It's supported by some CPUs: the linux kernel has likely/unlikely macros to indicate whether a branch is expected to be taken or not in the common case (which uses GCC attributes which allow this to be propagated through to codegen). It's just that it's generally less effective: firstly because not all branches get annotated, some that are annotated are annotated incorrectly, and in a lot of cases the branching behaviour is data-dependent enough that you can't make an optimal static prediction at all.