/L_DOUBLE_SIZE
       /L_DOUBLE_SIZE=option
       /L_DOUBLE_SIZE=128 (D)

    Determines how the compiler interprets the long double type. The
    qualifier options are 64 and 128.

    Specifying /L_DOUBLE_SIZE=64 treats all long double references
    as G_FLOAT, D_FLOAT, or T_FLOAT, depending on the value of the
    /FLOAT qualifier. Specifying /L_DOUBLE_SIZE=64 also defines the
    macro __X_FLOAT=0.

    Note: The /L_DOUBLE_SIZE=64 option is not available on I64
    systems. If it is specified, the compiler issues a warning
    message and uses /L_DOUBLE_SIZE=128.

    Specifying /L_DOUBLE_SIZE=128 treats all long double references
    as X_FLOAT. The /L_DOUBLE_SIZE=128 option also defines the macro
    __X_FLOAT=1. This is the default.