[PATCH 1/3] Compiler attributes: Introduce the __preserve_most function attribute

[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

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.

Introduce the attribute to compiler_attributes.h.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 include/linux/compiler_attributes.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 00efa35c350f..615a63ecfcf6 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -321,6 +321,17 @@
 # define __pass_object_size(type)
 #endif
 
+/*
+ * Optional: not supported by gcc.
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
+ */
+#if __has_attribute(__preserve_most__)
+# define __preserve_most __attribute__((__preserve_most__))
+#else
+# define __preserve_most
+#endif
+
 /*
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
  */
-- 
2.41.0.585.gd2178a4bd4-goog