VMS Help MACRO, /MIGRATION, Compiler Directives, .BRANCH UNLIKELY, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The .BRANCH_UNLIKELY directive instructs the compiler that
    the following conditional branch will likely not be taken. The
    compiler then generates code that incorporates that assumption.

    Alpha system hardware predicts that forward conditional branches
    are not taken. Therefore, if a .BRANCH_UNLIKELY directive
    precedes a branch that will be in the forward direction, it
    has no effect. However, if it precedes a branch that would be
    backward, code is generated to force the branch to be forward, to
    an out-of-line branch back to the actual branch destination.

    .BRANCH_UNLIKELY should be used only in cases where the branch is
    very unlikely, not just less frequent than the fall-through case.