Do not guess branch probabilities using heuristics. GCC will use heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by `__builtin_expect', then the heuristics will be used to guess branch probabilities for the rest of the control flow graph, taking the `__builtin_expect' info into account. The interactions between the heuristics and `__builtin_expect' can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of `__builtin_expect' are easier to understand.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.