x86_64: Ignore zero width bitfields in ABI and issue -Wpsabi warning about C zero...

x86_64: Ignore zero width bitfields in ABI and issue -Wpsabi warning about C zero width bitfield ABI changes [PR102024]

For zero-width bitfields current GCC classify_argument does:
                  if (DECL_BIT_FIELD (field))
                    {
                      for (i = (int_bit_position (field)
                                + (bit_offset % 64)) / 8 / 8;
                           i < ((int_bit_position (field) + (bit_offset % 64))
                                + tree_to_shwi (DECL_SIZE (field))
                                + 63) / 8 / 8; i++)
                        classes[i]
                          = merge_classes (X86_64_INTEGER_CLASS, classes[i]);
                    }
which I think means that if the zero-width bitfields are at bit-positions
(in the toplevel aggregate) which are multiples of 64 bits doesn't do
anything, (int_bit_position (field) + (bit_offset % 64)) / 64 and
(int_bit_position (field) + (bit_offset % 64) + 63) / 64 should be equal.
But for zero-width bitfields at other bit positions it will call
merge_classes once.  Now, the typical case is that the zero width bitfield
is surrounded by some bitfields and in that case, it doesn't change
anything, but it can be sandwitched in between floats too as the testcases
show.
In C we had this behavior, in C++ previously the FE was removing the
zero-width bitfields and therefore they were ignored.
LLVM and ICC seems to ignore those bitfields both in C and C++ (== passing
struct S in SSE register rather than in GPR).

The x86-64 psABI has been recently clarified by
https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/1aa4398d26c250b252a0c4a0f777216c9a6789ec
that zero width bitfield should be always ignored.

This patch implements that and emits a warning for C for cases where the ABI
changed from GCC 11.

2022-01-10  Jakub Jelinek  <jakub@redhat.com>

	PR target/102024
	* config/i386/i386.c (classify_argument): Add zero_width_bitfields
	argument, when seeing DECL_FIELD_CXX_ZERO_WIDTH_BIT_FIELD bitfields,
	always ignore them, when seeing other zero sized bitfields, either
	set zero_width_bitfields to 1 and ignore it or if equal to 2 process
	it.  Pass it to recursive calls.  Add wrapper
	with old arguments and diagnose ABI differences for C structures
	with zero width bitfields.  Formatting fixes.

	* gcc.target/i386/pr102024.c: New test.
	* g++.target/i386/pr102024.C: New test.