The often known as
unlikely macros help the compiler know whether an
if is usually going to be entered or skipped. Using it results in some (rather minor) performance improvements.
I started using them recently, and I'm not sure how often should such hints be used. I currently use it with error checking
ifs, which are usually marked as
unlikely. For example:
mem = malloc(size); if (unlikely(mem == NULL)) goto exit_no_mem;
It seems ok, but error-checking
ifs happen quite often and consequently the use of the said macros.
My question is, is it too much to have
unlikely macros on every error-checking
While we're at it, what other places are they often used?
In my current usage it's in a library that makes an abstraction from the real-time subsystem, so programs would become portable between RTAI, QNX and others. That said, most of the functions are rather small and directly call one or two other functions. Many are even
static inline functions.
So, first of all, it's not an application I could profile. It doesn't make sense to "identify bottle-necks" since it's a library, not a standalone application.
Second, it's kind of like "I know this is unlikely, I might as well tell it to the compiler". I don't actively try to optimize the