[PATCH 1/3] struct packed: Use gcc_struct on Windows

[Pedro Alves <pedro@palves.net>]

Building GDB on mingw/gcc hosts is currently broken, due to a static
assertion failure in gdbsupport/packed.h:

  In file included from ../../../../../binutils-gdb/gdb/../gdbsupport/common-defs.h:201,
		   from ../../../../../binutils-gdb/gdb/defs.h:28,
		   from ../../../../../binutils-gdb/gdb/dwarf2/read.c:31:
  ../../../../../binutils-gdb/gdb/../gdbsupport/packed.h: In instantiation of 'packed<T, Bytes>::packed(T) [with T = dwarf_unit_type; long long unsigned int Bytes = 1]':
  ../../../../../binutils-gdb/gdb/dwarf2/read.h:181:74:   required from here
  ../../../../../binutils-gdb/gdb/../gdbsupport/packed.h:41:40: error: static assertion failed
     41 |     gdb_static_assert (sizeof (packed) == Bytes);
	|                        ~~~~~~~~~~~~~~~~^~~~~~~~
  ../../../../../binutils-gdb/gdb/../gdbsupport/gdb_assert.h:27:48: note: in definition of macro 'gdb_static_assert'
     27 | #define gdb_static_assert(expr) static_assert (expr, "")
	|                                                ^~~~
  ../../../../../binutils-gdb/gdb/../gdbsupport/packed.h:41:40: note: the comparison reduces to '(4 == 1)'
     41 |     gdb_static_assert (sizeof (packed) == Bytes);
	|                        ~~~~~~~~~~~~~~~~^~~~~~~~


The issue is that mingw gcc defaults to "-mms-bitfields", which
affects how bitfields are laid out.  We can however tell GCC that we
want the regular GCC layout instead using attribute gcc_struct.

Attribute gcc_struct is not implemented in "clang -target
x86_64-pc-windows-gnu", so that will need a different fix.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29373

Change-Id: I023315ee03622c59c397bf4affc0b68179c32374
---
 gdbsupport/packed.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/gdbsupport/packed.h b/gdbsupport/packed.h
index 3468cf44207..53164a9e0c3 100644
--- a/gdbsupport/packed.h
+++ b/gdbsupport/packed.h
@@ -27,8 +27,16 @@
    bit-fields (and ENUM_BITFIELD), when the fields must have separate
    memory locations to avoid data races.  */
 
+/* We need gcc_struct on Windows GCC, as otherwise the size of e.g.,
+   "packed<int, 1>" will be larger than what we want.  */
+#if defined _WIN32
+# define ATTRIBUTE_GCC_STRUCT __attribute__((__gcc_struct__))
+#else
+# define ATTRIBUTE_GCC_STRUCT
+#endif
+
 template<typename T, size_t Bytes = sizeof (T)>
-struct packed
+struct ATTRIBUTE_GCC_STRUCT packed
 {
 public:
   packed () noexcept = default;
-- 
2.36.0