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#135 Efficient constants

open
nobody
None
5
2006-05-07
2006-05-07
No

I prefer to use constants of correct type instead of
#defines:

static const unsigned int X = 100;

instead of

#define X 100

When only the value of X, never it's address is used,
both should compile to euqivalent code.

sdcc generates rather compact code for the #define
(value "100" is used directly) and horribly inefficient
code for the constant ("100" lives in memory and each
time it is used it is loaded from there as if it wasn't
a constant).

I tried with sdcc -mz80

Philipp

Discussion

  • Logged In: NO

    Phillip, you misunderstood the C language specification.
    'static' is *storage class* specifier and 'const' is
    *storage* class* modifier. Either case, you need to spend
    some storage on X.

    Why is it useful and why there is difference between
    '#define X 100' and 'static const unsigned int X = 100;' ?

    Well, the former is ment as means to ease maintanence and
    control over "magic numbers" that may be sprinkled all over
    your code, while latter is useful if you need to change some
    localisation code in your program even *without
    recompilation*, i.e. right before you burn it in device (E,
    EE, Flash)PROM or something, provided you kept track of its
    address (symbol table). The obvious example is a serial
    number or version ID tag.

     
  • Raphael Neider
    Raphael Neider
    2006-05-24

    Logged In: YES
    user_id=1115835

    Well, the trick here is that *static* makes the variable
    locally visible within the (.o) file and does *not* produce
    a globally visible symbol that could be used to modify X's
    value outside of the current file. Thus I would rather agree
    with Philipp and propagate the constant through the expressions.
    Philipp even mentioned problem I can think of: pointers to X
    enforce current behaviour.

    Regards,
    Raphael

     
  • David Barnett
    David Barnett
    2007-01-02

    Logged In: YES
    user_id=896846
    Originator: NO

    I noticed one of the commercial compilers (Sourceboost) had the same issue. I was surprised to find that gcc doesn't seem to make that optimization, either (from what I could tell...I haven't used x86 asm much). I didn't follow the comments about the C language specification, but I haven't seen any good reasons not to propagate static consts except, as Philipp mentioned, when it needs an address. Is this optimization feasible/correct?

     
  • Graham  Hanson
    Graham Hanson
    2010-06-08

    I have just raised this point, specifically for the PIC14/PIC16 ports in the forums (I didn't find this originally because I filtered on "pic")

    I find that all the commercial compilers, and gcc I use in my day job on large *nix systems apply this optimisation as a matter of course (unless specifically disabled).

    An advantage of using static const is that the compiler can apply type checking rules whereas with #define it can't. It is also generally acknowledged that #define is evil (although sometimes unavoidable, especially when building for embedded systems)

    So given the above I vote in favour of optimising const statics as compile time constants.

    Graham