[dev.link] cmd/link: remove holes from global index space

In CL 217064, we made symbol's global index unique, but we still
reserve index space for each object file, which means we may
leave holes in the index space if the symbol is a dup or is
overwritten. In this CL, we stop reserving index spaces. Instead,
symbols are added one at a time, and only added if it does not
already exist. There is no more holes in the index space.

Change-Id: I3c4e67163c556ba1198e13065706510dac4692fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/217519
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/internal/ld/deadcode2.go

@@ -58,10 +58,8 @@ func (d *deadcodePass2) init() {
 		n := d.ldr.NDef()
 		for i := 1; i < n; i++ {
 			s := loader.Sym(i)
-			if !d.ldr.IsDup(s) {
 			d.mark(s, 0)
 		}
-		}
 		return
 	}
 

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

@@ -67,7 +67,6 @@ type oReader struct {
 type objIdx struct {
 	r *oReader
 	i Sym // start index
-	e Sym // end index
 }
 
 // objSym represents a symbol in an object file. It is a tuple of
@@ -130,9 +129,8 @@ func growBitmap(reqLen int, b bitmap) bitmap {
 // TODO: rework index space reservation.
 //
 // - Go object files are read before host object files; each Go object
-//   read allocates a new chunk of global index space of size P + NP,
-//   where P is the number of package defined symbols in the object and
-//   NP is the number of non-package defined symbols.
+//   read adds its defined (package + non-package) symbols to the global
+//   index space.
 //
 // - In loader.LoadRefs(), the loader makes a sweep through all of the
 //   non-package references in each object file and allocates sym indices
@@ -161,9 +159,6 @@ func growBitmap(reqLen int, b bitmap) bitmap {
 // - Each symbol gets a unique global index. For duplicated and
 //   overwriting/overwritten symbols, the second (or later) appearance
 //   of the symbol gets the same global index as the first appearance.
-//   This means, currently, there may be holes in the index space --
-//   the index reserved for a duplicated symbol does not actually
-//   point to any symbol.
 type Loader struct {
 	start       map[*oReader]Sym // map from object file to its start index
 	objs        []objIdx         // sorted by start index (i.e. objIdx.i)
@@ -297,11 +292,6 @@ func NewLoader(flags uint32, elfsetstring elfsetstringFunc) *Loader {
 	}
 }
 
-// Return the start index in the global index space for a given object file.
-func (l *Loader) startIndex(r *oReader) Sym {
-	return l.start[r]
-}
-
 // Add object file r, return the start index.
 func (l *Loader) addObj(pkg string, r *oReader) Sym {
 	if _, ok := l.start[r]; ok {
@@ -314,45 +304,50 @@ func (l *Loader) addObj(pkg string, r *oReader) Sym {
 	n := r.NSym() + r.NNonpkgdef()
 	i := l.max + 1
 	l.start[r] = i
-	l.objs = append(l.objs, objIdx{r, i, i + Sym(n) - 1})
-	l.max += Sym(n)
-	l.growValues(int(l.max))
+	l.objs = append(l.objs, objIdx{r, i})
+	l.growValues(int(l.max) + n)
 	return i
 }
 
-// Add a symbol with a given index, return the global index and whether it is added.
+// Add a symbol from an object file, return the global index and whether it is added.
 // If the symbol already exist, it returns the index of that symbol.
-func (l *Loader) AddSym(name string, ver int, i Sym, r *oReader, li int, dupok bool, typ sym.SymKind) (Sym, bool) {
+func (l *Loader) AddSym(name string, ver int, r *oReader, li int, dupok bool, typ sym.SymKind) (Sym, bool) {
 	if l.extStart != 0 {
 		panic("AddSym called after AddExtSym is called")
 	}
-	if int(i) != len(l.objSyms) {
-		fmt.Println(i, len(l.objSyms), name, ver)
-		panic("XXX AddSym inconsistency")
-	}
+	i := Sym(len(l.objSyms))
+	addToGlobal := func() {
+		l.max++
 		l.objSyms = append(l.objSyms, objSym{r, li})
+	}
 	if name == "" {
+		addToGlobal()
 		return i, true // unnamed aux symbol
 	}
 	if ver == r.version {
 		// Static symbol. Add its global index but don't
 		// add to name lookup table, as it cannot be
 		// referenced by name.
+		addToGlobal()
 		return i, true
 	}
-	if oldi, ok := l.symsByName[ver][name]; ok {
+	oldi, existed := l.symsByName[ver][name]
+	if !existed {
+		l.symsByName[ver][name] = i
+		addToGlobal()
+		return i, true
+	}
+	// symbol already exists
 	if dupok {
 		if l.flags&FlagStrictDups != 0 {
-				l.checkdup(name, i, r, oldi)
+			l.checkdup(name, r, li, oldi)
 		}
-			l.objSyms[i] = objSym{} // nil this out -- this is a hole in the index space
 		return oldi, false
 	}
 	oldr, oldli := l.toLocal(oldi)
 	oldsym := goobj2.Sym{}
 	oldsym.Read(oldr.Reader, oldr.SymOff(oldli))
 	if oldsym.Dupok() {
-			l.objSyms[i] = objSym{} // nil this out -- this is a hole in the index space
 		return oldi, false
 	}
 	overwrite := r.DataSize(li) != 0
@@ -363,19 +358,13 @@ func (l *Loader) AddSym(name string, ver int, i Sym, r *oReader, li int, dupok b
 			log.Fatalf("duplicated definition of symbol " + name)
 		}
 		l.objSyms[oldi] = objSym{r, li}
-			l.objSyms[i] = objSym{} // nil this out -- this is a hole in the index space
-			return oldi, true
 	} else {
 		// old symbol overwrites new symbol.
 		if !typ.IsData() { // only allow overwriting data symbol
 			log.Fatalf("duplicated definition of symbol " + name)
 		}
-			l.objSyms[i] = objSym{} // nil this out -- this is a hole in the index space
-			return oldi, false
 	}
-	}
-	l.symsByName[ver][name] = i
-	return i, true
+	return oldi, true
 }
 
 // newExtSym creates a new external sym with the specified
@@ -551,17 +540,8 @@ func (l *Loader) Lookup(name string, ver int) Sym {
 	return l.symsByName[ver][name]
 }
 
-// Returns whether i is a dup of another symbol, and i is not
-// "primary", i.e. i is a hole in the global index space.
-// TODO: get rid of the holes.
-func (l *Loader) IsDup(i Sym) bool {
-	r, _ := l.toLocal(i)
-	return r == nil
-}
-
 // Check that duplicate symbols have same contents.
-func (l *Loader) checkdup(name string, i Sym, r *oReader, dup Sym) {
-	li := int(i - l.startIndex(r))
+func (l *Loader) checkdup(name string, r *oReader, li int, dup Sym) {
 	p := r.Data(li)
 	if strings.HasPrefix(name, "go.info.") {
 		p, _ = patchDWARFName1(p, r)
@@ -1503,7 +1483,6 @@ func (l *Loader) Preload(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, lib *
 	}
 
 	istart := l.addObj(lib.Pkg, or)
-
 	l.growAttrBitmaps(int(istart) + ndef + nnonpkgdef)
 	for i, n := 0, ndef+nnonpkgdef; i < n; i++ {
 		osym := goobj2.Sym{}
@@ -1511,7 +1490,7 @@ func (l *Loader) Preload(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, lib *
 		name := strings.Replace(osym.Name, "\"\".", pkgprefix, -1)
 		v := abiToVer(osym.ABI, localSymVersion)
 		dupok := osym.Dupok()
-		gi, added := l.AddSym(name, v, istart+Sym(i), or, i, dupok, sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)])
+		gi, added := l.AddSym(name, v, or, i, dupok, sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)])
 		or.syms[i] = gi
 		if !added {
 			continue
@@ -1973,7 +1952,6 @@ func (l *Loader) CreateExtSym(name string) Sym {
 }
 
 func loadObjFull(l *Loader, r *oReader) {
-	istart := l.startIndex(r)
 	lib := r.unit.Lib
 	resolveSymRef := func(s goobj2.SymRef) *sym.Symbol {
 		i := l.resolve(r, s)
@@ -1986,38 +1964,39 @@ func loadObjFull(l *Loader, r *oReader) {
 	pcdataBase := r.PcdataBase()
 	rslice := []Reloc{}
 	for i, n := 0, r.NSym()+r.NNonpkgdef(); i < n; i++ {
+		// A symbol may be a dup or overwritten. In this case, its
+		// content will actually be provided by a different object
+		// (to which its global index points). Skip those symbols.
+		gi := l.toGlobal(r, i)
+		var isdup bool
+		if r2, i2 := l.toLocal(gi); r2 != r || i2 != i {
+			isdup = true
+		}
+
 		osym := goobj2.Sym{}
 		osym.Read(r.Reader, r.SymOff(i))
 		name := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
 		if name == "" {
 			continue
 		}
-		ver := abiToVer(osym.ABI, r.version)
 		dupok := osym.Dupok()
-		if dupok {
-			if dupsym := l.symsByName[ver][name]; dupsym != istart+Sym(i) {
-				if l.attrReachable.has(dupsym) {
+		if dupok && isdup {
+			if l.attrReachable.has(gi) {
 				// A dupok symbol is resolved to another package. We still need
 				// to record its presence in the current package, as the trampoline
 				// pass expects packages are laid out in dependency order.
-					s := l.Syms[dupsym]
+				s := l.Syms[gi]
 				if s.Type == sym.STEXT {
 					lib.DupTextSyms = append(lib.DupTextSyms, s)
-						lib.DupTextSyms2 = append(lib.DupTextSyms2, sym.LoaderSym(dupsym))
+					lib.DupTextSyms2 = append(lib.DupTextSyms2, sym.LoaderSym(gi))
 				}
 			}
 			continue
 		}
-		}
 
-		// A symbol may be a dup or overwritten. In this case, its
-		// content will actually be provided by a different object
-		// (to which its global index points). Skip those symbols.
-		gi := l.toGlobal(r, i)
-		if r2, i2 := l.toLocal(gi); r2 != r || i2 != i {
+		if isdup {
 			continue // come from a different object
 		}
-
 		s := l.Syms[gi]
 		if s == nil {
 			continue
@@ -2301,9 +2280,6 @@ func (l *Loader) UndefinedRelocTargets(limit int) []Sym {
 	result := []Sym{}
 	rslice := []Reloc{}
 	for si := Sym(1); si <= l.max; si++ {
-		if l.IsDup(si) {
-			continue
-		}
 		relocs := l.Relocs(si)
 		rslice = relocs.ReadAll(rslice)
 		for ri := 0; ri < relocs.Count; ri++ {
@@ -2342,10 +2318,6 @@ func (l *Loader) Dump() {
 		if s != nil {
 			fmt.Println(i, s, s.Type, pi)
 		} else {
-			if l.IsDup(i) {
-				fmt.Println(i, "<overwritten>")
-				continue
-			}
 			fmt.Println(i, l.SymName(i), "<not loaded>", pi)
 		}
 	}

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

@@ -20,8 +20,7 @@ import (
 // data or relocations).
 func addDummyObjSym(t *testing.T, ldr *Loader, or *oReader, name string) Sym {
 	idx := ldr.max + 1
-	ldr.max++
-	if _, ok := ldr.AddSym(name, 0, idx, or, int(idx-ldr.startIndex(or)), false, sym.SRODATA); !ok {
+	if _, ok := ldr.AddSym(name, 0, or, int(idx), false, sym.SRODATA); !ok {
 		t.Errorf("AddrSym failed for '" + name + "'")
 	}