In PPCReduceCRLogicals after splitting the original MBB into 2, the 2 impacted branches still use original branch probability. This is unreasonable. Suppose we have following code, and the probability of each successor is 50%.
condc = conda || condb
br condc, label %target, label %fallthrough
It can be transformed to following,
br conda, label %target, label %newbb
newbb:
br condb, label %target, label %fallthrough
Since each branch has a probability of 50% to each successor, the total probability to %fallthrough is 25% now, and the total probability to %target is 75%. This actually changed the original profiling data. A more reasonable probability can be set to 70% to the false side for each branch instruction, so the total probability to %fallthrough is close to 50%.
This patch assumes the two branches have same probability and computes new probability fore each target, and keep the total probability to original targets unchanged.