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Optimization question from a reader

  • Article

I received a question:

…I've always been curious about code like this:

IF wParam=ASC("a")
wParam=ASC("b")
ENDIF

Is it more appropriate to think of ASC("a") as the Fox equivalent of a C 'a', or is it an actual function call every time through?

It’s easy to test this.

Try running this code:

num=1e7 && 1e7 is 10 * 1e6 (3,6,9 are thousand, million, billion) = 10 * 1 million

wParam=ASC("a")

ns=SECONDS()

FOR i = 1 TO num

      IF wParam=ASC("a")

            wParam=ASC("b")

      ENDIF

ENDFOR

nd1=SECONDS()-ns

?nd1

ns=SECONDS()

FOR i = 1 TO num

      IF wParam=97

            wParam=98

      ENDIF

ENDFOR

nd2=SECONDS()-ns

?nd2,100*(nd1-nd2)/nd2

On my machine, the 2nd loop is 20-25% faster.

Of course an optimizing compiler could transform the 1st loop to the 2nd. In fact, a really good optimizing compiler would remove the loop completely!

In C and C++, the literal ‘a’ is exactly equal to 97, even at runtime, whereas the literal “a” (notice the single vs double quotes) is a string represented as a 2 byte array of 97 followed by a terminating 0.

The ASC() function takes any string expression as a parameter, including possibly a function call, so it cannot be optimized out in general.

Comments

  • Anonymous
    October 21, 2005
    I know the answer,
    but it can be interesting if you explain
    to the public because in the following's code
    the last loop it is 8 -10 % faster.


    num=1e7

    wParam=ASC("a")

    ns=SECONDS()
    FOR i = 1 TO num
    ENDFOR
    nd0=SECONDS()-ns

    ns=SECONDS()
    FOR i = 1 TO num
    IF wParam=97
    wParam = 98
    ENDIF
    ENDFOR
    nd1=SECONDS()-ns

    ns=SECONDS()
    FOR i = 1 TO num
    IF wParam#97
    LOOP
    ENDIF
    wParam = 98
    ENDFOR

    nd2=SECONDS()-ns
    ?nd1,nd2,100*(nd1-nd2)/(nd2-nd0)