Re: [PATCH] host-utils: Avoid using __builtin_subcll on buggy versions of Apple Clang

[Richard Henderson <richard.henderson@linaro.org>]

On 6/22/23 15:08, Peter Maydell wrote:
> 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
> 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 fix
> https://github.com/llvm/llvm-project/commit/fffb6e6afdbaba563189c1f715058ed401fbc88d
> but 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>
> ---
> I don't have a setup to test this, so this needs testing by the
> people who've encountered this compiler bug to confirm it does
> the right thing...
> ---
>   include/qemu/compiler.h   | 13 +++++++++++++
>   include/qemu/host-utils.h |  2 +-
>   2 files changed, 14 insertions(+), 1 deletion(-)

Ouch.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~