Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

From: Steven Rostedt
Date: Fri Aug 04 2023 - 11:58:53 EST


On Fri, 4 Aug 2023 11:02:56 +0200
Marco Elver <elver@xxxxxxxxxx> wrote:

> [1]: "On X86-64 and AArch64 targets, this attribute changes the calling
> convention of a function. The preserve_most calling convention attempts
> to make the code in the caller as unintrusive as possible. This
> convention behaves identically to the C calling convention on how
> arguments and return values are passed, but it uses a different set of
> caller/callee-saved registers. This alleviates the burden of saving and
> recovering a large register set before and after the call in the
> caller."
>
> [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
>
> Introduce the attribute to compiler_types.h as __preserve_most.
>
> Use of this attribute results in better code generation for calls to
> very rarely called functions, such as error-reporting functions, or
> rarely executed slow paths.
>
> Beware that the attribute conflicts with instrumentation calls inserted
> on function entry which do not use __preserve_most themselves. Notably,
> function tracing which assumes the normal C calling convention for the
> given architecture. Where the attribute is supported, __preserve_most
> will imply notrace. It is recommended to restrict use of the attribute
> to functions that should or already disable tracing.
>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> v2:
> * Imply notrace, to avoid any conflicts with tracing which is inserted
> on function entry. See added comments.

Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

-- Steve

> ---
> include/linux/compiler_types.h | 27 +++++++++++++++++++++++++++
> 1 file changed, 27 insertions(+)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 547ea1ff806e..12c4540335b7 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -106,6 +106,33 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
> #define __cold
> #endif
>
> +/*
> + * On x86-64 and arm64 targets, __preserve_most changes the calling convention
> + * of a function to make the code in the caller as unintrusive as possible. This
> + * convention behaves identically to the C calling convention on how arguments
> + * and return values are passed, but uses a different set of caller- and callee-
> + * saved registers.
> + *
> + * The purpose is to alleviates the burden of saving and recovering a large
> + * register set before and after the call in the caller. This is beneficial for
> + * rarely taken slow paths, such as error-reporting functions that may be called
> + * from hot paths.
> + *
> + * Note: This may conflict with instrumentation inserted on function entry which
> + * does not use __preserve_most or equivalent convention (if in assembly). Since
> + * function tracing assumes the normal C calling convention, where the attribute
> + * is supported, __preserve_most implies notrace.
> + *
> + * Optional: not supported by gcc.
> + *
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> + */
> +#if __has_attribute(__preserve_most__)
> +# define __preserve_most notrace __attribute__((__preserve_most__))
> +#else
> +# define __preserve_most
> +#endif
> +
> /* Builtins */
>
> /*