| Message ID | 20230808102049.465864-1-elver@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v3,1/3] compiler_types: Introduce the Clang __preserve_most function attribute | expand |
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@google.com> > Reviewed-by: Miguel Ojeda <ojeda@kernel.org> > Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org> So long as this implies notrace I believe it is safe, so: Acked-by: Mark Rutland <mark.rutland@arm.com> 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 >
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 */ /*