Re: Beware: Rep spec on an enumeration type causes code explosion

["Marin David Condic, 561.796.8997, M/S 731-96" <condicma@PWFL.COM>]

Tucker Taft <stt@HOUDINI.CAMB.INMET.COM> writes:
>language design principles.  Your enumeration rep clause simply reconfirmed
>the default representation, but suppose it didn't?  You would be
>stuck with the overhead you managed to eliminate by simply commenting
>out the rep clause.  Of course, the compiler could recognize various
>other specials cases, such as contiguous representation starting at
>something other than zero, or evenly distributed "holey" representation
>(e.g., even numbers only), or simply not "too" sparse, or ...
>
    I'll give you a good reason why getting too smart about rep clause
    implementation might be a Bad Thing. Suppose I implement an
    enumeral type with a "confirming" rep clause because I know I'm
    dealing with a real live hardware interface or message format and
    want to make it clear that the representation for this given
    enumeration is critical. I compile & run and my timing comes out a
    given way because your compiler was smart enough to detect that my
    representation was conveniently matching the positions. So far, so
    good. At a later date the hardware or message format changes to
    include some new bit patterns which create holes in the
    representation and now when I compile I get a significantly
    different implementation for the enumeration and a corresponding
    change in performance. This is a satisfactory definition of the
    technical term: "Bad Thing".

    Sometimes poor performance isn't as bad a Bad Thing as
    inconsistent performance can be.

    MDC

Marin David Condic, Senior Computer Engineer     Voice:     561.796.8997
Pratt & Whitney GESP, M/S 731-96, P.O.B. 109600  Fax:       561.796.4669
West Palm Beach, FL, 33410-9600                  Internet:  CONDICMA@PWFL.COM
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