Instructs the compiler that the following branch will likely
    not be taken. Therefore, the compiler generates code that
    incorporates that assumption.

    Format

      .BRANCH_UNLIKELY

    There are no parameters for this directive.

1  –  Description

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

    On OpenVMS I64 systems, when .BRANCH_UNLIKELY precedes a
    conditional backwards branch, the compiler will use the
    appropriate branch prediction flags on the generated Itanium
    instruction.

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

2  –  Example

  MOVL    #QUEUE,R0         ;Get queue header
  10$:    MOVL    (R0),R0   ;Get entry from queue
          BEQL    20$       ;Forward branch assumed unlikely
          .
          .                 ;Process queue entry
          .
          TSTL    (R0)      ;More than one entry (known to be unlikely)
          .BRANCH_UNLIKELY
          BNEQ    10$       ;This branch made into forward
  20$:                      ;conditional branch

      The .BRANCH_UNLIKELY directive is used here because the Alpha
      hardware would predict a backward branch to 10$ as likely to be
      taken. The programmer knows it is a rare case, so the directive
      is used to change the branch to a forward branch, which is
      predicted not taken.