Re: [PATCH v3 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

[Mark Rutland]

On Tue, Aug 08, 2023 at 12:17:25PM +0200, Marco Elver 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.
> If the arguments are passed in callee-saved registers, then they will be
> preserved by the callee across the call. This doesn't apply for values
> returned in callee-saved registers.
> 
>  * On X86-64 the callee preserves all general purpose registers, except
>    for R11. R11 can be used as a scratch register. Floating-point
>    registers (XMMs/YMMs) are not preserved and need to be saved by the
>    caller.
> 
>  * On AArch64 the callee preserve all general purpose registers, except
>    x0-X8 and X16-X18."
> 
> [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.
> 
> The attribute may be supported by a future GCC version (see
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).
> 
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> Reviewed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

So long as this implies notrace I believe it is safe, so:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
> v3:
> * Quote more from LLVM documentation about which registers are
>   callee/caller with preserve_most.
> * Code comment to restrict use where tracing is meant to be disabled.
> 
> v2:
> * Imply notrace, to avoid any conflicts with tracing which is inserted
>   on function entry. See added comments.
> ---
>  include/linux/compiler_types.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 547ea1ff806e..c88488715a39 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -106,6 +106,34 @@ 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.  It is recommended to restrict
> + * use of the attribute to functions that should or already disable tracing.
> + *
> + * 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 */
>  
>  /*
> -- 
> 2.41.0.640.ga95def55d0-goog
>