Hi! This patch documents the PR102024 ABI changes. The x86-64, ARM and AArch64 backends refer to this in their -Wpsabi diagnostics. Ok for wwwdocs?
diff --git a/htdocs/gcc-12/changes.html b/htdocs/gcc-12/changes.html index 689feeba..dc0e4074 100644 --- a/htdocs/gcc-12/changes.html +++ b/htdocs/gcc-12/changes.html @@ -28,6 +28,31 @@ a work-in-progress.</p> <!-- .................................................................. --> <h2>Caveats</h2> <ul> + <li> + An <a name="zero_width_bitfields">ABI</a> incompatibility between C and + C++ when passing or returning by value certain aggregates with zero + width bit-fields has been discovered on various targets. + As mentioned in <a href="https://gcc.gnu.org/PR102024";>PR102024</a>, + since the <a href="https://gcc.gnu.org/PR42217";>PR42217</a> 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. + <code>struct S { float a; int : 0; float b; }</code> or + <code>struct T { float c; int : 0; }</code> 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. + GCC 12 on the above targets will report such incompatibilities as + warnings or other diagnostics unless <code>-Wno-psabi</code> is used. + </li> <li> <strong>C:</strong> Computed gotos require a pointer type now. Jakub
