cmd/go, cmd/cgo, make.bash: cross compiling with cgo enabled

Introduce two new environment variables, CC_FOR_TARGET and CXX_FOR_TARGET.
CC_FOR_TARGET defaults to CC and is used when compiling for GOARCH, while
CC remains for compiling for GOHOSTARCH.
CXX_FOR_TARGET defaults to CXX and is used when compiling C++ code for
GOARCH.

CGO_ENABLED defaults to disabled when cross compiling and has to be
explicitly enabled.

Update #4714

LGTM=minux.ma, iant
R=golang-codereviews, minux.ma, iant, rsc, dominik.honnef
CC=golang-codereviews
https://golang.org/cl/57100043

src/cmd/dist/a.h

@@ -75,7 +75,8 @@ extern char *goroot_final;
 extern char *goextlinkenabled;
 extern char *goversion;
 extern char *defaultcc;
-extern char *defaultcxx;
+extern char *defaultcxxtarget;
+extern char *defaultcctarget;
 extern char *workdir;
 extern char *tooldir;
 extern char *slash;

src/cmd/dist/build.c

@@ -27,7 +27,8 @@ char *gochar;
 char *goversion;
 char *slash;	// / for unix, \ for windows
 char *defaultcc;
-char *defaultcxx;
+char *defaultcxxtarget;
+char *defaultcctarget;
 bool	rebuildall;
 bool defaultclang;
 
@@ -166,6 +167,14 @@ init(void)
 	}
 	defaultcc = btake(&b);
 
+	xgetenv(&b, "CC_FOR_TARGET");
+	if(b.len == 0) {
+		bprintf(&b, defaultcc);
+	}
+	defaultcctarget = btake(&b);
+
+	xgetenv(&b, "CXX_FOR_TARGET");
+	if(b.len == 0) {
 		xgetenv(&b, "CXX");
 		if(b.len == 0) {
 			if(defaultclang)
@@ -173,7 +182,8 @@ init(void)
 			else
 				bprintf(&b, "g++");
 		}
-	defaultcxx = btake(&b);
+	}
+	defaultcxxtarget = btake(&b);
 
 	xsetenv("GOROOT", goroot);
 	xsetenv("GOARCH", goarch);
@@ -1537,6 +1547,7 @@ cmdenv(int argc, char **argv)
 		usage();
 
 	xprintf(format, "CC", defaultcc);
+	xprintf(format, "CC_FOR_TARGET", defaultcctarget);
 	xprintf(format, "GOROOT", goroot);
 	xprintf(format, "GOBIN", gobin);
 	xprintf(format, "GOARCH", goarch);

src/cmd/dist/buildgo.c

@@ -31,7 +31,7 @@ mkzdefaultcc(char *dir, char *file)
 		"\n"
 		"const defaultCC = `%s`\n"
 		"const defaultCXX = `%s`\n",
-		defaultcc, defaultcxx);
+		defaultcctarget, defaultcxxtarget);
 
 	writefile(&out, file, 0);
 

src/cmd/go/build.go

@@ -1708,11 +1708,10 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 	if buildContext.InstallSuffix != "" {
 		ldflags = append(ldflags, "-installsuffix", buildContext.InstallSuffix)
 	}
-	if cxx {
+	// If the user has not specified the -extld option, then specify the
-		// The program includes C++ code.  If the user has not
+	// appropriate linker. In case of C++ code, use the compiler named
-		// specified the -extld option, then default to
+	// by the CXX environment variable or defaultCXX if CXX is not set.
-		// linking with the compiler named by the CXX
+	// Else, use the CC environment variable and defaultCC as fallback.
-		// environment variable, or g++ if CXX is not set.
 	extld := false
 	for _, f := range ldflags {
 		if f == "-extld" || strings.HasPrefix(f, "-extld=") {
@@ -1721,9 +1720,11 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 		}
 	}
 	if !extld {
-			compiler := strings.Fields(os.Getenv("CXX"))
+		var compiler []string
-			if len(compiler) == 0 {
+		if cxx {
-				compiler = []string{"g++"}
+			compiler = ccompilerPath("CXX", defaultCXX)
+		} else {
+			compiler = ccompilerPath("CC", defaultCC)
 		}
 		ldflags = append(ldflags, "-extld="+compiler[0])
 		if len(compiler) > 1 {
@@ -1745,7 +1746,6 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 			}
 		}
 	}
-	}
 	return b.run(".", p.ImportPath, nil, tool(archChar+"l"), "-o", out, importArgs, swigArg, ldflags, mainpkg)
 }
 
@@ -1973,15 +1973,12 @@ func (b *builder) gxxCmd(objdir string) []string {
 }
 
 // ccompilerCmd returns a command line prefix for the given environment
-// variable and using the default command when the variable is empty
+// variable and using the default command when the variable is empty.
 func (b *builder) ccompilerCmd(envvar, defcmd, objdir string) []string {
 	// NOTE: env.go's mkEnv knows that the first three
 	// strings returned are "gcc", "-I", objdir (and cuts them off).
 
-	compiler := strings.Fields(os.Getenv(envvar))
+	compiler := ccompilerPath(envvar, defcmd)
-	if len(compiler) == 0 {
-		compiler = strings.Fields(defcmd)
-	}
 	a := []string{compiler[0], "-I", objdir, "-g", "-O2"}
 	a = append(a, compiler[1:]...)
 
@@ -2039,6 +2036,16 @@ func envList(key string) []string {
 	return strings.Fields(os.Getenv(key))
 }
 
+// ccompilerCmd returns the compilerpath for the given environment
+// variable and using the default command when the variable is empty.
+func ccompilerPath(envvar, defcmd string) []string {
+	compiler := envList(envvar)
+	if len(compiler) == 0 {
+		compiler = strings.Fields(defcmd)
+	}
+	return compiler
+}
+
 var cgoRe = regexp.MustCompile(`[/\\:]`)
 
 var (

src/make.bash

@@ -35,10 +35,15 @@
 # controls the default behavior of the linker's -linkmode option.  The
 # default value depends on the system.
 #
-# CC: Command line to run to get at host C compiler.
+# CC: Command line to run to compile C code for GOHOSTARCH.
 # Default is "gcc". Also supported: "clang".
-# CXX: Command line to run to get at host C++ compiler, only recorded
+#
-# for cgo use. Default is "g++". Also supported: "clang++".
+# CC_FOR_TARGET: Command line to run to compile C code for GOARCH.
+# This is used by cgo.  Default is CC.
+#
+# CXX_FOR_TARGET: Command line to run to compile C++ code for GOARCH.
+# This is used by cgo. Default is CXX, or, if that is not set, 
+# "g++" or "clang++".
 #
 # GO_DISTFLAGS: extra flags to provide to "dist bootstrap". Use "-s"
 # to build a statically linked toolchain.
@@ -153,13 +158,15 @@ echo
 
 if [ "$GOHOSTARCH" != "$GOARCH" -o "$GOHOSTOS" != "$GOOS" ]; then
 	echo "# Building packages and commands for host, $GOHOSTOS/$GOHOSTARCH."
-	GOOS=$GOHOSTOS GOARCH=$GOHOSTARCH \
+	# CC_FOR_TARGET is recorded as the default compiler for the go tool. When building for the host, however,
+	# use the host compiler, CC, from `cmd/dist/dist env` instead.
+	CC=$CC GOOS=$GOHOSTOS GOARCH=$GOHOSTARCH \
 		"$GOTOOLDIR"/go_bootstrap install -ccflags "$GO_CCFLAGS" -gcflags "$GO_GCFLAGS" -ldflags "$GO_LDFLAGS" -v std
 	echo
 fi
 
 echo "# Building packages and commands for $GOOS/$GOARCH."
-"$GOTOOLDIR"/go_bootstrap install $GO_FLAGS -ccflags "$GO_CCFLAGS" -gcflags "$GO_GCFLAGS" -ldflags "$GO_LDFLAGS" -v std
+CC=$CC_FOR_TARGET "$GOTOOLDIR"/go_bootstrap install $GO_FLAGS -ccflags "$GO_CCFLAGS" -gcflags "$GO_GCFLAGS" -ldflags "$GO_LDFLAGS" -v std
 echo
 
 rm -f "$GOTOOLDIR"/go_bootstrap

src/pkg/go/build/build.go

@@ -303,8 +303,7 @@ func defaultContext() Context {
 	case "0":
 		c.CgoEnabled = false
 	default:
-		// golang.org/issue/5141
+		// cgo must be explicitly enabled for cross compilation builds
-		// cgo should be disabled for cross compilation builds
 		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
 			c.CgoEnabled = cgoEnabled[c.GOOS+"/"+c.GOARCH]
 			break