Re: [Tinycc-devel] Fwd: TCC VLA bug?

[Stefanos]

On Fri, 3 Dec 2021 01:06:35 +0200
Stefanos via Tinycc-devel <tinycc-devel@nongnu.org> wrote:

> With `gcc -std=c99` and `gcc -std=c11` I get the - identical in both cases - 
> following output:
> 
>    1   2   3   4   5
>    6   7   8   9  10
>   11  12  13  14  15
>   16  17  18  19  20
> 
>   21  22  23  24  25
>   26  27  28  29  30
>   31  32  33  34  35
>   36  37  38  39  40
> 
>   41  42  43  44  45
>   46  47  48  49  50
>   51  52  53  54  55
>   56  57  58  59  60
> 
>    1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  
> 20
>   21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  
> 40
>   41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  
> 60
> 
> With `tcc -std=c99` though and `tcc -std=c11` I get completely different 
> results:
> 
> C99 output
> ----------
> 
>    1   2  21  22  41
>   42  43  44  45  46
>   47  48  49  50  51
>   52  53  54  55  56
> 
>   21  22  41  42  43
>   44  45  46  47  48
>   49  50  51  52  53
>   54  55  56  57  58
> 
>   41  42  43  44  45
>   46  47  48  49  50
>   51  52  53  54  55
>   56  57  58  59  60
> 
>    1   2  21  22  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  
> 56
>   57  58  59  60   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0  
>  0
>    0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0  
>  0
> 
> 
> C11 output
> ----------
> 
>    1   2  21  22  41
>   42  43  44  45  46
>   47  48  49  50  51
>   52  53  54  55  56
> 
>   21  22  41  42  43
>   44  45  46  47  48
>   49  50  51  52  53
>   54  55  56  57  58
> 
>   41  42  43  44  45
>   46  47  48  49  50
>   51  52  53  54  55
>   56  57  58  59  60
> 
>    1   2  21  22  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  
> 56
>   57  58  59  60   0   0 16391   0-107212368 22035-107212704 22035-107212592 
> 22035   0   0 536872934   0   8   0
>    5   0-107212536 22035-107212648 22035-107212536 22035   0   0 536870912   
> 0  -1   0   7   0-107212368 22035-107212592 22035
> 
> 
> For some reason, it gives me the impression it seems '21' as `2+1` thus '3', 
> `2+2` thus '4', and so forth; I could be wrong though...
>

Seems like it's related to `malloc()` somehow; with fix-sized array it behaves 
as expected.

Here's the code, a bit modified for debugging purposes:

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

int main(void)
{
    const int w = 5;
    const int h = 4;
    const int d = 3;

    int (*arr)[h][w] = malloc(sizeof(int) * d*h*w);
    int c = 1;
    int starr[3][4][5];

    /* fill with consecutive numbers */
    for (int z=0; z<d; z++) {
        for (int y=0; y<h; y++) {
            for (int x=0; x<w; x++) {
                starr[z][y][x] = c++;
                printf(
                    "(%p) starr[%i][%i][%i]: % 4i\n", 
                    &starr[z][y][x], z, y, x, starr[z][y][x]
                );
            }
            puts("");
        }
        puts("");
    }
    putchar('\n');

    /* print structured */
    for (int z=0; z<d; z++) {
        for (int y=0; y<h; y++) {
            for (int x=0; x<w; x++) {
                printf(
                    "(%p) starr[%i][%i][%i]: % 4i\n", 
                    &starr[z][y][x], z, y, x, starr[z][y][x]
                );
            }
            puts("");
        }
        puts("");
    }

    /* reset c to 1 */
    c = 1;
    /* fill with consecutive numbers */
    for (int z=0; z<d; z++) {
        for (int y=0; y<h; y++) {
            for (int x=0; x<w; x++) {
                arr[z][y][x] = c++;
                printf(
                    "(%p) arr[%i][%i][%i]: % 4i\n", 
                    &arr[z][y][x], z, y, x, arr[z][y][x]
                );
            }
            puts("");
        }
        puts("");
    }
    putchar('\n');

    /* print structured */
    for (int z=0; z<d; z++) {
        for (int y=0; y<h; y++) {
            for (int x=0; x<w; x++) {
                printf(
                    "(%p) arr[%i][%i][%i]: % 4i\n", 
                    &arr[z][y][x], z, y, x, arr[z][y][x]
                );
            }
            puts("");
        }
        puts("");
    }

    /* print plain */
    for (int i=0; i<d*h*w; i++) {
        printf("% 4i", ((int*)arr)[i]);
        if ((i+1) % (w*h) == 0) puts("");
    }
}