cmd/cgo: use gcc_struct attribute for packed structs to work around GCC PR52991.

Fixes #5603.

R=iant, dave
CC=gobot, golang-dev
https://golang.org/cl/9895043

misc/cgo/test/cgo_test.go

@@ -41,5 +41,6 @@ func Test5227(t *testing.T)                { test5227(t) }
 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 BenchmarkCgoCall(b *testing.B) { benchCgoCall(b) }

misc/cgo/test/issue5603.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
+
+/*
+const long long issue5603exp = 0x12345678;
+long long issue5603foo0() { return issue5603exp; }
+long long issue5603foo1(void *p) { return issue5603exp; }
+long long issue5603foo2(void *p, void *q) { return issue5603exp; }
+long long issue5603foo3(void *p, void *q, void *r) { return issue5603exp; }
+long long issue5603foo4(void *p, void *q, void *r, void *s) { return issue5603exp; }
+*/
+import "C"
+
+import "testing"
+
+func test5603(t *testing.T) {
+	var x [5]int64
+	exp := int64(C.issue5603exp)
+	x[0] = int64(C.issue5603foo0())
+	x[1] = int64(C.issue5603foo1(nil))
+	x[2] = int64(C.issue5603foo2(nil, nil))
+	x[3] = int64(C.issue5603foo3(nil, nil, nil))
+	x[4] = int64(C.issue5603foo4(nil, nil, nil, nil))
+	for i, v := range x {
+		if v != exp {
+			t.Errorf("issue5603foo%d() returns %v, expected %v", i, v, exp)
+		}
+	}
+}

src/cmd/cgo/doc.go

@@ -310,7 +310,7 @@ file compiled by gcc, the file x.cgo2.c:
 			char* p0;
 			int r;
 			char __pad12[4];
-		} __attribute__((__packed__)) *a = v;
+		} __attribute__((__packed__, __gcc_struct__)) *a = v;
 		a->r = puts((void*)a->p0);
 	}
 

src/cmd/cgo/out.go

@@ -485,7 +485,13 @@ func (p *Package) writeOutputFunc(fgcc *os.File, n *Name) {
 	// 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.
-	fmt.Fprintf(fgcc, "\t%s __attribute__((__packed__)) *a = v;\n", ctype)
+	// Use __gcc_struct__ to work around http://gcc.gnu.org/PR52991 on x86,
+	// and http://golang.org/issue/5603.
+	extraAttr := ""
+	if goarch == "amd64" || goarch == "386" {
+		extraAttr = ", __gcc_struct__"
+	}
+	fmt.Fprintf(fgcc, "\t%s __attribute__((__packed__%v)) *a = v;\n", ctype, extraAttr)
 	fmt.Fprintf(fgcc, "\t")
 	if t := n.FuncType.Result; t != nil {
 		fmt.Fprintf(fgcc, "a->r = ")