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I came across these 2 macros in Linux kernel code. I know they are instructions to compiler (gcc) for optimizations in case of branching. My question is, can we use these macros in user space code? Will it give any optimization? Any example will be very helpful.
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A look at the Linux kernel code will show many if conditions enclosed in likely and unlikely macros. These macros invoke compiler directives that give the compiler a hint on the code leg that should be optimized for performance.
C++ attribute: likely, unlikely (since C++20) 1) Applies to a statement to allow the compiler to optimize for the case where paths of execution including that statement are more likely than any alternative path of execution that does not include such a statement.
You can use the __builtin_expect built-in function to indicate that an expression is likely to evaluate to a specified value. The compiler can use this knowledge to direct optimizations. This built-in function is portable with the GNU C/C++ __builtin_expect function.
Yes they can. In the Linux kernel, they are defined as
#define likely(x) __builtin_expect(!!(x), 1) #define unlikely(x) __builtin_expect(!!(x), 0) The __builtin_expect macros are GCC specific macros that use the branch prediction; they tell the processor whether a condition is likely to be true, so that the processor can prefetch instructions on the correct "side" of the branch.
You should wrap the defines in an ifdef to ensure compilation on other compilers:
#ifdef __GNUC__ #define likely(x) __builtin_expect(!!(x), 1) #define unlikely(x) __builtin_expect(!!(x), 0) #else #define likely(x) (x) #define unlikely(x) (x) #endif It will definitely give you optimizations if you use it for correct branch predictions.
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