cmd/cgo: use __gcc_struct__ for go exported functions

Fixes #6833

R=minux.ma, iant
CC=golang-dev
https://golang.org/cl/35790045

misc/cgo/test/cgo_test.go

@@ -43,6 +43,7 @@ func TestCflags(t *testing.T)              { testCflags(t) }
 func Test5337(t *testing.T)                { test5337(t) }
 func Test5548(t *testing.T)                { test5548(t) }
 func Test5603(t *testing.T)                { test5603(t) }
+func Test6833(t *testing.T)                { test6833(t) }
 func Test3250(t *testing.T)                { test3250(t) }
 func TestCallbackStack(t *testing.T)       { testCallbackStack(t) }
 func TestFpVar(t *testing.T)               { testFpVar(t) }

misc/cgo/test/issue6833.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cgotest
+
+/*
+extern unsigned long long issue6833Func(unsigned int, unsigned long long);
+*/
+import "C"
+
+import "testing"
+
+//export GoIssue6833Func
+func GoIssue6833Func(aui uint, aui64 uint64) uint64 {
+	return aui64 + uint64(aui)
+}
+
+func test6833(t *testing.T) {
+	ui := 7
+	ull := uint64(0x4000300020001000)
+	v := uint64(C.issue6833Func(C.uint(ui), C.ulonglong(ull)))
+	exp := uint64(ui) + ull
+	if v != exp {
+		t.Errorf("issue6833Func() returns %x, expected %x", v, exp)
+	}
+}

misc/cgo/test/issue6833_c.c

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "_cgo_export.h"
+ 
+unsigned long long
+issue6833Func(unsigned int aui, unsigned long long aull) {
+	return GoIssue6833Func(aui, aull);
+}

src/cmd/cgo/out.go

@@ -529,15 +529,8 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 	}
 	// We're trying to write a gcc struct that matches 6c/8c/5c's layout.
 	// Use packed attribute to force no padding in this struct in case
-	// gcc has different packing requirements.  For example,
-	// on 386 Windows, gcc wants to 8-align int64s, but 8c does not.
-	// Use __gcc_struct__ to work around http://gcc.gnu.org/PR52991 on x86,
-	// and http://golang.org/issue/5603.
-	extraAttr := ""
-	if !strings.Contains(p.gccBaseCmd()[0], "clang") && (goarch == "amd64" || goarch == "386") {
-		extraAttr = ", __gcc_struct__"
-	}
-	fmt.Fprintf(fgcc, "\t%s __attribute__((__packed__%v)) *a = v;\n", ctype, extraAttr)
+	// gcc has different packing requirements.
+	fmt.Fprintf(fgcc, "\t%s %v *a = v;\n", ctype, p.packedAttribute())
 	fmt.Fprintf(fgcc, "\t")
 	if t := n.FuncType.Result; t != nil {
 		fmt.Fprintf(fgcc, "a->r = ")
@@ -618,6 +611,19 @@ func (p *Package) writeGccgoOutputFunc(fgcc *os.File, n *Name) {
 	fmt.Fprintf(fgcc, "\n")
 }
 
+// packedAttribute returns host compiler struct attribute that will be
+// used to match 6c/8c/5c's struct layout. For example, on 386 Windows,
+// gcc wants to 8-align int64s, but 8c does not.
+// Use __gcc_struct__ to work around http://gcc.gnu.org/PR52991 on x86,
+// and http://golang.org/issue/5603.
+func (p *Package) packedAttribute() string {
+	s := "__attribute__((__packed__"
+	if !strings.Contains(p.gccBaseCmd()[0], "clang") && (goarch == "amd64" || goarch == "386") {
+		s += ", __gcc_struct__"
+	}
+	return s + "))"
+}
+
 // Write out the various stubs we need to support functions exported
 // from Go so that they are callable from C.
 func (p *Package) writeExports(fgo2, fc, fm *os.File) {
@@ -727,7 +733,7 @@ func (p *Package) writeExports(fgo2, fc, fm *os.File) {
 		fmt.Fprintf(fgcc, "extern void _cgoexp%s_%s(void *, int);\n", cPrefix, exp.ExpName)
 		fmt.Fprintf(fgcc, "\n%s\n", s)
 		fmt.Fprintf(fgcc, "{\n")
-		fmt.Fprintf(fgcc, "\t%s __attribute__((packed)) a;\n", ctype)
+		fmt.Fprintf(fgcc, "\t%s %v a;\n", ctype, p.packedAttribute())
 		if gccResult != "void" && (len(fntype.Results.List) > 1 || len(fntype.Results.List[0].Names) > 1) {
 			fmt.Fprintf(fgcc, "\t%s r;\n", gccResult)
 		}