cmd/internal/ld: add -buildmode=c-shared as an alternative to -shared

The linker currently (on some platforms) takes a -shared flag, which means
approximately what -buildmode=c-shared means in the in the proposed "Go
Execution Modes" document. As part of implementing other modes, the term
"shared" becomes horribly overloaded, so this replaces -shared with a
-buildmode argument instead (which currently only handles -buildmode=c-shared
and the default -buildmode=exe -- no new behaviour here).

As the linker support for -shared was in 1.4 this retains it as an alias.

Change-Id: Id2ebb8e05ee07f46208a554bc2622d0e67b47082
Reviewed-on: https://go-review.googlesource.com/8304
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/5l/asm.go

@@ -245,7 +245,7 @@ func elfreloc1(r *ld.Reloc, sectoff int64) int {
 
 	case ld.R_TLS:
 		if r.Siz == 4 {
-			if ld.Flag_shared != 0 {
+			if ld.Buildmode == ld.BuildmodeCShared {
 				ld.Thearch.Lput(ld.R_ARM_TLS_IE32 | uint32(elfsym)<<8)
 			} else {
 				ld.Thearch.Lput(ld.R_ARM_TLS_LE32 | uint32(elfsym)<<8)

src/cmd/6l/asm.go

@@ -330,7 +330,7 @@ func elfreloc1(r *ld.Reloc, sectoff int64) int {
 
 	case ld.R_TLS:
 		if r.Siz == 4 {
-			if ld.Flag_shared != 0 {
+			if ld.Buildmode == ld.BuildmodeCShared {
 				ld.Thearch.Vput(ld.R_X86_64_GOTTPOFF | uint64(elfsym)<<32)
 			} else {
 				ld.Thearch.Vput(ld.R_X86_64_TPOFF32 | uint64(elfsym)<<32)

src/cmd/6l/obj.go

@@ -91,7 +91,7 @@ func archinit() {
 		ld.Linkmode = ld.LinkInternal
 	}
 
-	if ld.Flag_shared != 0 {
+	if ld.Buildmode == ld.BuildmodeCShared {
 		ld.Linkmode = ld.LinkExternal
 	}
 

src/cmd/internal/ld/data.go

@@ -960,7 +960,7 @@ func dosymtype() {
 		}
 		// Create a new entry in the .init_array section that points to the
 		// library initializer function.
-		if Flag_shared != 0 && s.Name == INITENTRY {
+		if Buildmode == BuildmodeCShared && s.Name == INITENTRY {
 			addinitarrdata(s)
 		}
 	}
@@ -1322,7 +1322,7 @@ func dodata() {
 	sect.Length = uint64(datsize) - sect.Vaddr
 
 	/* shared library initializer */
-	if Flag_shared != 0 {
+	if Buildmode == BuildmodeCShared {
 		sect := addsection(&Segdata, ".init_array", 06)
 		sect.Align = maxalign(s, SINITARR)
 		datsize = Rnd(datsize, int64(sect.Align))

src/cmd/internal/ld/elf.go

@@ -1658,7 +1658,7 @@ func doelf() {
 		Addstring(shstrtab, ".note.GNU-stack")
 	}
 
-	if Flag_shared != 0 {
+	if Buildmode == BuildmodeCShared {
 		Addstring(shstrtab, ".init_array")
 		switch Thearch.Thechar {
 		case '6', '7', '9':

src/cmd/internal/ld/go.go

@@ -474,7 +474,7 @@ func loadcgo(file string, pkg string, p string) {
 			local = expandpkg(local, pkg)
 			s = Linklookup(Ctxt, local, 0)
 
-			if Flag_shared != 0 && s == Linklookup(Ctxt, "main", 0) {
+			if Buildmode == BuildmodeCShared && s == Linklookup(Ctxt, "main", 0) {
 				continue
 			}
 

src/cmd/internal/ld/lib.go

@@ -33,6 +33,7 @@ package ld
 import (
 	"bytes"
 	"cmd/internal/obj"
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"log"
@@ -160,7 +161,7 @@ var (
 	elfglobalsymndx    int
 	flag_installsuffix string
 	flag_race          int
-	Flag_shared        int
+	Buildmode          BuildMode
 	tracksym           string
 	interpreter        string
 	tmpdir             string
@@ -234,6 +235,44 @@ func Lflag(arg string) {
 	Ctxt.Libdir = append(Ctxt.Libdir, arg)
 }
 
+// A BuildMode indicates the sort of object we are building:
+//   "exe": build a main package and everything it imports into an executable.
+//   "c-shared": build a main package, plus all packages that it imports, into a
+//     single C shared library. The only callable symbols will be those functions
+//     marked as exported.
+type BuildMode uint8
+
+const (
+	BuildmodeExe BuildMode = iota
+	BuildmodeCShared
+)
+
+func (mode *BuildMode) Set(s string) error {
+	switch s {
+	default:
+		return errors.New("invalid mode")
+	case "exe":
+		*mode = BuildmodeExe
+	case "c-shared":
+		goarch := obj.Getgoarch()
+		if goarch != "amd64" && goarch != "arm" {
+			return fmt.Errorf("not supported on %s", goarch)
+		}
+		*mode = BuildmodeCShared
+	}
+	return nil
+}
+
+func (mode *BuildMode) String() string {
+	switch *mode {
+	case BuildmodeExe:
+		return "exe"
+	case BuildmodeCShared:
+		return "c-shared"
+	}
+	return fmt.Sprintf("BuildMode(%d)", uint8(*mode))
+}
+
 /*
  * Unix doesn't like it when we write to a running (or, sometimes,
  * recently run) binary, so remove the output file before writing it.
@@ -276,10 +315,13 @@ func libinit() {
 	coutbuf = *Binitw(f)
 
 	if INITENTRY == "" {
-		if Flag_shared == 0 {
+		switch Buildmode {
-			INITENTRY = fmt.Sprintf("_rt0_%s_%s", goarch, goos)
+		case BuildmodeCShared:
-		} else {
 			INITENTRY = fmt.Sprintf("_rt0_%s_%s_lib", goarch, goos)
+		case BuildmodeExe:
+			INITENTRY = fmt.Sprintf("_rt0_%s_%s", goarch, goos)
+		default:
+			Diag("unknown INITENTRY for buildmode %v", Buildmode)
 		}
 	}
 
@@ -324,7 +366,7 @@ func loadinternal(name string) {
 }
 
 func loadlib() {
-	if Flag_shared != 0 {
+	if Buildmode == BuildmodeCShared {
 		s := Linklookup(Ctxt, "runtime.islibrary", 0)
 		s.Dupok = 1
 		Adduint8(Ctxt, s, 1)
@@ -454,7 +496,7 @@ func loadlib() {
 	// binaries, so leave it enabled on OS X (Mach-O) binaries.
 	// Also leave it enabled on Solaris which doesn't support
 	// statically linked binaries.
-	if Flag_shared == 0 && havedynamic == 0 && HEADTYPE != Hdarwin && HEADTYPE != Hsolaris {
+	if Buildmode == BuildmodeExe && havedynamic == 0 && HEADTYPE != Hdarwin && HEADTYPE != Hsolaris {
 		Debug['d'] = 1
 	}
 
@@ -746,7 +788,7 @@ func hostlink() {
 		argv = append(argv, "-Wl,--rosegment")
 	}
 
-	if Flag_shared != 0 {
+	if Buildmode == BuildmodeCShared {
 		argv = append(argv, "-Wl,-Bsymbolic")
 		argv = append(argv, "-shared")
 	}

src/cmd/internal/ld/pobj.go

@@ -102,6 +102,7 @@ func Ldmain() {
 	obj.Flagfn1("X", "name value: define string data", addstrdata1)
 	obj.Flagcount("Z", "clear stack frame on entry", &Debug['Z'])
 	obj.Flagcount("a", "disassemble output", &Debug['a'])
+	flag.Var(&Buildmode, "buildmode", "build mode to use")
 	obj.Flagcount("c", "dump call graph", &Debug['c'])
 	obj.Flagcount("d", "disable dynamic executable", &Debug['d'])
 	obj.Flagstr("extld", "ld: linker to run in external mode", &extld)
@@ -116,8 +117,9 @@ func Ldmain() {
 	obj.Flagstr("r", "dir1:dir2:...: set ELF dynamic linker search path", &rpath)
 	obj.Flagcount("race", "enable race detector", &flag_race)
 	obj.Flagcount("s", "disable symbol table", &Debug['s'])
+	var flagShared int
 	if Thearch.Thechar == '5' || Thearch.Thechar == '6' {
-		obj.Flagcount("shared", "generate shared object (implies -linkmode external)", &Flag_shared)
+		obj.Flagcount("shared", "generate shared object (implies -linkmode external)", &flagShared)
 	}
 	obj.Flagstr("tmpdir", "dir: leave temporary files in this directory", &tmpdir)
 	obj.Flagcount("u", "reject unsafe packages", &Debug['u'])
@@ -141,6 +143,14 @@ func Ldmain() {
 	startProfile()
 	Ctxt.Bso = &Bso
 	Ctxt.Debugvlog = int32(Debug['v'])
+	if flagShared != 0 {
+		if Buildmode == BuildmodeExe {
+			Buildmode = BuildmodeCShared
+		} else if Buildmode != BuildmodeCShared {
+			Diag("-shared and -buildmode=%s are incompatible\n", Buildmode.String())
+			Errorexit()
+		}
+	}
 
 	if flag.NArg() != 1 {
 		usage()