Messages in this thread | | | Date | Fri, 4 Aug 2023 11:58:43 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute |
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On Fri, 4 Aug 2023 11:02:56 +0200 Marco Elver <elver@google.com> 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@google.com> > --- > v2: > * Imply notrace, to avoid any conflicts with tracing which is inserted > on function entry. See added comments.
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
-- 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 */ > > /*
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