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

    This directive associates an alignment attribute with a symbol
    definition for a register offset. You can use this directive
    when you know the alignment of the base register. This attribute
    guarantees to the compiler that the base register has the same
    alignment, which enables the compiler to generate optimal code.

    Format

      .SYMBOL_ALIGNMENT  argument-list

1 - Parameters

 argument-list

    One of the arguments listed in the following table.

    Option   Description

    long     Declares longword alignment for any symbol that you
             declare after this directive.
    quad     Declares quadword alignment for any symbol that you
             declare after this directive.
    none     Turns off the alignment specified by the preceding
             .SYMBOL_ALIGNMENT directive.

2 - Description

    The .SYMBOL_ALIGNMENT directive is used to associate an alignment
    attribute with the fields in a structure when you know the base
    alignment. It is used in pairs. The first .SYMBOL_ALIGNMENT
    directive associates either longword (long) or quadword (quad)
    alignment with the symbol or symbols that follow. The second
    directive, .SYMBOL_ALIGNMENT none, turns it off.

    Any time a reference is made with a symbol with an alignment
    attribute, the base register of that reference, in effect,
    inherits the symbol's alignment. The compiler also resets the
    base register's alignment to longword for subsequent alignment
    tracking. This alignment guarantee enables the compiler to
    produce more efficient code sequences.

3 - Example

  OFFSET1 = 4
  .SYMBOL_ALIGNMENT LONG
  OFFSET2 = 8
  OFFSET3 = 12
  .SYMBOL_ALIGNMENT QUAD
  OFFSET4 = 16
  .SYMBOL_ALIGNMENT NONE
  OFFSET5 = 20
      .
      .
      .
  CLR1 OFFSET2(R8)
      .
      .
      .
  MOVL R2, OFFSET4(R6)

      For OFFSET1 and OFFSET5, the compiler will use only its
      tracking information for deciding if Rn in OFFSET1(Rn) is
      aligned or not. For the other references, the base register
      will be treated as longword (OFFSET2 and OFFSET3) or quadword
      (OFFSET4) aligned.

      After each use of OFFSET2 or OFFSET4, the base register in the
      reference is reset to longword alignment. In this example, the
      alignment of R8 and R6 will be reset to longword, although the
      reference to OFFSET4 will use the stronger quadword alignment.