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From: Pedro Alves <palves@sourceware.org> To: gdb-cvs@sourceware.org Subject: [binutils-gdb] struct packed: Use gcc_struct on Windows Date: Mon, 25 Jul 2022 15:12:10 +0000 (GMT) [thread overview] Message-ID: <20220725151210.8894C3857809@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=4ca26ad7dec88ab6fa8507ba069e9f1b3c5196da commit 4ca26ad7dec88ab6fa8507ba069e9f1b3c5196da Author: Pedro Alves <pedro@palves.net> Date: Tue Jul 19 00:26:33 2022 +0100 struct packed: Use gcc_struct on Windows 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 Diff: --- 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;
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