gtsocial-umbx

dwarf.go (22513B)

---

 // Copyright 2019 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.
 
 // Writes dwarf information to object files.
 
 package obj
 
 import (
 	"github.com/twitchyliquid64/golang-asm/dwarf"
 	"github.com/twitchyliquid64/golang-asm/objabi"
 	"github.com/twitchyliquid64/golang-asm/src"
 	"fmt"
 	"sort"
 	"sync"
 )
 
 // Generate a sequence of opcodes that is as short as possible.
 // See section 6.2.5
 const (
 	LINE_BASE   = -4
 	LINE_RANGE  = 10
 	PC_RANGE    = (255 - OPCODE_BASE) / LINE_RANGE
 	OPCODE_BASE = 11
 )
 
 // generateDebugLinesSymbol fills the debug lines symbol of a given function.
 //
 // It's worth noting that this function doesn't generate the full debug_lines
 // DWARF section, saving that for the linker. This function just generates the
 // state machine part of debug_lines. The full table is generated by the
 // linker.  Also, we use the file numbers from the full package (not just the
 // function in question) when generating the state machine. We do this so we
 // don't have to do a fixup on the indices when writing the full section.
 func (ctxt *Link) generateDebugLinesSymbol(s, lines *LSym) {
 	dctxt := dwCtxt{ctxt}
 
 	// Emit a LNE_set_address extended opcode, so as to establish the
 	// starting text address of this function.
 	dctxt.AddUint8(lines, 0)
 	dwarf.Uleb128put(dctxt, lines, 1+int64(ctxt.Arch.PtrSize))
 	dctxt.AddUint8(lines, dwarf.DW_LNE_set_address)
 	dctxt.AddAddress(lines, s, 0)
 
 	// Set up the debug_lines state machine to the default values
 	// we expect at the start of a new sequence.
 	stmt := true
 	line := int64(1)
 	pc := s.Func.Text.Pc
 	var lastpc int64 // last PC written to line table, not last PC in func
 	name := ""
 	prologue, wrotePrologue := false, false
 	// Walk the progs, generating the DWARF table.
 	for p := s.Func.Text; p != nil; p = p.Link {
 		prologue = prologue || (p.Pos.Xlogue() == src.PosPrologueEnd)
 		// If we're not at a real instruction, keep looping!
 		if p.Pos.Line() == 0 || (p.Link != nil && p.Link.Pc == p.Pc) {
 			continue
 		}
 		newStmt := p.Pos.IsStmt() != src.PosNotStmt
 		newName, newLine := linkgetlineFromPos(ctxt, p.Pos)
 
 		// Output debug info.
 		wrote := false
 		if name != newName {
 			newFile := ctxt.PosTable.FileIndex(newName) + 1 // 1 indexing for the table.
 			dctxt.AddUint8(lines, dwarf.DW_LNS_set_file)
 			dwarf.Uleb128put(dctxt, lines, int64(newFile))
 			name = newName
 			wrote = true
 		}
 		if prologue && !wrotePrologue {
 			dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_set_prologue_end))
 			wrotePrologue = true
 			wrote = true
 		}
 		if stmt != newStmt {
 			dctxt.AddUint8(lines, uint8(dwarf.DW_LNS_negate_stmt))
 			stmt = newStmt
 			wrote = true
 		}
 
 		if line != int64(newLine) || wrote {
 			pcdelta := p.Pc - pc
 			lastpc = p.Pc
 			putpclcdelta(ctxt, dctxt, lines, uint64(pcdelta), int64(newLine)-line)
 			line, pc = int64(newLine), p.Pc
 		}
 	}
 
 	// Because these symbols will be concatenated together by the
 	// linker, we need to reset the state machine that controls the
 	// debug symbols. Do this using an end-of-sequence operator.
 	//
 	// Note: at one point in time, Delve did not support multiple end
 	// sequence ops within a compilation unit (bug for this:
 	// https://github.com/go-delve/delve/issues/1694), however the bug
 	// has since been fixed (Oct 2019).
 	//
 	// Issue 38192: the DWARF standard specifies that when you issue
 	// an end-sequence op, the PC value should be one past the last
 	// text address in the translation unit, so apply a delta to the
 	// text address before the end sequence op. If this isn't done,
 	// GDB will assign a line number of zero the last row in the line
 	// table, which we don't want.
 	lastlen := uint64(s.Size - (lastpc - s.Func.Text.Pc))
 	putpclcdelta(ctxt, dctxt, lines, lastlen, 0)
 	dctxt.AddUint8(lines, 0) // start extended opcode
 	dwarf.Uleb128put(dctxt, lines, 1)
 	dctxt.AddUint8(lines, dwarf.DW_LNE_end_sequence)
 }
 
 func putpclcdelta(linkctxt *Link, dctxt dwCtxt, s *LSym, deltaPC uint64, deltaLC int64) {
 	// Choose a special opcode that minimizes the number of bytes needed to
 	// encode the remaining PC delta and LC delta.
 	var opcode int64
 	if deltaLC < LINE_BASE {
 		if deltaPC >= PC_RANGE {
 			opcode = OPCODE_BASE + (LINE_RANGE * PC_RANGE)
 		} else {
 			opcode = OPCODE_BASE + (LINE_RANGE * int64(deltaPC))
 		}
 	} else if deltaLC < LINE_BASE+LINE_RANGE {
 		if deltaPC >= PC_RANGE {
 			opcode = OPCODE_BASE + (deltaLC - LINE_BASE) + (LINE_RANGE * PC_RANGE)
 			if opcode > 255 {
 				opcode -= LINE_RANGE
 			}
 		} else {
 			opcode = OPCODE_BASE + (deltaLC - LINE_BASE) + (LINE_RANGE * int64(deltaPC))
 		}
 	} else {
 		if deltaPC <= PC_RANGE {
 			opcode = OPCODE_BASE + (LINE_RANGE - 1) + (LINE_RANGE * int64(deltaPC))
 			if opcode > 255 {
 				opcode = 255
 			}
 		} else {
 			// Use opcode 249 (pc+=23, lc+=5) or 255 (pc+=24, lc+=1).
 			//
 			// Let x=deltaPC-PC_RANGE.  If we use opcode 255, x will be the remaining
 			// deltaPC that we need to encode separately before emitting 255.  If we
 			// use opcode 249, we will need to encode x+1.  If x+1 takes one more
 			// byte to encode than x, then we use opcode 255.
 			//
 			// In all other cases x and x+1 take the same number of bytes to encode,
 			// so we use opcode 249, which may save us a byte in encoding deltaLC,
 			// for similar reasons.
 			switch deltaPC - PC_RANGE {
 			// PC_RANGE is the largest deltaPC we can encode in one byte, using
 			// DW_LNS_const_add_pc.
 			//
 			// (1<<16)-1 is the largest deltaPC we can encode in three bytes, using
 			// DW_LNS_fixed_advance_pc.
 			//
 			// (1<<(7n))-1 is the largest deltaPC we can encode in n+1 bytes for
 			// n=1,3,4,5,..., using DW_LNS_advance_pc.
 			case PC_RANGE, (1 << 7) - 1, (1 << 16) - 1, (1 << 21) - 1, (1 << 28) - 1,
 				(1 << 35) - 1, (1 << 42) - 1, (1 << 49) - 1, (1 << 56) - 1, (1 << 63) - 1:
 				opcode = 255
 			default:
 				opcode = OPCODE_BASE + LINE_RANGE*PC_RANGE - 1 // 249
 			}
 		}
 	}
 	if opcode < OPCODE_BASE || opcode > 255 {
 		panic(fmt.Sprintf("produced invalid special opcode %d", opcode))
 	}
 
 	// Subtract from deltaPC and deltaLC the amounts that the opcode will add.
 	deltaPC -= uint64((opcode - OPCODE_BASE) / LINE_RANGE)
 	deltaLC -= (opcode-OPCODE_BASE)%LINE_RANGE + LINE_BASE
 
 	// Encode deltaPC.
 	if deltaPC != 0 {
 		if deltaPC <= PC_RANGE {
 			// Adjust the opcode so that we can use the 1-byte DW_LNS_const_add_pc
 			// instruction.
 			opcode -= LINE_RANGE * int64(PC_RANGE-deltaPC)
 			if opcode < OPCODE_BASE {
 				panic(fmt.Sprintf("produced invalid special opcode %d", opcode))
 			}
 			dctxt.AddUint8(s, dwarf.DW_LNS_const_add_pc)
 		} else if (1<<14) <= deltaPC && deltaPC < (1<<16) {
 			dctxt.AddUint8(s, dwarf.DW_LNS_fixed_advance_pc)
 			dctxt.AddUint16(s, uint16(deltaPC))
 		} else {
 			dctxt.AddUint8(s, dwarf.DW_LNS_advance_pc)
 			dwarf.Uleb128put(dctxt, s, int64(deltaPC))
 		}
 	}
 
 	// Encode deltaLC.
 	if deltaLC != 0 {
 		dctxt.AddUint8(s, dwarf.DW_LNS_advance_line)
 		dwarf.Sleb128put(dctxt, s, deltaLC)
 	}
 
 	// Output the special opcode.
 	dctxt.AddUint8(s, uint8(opcode))
 }
 
 // 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) AddUint16(s dwarf.Sym, i uint16) {
 	c.AddInt(s, 2, int64(i))
 }
 func (c dwCtxt) AddUint8(s dwarf.Sym, i uint8) {
 	b := []byte{byte(i)}
 	c.AddBytes(s, b)
 }
 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) AddAddress(s dwarf.Sym, data interface{}, value int64) {
 	ls := s.(*LSym)
 	size := c.PtrSize()
 	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) AddCURelativeAddress(s dwarf.Sym, data interface{}, value int64) {
 	ls := s.(*LSym)
 	rsym := data.(*LSym)
 	ls.WriteCURelativeAddr(c.Link, ls.Size, rsym, value)
 }
 func (c dwCtxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64) {
 	panic("should be used only in the linker")
 }
 func (c dwCtxt) AddDWARFAddrSectionOffset(s dwarf.Sym, t interface{}, ofs int64) {
 	size := 4
 	if isDwarf64(c.Link) {
 		size = 8
 	}
 
 	ls := s.(*LSym)
 	rsym := t.(*LSym)
 	ls.WriteAddr(c.Link, ls.Size, size, rsym, ofs)
 	r := &ls.R[len(ls.R)-1]
 	r.Type = objabi.R_DWARFSECREF
 }
 
 func (c dwCtxt) AddFileRef(s dwarf.Sym, f interface{}) {
 	ls := s.(*LSym)
 	rsym := f.(*LSym)
 	fidx := c.Link.PosTable.FileIndex(rsym.Name)
 	// Note the +1 here -- the value we're writing is going to be an
 	// index into the DWARF line table file section, whose entries
 	// are numbered starting at 1, not 0.
 	ls.WriteInt(c.Link, ls.Size, 4, int64(fidx+1))
 }
 
 func (c dwCtxt) CurrentOffset(s dwarf.Sym) int64 {
 	ls := s.(*LSym)
 	return ls.Size
 }
 
 // Here "from" is a symbol corresponding to an inlined or concrete
 // function, "to" is the symbol for the corresponding abstract
 // function, and "dclIdx" is the index of the symbol of interest with
 // respect to the Dcl slice of the original pre-optimization version
 // of the inlined function.
 func (c dwCtxt) RecordDclReference(from dwarf.Sym, to dwarf.Sym, dclIdx int, inlIndex int) {
 	ls := from.(*LSym)
 	tls := to.(*LSym)
 	ridx := len(ls.R) - 1
 	c.Link.DwFixups.ReferenceChildDIE(ls, ridx, tls, dclIdx, inlIndex)
 }
 
 func (c dwCtxt) RecordChildDieOffsets(s dwarf.Sym, vars []*dwarf.Var, offsets []int32) {
 	ls := s.(*LSym)
 	c.Link.DwFixups.RegisterChildDIEOffsets(ls, vars, offsets)
 }
 
 func (c dwCtxt) Logf(format string, args ...interface{}) {
 	c.Link.Logf(format, args...)
 }
 
 func isDwarf64(ctxt *Link) bool {
 	return ctxt.Headtype == objabi.Haix
 }
 
 func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym, dwarfAbsFnSym, dwarfDebugLines *LSym) {
 	if s.Type != objabi.STEXT {
 		ctxt.Diag("dwarfSym of non-TEXT %v", s)
 	}
 	if s.Func.dwarfInfoSym == nil {
 		s.Func.dwarfInfoSym = &LSym{
 			Type: objabi.SDWARFFCN,
 		}
 		if ctxt.Flag_locationlists {
 			s.Func.dwarfLocSym = &LSym{
 				Type: objabi.SDWARFLOC,
 			}
 		}
 		s.Func.dwarfRangesSym = &LSym{
 			Type: objabi.SDWARFRANGE,
 		}
 		s.Func.dwarfDebugLinesSym = &LSym{
 			Type: objabi.SDWARFLINES,
 		}
 		if s.WasInlined() {
 			s.Func.dwarfAbsFnSym = ctxt.DwFixups.AbsFuncDwarfSym(s)
 		}
 	}
 	return s.Func.dwarfInfoSym, s.Func.dwarfLocSym, s.Func.dwarfRangesSym, s.Func.dwarfAbsFnSym, s.Func.dwarfDebugLinesSym
 }
 
 func (s *LSym) Length(dwarfContext interface{}) int64 {
 	return s.Size
 }
 
 // fileSymbol returns a symbol corresponding to the source file of the
 // first instruction (prog) of the specified function. This will
 // presumably be the file in which the function is defined.
 func (ctxt *Link) fileSymbol(fn *LSym) *LSym {
 	p := fn.Func.Text
 	if p != nil {
 		f, _ := linkgetlineFromPos(ctxt, p.Pos)
 		fsym := ctxt.Lookup(f)
 		return fsym
 	}
 	return nil
 }
 
 // populateDWARF fills in the DWARF Debugging Information Entries for
 // TEXT symbol 's'. The various DWARF symbols must already have been
 // initialized in InitTextSym.
 func (ctxt *Link) populateDWARF(curfn interface{}, s *LSym, myimportpath string) {
 	info, loc, ranges, absfunc, lines := ctxt.dwarfSym(s)
 	if info.Size != 0 {
 		ctxt.Diag("makeFuncDebugEntry double process %v", s)
 	}
 	var scopes []dwarf.Scope
 	var inlcalls dwarf.InlCalls
 	if ctxt.DebugInfo != nil {
 		scopes, inlcalls = ctxt.DebugInfo(s, info, curfn)
 	}
 	var err error
 	dwctxt := dwCtxt{ctxt}
 	filesym := ctxt.fileSymbol(s)
 	fnstate := &dwarf.FnState{
 		Name:          s.Name,
 		Importpath:    myimportpath,
 		Info:          info,
 		Filesym:       filesym,
 		Loc:           loc,
 		Ranges:        ranges,
 		Absfn:         absfunc,
 		StartPC:       s,
 		Size:          s.Size,
 		External:      !s.Static(),
 		Scopes:        scopes,
 		InlCalls:      inlcalls,
 		UseBASEntries: ctxt.UseBASEntries,
 	}
 	if absfunc != nil {
 		err = dwarf.PutAbstractFunc(dwctxt, fnstate)
 		if err != nil {
 			ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
 		}
 		err = dwarf.PutConcreteFunc(dwctxt, fnstate)
 	} else {
 		err = dwarf.PutDefaultFunc(dwctxt, fnstate)
 	}
 	if err != nil {
 		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
 	}
 	// Fill in the debug lines symbol.
 	ctxt.generateDebugLinesSymbol(s, lines)
 }
 
 // DwarfIntConst creates a link symbol for an integer constant with the
 // given name, type and value.
 func (ctxt *Link) DwarfIntConst(myimportpath, name, typename string, val int64) {
 	if myimportpath == "" {
 		return
 	}
 	s := ctxt.LookupInit(dwarf.ConstInfoPrefix+myimportpath, func(s *LSym) {
 		s.Type = objabi.SDWARFCONST
 		ctxt.Data = append(ctxt.Data, s)
 	})
 	dwarf.PutIntConst(dwCtxt{ctxt}, s, ctxt.Lookup(dwarf.InfoPrefix+typename), myimportpath+"."+name, val)
 }
 
 func (ctxt *Link) DwarfAbstractFunc(curfn interface{}, s *LSym, myimportpath string) {
 	absfn := ctxt.DwFixups.AbsFuncDwarfSym(s)
 	if absfn.Size != 0 {
 		ctxt.Diag("internal error: DwarfAbstractFunc double process %v", s)
 	}
 	if s.Func == nil {
 		s.Func = new(FuncInfo)
 	}
 	scopes, _ := ctxt.DebugInfo(s, absfn, curfn)
 	dwctxt := dwCtxt{ctxt}
 	filesym := ctxt.fileSymbol(s)
 	fnstate := dwarf.FnState{
 		Name:          s.Name,
 		Importpath:    myimportpath,
 		Info:          absfn,
 		Filesym:       filesym,
 		Absfn:         absfn,
 		External:      !s.Static(),
 		Scopes:        scopes,
 		UseBASEntries: ctxt.UseBASEntries,
 	}
 	if err := dwarf.PutAbstractFunc(dwctxt, &fnstate); err != nil {
 		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
 	}
 }
 
 // This table is designed to aid in the creation of references between
 // DWARF subprogram DIEs.
 //
 // In most cases when one DWARF DIE has to refer to another DWARF DIE,
 // the target of the reference has an LSym, which makes it easy to use
 // the existing relocation mechanism. For DWARF inlined routine DIEs,
 // however, the subprogram DIE has to refer to a child
 // parameter/variable DIE of the abstract subprogram. This child DIE
 // doesn't have an LSym, and also of interest is the fact that when
 // DWARF generation is happening for inlined function F within caller
 // G, it's possible that DWARF generation hasn't happened yet for F,
 // so there is no way to know the offset of a child DIE within F's
 // abstract function. Making matters more complex, each inlined
 // instance of F may refer to a subset of the original F's variables
 // (depending on what happens with optimization, some vars may be
 // eliminated).
 //
 // The fixup table below helps overcome this hurdle. At the point
 // where a parameter/variable reference is made (via a call to
 // "ReferenceChildDIE"), a fixup record is generate that records
 // the relocation that is targeting that child variable. At a later
 // point when the abstract function DIE is emitted, there will be
 // a call to "RegisterChildDIEOffsets", at which point the offsets
 // needed to apply fixups are captured. Finally, once the parallel
 // portion of the compilation is done, fixups can actually be applied
 // during the "Finalize" method (this can't be done during the
 // parallel portion of the compile due to the possibility of data
 // races).
 //
 // This table is also used to record the "precursor" function node for
 // each function that is the target of an inline -- child DIE references
 // have to be made with respect to the original pre-optimization
 // version of the function (to allow for the fact that each inlined
 // body may be optimized differently).
 type DwarfFixupTable struct {
 	ctxt      *Link
 	mu        sync.Mutex
 	symtab    map[*LSym]int // maps abstract fn LSYM to index in svec
 	svec      []symFixups
 	precursor map[*LSym]fnState // maps fn Lsym to precursor Node, absfn sym
 }
 
 type symFixups struct {
 	fixups   []relFixup
 	doffsets []declOffset
 	inlIndex int32
 	defseen  bool
 }
 
 type declOffset struct {
 	// Index of variable within DCL list of pre-optimization function
 	dclIdx int32
 	// Offset of var's child DIE with respect to containing subprogram DIE
 	offset int32
 }
 
 type relFixup struct {
 	refsym *LSym
 	relidx int32
 	dclidx int32
 }
 
 type fnState struct {
 	// precursor function (really *gc.Node)
 	precursor interface{}
 	// abstract function symbol
 	absfn *LSym
 }
 
 func NewDwarfFixupTable(ctxt *Link) *DwarfFixupTable {
 	return &DwarfFixupTable{
 		ctxt:      ctxt,
 		symtab:    make(map[*LSym]int),
 		precursor: make(map[*LSym]fnState),
 	}
 }
 
 func (ft *DwarfFixupTable) GetPrecursorFunc(s *LSym) interface{} {
 	if fnstate, found := ft.precursor[s]; found {
 		return fnstate.precursor
 	}
 	return nil
 }
 
 func (ft *DwarfFixupTable) SetPrecursorFunc(s *LSym, fn interface{}) {
 	if _, found := ft.precursor[s]; found {
 		ft.ctxt.Diag("internal error: DwarfFixupTable.SetPrecursorFunc double call on %v", s)
 	}
 
 	// initialize abstract function symbol now. This is done here so
 	// as to avoid data races later on during the parallel portion of
 	// the back end.
 	absfn := ft.ctxt.LookupDerived(s, dwarf.InfoPrefix+s.Name+dwarf.AbstractFuncSuffix)
 	absfn.Set(AttrDuplicateOK, true)
 	absfn.Type = objabi.SDWARFABSFCN
 	ft.ctxt.Data = append(ft.ctxt.Data, absfn)
 
 	// In the case of "late" inlining (inlines that happen during
 	// wrapper generation as opposed to the main inlining phase) it's
 	// possible that we didn't cache the abstract function sym for the
 	// text symbol -- do so now if needed. See issue 38068.
 	if s.Func != nil && s.Func.dwarfAbsFnSym == nil {
 		s.Func.dwarfAbsFnSym = absfn
 	}
 
 	ft.precursor[s] = fnState{precursor: fn, absfn: absfn}
 }
 
 // Make a note of a child DIE reference: relocation 'ridx' within symbol 's'
 // is targeting child 'c' of DIE with symbol 'tgt'.
 func (ft *DwarfFixupTable) ReferenceChildDIE(s *LSym, ridx int, tgt *LSym, dclidx int, inlIndex int) {
 	// Protect against concurrent access if multiple backend workers
 	ft.mu.Lock()
 	defer ft.mu.Unlock()
 
 	// Create entry for symbol if not already present.
 	idx, found := ft.symtab[tgt]
 	if !found {
 		ft.svec = append(ft.svec, symFixups{inlIndex: int32(inlIndex)})
 		idx = len(ft.svec) - 1
 		ft.symtab[tgt] = idx
 	}
 
 	// Do we have child DIE offsets available? If so, then apply them,
 	// otherwise create a fixup record.
 	sf := &ft.svec[idx]
 	if len(sf.doffsets) > 0 {
 		found := false
 		for _, do := range sf.doffsets {
 			if do.dclIdx == int32(dclidx) {
 				off := do.offset
 				s.R[ridx].Add += int64(off)
 				found = true
 				break
 			}
 		}
 		if !found {
 			ft.ctxt.Diag("internal error: DwarfFixupTable.ReferenceChildDIE unable to locate child DIE offset for dclIdx=%d src=%v tgt=%v", dclidx, s, tgt)
 		}
 	} else {
 		sf.fixups = append(sf.fixups, relFixup{s, int32(ridx), int32(dclidx)})
 	}
 }
 
 // Called once DWARF generation is complete for a given abstract function,
 // whose children might have been referenced via a call above. Stores
 // the offsets for any child DIEs (vars, params) so that they can be
 // consumed later in on DwarfFixupTable.Finalize, which applies any
 // outstanding fixups.
 func (ft *DwarfFixupTable) RegisterChildDIEOffsets(s *LSym, vars []*dwarf.Var, coffsets []int32) {
 	// Length of these two slices should agree
 	if len(vars) != len(coffsets) {
 		ft.ctxt.Diag("internal error: RegisterChildDIEOffsets vars/offsets length mismatch")
 		return
 	}
 
 	// Generate the slice of declOffset's based in vars/coffsets
 	doffsets := make([]declOffset, len(coffsets))
 	for i := range coffsets {
 		doffsets[i].dclIdx = vars[i].ChildIndex
 		doffsets[i].offset = coffsets[i]
 	}
 
 	ft.mu.Lock()
 	defer ft.mu.Unlock()
 
 	// Store offsets for this symbol.
 	idx, found := ft.symtab[s]
 	if !found {
 		sf := symFixups{inlIndex: -1, defseen: true, doffsets: doffsets}
 		ft.svec = append(ft.svec, sf)
 		ft.symtab[s] = len(ft.svec) - 1
 	} else {
 		sf := &ft.svec[idx]
 		sf.doffsets = doffsets
 		sf.defseen = true
 	}
 }
 
 func (ft *DwarfFixupTable) processFixups(slot int, s *LSym) {
 	sf := &ft.svec[slot]
 	for _, f := range sf.fixups {
 		dfound := false
 		for _, doffset := range sf.doffsets {
 			if doffset.dclIdx == f.dclidx {
 				f.refsym.R[f.relidx].Add += int64(doffset.offset)
 				dfound = true
 				break
 			}
 		}
 		if !dfound {
 			ft.ctxt.Diag("internal error: DwarfFixupTable has orphaned fixup on %v targeting %v relidx=%d dclidx=%d", f.refsym, s, f.relidx, f.dclidx)
 		}
 	}
 }
 
 // return the LSym corresponding to the 'abstract subprogram' DWARF
 // info entry for a function.
 func (ft *DwarfFixupTable) AbsFuncDwarfSym(fnsym *LSym) *LSym {
 	// Protect against concurrent access if multiple backend workers
 	ft.mu.Lock()
 	defer ft.mu.Unlock()
 
 	if fnstate, found := ft.precursor[fnsym]; found {
 		return fnstate.absfn
 	}
 	ft.ctxt.Diag("internal error: AbsFuncDwarfSym requested for %v, not seen during inlining", fnsym)
 	return nil
 }
 
 // Called after all functions have been compiled; the main job of this
 // function is to identify cases where there are outstanding fixups.
 // This scenario crops up when we have references to variables of an
 // inlined routine, but that routine is defined in some other package.
 // This helper walks through and locate these fixups, then invokes a
 // helper to create an abstract subprogram DIE for each one.
 func (ft *DwarfFixupTable) Finalize(myimportpath string, trace bool) {
 	if trace {
 		ft.ctxt.Logf("DwarfFixupTable.Finalize invoked for %s\n", myimportpath)
 	}
 
 	// Collect up the keys from the precursor map, then sort the
 	// resulting list (don't want to rely on map ordering here).
 	fns := make([]*LSym, len(ft.precursor))
 	idx := 0
 	for fn := range ft.precursor {
 		fns[idx] = fn
 		idx++
 	}
 	sort.Sort(BySymName(fns))
 
 	// Should not be called during parallel portion of compilation.
 	if ft.ctxt.InParallel {
 		ft.ctxt.Diag("internal error: DwarfFixupTable.Finalize call during parallel backend")
 	}
 
 	// Generate any missing abstract functions.
 	for _, s := range fns {
 		absfn := ft.AbsFuncDwarfSym(s)
 		slot, found := ft.symtab[absfn]
 		if !found || !ft.svec[slot].defseen {
 			ft.ctxt.GenAbstractFunc(s)
 		}
 	}
 
 	// Apply fixups.
 	for _, s := range fns {
 		absfn := ft.AbsFuncDwarfSym(s)
 		slot, found := ft.symtab[absfn]
 		if !found {
 			ft.ctxt.Diag("internal error: DwarfFixupTable.Finalize orphan abstract function for %v", s)
 		} else {
 			ft.processFixups(slot, s)
 		}
 	}
 }
 
 type BySymName []*LSym
 
 func (s BySymName) Len() int           { return len(s) }
 func (s BySymName) Less(i, j int) bool { return s[i].Name < s[j].Name }
 func (s BySymName) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }