See the description of the .BRANCH_LIKELY directive for the
    assumptions used by the compiler when predicting branches.

    On OpenVMS Alpha systems, when .BRANCH_UNLIKELY precedes a
    conditional backwards branch, the compiler will change the
    conditional branch and the code such that the branch is a forward
    branch to an unconditional backwards branch. .BRANCH_UNLIKELY
    should only be used in cases where the branch is very unlikely,
    not just less frequent than the fall-through case.

    .BRANCH_UNLIKELY has no effect if it precedes a conditional
    forwards branch.