| 3 | 3 | Excerpt: "An ABI incompatibility between C and C++ when passing or returning by value certain aggregates containing zero width bit-fields has been discovered on various targets. As mentioned in PR102024, since the PR42217 fix in GCC 4.5 the C++ front-end has been removing zero width bit-fields from the internal representation of the aggregates after the layout of those aggregates, but the C front-end kept them, so passing e.g. struct S { float a; int : 0; float b; } or struct T { float c; int : 0; } by value could differ between C and C++. Starting with GCC 12 the C++ front-end no longer removes those bit-fields from the internal representation and per clarified psABI some targets have been changed, so that they either ignore those bit-fields in the argument passing by value decisions in both C and C++, or they always take them into account. x86-64, ARM and AArch64 will always ignore them (so there is a C ABI incompatibility between GCC 11 and earlier with GCC 12 or later), PowerPC64 ELFv2 and S/390 always take them into account (so there is a C++ ABI incompatibility, GCC 4.4 and earlier compatible with GCC 12 or later, incompatible with GCC 4.5 through GCC 11). RISC-V has changed the handling of these already starting with GCC 10. As the ABI requires, MIPS takes them into account handling function return values so there is a C++ ABI incompatibility with GCC 4.5 through 11. For function arguments on MIPS, refer to the MIPS specific entry. GCC 12 on the above targets will report such incompatibilities as warnings or other diagnostics unless -Wno-psabi is used." |
