[dev.link] cmd/link: hoist dwarfGenerateDebugSyms out of dodata()

Hoist dwarfGenerateDebugSyms call up out of dodata to before
loadlibfull. This required a couple of small tweaks to the
loader and to loadlibfull.

Change-Id: I48ffb450d2e48b9e55775b73a6debcd27dbb7b9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/228221
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1923,8 +1923,6 @@ func (ctxt *Link) dodata() {
 		ctxt.datap = append(ctxt.datap, data[symn]...)
 	}
 
-	dwarfGenerateDebugSyms(ctxt)
-
 	var i int
 	for ; i < len(dwarfp); i++ {
 		s := dwarfp[i]

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

@@ -1970,17 +1970,6 @@ func dwarfGenerateDebugSyms(ctxt *Link) {
 }
 
 func (d *dwctxt2) dwarfGenerateDebugSyms() {
-
-	// Hack: because the "wavefront" hasn't been pushed all the way
-	// up to dodata(), there will have been changes made to the sym.Symbol's
-	// that are not yet reflected in the loader. Call a temporary
-	// loader routine that copies any changes back.
-	// WARNING: changing a symbol's content will usually require
-	// calling the loader cloneToExternal method, meaning that there
-	// can be an increase in memory, so this is likely to mess up any
-	// benchmarking runs.
-	d.ldr.PropagateSymbolChangesBackToLoader()
-
 	abbrev := d.writeabbrev()
 	syms := []loader.Sym{abbrev}
 
@@ -2036,8 +2025,6 @@ func (d *dwctxt2) dwarfGenerateDebugSyms() {
 		}
 	}
 	dwarfp2 = syms
-	anonVerReplacement := d.linkctxt.Syms.IncVersion()
-	dwarfp = d.ldr.PropagateLoaderChangesToSymbols(dwarfp2, anonVerReplacement)
 }
 
 func (d *dwctxt2) collectlocs(syms []loader.Sym, units []*sym.CompilationUnit) []loader.Sym {

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

@@ -2801,6 +2801,16 @@ func (ctxt *Link) loadlibfull() {
 	// Convert special symbols created by pcln.
 	pclntabFirstFunc = ctxt.loader.Syms[pclntabFirstFunc2]
 	pclntabLastFunc = ctxt.loader.Syms[pclntabLastFunc2]
+
+	// Populate dwarfp from dwarfp2. If we see a symbol index on dwarfp2
+	// whose loader.Syms entry is nil, something went wrong.
+	for _, symIdx := range dwarfp2 {
+		s := ctxt.loader.Syms[symIdx]
+		if s == nil {
+			panic(fmt.Sprintf("nil sym for dwarfp2 element %d", symIdx))
+		}
+		dwarfp = append(dwarfp, s)
+	}
 }
 
 func (ctxt *Link) dumpsyms() {

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

@@ -298,6 +298,8 @@ func Main(arch *sys.Arch, theArch Arch) {
 	container := ctxt.pclntab()
 	bench.Start("findfunctab")
 	ctxt.findfunctab(container)
+	bench.Start("dwarfGenerateDebugSyms")
+	dwarfGenerateDebugSyms(ctxt)
 	bench.Start("loadlibfull")
 	ctxt.loadlibfull() // XXX do it here for now
 	bench.Start("symtab")

src/cmd/link/internal/loader/loader.go

@@ -2204,13 +2204,15 @@ func loadObjSyms(l *Loader, syms *sym.Symbols, r *oReader) int {
 		name := strings.Replace(osym.Name(r.Reader), "\"\".", r.pkgprefix, -1)
 		t := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type())]
 		// NB: for the test below, we can skip most anonymous symbols
-		// since they will never be turned into sym.Symbols (ex:
-		// funcdata), however DWARF subprogram DIE symbols (which are
-		// nameless) will eventually need to be turned into
-		// sym.Symbols (with relocations), so the simplest thing to do
-		// is include them as part of this loop.
-		if name == "" && t != sym.SDWARFINFO {
-			continue
+		// since they will never be turned into sym.Symbols (eg:
+		// funcdata). DWARF symbols are an exception however -- we
+		// want to include all reachable but nameless DWARF symbols.
+		if name == "" {
+			switch t {
+			case sym.SDWARFINFO, sym.SDWARFRANGE, sym.SDWARFLOC, sym.SDWARFLINES:
+			default:
+				continue
+			}
 		}
 		ver := abiToVer(osym.ABI(), r.version)
 		if t == sym.SXREF {