c++, abi: Set DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on C++ zero width bitfields [PR102024]

c++, abi: Set DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on C++ zero width bitfields [PR102024]

The removal of remove_zero_width_bitfields function and its call from
C++ FE layout_class_type (which I've done in the P0466R5
layout-compatible helper intrinsics patch, so that the FE can actually
determine what is and isn't layout-compatible according to the spec)
unfortunately changed the ABI on various platforms.
The C FE has been keeping zero-width bitfields in the types, while
the C++ FE has been removing them after structure layout, so in various
cases when passing such structures in registers we had different ABI
between C and C++.

While both the C and C++ FE had some code to remove zero width bitfields
after structure layout, in both FEs it was buggy and didn't really remove
any.  In the C FE that code has been removed later on, while in the C++ FE
for GCC 4.5 in PR42217 it has been actually fixed, so the C++ FE started
to remove those bitfields.

The following patch doesn't change anything ABI-wise, but allows the
targets to decide what to do, emit -Wpsabi warnings etc.
Non-C zero width bitfields will be seen by the backends as normal
zero width bitfields, C++ zero width bitfields that used to be previously
removed will have DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD flag set.
I've reused the DECL_FIELD_ABI_IGNORED flag which is only used on non-bitfield
FIELD_DECLs right now, but the macros now check DECL_BIT_FIELD flag.

Each backend can then decide what it wants, whether it wants to keep
different ABI between C and C++ as in GCC 11 and older (i.e. incompatible
with G++ <= 4.4, compatible with G++ 4.5 .. 11), for that it would
ignore for the aggregate passing/returning decisions all
DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD FIELD_DECLs), whether it wants to never
ignore zero width bitfields (no changes needed for that case, except perhaps
-Wpsabi warning should be added and for that DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD
can be tested), or whether it wants to always ignore zero width bitfields
(I think e.g. riscv in GCC 10+ does that).

All this patch does is set the flag which the backends can then use.

2021-09-03  Jakub Jelinek  <jakub@redhat.com>

	PR target/102024
gcc/
	* tree.h (DECL_FIELD_ABI_IGNORED): Changed into rvalue only macro
	that is false if DECL_BIT_FIELD.
	(SET_DECL_FIELD_ABI_IGNORED, DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD,
	SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD): Define.
	* tree-streamer-out.c (pack_ts_decl_common_value_fields): For
	DECL_BIT_FIELD stream DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead
	of DECL_FIELD_ABI_IGNORED.
	* tree-streamer-in.c (unpack_ts_decl_common_value_fields): Use
	SET_DECL_FIELD_ABI_IGNORED instead of writing to
	DECL_FIELD_ABI_IGNORED and for DECL_BIT_FIELD use
	SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead.
	* lto-streamer-out.c (hash_tree): For DECL_BIT_FIELD hash
	DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD instead of DECL_FIELD_ABI_IGNORED.
gcc/cp/
	* class.c (build_base_field): Use SET_DECL_FIELD_ABI_IGNORED
	instead of writing to DECL_FIELD_ABI_IGNORED.
	(layout_class_type): Likewise.  In the place where zero-width
	bitfields used to be removed, use
	SET_DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD on those fields instead.
gcc/lto/
	* lto-common.c (compare_tree_sccs_1): Also compare
	DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD values.