shared library
 			// these symbols are undefined and that's OK.
-			if Buildmode == BuildmodeShared && (r.Sym.Name == "main.main" || r.Sym.Name == "main.init") {
-				r.Sym.Type = obj.SDYNIMPORT
+			if Buildmode == BuildmodeShared {
+				if r.Sym.Name == "main.main" || r.Sym.Name == "main.init" {
+					r.Sym.Type = obj.SDYNIMPORT
+				} else if strings.HasPrefix(r.Sym.Name, "go.info.") {
+					// Skip go.info symbols. They are only needed to communicate
+					// DWARF info between the compiler and linker.
+					continue
+				}
 			} else {
 				Diag("%s: not defined", r.Sym.Name)
 				continue

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

@@ -111,6 +111,7 @@ import (
 	"bufio"
 	"bytes"
 	"cmd/internal/bio"
+	"cmd/internal/dwarf"
 	"cmd/internal/obj"
 	"crypto/sha1"
 	"encoding/base64"
@@ -401,6 +402,37 @@ overwrite:
 			r.ctxt.Textp = append(r.ctxt.Textp, s)
 		}
 	}
+	if s.Type == obj.SDWARFINFO {
+		r.patchDWARFName(s)
+	}
+}
+
+func (r *objReader) patchDWARFName(s *LSym) {
+	// This is kind of ugly. Really the package name should not
+	// even be included here.
+	if s.Size < 1 || s.P[0] != dwarf.DW_ABRV_FUNCTION {
+		return
+	}
+	e := bytes.IndexByte(s.P, 0)
+	if e == -1 {
+		return
+	}
+	p := bytes.Index(s.P[:e], emptyPkg)
+	if p == -1 {
+		return
+	}
+	pkgprefix := []byte(r.pkg + ".")
+	patched := bytes.Replace(s.P[:e], emptyPkg, pkgprefix, -1)
+
+	s.P = append(patched, s.P[e:]...)
+	delta := int64(len(s.P)) - s.Size
+	s.Size = int64(len(s.P))
+	for i := range s.R {
+		r := &s.R[i]
+		if r.Off > int32(e) {
+			r.Off += int32(delta)
+		}
+	}
 }
 
 func (r *objReader) readFull(b []byte) {

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

@@ -269,6 +269,8 @@ func Asmplan9sym() {
 
 var symt *LSym
 
+var encbuf [10]byte
+
 func Wputb(w uint16) { Cwrite(Append16b(encbuf[:0], w)) }
 func Lputb(l uint32) { Cwrite(Append32b(encbuf[:0], l)) }
 func Vputb(v uint64) { Cwrite(Append64b(encbuf[:0], v)) }