test1.c

/* gives wrong result at -O3 */

#include <stdio.h>

static void ac3_excite(int *excitep, int *band_psd,
                       int lowcomp, int is_lfe, int end)
{
    int excite[1] = { 0 };
    int band;

    for (band = 0; band < end; band++) {
        if (!(is_lfe && band == 6))
            lowcomp = 2;
        excite[band] = lowcomp;
        if (!(is_lfe && band == 6)) {
            if (band_psd[band] <= band_psd[band+1]) {
                break;
            }
        }
    }

    excitep[0] = excite[0];
}

static int psd[2];

int main(void)
{
    int excite;

    ac3_excite(&excite, psd, 1, 0, 1);
    printf("%d\n", excite);

    return 0;
}

test2.c

/* crashes at -O2 or higher */

int foo(char *a, unsigned b, int c)
{
    return a[b + 1 - c];
}

int main(void)
{
    char a = 0;
    foo(&a, 0, 1);
    return 0;
}

test3.c

/* wrong result at -O3 */

#include <stdio.h>

float dprod(int *a, float *b, int n)
{
    float sum = 0.0;
    int i;

    for (i = 0; i < n; i++)
        sum += a[i] * b[i];

    return sum;
}

int main(void)
{
    int a[32];
    float b[32];
    float s;
    int i;

    for (i = 0; i < 32; i++) {
        a[i] = i;
        b[i] = i;
    }

    s = dprod(a, b, 32);

    printf("%f\n", s);

    return 0;
}

test4.c

/*
   wrong output at any optimisation level
   correct output:
   1 4
   1 1
*/

#include <stdio.h>
#include <stdlib.h>

int foo(char *a, unsigned b, int c)
{
    return a[b - c + 1];
}

int bar(char *a, int b, int c)
{
    return a[b - c + 1];
}

int main(void)
{
    char *a;
    int b, c;

    a = malloc(0x100000001);

    a[          0] = 1;
    a[          1] = 2;
    a[ 0xfffffffe] = 3;
    a[ 0xffffffff] = 4;
    a[0x100000000] = 5;

    b = foo(a, 0, 1);
    c = foo(a, 0, 2);
    printf("%d %d\n", b, c);

    b = bar(a,     0, 1);
    c = bar(a + 1, 0, 2);
    printf("%d %d\n", b, c);

    return 0;
}