Merge tag 'compiler-attributes-for-linus-4.20-rc1' of https://github.…

…com/ojeda/linux

Pull compiler attribute updates from Miguel Ojeda:
 "This is an effort to disentangle the include/linux/compiler*.h headers
  and bring them up to date.

  The main idea behind the series is to use feature checking macros
  (i.e. __has_attribute) instead of compiler version checks (e.g.
  GCC_VERSION), which are compiler-agnostic (so they can be shared,
  reducing the size of compiler-specific headers) and version-agnostic.

  Other related improvements have been performed in the headers as well,
  which on top of the use of __has_attribute it has amounted to a
  significant simplification of these headers (e.g. GCC_VERSION is now
  only guarding a few non-attribute macros).

  This series should also help the efforts to support compiling the
  kernel with clang and icc. A fair amount of documentation and comments
  have also been added, clarified or removed; and the headers are now
  more readable, which should help kernel developers in general.

  The series was triggered due to the move to gcc >= 4.6. In turn, this
  series has also triggered Sparse to gain the ability to recognize
  __has_attribute on its own.

  Finally, the __nonstring variable attribute series has been also
  applied on top; plus two related patches from Nick Desaulniers for
  unreachable() that came a bit afterwards"

* tag 'compiler-attributes-for-linus-4.20-rc1' of https://github.com/ojeda/linux:
  compiler-gcc: remove comment about gcc 4.5 from unreachable()
  compiler.h: update definition of unreachable()
  Compiler Attributes: ext4: remove local __nonstring definition
  Compiler Attributes: auxdisplay: panel: use __nonstring
  Compiler Attributes: enable -Wstringop-truncation on W=1 (gcc >= 8)
  Compiler Attributes: add support for __nonstring (gcc >= 8)
  Compiler Attributes: add MAINTAINERS entry
  Compiler Attributes: add Doc/process/programming-language.rst
  Compiler Attributes: remove uses of __attribute__ from compiler.h
  Compiler Attributes: KENTRY used twice the "used" attribute
  Compiler Attributes: use feature checks instead of version checks
  Compiler Attributes: add missing SPDX ID in compiler_types.h
  Compiler Attributes: remove unneeded sparse (__CHECKER__) tests
  Compiler Attributes: homogenize __must_be_array
  Compiler Attributes: remove unneeded tests
  Compiler Attributes: always use the extra-underscores syntax
  Compiler Attributes: remove unused attributes

Documentation/process/index.rst

@@ -25,6 +25,7 @@ Below are the essential guides that every developer should read.
    code-of-conduct-interpretation
    development-process
    submitting-patches
+   programming-language
    coding-style
    maintainer-pgp-guide
    email-clients

Documentation/process/programming-language.rst

@@ -0,0 +1,45 @@
+.. _programming_language:
+
+Programming Language
+====================
+
+The kernel is written in the C programming language [c-language]_.
+More precisely, the kernel is typically compiled with ``gcc`` [gcc]_
+under ``-std=gnu89`` [gcc-c-dialect-options]_: the GNU dialect of ISO C90
+(including some C99 features).
+
+This dialect contains many extensions to the language [gnu-extensions]_,
+and many of them are used within the kernel as a matter of course.
+
+There is some support for compiling the kernel with ``clang`` [clang]_
+and ``icc`` [icc]_ for several of the architectures, although at the time
+of writing it is not completed, requiring third-party patches.
+
+Attributes
+----------
+
+One of the common extensions used throughout the kernel are attributes
+[gcc-attribute-syntax]_. Attributes allow to introduce
+implementation-defined semantics to language entities (like variables,
+functions or types) without having to make significant syntactic changes
+to the language (e.g. adding a new keyword) [n2049]_.
+
+In some cases, attributes are optional (i.e. a compiler not supporting them
+should still produce proper code, even if it is slower or does not perform
+as many compile-time checks/diagnostics).
+
+The kernel defines pseudo-keywords (e.g. ``__pure``) instead of using
+directly the GNU attribute syntax (e.g. ``__attribute__((__pure__))``)
+in order to feature detect which ones can be used and/or to shorten the code.
+
+Please refer to ``include/linux/compiler_attributes.h`` for more information.
+
+.. [c-language] http://www.open-std.org/jtc1/sc22/wg14/www/standards
+.. [gcc] https://gcc.gnu.org
+.. [clang] https://clang.llvm.org
+.. [icc] https://software.intel.com/en-us/c-compilers
+.. [gcc-c-dialect-options] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
+.. [gnu-extensions] https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
+.. [gcc-attribute-syntax] https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html
+.. [n2049] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2049.pdf
+

MAINTAINERS

@@ -3737,6 +3737,11 @@ L:	[email protected]
 S:	Maintained
 F:	drivers/platform/x86/compal-laptop.c
 
+COMPILER ATTRIBUTES
+M:	Miguel Ojeda <[email protected]>
+S:	Maintained
+F:	include/linux/compiler_attributes.h
+
 CONEXANT ACCESSRUNNER USB DRIVER
 L:	[email protected]
 W:	http://accessrunner.sourceforge.net/

drivers/auxdisplay/panel.c

@@ -155,10 +155,9 @@ struct logical_input {
 			int release_data;
 		} std;
 		struct {	/* valid when type == INPUT_TYPE_KBD */
-			/* strings can be non null-terminated */
-			char press_str[sizeof(void *) + sizeof(int)];
-			char repeat_str[sizeof(void *) + sizeof(int)];
-			char release_str[sizeof(void *) + sizeof(int)];
+			char press_str[sizeof(void *) + sizeof(int)] __nonstring;
+			char repeat_str[sizeof(void *) + sizeof(int)] __nonstring;
+			char release_str[sizeof(void *) + sizeof(int)] __nonstring;
 		} kbd;
 	} u;
 };

fs/ext4/ext4.h

@@ -45,15 +45,6 @@
 
 #include <linux/compiler.h>
 
-/* Until this gets included into linux/compiler-gcc.h */
-#ifndef __nonstring
-#if defined(GCC_VERSION) && (GCC_VERSION >= 80000)
-#define __nonstring __attribute__((nonstring))
-#else
-#define __nonstring
-#endif
-#endif
-
 /*
  * The fourth extended filesystem constants/structures
  */

include/linux/compiler-clang.h

@@ -21,8 +21,6 @@
 #define __SANITIZE_ADDRESS__
 #endif
 
-#define __no_sanitize_address __attribute__((no_sanitize("address")))
-
 /*
  * Not all versions of clang implement the the type-generic versions
  * of the builtin overflow checkers. Fortunately, clang implements
@@ -41,6 +39,3 @@
  * compilers, like ICC.
  */
 #define barrier() __asm__ __volatile__("" : : : "memory")
-#define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
-#define __assume_aligned(a, ...)	\
-	__attribute__((__assume_aligned__(a, ## __VA_ARGS__)))

include/linux/compiler-gcc.h

@@ -68,31 +68,20 @@
  */
 #define uninitialized_var(x) x = x
 
-#ifdef __CHECKER__
-#define __must_be_array(a)	0
-#else
-/* &a[0] degrades to a pointer: a different type from an array */
-#define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
-#endif
-
 #ifdef RETPOLINE
-#define __noretpoline __attribute__((indirect_branch("keep")))
+#define __noretpoline __attribute__((__indirect_branch__("keep")))
 #endif
 
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
 
-#define __optimize(level)	__attribute__((__optimize__(level)))
-
 #define __compiletime_object_size(obj) __builtin_object_size(obj, 0)
 
-#ifndef __CHECKER__
-#define __compiletime_warning(message) __attribute__((warning(message)))
-#define __compiletime_error(message) __attribute__((error(message)))
+#define __compiletime_warning(message) __attribute__((__warning__(message)))
+#define __compiletime_error(message) __attribute__((__error__(message)))
 
-#ifdef LATENT_ENTROPY_PLUGIN
+#if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
 #define __latent_entropy __attribute__((latent_entropy))
 #endif
-#endif /* __CHECKER__ */
 
 /*
  * calling noreturn functions, __builtin_unreachable() and __builtin_trap()
@@ -107,10 +96,6 @@
  * Mark a position in code as unreachable.  This can be used to
  * suppress control flow warnings after asm blocks that transfer
  * control elsewhere.
- *
- * Early snapshots of gcc 4.5 don't support this and we can't detect
- * this in the preprocessor, but we can live with this because they're
- * unreleased.  Really, we need to have autoconf for the kernel.
  */
 #define unreachable() \
 	do {					\
@@ -119,9 +104,6 @@
 		__builtin_unreachable();	\
 	} while (0)
 
-/* Mark a function definition as prohibited from being cloned. */
-#define __noclone	__attribute__((__noclone__, __optimize__("no-tracer")))
-
 #if defined(RANDSTRUCT_PLUGIN) && !defined(__CHECKER__)
 #define __randomize_layout __attribute__((randomize_layout))
 #define __no_randomize_layout __attribute__((no_randomize_layout))
@@ -130,32 +112,6 @@
 #define randomized_struct_fields_end	} __randomize_layout;
 #endif
 
-/*
- * When used with Link Time Optimization, gcc can optimize away C functions or
- * variables which are referenced only from assembly code.  __visible tells the
- * optimizer that something else uses this function or variable, thus preventing
- * this.
- */
-#define __visible	__attribute__((externally_visible))
-
-/* gcc version specific checks */
-
-#if GCC_VERSION >= 40900 && !defined(__CHECKER__)
-/*
- * __assume_aligned(n, k): Tell the optimizer that the returned
- * pointer can be assumed to be k modulo n. The second argument is
- * optional (default 0), so we use a variadic macro to make the
- * shorthand.
- *
- * Beware: Do not apply this to functions which may return
- * ERR_PTRs. Also, it is probably unwise to apply it to functions
- * returning extra information in the low bits (but in that case the
- * compiler should see some alignment anyway, when the return value is
- * massaged by 'flags = ptr & 3; ptr &= ~3;').
- */
-#define __assume_aligned(a, ...) __attribute__((__assume_aligned__(a, ## __VA_ARGS__)))
-#endif
-
 /*
  * GCC 'asm goto' miscompiles certain code sequences:
  *
@@ -187,39 +143,22 @@
 #define KASAN_ABI_VERSION 3
 #endif
 
-#if GCC_VERSION >= 40902
 /*
- * Tell the compiler that address safety instrumentation (KASAN)
- * should not be applied to that function.
- * Conflicts with inlining: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
+ * Because __no_sanitize_address conflicts with inlining:
+ *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
+ * we do one or the other. 
  */
-#define __no_sanitize_address __attribute__((no_sanitize_address))
 #ifdef CONFIG_KASAN
 #define __no_sanitize_address_or_inline					\
 	__no_sanitize_address __maybe_unused notrace
 #else
 #define __no_sanitize_address_or_inline inline
 #endif
-#endif
 
 #if GCC_VERSION >= 50100
-/*
- * Mark structures as requiring designated initializers.
- * https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html
- */
-#define __designated_init __attribute__((designated_init))
 #define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
 #endif
 
-#if !defined(__noclone)
-#define __noclone	/* not needed */
-#endif
-
-#if !defined(__no_sanitize_address)
-#define __no_sanitize_address
-#define __no_sanitize_address_or_inline inline
-#endif
-
 /*
  * Turn individual warnings and errors on and off locally, depending
  * on version.

include/linux/compiler-intel.h

@@ -29,17 +29,8 @@
  */
 #define OPTIMIZER_HIDE_VAR(var) barrier()
 
-/* Intel ECC compiler doesn't support __builtin_types_compatible_p() */
-#define __must_be_array(a) 0
-
 #endif
 
 /* icc has this, but it's called _bswap16 */
 #define __HAVE_BUILTIN_BSWAP16__
 #define __builtin_bswap16 _bswap16
-
-/* The following are for compatibility with GCC, from compiler-gcc.h,
- * and may be redefined here because they should not be shared with other
- * compilers, like clang.
- */
-#define __visible	__attribute__((externally_visible))

include/linux/compiler.h

@@ -23,8 +23,8 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 #define __branch_check__(x, expect, is_constant) ({			\
 			long ______r;					\
 			static struct ftrace_likely_data		\
-				__attribute__((__aligned__(4)))		\
-				__attribute__((section("_ftrace_annotated_branch"))) \
+				__aligned(4)				\
+				__section("_ftrace_annotated_branch")	\
 				______f = {				\
 				.data.func = __func__,			\
 				.data.file = __FILE__,			\
@@ -59,8 +59,8 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	({								\
 		int ______r;						\
 		static struct ftrace_branch_data			\
-			__attribute__((__aligned__(4)))			\
-			__attribute__((section("_ftrace_branch")))	\
+			__aligned(4)					\
+			__section("_ftrace_branch")			\
 			______f = {					\
 				.func = __func__,			\
 				.file = __FILE__,			\
@@ -115,7 +115,10 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define ASM_UNREACHABLE
 #endif
 #ifndef unreachable
-# define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
+# define unreachable() do {		\
+	annotate_unreachable();		\
+	__builtin_unreachable();	\
+} while (0)
 #endif
 
 /*
@@ -137,7 +140,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	extern typeof(sym) sym;					\
 	static const unsigned long __kentry_##sym		\
 	__used							\
-	__attribute__((section("___kentry" "+" #sym ), used))	\
+	__section("___kentry" "+" #sym )			\
 	= (unsigned long)&sym;
 #endif
 
@@ -278,7 +281,7 @@ unsigned long read_word_at_a_time(const void *addr)
  * visible to the compiler.
  */
 #define __ADDRESSABLE(sym) \
-	static void * __attribute__((section(".discard.addressable"), used)) \
+	static void * __section(".discard.addressable") __used \
 		__PASTE(__addressable_##sym, __LINE__) = (void *)&sym;
 
 /**
@@ -331,10 +334,6 @@ static inline void *offset_to_ptr(const int *off)
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 
-#ifndef __optimize
-# define __optimize(level)
-#endif
-
 /* Compile time object size, -1 for unknown */
 #ifndef __compiletime_object_size
 # define __compiletime_object_size(obj) -1
@@ -376,4 +375,7 @@ static inline void *offset_to_ptr(const int *off)
 	compiletime_assert(__native_word(t),				\
 		"Need native word sized stores/loads for atomicity.")
 
+/* &a[0] degrades to a pointer: a different type from an array */
+#define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
+
 #endif /* __LINUX_COMPILER_H */