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struct packed: Use gcc_struct on Windows

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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
This commit is contained in:
Pedro Alves
2022-07-19 00:26:33 +01:00
parent f7f904e4fd
commit 4ca26ad7de
1 changed files with 9 additions and 1 deletions
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10
gdbsupport/packed.h
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@ -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;