cmd/link: improved host archive debug trace output

When ctxt.Debugvlog > 1, produce additional trace output to describe
which object files are being pulled out of host archive libraries and
why they were pulled (e.g. which symbol had a reference to something
in a library). Intended to make it easier to debug problems with cgo
internal linking.

Change-Id: Icd64aff244b9145162a00cb51642ef32f26adfba
Reviewed-on: https://go-review.googlesource.com/c/go/+/451736
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -110,12 +110,15 @@ func hostArchive(ctxt *Link, name string) {
 	for any {
 		var load []uint64
 		returnAllUndefs := -1
-		undefs := ctxt.loader.UndefinedRelocTargets(returnAllUndefs)
+		undefs, froms := ctxt.loader.UndefinedRelocTargets(returnAllUndefs)
-		for _, symIdx := range undefs {
+		for k, symIdx := range undefs {
-			name := ctxt.loader.SymName(symIdx)
+			sname := ctxt.loader.SymName(symIdx)
-			if off := armap[name]; off != 0 && !loaded[off] {
+			if off := armap[sname]; off != 0 && !loaded[off] {
 				load = append(load, off)
 				loaded[off] = true
+				if ctxt.Debugvlog > 1 {
+					ctxt.Logf("hostArchive(%s): selecting object at offset %x to resolve %s [%d] reference from %s [%d]\n", name, off, sname, symIdx, ctxt.loader.SymName(froms[k]), froms[k])
+				}
 			}
 		}
 

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

@@ -615,9 +615,14 @@ func (ctxt *Link) loadlib() {
 		// If we have any undefined symbols in external
 		// objects, try to read them from the libgcc file.
 		any := false
-		undefs := ctxt.loader.UndefinedRelocTargets(1)
+		undefs, froms := ctxt.loader.UndefinedRelocTargets(1)
 		if len(undefs) > 0 {
 			any = true
+			if ctxt.Debugvlog > 1 {
+				ctxt.Logf("loadlib: first unresolved is %s [%d] from %s [%d]\n",
+					ctxt.loader.SymName(undefs[0]), undefs[0],
+					ctxt.loader.SymName(froms[0]), froms[0])
+			}
 		}
 		if any {
 			if *flagLibGCC == "" {
@@ -681,9 +686,14 @@ func loadWindowsHostArchives(ctxt *Link) {
 			hostArchive(ctxt, p)
 		}
 		any = false
-		undefs := ctxt.loader.UndefinedRelocTargets(1)
+		undefs, froms := ctxt.loader.UndefinedRelocTargets(1)
 		if len(undefs) > 0 {
 			any = true
+			if ctxt.Debugvlog > 1 {
+				ctxt.Logf("loadWindowsHostArchives: remaining unresolved is %s [%d] from %s [%d]\n",
+					ctxt.loader.SymName(undefs[0]), undefs[0],
+					ctxt.loader.SymName(froms[0]), froms[0])
+			}
 		}
 	}
 	// If needed, create the __CTOR_LIST__ and __DTOR_LIST__
@@ -2141,7 +2151,7 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 // to true if there is an unresolved reference to the symbol in want[K].
 func symbolsAreUnresolved(ctxt *Link, want []string) []bool {
 	returnAllUndefs := -1
-	undefs := ctxt.loader.UndefinedRelocTargets(returnAllUndefs)
+	undefs, _ := ctxt.loader.UndefinedRelocTargets(returnAllUndefs)
 	seen := make(map[loader.Sym]struct{})
 	rval := make([]bool, len(want))
 	wantm := make(map[string]int)

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

@@ -2392,12 +2392,15 @@ func (l *Loader) RelocVariant(s Sym, ri int) sym.RelocVariant {
 // space, looking for symbols with relocations targeting undefined
 // references. The linker's loadlib method uses this to determine if
 // there are unresolved references to functions in system libraries
-// (for example, libgcc.a), presumably due to CGO code. Return
+// (for example, libgcc.a), presumably due to CGO code. Return value
-// value is a list of loader.Sym's corresponding to the undefined
+// is a pair of lists of loader.Sym's. First list corresponds to the
-// cross-refs. The "limit" param controls the maximum number of
+// corresponding to the undefined symbols themselves, the second list
-// results returned; if "limit" is -1, then all undefs are returned.
+// is the symbol that is making a reference to the undef. The "limit"
-func (l *Loader) UndefinedRelocTargets(limit int) []Sym {
+// param controls the maximum number of results returned; if "limit"
-	result := []Sym{}
+// is -1, then all undefs are returned.
+func (l *Loader) UndefinedRelocTargets(limit int) ([]Sym, []Sym) {
+	result, fromr := []Sym{}, []Sym{}
+outerloop:
 	for si := Sym(1); si < Sym(len(l.objSyms)); si++ {
 		relocs := l.Relocs(si)
 		for ri := 0; ri < relocs.Count(); ri++ {
@@ -2405,13 +2408,14 @@ func (l *Loader) UndefinedRelocTargets(limit int) []Sym {
 			rs := r.Sym()
 			if rs != 0 && l.SymType(rs) == sym.SXREF && l.SymName(rs) != ".got" {
 				result = append(result, rs)
+				fromr = append(fromr, si)
 				if limit != -1 && len(result) >= limit {
-					break
+					break outerloop
 				}
 			}
 		}
 	}
-	return result
+	return result, fromr
 }
 
 // AssignTextSymbolOrder populates the Textp slices within each