https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102024
--- Comment #17 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:3159da6c46568a7c600f78fb3a3b76e2ea4bf4cc commit r12-6418-g3159da6c46568a7c600f78fb3a3b76e2ea4bf4cc Author: Jakub Jelinek <ja...@redhat.com> Date: Mon Jan 10 17:43:23 2022 +0100 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 <ja...@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.
