Can likely/unlikely macros be used in user-space code?

26
votes

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.

3
clinuxgccoptimizationlikely-unlikely
kerneltrap.org/node/4705 – pmg
duplicate? stackoverflow.com/questions/109710/… – Aleksei Potov
I checked these posts, but both again talks about kernel related stuff. I wanted to know whether same can be used in user code. – Vinit Dhatrak
If you are programming for any reasonably powerful processor, you are unlikely to get any performance benefit. Modern dynamic branch predictors are quite good. – Jay Conrod
@Jay I think programmer should not assume power of processor. Dynamic branch detection would be easier if programmer explicitly provide the information. – Vinit Dhatrak

3 Answers

44
votes

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.

12
votes

Take a look into What Every Programmer Should Know About Memory under "6.2.2 Optimizing Level 1 Instruction Cache Access" - there's a section about exactly this.

4
votes

The likely() and unlikely() macros are pretty names defined in the kernel headers for something that is a real gcc feature