enhancement/580: Add some system functions for use in constant expressions

From: sharp@cadence.com
Date: Wed May 05 2004 - 11:10:58 PDT

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    >Number: 580
    >Notify-List:
    >Category: enhancement
    >Originator: sharp@cadence.com (Behavioral Task Force)
    >Environment:
    >Description:

    If enhancement/387 is approved, allowing certain system
    functions to be used in constant expressions, there may
    be some new system functions that would be useful with
    this. The most obvious example would be to add a
    ceiling-of-log-base-2 function, for use in setting address
    bus widths based on memory size, or data width needed to
    represent a certain maximum value.

    We would need to solicit other suggestions from users.

    I could suggest others that are difficult to compute
    generally with the existing operators, but I don't know
    if they are actually useful. Shalom seemed to think that
    some of them might be, though I don't know whether he
    meant in general or in constant expressions.

    Possible examples might include find-first-bit-set,
    though I think this could be computed fairly easily given
    a log-base-2 function. A find-first-bit-clear would be
    trivial to compute given a find-first-bit-set, using ~.
    These would find the MSB. Shalom also mentioned that
    finding the LSB set would be useful. It turns out that
    this can be computed given a function that finds the MSB
    set plus some additional math, though the tricks involved
    may not be widely known.

    Another related example might be a population count, which
    counts the number of set bits in a value. There are also
    some other enhancement requests that could be satisfied by
    new system functions (e.g. rotates, bit-reversal), and such
    functions would also be suitable for constant expressions
    (though they might not be useful there).

    Again, I don't think we should put any of these in unless
    they would actually be useful to users.

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