Re: [PATCH] tcg: Mark tcg helpers noinline to avoid an issue with LTO

[Peter Maydell]

On Thu, 19 Jan 2023 at 16:50, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Marking helpers __attribute__((noinline)) prevents an issue
> with GCC's ipa-split pass under --enable-lto.
>
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1454
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>
> Idan, please give this a try.  By inspection, it fixes the issue
> with s390x's helper_divs32 function.
>
>
> r~
>
> ---
>  include/exec/helper-proto.h | 32 ++++++++++++++++++++++++--------
>  1 file changed, 24 insertions(+), 8 deletions(-)
>
> diff --git a/include/exec/helper-proto.h b/include/exec/helper-proto.h
> index c4b1bda632..7a3f04b58c 100644
> --- a/include/exec/helper-proto.h
> +++ b/include/exec/helper-proto.h
> @@ -6,34 +6,49 @@
>
>  #include "exec/helper-head.h"
>
> +/*
> + * Work around an issue with --enable-lto, in which GCC's ipa-split pass
> + * decides to split out the noreturn code paths that raise an exception,
> + * taking the __builtin_return_address() along into the new function,
> + * where it no longer computes a value that returns to TCG generated code.
> + * Despite the name, the noinline attribute affects splitter, so this
> + * prevents the optimization in question.  Given that helpers should not
> + * otherwise be called directly, this should have any other visible effect.
> + *
> + * See https://gitlab.com/qemu-project/qemu/-/issues/1454
> + */

Do we have any chance of persuading the gcc folks to document
that 'noinline' also affects function splitting ? That would
reduce the chance that they decide to take out the attribute
check at some future date...

thanks
-- PMM