cmd/cgo: add support for function export for gccgo.

A "gccgoprefix" flag is added and used by the go tool,
to mirror the -fgo-prefix flag for gccgo, whose value
is required to know how to access functions from C.

Trying to export Go methods or unexported Go functions
will not work.

Also fix go test on "main" packages.

Updates #2313.
Fixes #3262.

R=mpimenov, rsc, iant
CC=golang-dev
https://golang.org/cl/5797046

src/cmd/cgo/main.go

@@ -136,6 +136,7 @@ var cdefs = flag.Bool("cdefs", false, "for bootstrap: write C definitions for C
 var objDir = flag.String("objdir", "", "object directory")
 
 var gccgo = flag.Bool("gccgo", false, "generate files for use with gccgo")
+var gccgoprefix = flag.String("gccgoprefix", "go", "prefix of symbols generated by gccgo")
 var importRuntimeCgo = flag.Bool("import_runtime_cgo", true, "import runtime/cgo in generated code")
 var goarch, goos string
 

src/cmd/cgo/out.go

@@ -107,7 +107,11 @@ func (p *Package) writeDefs() {
 		}
 	}
 
+	if *gccgo {
+		p.writeGccgoExports(fgo2, fc, fm)
+	} else {
 		p.writeExports(fgo2, fc, fm)
+	}
 
 	fgo2.Close()
 	fc.Close()
@@ -624,6 +628,83 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 	}
 }
 
+// Write out the C header allowing C code to call exported gccgo functions.
+func (p *Package) writeGccgoExports(fgo2, fc, fm *os.File) {
+	fgcc := creat(*objDir + "_cgo_export.c")
+	fgcch := creat(*objDir + "_cgo_export.h")
+	_ = fgcc
+
+	fmt.Fprintf(fgcch, "/* Created by cgo - DO NOT EDIT. */\n")
+	fmt.Fprintf(fgcch, "%s\n", p.Preamble)
+	fmt.Fprintf(fgcch, "%s\n", gccExportHeaderProlog)
+	fmt.Fprintf(fm, "#include \"_cgo_export.h\"\n")
+
+	clean := func(r rune) rune {
+		switch {
+		case 'A' <= r && r <= 'Z', 'a' <= r && r <= 'z',
+			'0' <= r && r <= '9':
+			return r
+		}
+		return '_'
+	}
+	gccgoSymbolPrefix := strings.Map(clean, *gccgoprefix)
+
+	for _, exp := range p.ExpFunc {
+		// TODO: support functions with receivers.
+		fn := exp.Func
+		fntype := fn.Type
+
+		if !ast.IsExported(fn.Name.Name) {
+			fatalf("cannot export unexported function %s with gccgo", fn.Name)
+		}
+
+		cdeclBuf := new(bytes.Buffer)
+		resultCount := 0
+		forFieldList(fntype.Results,
+			func(i int, atype ast.Expr) { resultCount++ })
+		switch resultCount {
+		case 0:
+			fmt.Fprintf(cdeclBuf, "void")
+		case 1:
+			forFieldList(fntype.Results,
+				func(i int, atype ast.Expr) {
+					t := p.cgoType(atype)
+					fmt.Fprintf(cdeclBuf, "%s", t.C)
+				})
+		default:
+			// Declare a result struct.
+			fmt.Fprintf(fgcch, "struct %s_result {\n", exp.ExpName)
+			forFieldList(fntype.Results,
+				func(i int, atype ast.Expr) {
+					t := p.cgoType(atype)
+					fmt.Fprintf(fgcch, "\t%s r%d;\n", t.C, i)
+				})
+			fmt.Fprintf(fgcch, "};\n")
+			fmt.Fprintf(cdeclBuf, "struct %s_result", exp.ExpName)
+		}
+
+		// The function name.
+		fmt.Fprintf(cdeclBuf, " "+exp.ExpName)
+		gccgoSymbol := fmt.Sprintf("%s.%s.%s", gccgoSymbolPrefix, p.PackageName, exp.Func.Name)
+		fmt.Fprintf(cdeclBuf, " (")
+		// Function parameters.
+		forFieldList(fntype.Params,
+			func(i int, atype ast.Expr) {
+				if i > 0 {
+					fmt.Fprintf(cdeclBuf, ", ")
+				}
+				t := p.cgoType(atype)
+				fmt.Fprintf(cdeclBuf, "%s p%d", t.C, i)
+			})
+		fmt.Fprintf(cdeclBuf, ")")
+		cdecl := cdeclBuf.String()
+
+		fmt.Fprintf(fgcch, "extern %s __asm__(\"%s\");\n", cdecl, gccgoSymbol)
+		// Dummy declaration for _cgo_main.c
+		fmt.Fprintf(fm, "%s {}\n", cdecl)
+	}
+}
+
 // Call a function for each entry in an ast.FieldList, passing the
 // index into the list and the type.
 func forFieldList(fl *ast.FieldList, fn func(int, ast.Expr)) {

src/cmd/go/build.go

@@ -661,7 +661,7 @@ func (b *builder) build(a *action) (err error) {
 		}
 
 		cgoExe := tool("cgo")
-		if a.cgo != nil {
+		if a.cgo != nil && a.cgo.target != "" {
 			cgoExe = a.cgo.target
 		}
 		outGo, outObj, err := b.cgo(a.p, cgoExe, obj, gccfiles)
@@ -1239,12 +1239,8 @@ func (gccgcToolchain) gc(b *builder, p *Package, obj string, importArgs []string
 	out := p.Name + ".o"
 	ofile = obj + out
 	gcargs := []string{"-g"}
-	if p.Name != "main" {
-		if p.fake {
-			gcargs = append(gcargs, "-fgo-prefix=fake_"+p.ImportPath)
-		} else {
-			gcargs = append(gcargs, "-fgo-prefix=go_"+p.ImportPath)
-		}
+	if prefix := gccgoPrefix(p); prefix != "" {
+		gcargs = append(gcargs, "-fgo-prefix="+gccgoPrefix(p))
 	}
 	args := stringList("gccgo", importArgs, "-c", gcargs, "-o", ofile, buildGccgoflags)
 	for _, f := range gofiles {
@@ -1304,6 +1300,16 @@ func (gccgcToolchain) cc(b *builder, p *Package, objdir, ofile, cfile string) er
 		"-DGOOS_"+goos, "-DGOARCH_"+goarch, "-c", cfile)
 }
 
+func gccgoPrefix(p *Package) string {
+	switch {
+	case p.build.IsCommand() && !p.forceLibrary:
+		return ""
+	case p.fake:
+		return "fake_" + p.ImportPath
+	}
+	return "go_" + p.ImportPath
+}
+
 // gcc runs the gcc C compiler to create an object from a single C file.
 func (b *builder) gcc(p *Package, out string, flags []string, cfile string) error {
 	cfile = mkAbs(p.Dir, cfile)
@@ -1404,6 +1410,9 @@ func (b *builder) cgo(p *Package, cgoExe, obj string, gccfiles []string) (outGo,
 	}
 	if _, ok := buildToolchain.(gccgcToolchain); ok {
 		cgoflags = append(cgoflags, "-gccgo")
+		if prefix := gccgoPrefix(p); prefix != "" {
+			cgoflags = append(cgoflags, "-gccgoprefix="+gccgoPrefix(p))
+		}
 	}
 	if err := b.run(p.Dir, p.ImportPath, cgoExe, "-objdir", obj, cgoflags, "--", cgoCFLAGS, p.CgoFiles); err != nil {
 		return nil, nil, err

src/cmd/go/pkg.go

@@ -72,6 +72,7 @@ type Package struct {
 	target       string   // installed file for this package (may be executable)
 	fake         bool     // synthesized package
 	forceBuild   bool     // this package must be rebuilt
+	forceLibrary bool     // this package is a library (even if named "main")
 	local        bool     // imported via local path (./ or ../)
 	localPrefix  string   // interpret ./ and ../ imports relative to this prefix
 }

src/cmd/go/test.go

@@ -446,6 +446,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		ptest.imports = append(append([]*Package{}, p.imports...), imports...)
 		ptest.pkgdir = testDir
 		ptest.fake = true
+		ptest.forceLibrary = true
 		ptest.Stale = true
 		ptest.build = new(build.Package)
 		*ptest.build = *p.build
@@ -489,7 +490,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		ImportPath: "testmain",
 		Root:       p.Root,
 		imports:    []*Package{ptest},
-		build:      &build.Package{},
+		build:      &build.Package{Name: "main"},
 		fake:       true,
 		Stale:      true,
 	}