29191ea6a5
We use __builtin_subcll() to do a 64-bit subtract with borrow-in and borrow-out when the host compiler supports it. Unfortunately some versions of Apple Clang have a bug in their implementation of this intrinsic which means it returns the wrong value. The effect is that a QEMU built with the affected compiler will hang when emulating x86 or m68k float80 division. The upstream LLVM issue is: https://github.com/llvm/llvm-project/issues/55253 The commit that introduced the bug apparently never made it into an upstream LLVM release without the subsequent fixfffb6e6afdbut unfortunately it did make it into Apple Clang 14.0, as shipped in Xcode 14.3 (14.2 is reported to be OK). The Apple bug number is FB12210478. Add ifdefs to avoid use of __builtin_subcll() on Apple Clang version 14 or greater. There is not currently a version of Apple Clang which has the bug fix -- when one appears we should be able to add an upper bound to the ifdef condition so we can start using the builtin again. We make the lower bound a conservative "any Apple clang with major version 14 or greater" because the consequences of incorrectly disabling the builtin when it would work are pretty small and the consequences of not disabling it when we should are pretty bad. Many thanks to those users who both reported this bug and also did a lot of work in identifying the root cause; in particular to Daniel Bertalan and osy. Cc: qemu-stable@nongnu.org Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1631 Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1659 Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Tested-by: Daniel Bertalan <dani@danielbertalan.dev> Tested-by: Tested-By: Solra Bizna <solra@bizna.name> Message-id: 20230622130823.1631719-1-peter.maydell@linaro.org (cherry picked from commitb0438861ef) Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>