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You can use the __builtin_expect keyword on GCC and clang. It does make a difference (HPC code uses it quite a bit because of this), but as it's done manually,

by berkut 5y ago

You can use the __builtin_expect keyword on GCC and clang. It does make a difference (HPC code uses it quite a bit because of this), but as it's done manually, results obviously vary depending on processor, and there's a limit to how far you can take it. Trying not to branch (not always possible) is best... Basically, it's like manually doing profile-guided optimisation - you look at perf or vtune, and add the above to branches which have a high penalty and which are rarely or often taken, and see if it makes a difference.

Veedrac 5y ago
Note that this is not targeted to branch prediction, but compiler optimizations. It should be done carefully, because it tends to have a larger downside than its upside, and the changes the compiler makes based on it can be unpredictable, and unstable. > Trying not to branch (not always possible) is best... Branches are often faster than branch-free methods, since they can speculate certain data dependencies away.
- berkut 5y ago
  > Branches are often faster than branch-free methods, since they can speculate certain data dependencies away. And often they're slower, since predicting some branches is often impossible due to the data :) So the processor ends up miss-predicting most of the time, thinking it knows what happened last time. It all depends on what you're doing.