__has_attribute(__preserve_most__) returns true even when a target doesn't support it #1908
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nickdesaulniers
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akiyks
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[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. Note: The additional preprocessor check against architecture should not be necessary if __has_attribute() only returns true where supported; also see ClangBuiltLinux#1908. But until __has_attribute() does the right thing, we also guard by known-supported architectures to avoid build warnings on other architectures. 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> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Acked-by: "Steven Rostedt (Google)" <rostedt@goodmis.org> Acked-by: Mark Rutland <mark.rutland@arm.com> Link: https://lore.kernel.org/r/20230811151847.1594958-1-elver@google.com Signed-off-by: Kees Cook <keescook@chromium.org>
fadlyas07
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Mar 30, 2024
…st function attribute [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. Note: The additional preprocessor check against architecture should not be necessary if __has_attribute() only returns true where supported; also see ClangBuiltLinux/linux#1908. But until __has_attribute() does the right thing, we also guard by known-supported architectures to avoid build warnings on other architectures. 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> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Acked-by: "Steven Rostedt (Google)" <rostedt@goodmis.org> Acked-by: Mark Rutland <mark.rutland@arm.com> Link: https://lore.kernel.org/r/20230811151847.1594958-1-elver@google.com Signed-off-by: Kees Cook <keescook@chromium.org> (cherry picked from commit 7a0fd5e) Conflicts: include/linux/compiler_types.h We don't have commit c27cd08 ("Compiler attributes: GCC cold function alignment workarounds"), which is a context conflict and not functionally required. BUG=b:330784849 TEST=CQ Change-Id: Ie123c30196a61efa030fbc49138d4f1aafa47b07 Signed-off-by: Stephen Boyd <swboyd@chromium.org> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/5385690 Reviewed-by: Douglas Anderson <dianders@chromium.org>
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via https://lore.kernel.org/llvm/ZNN1WdZ5J%2Fe5OPAt@elver.google.com/
cc @melver
Long term, perhaps we can fix in clang what __has_attribute evaluates to for this attribute depending on the target.
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