[ILUG] gcc optimisation weirdness?

[Brady, Padraig]

Few points.

1. Shouldn't it be: int frequency[10];
2. Try to get assembler output for the various optimisation options and diff
them.
3. C compilers can init arrays @ 3 different times.
	1. Compile time. Use static arrays of appropriate. All values are
set to 0
	2. Program startup code, sets module level initialised arrays. I.E.
int arr[]={1,2};
	3. Runtime code generated by compiler inits local initialised arrays
(using
         the same construct as in 2).

p.s. a handy macro when working with arrays is
#define ARRAY_SIZE(array) (sizeof(array)/sizeof(array[0]))
This will be worked out by the compiler at compile time.
So this tidys things a lot when you don't have to explicitly
specify (configure) the number of elements (99% of time).
e.g.

int arr[]={1,2,3,4,5};
int arr_idx=0;
for(;arr_idx<ARRAY_SIZE(arr);arr_idx++) printf("%d,",arr[arr_idx]);

> -----Original Message-----
> From: Paul Jakma [mailto:paulj at itg.ie]
> Sent: 10 May 2000 17:39
> To: ilug at linux.ie
> Subject: [ILUG] gcc optimisation weirdness?
> 
> 
> gcc seems to do weird things to the following:
> 
> int input, frequency[9];
> 
> //frequency[] initialised to 0.
> 
> do {
>                 scanf("%d", &input);
>                 if (input > 0 && input < 11) {
> 			//point 1
>                         (frequency[input-1])++;
> 			//point 2
>                         }
>         } while (input);
> 
> if i compile with -O2 or -O3 it works as i expect it to, but 
> if i compile
> with "gcc -o test test.c" then:
> 
> it works correctly for digits 0 thru 9, however if i enter 10 it goes
> wrong: at point 1 input == 10, at point 2 input == 11! 
> 
> seems like gcc is applying the increment to variable input,
> despite my bracketing of frequency[input-1]! which i don't
> want. why???
> 
> (even without the brackets it should not increment input)
> 
> --paulj.
> 
> PS: is there anyway to initialise an array at declaration time, rather
> than iterate thru the array and set each element manually? eg 
> something
> like: int array[9]=0; 
> 
> 
> 
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