cmd/link: add and use new SymKind SFirstUnallocated

The linker sources in several places used SXREF to mark the first
SymKind which is not allocated in memory. This is cryptic.
Instead use SFirstUnallocated, following the example of the
existing SFirstWritable.

Change-Id: If326ad63027402699094bcc49ef860db3772f82a
Reviewed-on: https://go-review.googlesource.com/c/go/+/715623
Reviewed-by: Than McIntosh <thanm@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ianlancetaylor

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

@@ -1628,9 +1628,9 @@ type dodataState struct {
 	// Link context
 	ctxt *Link
 	// Data symbols bucketed by type.
-	data [sym.SXREF][]loader.Sym
+	data [sym.SFirstUnallocated][]loader.Sym
 	// Max alignment for each flavor of data symbol.
-	dataMaxAlign [sym.SXREF]int32
+	dataMaxAlign [sym.SFirstUnallocated]int32
 	// Overridden sym type
 	symGroupType []sym.SymKind
 	// Current data size so far.
@@ -1687,7 +1687,7 @@ func (ctxt *Link) dodata(symGroupType []sym.SymKind) {
 
 		st := state.symType(s)
 
-		if st <= sym.STEXTFIPSEND || st >= sym.SXREF {
+		if st <= sym.STEXTEND || st >= sym.SFirstUnallocated {
 			continue
 		}
 		state.data[st] = append(state.data[st], s)
@@ -2264,11 +2264,11 @@ func (state *dodataState) allocateDataSections(ctxt *Link) {
 	}
 
 	siz := 0
-	for symn := sym.SELFRXSECT; symn < sym.SXREF; symn++ {
+	for symn := sym.SELFRXSECT; symn < sym.SFirstUnallocated; symn++ {
 		siz += len(state.data[symn])
 	}
 	ctxt.datap = make([]loader.Sym, 0, siz)
-	for symn := sym.SELFRXSECT; symn < sym.SXREF; symn++ {
+	for symn := sym.SELFRXSECT; symn < sym.SFirstUnallocated; symn++ {
 		ctxt.datap = append(ctxt.datap, state.data[symn]...)
 	}
 }

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

@@ -919,7 +919,7 @@ func collectmachosyms(ctxt *Link) {
 			continue
 		}
 		t := ldr.SymType(s)
-		if t >= sym.SELFRXSECT && t < sym.SXREF { // data sections handled in dodata
+		if t >= sym.SELFRXSECT && t < sym.SFirstUnallocated { // data sections handled in dodata
 			if t == sym.STLSBSS {
 				// TLSBSS is not used on darwin. See data.go:allocateDataSections
 				continue

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

@@ -854,7 +854,7 @@ func (f *peFile) writeSymbols(ctxt *Link) {
 			continue
 		}
 		t := ldr.SymType(s)
-		if t >= sym.SELFRXSECT && t < sym.SXREF { // data sections handled in dodata
+		if t >= sym.SELFRXSECT && t < sym.SFirstUnallocated { // data sections handled in dodata
 			if t == sym.STLSBSS {
 				continue
 			}

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

@@ -244,7 +244,7 @@ func genelfsym(ctxt *Link, elfbind elf.SymBind) {
 			continue
 		}
 		st := ldr.SymType(s)
-		if st >= sym.SELFRXSECT && st < sym.SXREF {
+		if st >= sym.SELFRXSECT && st < sym.SFirstUnallocated {
 			typ := elf.STT_OBJECT
 			if st == sym.STLSBSS {
 				if ctxt.IsInternal() {
@@ -345,7 +345,7 @@ func asmbPlan9Sym(ctxt *Link) {
 			continue
 		}
 		t := ldr.SymType(s)
-		if t >= sym.SELFRXSECT && t < sym.SXREF { // data sections handled in dodata
+		if t >= sym.SELFRXSECT && t < sym.SFirstUnallocated { // data sections handled in dodata
 			if t == sym.STLSBSS {
 				continue
 			}
@@ -843,7 +843,7 @@ func (ctxt *Link) symtab(pcln *pclntab) []sym.SymKind {
 }
 
 // CarrierSymByType tracks carrier symbols and their sizes.
-var CarrierSymByType [sym.SXREF]struct {
+var CarrierSymByType [sym.SFirstUnallocated]struct {
 	Sym  loader.Sym
 	Size int64
 }

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

@@ -1121,7 +1121,7 @@ func (f *xcoffFile) asmaixsym(ctxt *Link) {
 				putaixsym(ctxt, s, BSSSym)
 			}
 
-		case st >= sym.SELFRXSECT && st < sym.SXREF: // data sections handled in dodata
+		case st >= sym.SELFRXSECT && st < sym.SFirstUnallocated: // data sections handled in dodata
 			if ldr.AttrReachable(s) {
 				putaixsym(ctxt, s, DataSym)
 			}

src/cmd/link/internal/sym/symkind.go

@@ -125,6 +125,7 @@ const (
 	STLSBSS                 // Thread-local zeroed data.
 
 	// Unallocated segment.
+	SFirstUnallocated
 	SXREF             // Reference from non-Go object file.
 	SMACHOSYMSTR      // Mach-O string table.
 	SMACHOSYMTAB      // Mach-O symbol table.