Messages in this thread Patch in this message | | | Date | Fri, 4 Aug 2023 11:02:56 +0200 | Subject | [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute | From | Marco Elver <> |
| |
[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. --- 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 */ /* -- 2.41.0.640.ga95def55d0-goog
| |