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efe3a85eab
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> |
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