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kernel/include/linux/compiler_attributes.h
Yury Norov efe3a85eab Compiler Attributes: Add __always_used macro 
In some cases like performance benchmarking, we need to call a
function, but don't need to read the returned value. If compiler
recognizes the function as pure or const, it can remove the function
invocation, which is not what we want.

To prevent that, the common practice is assigning the return value to
a temporary static volatile variable. From compiler's point of view,
the variable is unused because never read back after been assigned.
To make sure the variable is always emitted, we provide a __used
attribute. This works with GCC, but clang still emits
Wunused-but-set-variable. To suppress that warning, we need to teach
clang to do that with the 'unused' attribute.

Nathan Chancellor explained that in details:

  While having used and unused attributes together might look unusual,
  reading the GCC attribute manual makes it seem like these attributes
  fulfill similar yet different roles, __unused__ prevents any unused
  warnings while __used__ forces the variable to be emitted. A strict
  reading of that does not make it seem like __used__ implies disabling
  unused warnings

The compiler documentation makes it clear what happens behind the 'used'
and 'unused' attributes, but the chosen names may confuse readers if
such combination catches an eye in a random code.

This patch adds __always_used macro, which combines both attributes
and comments on what happens for those interested in details.

Suggested-by: Nathan Chancellor <nathan@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/oe-kbuild-all/202405030808.UsoMKFNP-lkp@intel.com/
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
2024-05-09 09:25:08 -07:00

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/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_COMPILER_ATTRIBUTES_H
#define __LINUX_COMPILER_ATTRIBUTES_H
/*
* The attributes in this file are unconditionally defined and they directly
* map to compiler attribute(s), unless one of the compilers does not support
* the attribute. In that case, __has_attribute is used to check for support
* and the reason is stated in its comment ("Optional: ...").
*
* Any other "attributes" (i.e. those that depend on a configuration option,
* on a compiler, on an architecture, on plugins, on other attributes...)
* should be defined elsewhere (e.g. compiler_types.h or compiler-*.h).
* The intention is to keep this file as simple as possible, as well as
* compiler- and version-agnostic (e.g. avoiding GCC_VERSION checks).
*
* This file is meant to be sorted (by actual attribute name,
* not by #define identifier). Use the __attribute__((__name__)) syntax
* (i.e. with underscores) to avoid future collisions with other macros.
* Provide links to the documentation of each supported compiler, if it exists.
*/
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alias-function-attribute
*/
#define __alias(symbol) __attribute__((__alias__(#symbol)))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-aligned-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-aligned-type-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-aligned-variable-attribute
*/
#define __aligned(x) __attribute__((__aligned__(x)))
#define __aligned_largest __attribute__((__aligned__))
/*
* Note: do not use this directly. Instead, use __alloc_size() since it is conditionally
* available and includes other attributes. For GCC < 9.1, __alloc_size__ gets undefined
* in compiler-gcc.h, due to misbehaviors.
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size
*/
#define __alloc_size__(x, ...) __attribute__((__alloc_size__(x, ## __VA_ARGS__)))
/*
* Note: users of __always_inline currently do not write "inline" themselves,
* which seems to be required by gcc to apply the attribute according
* to its docs (and also "warning: always_inline function might not be
* inlinable [-Wattributes]" is emitted).
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-always_005finline-function-attribute
* clang: mentioned
*/
#define __always_inline inline __attribute__((__always_inline__))
/*
* 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;').
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-assume_005faligned-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#assume-aligned
*/
#define __assume_aligned(a, ...) __attribute__((__assume_aligned__(a, ## __VA_ARGS__)))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-cleanup-variable-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#cleanup
*/
#define __cleanup(func) __attribute__((__cleanup__(func)))
/*
* Note the long name.
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-const-function-attribute
*/
#define __attribute_const__ __attribute__((__const__))
/*
* Optional: only supported since gcc >= 9
* Optional: not supported by clang
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-copy-function-attribute
*/
#if __has_attribute(__copy__)
# define __copy(symbol) __attribute__((__copy__(symbol)))
#else
# define __copy(symbol)
#endif
/*
* Optional: only supported since gcc >= 15
* Optional: only supported since clang >= 18
*
* gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
* clang: https://github.com/llvm/llvm-project/pull/76348
*/
#if __has_attribute(__counted_by__)
# define __counted_by(member) __attribute__((__counted_by__(member)))
#else
# define __counted_by(member)
#endif
/*
* Optional: not supported by gcc
* Optional: only supported since clang >= 14.0
*
* clang: https://clang.llvm.org/docs/AttributeReference.html#diagnose_as_builtin
*/
#if __has_attribute(__diagnose_as_builtin__)
# define __diagnose_as(builtin...) __attribute__((__diagnose_as_builtin__(builtin)))
#else
# define __diagnose_as(builtin...)
#endif
/*
* Don't. Just don't. See commit 771c035372a0 ("deprecate the '__deprecated'
* attribute warnings entirely and for good") for more information.
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-deprecated-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-deprecated-type-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-deprecated-variable-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Enumerator-Attributes.html#index-deprecated-enumerator-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#deprecated
*/
#define __deprecated
/*
* Optional: not supported by clang
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-designated_005finit-type-attribute
*/
#if __has_attribute(__designated_init__)
# define __designated_init __attribute__((__designated_init__))
#else
# define __designated_init
#endif
/*
* Optional: only supported since clang >= 14.0
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-error-function-attribute
*/
#if __has_attribute(__error__)
# define __compiletime_error(msg) __attribute__((__error__(msg)))
#else
# define __compiletime_error(msg)
#endif
/*
* Optional: not supported by clang
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-externally_005fvisible-function-attribute
*/
#if __has_attribute(__externally_visible__)
# define __visible __attribute__((__externally_visible__))
#else
# define __visible
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-format-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#format
*/
#define __printf(a, b) __attribute__((__format__(printf, a, b)))
#define __scanf(a, b) __attribute__((__format__(scanf, a, b)))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-gnu_005finline-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#gnu-inline
*/
#define __gnu_inline __attribute__((__gnu_inline__))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-malloc-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#malloc
*/
#define __malloc __attribute__((__malloc__))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-mode-type-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-mode-variable-attribute
*/
#define __mode(x) __attribute__((__mode__(x)))
/*
* Optional: only supported since gcc >= 7
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html#index-no_005fcaller_005fsaved_005fregisters-function-attribute_002c-x86
* clang: https://clang.llvm.org/docs/AttributeReference.html#no-caller-saved-registers
*/
#if __has_attribute(__no_caller_saved_registers__)
# define __no_caller_saved_registers __attribute__((__no_caller_saved_registers__))
#else
# define __no_caller_saved_registers
#endif
/*
* Optional: not supported by clang
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noclone-function-attribute
*/
#if __has_attribute(__noclone__)
# define __noclone __attribute__((__noclone__))
#else
# define __noclone
#endif
/*
* Add the pseudo keyword 'fallthrough' so case statement blocks
* must end with any of these keywords:
* break;
* fallthrough;
* continue;
* goto <label>;
* return [expression];
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
*/
#if __has_attribute(__fallthrough__)
# define fallthrough __attribute__((__fallthrough__))
#else
# define fallthrough do {} while (0) /* fallthrough */
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
* clang: https://clang.llvm.org/docs/AttributeReference.html#flatten
*/
# define __flatten __attribute__((flatten))
/*
* Note the missing underscores.
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noinline-function-attribute
* clang: mentioned
*/
#define noinline __attribute__((__noinline__))
/*
* Optional: only supported since gcc >= 8
* Optional: not supported by clang
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute
*/
#if __has_attribute(__nonstring__)
# define __nonstring __attribute__((__nonstring__))
#else
# define __nonstring
#endif
/*
* Optional: only supported since GCC >= 7.1, clang >= 13.0.
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fprofile_005finstrument_005ffunction-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#no-profile-instrument-function
*/
#if __has_attribute(__no_profile_instrument_function__)
# define __no_profile __attribute__((__no_profile_instrument_function__))
#else
# define __no_profile
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#noreturn
* clang: https://clang.llvm.org/docs/AttributeReference.html#id1
*/
#define __noreturn __attribute__((__noreturn__))
/*
* Optional: only supported since GCC >= 11.1, clang >= 7.0.
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fstack_005fprotector-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#no-stack-protector-safebuffers
*/
#if __has_attribute(__no_stack_protector__)
# define __no_stack_protector __attribute__((__no_stack_protector__))
#else
# define __no_stack_protector
#endif
/*
* Optional: not supported by gcc.
*
* clang: https://clang.llvm.org/docs/AttributeReference.html#overloadable
*/
#if __has_attribute(__overloadable__)
# define __overloadable __attribute__((__overloadable__))
#else
# define __overloadable
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-packed-type-attribute
* clang: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-packed-variable-attribute
*/
#define __packed __attribute__((__packed__))
/*
* Note: the "type" argument should match any __builtin_object_size(p, type) usage.
*
* Optional: not supported by gcc.
*
* clang: https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size
*/
#if __has_attribute(__pass_dynamic_object_size__)
# define __pass_dynamic_object_size(type) __attribute__((__pass_dynamic_object_size__(type)))
#else
# define __pass_dynamic_object_size(type)
#endif
#if __has_attribute(__pass_object_size__)
# define __pass_object_size(type) __attribute__((__pass_object_size__(type)))
#else
# define __pass_object_size(type)
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-pure-function-attribute
*/
#define __pure __attribute__((__pure__))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-section-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-section-variable-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#section-declspec-allocate
*/
#define __section(section) __attribute__((__section__(section)))
/*
* Optional: only supported since gcc >= 12
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-uninitialized-variable-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#uninitialized
*/
#if __has_attribute(__uninitialized__)
# define __uninitialized __attribute__((__uninitialized__))
#else
# define __uninitialized
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-unused-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-unused-type-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-unused-variable-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html#index-unused-label-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
*/
#define __always_unused __attribute__((__unused__))
#define __maybe_unused __attribute__((__unused__))
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-used-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-used-variable-attribute
*/
#define __used __attribute__((__used__))
/*
* The __used attribute guarantees that the attributed variable will be
* always emitted by a compiler. It doesn't prevent the compiler from
* throwing 'unused' warnings when it can't detect how the variable is
* actually used. It's a compiler implementation details either emit
* the warning in that case or not.
*
* The combination of both 'used' and 'unused' attributes ensures that
* the variable would be emitted, and will not trigger 'unused' warnings.
* The attribute is applicable for functions, static and global variables.
*/
#define __always_used __used __maybe_unused
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-warn_005funused_005fresult-function-attribute
* clang: https://clang.llvm.org/docs/AttributeReference.html#nodiscard-warn-unused-result
*/
#define __must_check __attribute__((__warn_unused_result__))
/*
* Optional: only supported since clang >= 14.0
*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-warning-function-attribute
*/
#if __has_attribute(__warning__)
# define __compiletime_warning(msg) __attribute__((__warning__(msg)))
#else
# define __compiletime_warning(msg)
#endif
/*
* Optional: only supported since clang >= 14.0
*
* clang: https://clang.llvm.org/docs/AttributeReference.html#disable-sanitizer-instrumentation
*
* disable_sanitizer_instrumentation is not always similar to
* no_sanitize((<sanitizer-name>)): the latter may still let specific sanitizers
* insert code into functions to prevent false positives. Unlike that,
* disable_sanitizer_instrumentation prevents all kinds of instrumentation to
* functions with the attribute.
*/
#if __has_attribute(disable_sanitizer_instrumentation)
# define __disable_sanitizer_instrumentation \
__attribute__((disable_sanitizer_instrumentation))
#else
# define __disable_sanitizer_instrumentation
#endif
/*
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-weak-function-attribute
* gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-weak-variable-attribute
*/
#define __weak __attribute__((__weak__))
/*
* Used by functions that use '__builtin_return_address'. These function
* don't want to be splited or made inline, which can make
* the '__builtin_return_address' get unexpected address.
*/
#define __fix_address noinline __noclone
#endif /* __LINUX_COMPILER_ATTRIBUTES_H */
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