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SME uses a lazy save system to manage ZA. The idea is that, if a function with ZA state wants to call a "normal" function, it can leave its state in ZA and instead set up a lazy save buffer. If, unexpectedly, that normal function contains a nested use of ZA, that nested use of ZA must commit the lazy save first. This lazy save system uses a special system register called TPIDR2_EL0. See: https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#66the-za-lazy-saving-scheme for details. The ABI specifies that, on entry to an exception handler, the following things must be true: * PSTATE.SM must be 0 (the processor must be in non-streaming mode) * PSTATE.ZA must be 0 (ZA must be off) * TPIDR2_EL0 must be 0 (there must be no uncommitted lazy save) This is normally done by making _Unwind_RaiseException & friends commit any lazy save before they unwind. This also has the side effect of ensuring that TPIDR2_EL0 is never left pointing to a lazy save buffer that has been unwound. However, things get more complicated with signals. If: (a) a signal is raised while ZA is dormant (that is, while there is an uncommitted lazy save); (b) the signal handler throws an exception; and (c) that exception is caught outside the signal handler something must ensure that the lazy save from (a) is committed. This would be simple if the signal handler was entered with ZA and TPIDR2_EL0 intact. However, for various good reasons that are out of scope here, this is not done. Instead, Linux now clears both TPIDR2_EL0 and PSTATE.ZA before entering a signal handler, see: https://lore.kernel.org/all/20250417190113.3778111-1-mark.rutland@arm.com/ for details. Therefore, it is the unwinder that must simulate a commit of the lazy save from (a). It can do this by reading the previous values of TPIDR2_EL0 and ZA from the sigcontext. The SME-related sigcontext structures were only added to linux's asm/sigcontext.h relatively recently and we can't rely on GCC being built against such recent kernel header files. The patch therefore uses defines relevant macros if they are not defined and provide types that comply with ABI layout of the corresponding linux types. The patch includes some ugly casting in an attempt to support big-endian ILP32, even though SME on big-endian ILP32 linux should never be a thing. We can remove it if we also remove ILP32 support from GCC. Co-authored-by: Yury Khrustalev <yury.khrustalev@arm.com> Reviewed-by: Tamar Christina <tamar.christina@arm.com> gcc/ * doc/sourcebuild.texi (aarch64_sme_hw): Document. gcc/testsuite/ * lib/target-supports.exp (add_options_for_aarch64_sme) (check_effective_target_aarch64_sme_hw): New procedures. * g++.target/aarch64/sme/sme_throw_1.C: New test. * g++.target/aarch64/sme/sme_throw_2.C: Likewise. libgcc/ * config/aarch64/linux-unwind.h (aarch64_fallback_frame_state): If a signal was raised while there was an uncommitted lazy save, commit the save as part of the unwind process.