cmd/link: abstract DWARF metadata symbol lookup

The compiler passes a lot of DWARF metadata about functions to the
linker via symbols whose names are derived from the function's own
symbol name. We look up these symbols in several places. This is about
to get slightly more complex as we introduce ABIs as symbol versions,
so abstract this lookup pattern into a helper function.

For #27539.

Change-Id: Ic71f6b5dc6608a5a5f5f515808981e6d6f5d728e
Reviewed-on: https://go-review.googlesource.com/c/146858
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

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

@@ -342,6 +342,16 @@ func lookupOrDiag(ctxt *Link, n string) *sym.Symbol {
 	return s
 }
 
+// dwarfFuncSym looks up a DWARF metadata symbol for function symbol s.
+// If the symbol does not exist, it creates it if create is true,
+// or returns nil otherwise.
+func dwarfFuncSym(ctxt *Link, s *sym.Symbol, meta string, create bool) *sym.Symbol {
+	if create {
+		return ctxt.Syms.Lookup(meta+s.Name, int(s.Version))
+	}
+	return ctxt.Syms.ROLookup(meta+s.Name, int(s.Version))
+}
+
 func dotypedef(ctxt *Link, parent *dwarf.DWDie, name string, def *dwarf.DWDie) *dwarf.DWDie {
 	// Only emit typedefs for real names.
 	if strings.HasPrefix(name, "map[") {
@@ -1146,7 +1156,7 @@ func writelines(ctxt *Link, unit *compilationUnit, ls *sym.Symbol) {
 	// indexes (created by numberfile) to CU-local indexes.
 	fileNums := make(map[int]int)
 	for _, s := range unit.lib.Textp { // textp has been dead-code-eliminated already.
-		dsym := ctxt.Syms.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
+		dsym := dwarfFuncSym(ctxt, s, dwarf.InfoPrefix, true)
 		for _, f := range s.FuncInfo.File {
 			if _, ok := fileNums[int(f.Value)]; ok {
 				continue
@@ -1756,12 +1766,12 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 		// referenced abstract functions.
 		// Collect all debug_range symbols in unit.rangeSyms
 		for _, s := range lib.Textp { // textp has been dead-code-eliminated already.
-			dsym := ctxt.Syms.ROLookup(dwarf.InfoPrefix+s.Name, int(s.Version))
+			dsym := dwarfFuncSym(ctxt, s, dwarf.InfoPrefix, false)
 			dsym.Attr |= sym.AttrNotInSymbolTable | sym.AttrReachable
 			dsym.Type = sym.SDWARFINFO
 			unit.funcDIEs = append(unit.funcDIEs, dsym)
 
-			rangeSym := ctxt.Syms.ROLookup(dwarf.RangePrefix+s.Name, int(s.Version))
+			rangeSym := dwarfFuncSym(ctxt, s, dwarf.RangePrefix, false)
 			if rangeSym != nil && rangeSym.Size > 0 {
 				rangeSym.Attr |= sym.AttrReachable | sym.AttrNotInSymbolTable
 				rangeSym.Type = sym.SDWARFRANGE