linux-kernel - [PATCH 1/3] Compiler Attributes: Add _

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Message-ID: <20250910151216.646600-2-hca@linux.ibm.com>
Date: Wed, 10 Sep 2025 17:12:14 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Nathan Chancellor <nathan@...nel.org>, Miguel Ojeda <ojeda@...nel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Juergen Christ <jchrist@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Sven Schnelle <svens@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: [PATCH 1/3] Compiler Attributes: Add __assume macro

Make the statement attribute "assume" with a new __assume macro available.

This allows compilers to generate better code, however code which makes use
of __assume must be written as if the compiler ignores the hint. Otherwise
this may lead to subtle bugs if code is compiled with compilers which do
not support the attribute.

Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>
---
 include/linux/compiler_attributes.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index c16d4199bf92..16c3d4a865e2 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -54,6 +54,22 @@
  */
 #define __always_inline                 inline __attribute__((__always_inline__))
 
+/*
+ * Beware: Code which makes use of __assume must be written as if the compiler
+ * ignores the hint. Otherwise this may lead to subtle bugs if code is compiled
+ * with compilers which do not support the attribute.
+ *
+ * Optional: only supported since GCC >= 13.1, clang >= 12.0
+ *
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#index-assume-statement-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#assume
+ */
+#if __has_attribute(assume)
+# define __assume(expr)                 __attribute__((__assume__(expr)))
+#else
+# define __assume(expr)
+#endif
+
 /*
  * The second argument is optional (default 0), so we use a variadic macro
  * to make the shorthand.
-- 
2.48.1