+ compiler-attributes-introduce-the-__preserve_most-function-attribute.patch added to mm-nonmm-unstable branch

+ compiler-attributes-introduce-the-__preserve_most-function-attribute.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: Compiler attributes: introduce the LLVM __preserve_most function attribute
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     compiler-attributes-introduce-the-__preserve_most-function-attribute.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/compiler-attributes-introduce-the-__preserve_most-function-attribute.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Marco Elver <elver@google.com>
Subject: Compiler attributes: introduce the LLVM __preserve_most function attribute
Date: Wed, 2 Aug 2023 17:06:37 +0200

[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.

Link: https://lkml.kernel.org/r/20230802150712.3583252-1-elver@google.com
Signed-off-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Tom Rix <trix@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/compiler_attributes.h |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/include/linux/compiler_attributes.h~compiler-attributes-introduce-the-__preserve_most-function-attribute
+++ a/include/linux/compiler_attributes.h
@@ -322,6 +322,17 @@
 #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
  */
 #define __pure                          __attribute__((__pure__))
_

Patches currently in -mm which might be from elver@google.com are

compiler-attributes-introduce-the-__preserve_most-function-attribute.patch
list_debug-introduce-inline-wrappers-for-debug-checks.patch
list_debug-introduce-config_debug_list_minimal.patch