Re: Can compilers do this?

["Marin David Condic, 407.796.8997, M/S 731-93" <condicma@PWFL.COM>]

Robert Dewar <dewar@CS.NYU.EDU> writes:
>Subject: Re: Can compilers do this?
>
>BwB says
>
>"Also, suppose Y and X are floating point variables, and M and B are
>CONSTANT floating point variables (not named numbers) initialized to
>1.0  and  0.0  respectively. If one writes
>
>     Y := M * X  +  B ;
>
>can (and will) any compiler reduce this to
>
>     Y := X ;
>
>
>Sure, this is just standard constant propagation, a very common
>optimization.
>
    Except that you might not always want to. Constant folding can be
    a "good" optimization from a speed standpoint, but a "bad"
    optimization from a test standpoint.

    When we build control systems, we typically like to keep tables
    and assorted "constants" as "rubber constants". They are declared
    constants so that the program can't change the values, but when
    they're loaded into the box, we can change them with test
    equipment in order to affect "trim" on the control or accomplish
    other test stuff. If they were optimized away, you couldn't do
    this.

    The EDS-Scicon compiler we are working with gives us a "pragma
    VOLATILE" command to make sure that the constants aren't optimized
    away. I personally think anyone making a compiler for embedded
    systems should provide a similar option

    Pax,
    MDC

Marin David Condic, Senior Computer Engineer    ATT:        407.796.8997
M/S 731-93                                      Technet:    796.8997
Pratt & Whitney, GESP                           Fax:        407.796.4669
P.O. Box 109600                                 Internet:   CONDICMA@PWFL.COM
West Palm Beach, FL 33410-9600                  Internet:   MDCONDIC@AOL.COM
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