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Subject[patch] compiler: introduce __used and __maybe_unused
__used is defined to be __attribute__((unused)) for all pre-3.4 gcc
compilers to suppress warnings for unused functions because perhaps they
are referenced only in inline assembly. It is defined to be
__attribute__((used)) for gcc 3.4 and later so that the code is still
emitted for such functions.

There was a mistake in the current implementation of __attribute_used__
whereas it would be defined to be __attribute__((used)) incorrectly for
gcc 3.3 and later. The unit-at-a-time compilation scheme was only
introduced in gcc 3.4 and later versions as specified in
http://www.gnu.org/software/gcc/gcc-3.4/changes.html.

__maybe_unused is defined to be __attribute__((unused)) for both function
and variable use if it could possibly be unreferenced due to the
evaluation of preprocessor macros. Function prototypes shall be marked
with __maybe_unused if the actual definition of the function is dependant
on preprocessor macros.

No update to compiler-intel.h is necessary because ICC supports both
__attribute__((used)) and __attribute__((unused)) as specified by the
gcc manual.

__attribute_used__ is deprecated and will be removed once all current
code is converted to using __used.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andrian Bunk <bunk@stusta.de>
Signed-off-by: David Rientjes <rientjes@google.com>
---
include/linux/compiler-gcc.h | 1 +
include/linux/compiler-gcc3.h | 13 ++++++-------
include/linux/compiler-gcc4.h | 3 ++-
include/linux/compiler.h | 17 ++++++++++++++---
4 files changed, 23 insertions(+), 11 deletions(-)
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -37,3 +37,4 @@
#define noinline __attribute__((noinline))
#define __attribute_pure__ __attribute__((pure))
#define __attribute_const__ __attribute__((__const__))
+#define __maybe_unused __attribute__((unused))
diff --git a/include/linux/compiler-gcc3.h b/include/linux/compiler-gcc3.h
--- a/include/linux/compiler-gcc3.h
+++ b/include/linux/compiler-gcc3.h
@@ -3,14 +3,13 @@
/* These definitions are for GCC v3.x. */
#include <linux/compiler-gcc.h>

-#if __GNUC_MINOR__ >= 3
-# define __attribute_used__ __attribute__((__used__))
-#else
-# define __attribute_used__ __attribute__((__unused__))
-#endif
-
#if __GNUC_MINOR__ >= 4
-#define __must_check __attribute__((warn_unused_result))
+# define __used __attribute__((__used__))
+# define __attribute_used__ __used /* deprecated */
+# define __must_check __attribute__((warn_unused_result))
+#else
+# define __used __attribute__((__unused__))
+# define __attribute_used__ __used /* deprecated */
#endif

#define __always_inline inline __attribute__((always_inline))
diff --git a/include/linux/compiler-gcc4.h b/include/linux/compiler-gcc4.h
--- a/include/linux/compiler-gcc4.h
+++ b/include/linux/compiler-gcc4.h
@@ -12,7 +12,8 @@
# define __inline __inline __attribute__((always_inline))
#endif

-#define __attribute_used__ __attribute__((__used__))
+#define __used __attribute__((__used__))
+#define __attribute_used__ __used /* deprecated */
#define __must_check __attribute__((warn_unused_result))
#define __compiler_offsetof(a,b) __builtin_offsetof(a,b)
#define __always_inline inline __attribute__((always_inline))
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -108,15 +108,26 @@ extern void __chk_io_ptr(const void __iomem *);
* Allow us to avoid 'defined but not used' warnings on functions and data,
* as well as force them to be emitted to the assembly file.
*
- * As of gcc 3.3, static functions that are not marked with attribute((used))
- * may be elided from the assembly file. As of gcc 3.3, static data not so
+ * As of gcc 3.4, static functions that are not marked with attribute((used))
+ * may be elided from the assembly file. As of gcc 3.4, static data not so
* marked will not be elided, but this may change in a future gcc version.
*
* In prior versions of gcc, such functions and data would be emitted, but
* would be warned about except with attribute((unused)).
+ *
+ * Mark functions that are referenced only in inline assembly as __used so
+ * the code is emitted even though it appears to be unreferenced.
*/
#ifndef __attribute_used__
-# define __attribute_used__ /* unimplemented */
+# define __attribute_used__ /* deprecated */
+#endif
+
+#ifndef __used
+# define __used /* unimplemented */
+#endif
+
+#ifndef __maybe_unused
+# define __maybe_unused /* unimplemented */
#endif

/*
-
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