How much usage of “likely” and “unlikely” macros is too much

cclean codeperformance

The often known as likely and 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 likely and unlikely macros on every error-checking if?

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 if.

Best Answer

Do you need performance that badly that you're willing to pollute your code with that? It's a minor optimization.

  • Does the code run in a tight loop?
  • Does your application have performance problems?
  • Have you profiled your application and determined that this particular loop costs a lot of CPU time?

Unless you can answer yes to all the above, don't bother with stuff like this.

Edit: in response to the edit. Even when you can't profile, you can usually estimate hotspots. A memory allocation function that is called by everyone is a good candidate, especially since it requires only a single use of the macro to work for the whole library.

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