[Clang] C2Y `_Countof` weird sub-array count with multidimensional VLA

[fuhsnn]

This snippet:

#include <stdio.h>
int main(int i, char**) {
    int arr[11][++i][33][++i];

    printf("%lu\t", _Countof arr);
    printf("%lu\t", _Countof *arr);
    printf("%lu\t", _Countof **arr);
    printf("%lu\t", _Countof ***arr);
    printf("\n");
    printf("%lu\t",  sizeof arr / sizeof *arr);
    printf("%lu\t",  sizeof *arr / sizeof **arr);
    printf("%lu\t",  sizeof **arr / sizeof ***arr);
    printf("%lu\t",  sizeof ***arr / sizeof ****arr);
    printf("\n\t^ here\n");
}

prints the following with current Godbolt trunk and clang-21 in LLVM Debian nightly:

11	198	33	3	
11	2	33	3	
	^ here

The second output looks funny, if I understand _Countof correctly it should be 2 instead of 2*33*3.
Godbolt: https://godbolt.org/z/o1c6bWosM

[frederick-vs-ja]

Slight changed example (with side effect in ++i avoided):

#include <stdio.h>

int main() {
    int j = 2;
    int k = 3;
    int arr[11][j][33][k];

    printf("%lu\t", _Countof arr);
    printf("%lu\t", _Countof *arr);
    printf("%lu\t", _Countof **arr);
    printf("%lu\t", _Countof ***arr);
    printf("\n");
    printf("%lu\t",  sizeof arr / sizeof *arr);
    printf("%lu\t",  sizeof *arr / sizeof **arr);
    printf("%lu\t",  sizeof **arr / sizeof ***arr);
    printf("%lu\t",  sizeof ***arr / sizeof ****arr);
    printf("\n\t^ here\n");

    constexpr int l = 2;
    int brr[11][l][33][k];

    printf("%lu\t", _Countof brr);
    printf("%lu\t", _Countof *brr);
    printf("%lu\t", _Countof **brr);
    printf("%lu\t", _Countof ***brr);
    printf("\n");
    printf("%lu\t",  sizeof brr / sizeof *brr);
    printf("%lu\t",  sizeof *brr / sizeof **brr);
    printf("%lu\t",  sizeof **brr / sizeof ***brr);
    printf("%lu\t",  sizeof ***brr / sizeof ****brr);
    printf("\n\t^ here\n");
}

Output:

11	198	33	3	
11	2	33	3	
	^ here
11	2	33	3	
11	2	33	3	
	^ here

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Hsiang-Ying Fu (fuhsnn)

This snippet: ``` #include <stdio.h> int main(int i, char**) { int arr[11][++i][33][++i];

printf("%lu\t", _Countof arr);
printf("%lu\t", _Countof *arr);
printf("%lu\t", _Countof **arr);
printf("%lu\t", _Countof ***arr);
printf("\n");
printf("%lu\t",  sizeof arr / sizeof *arr);
printf("%lu\t",  sizeof *arr / sizeof **arr);
printf("%lu\t",  sizeof **arr / sizeof ***arr);
printf("%lu\t",  sizeof ***arr / sizeof ****arr);
printf("\n\t^ here\n");

}

prints the following with current Godbolt trunk and clang-21 in LLVM Debian nightly:

11 198 33 3
11 2 33 3
^ here

The second output looks funny, if I understand _Countof correctly it should be `2` instead of `2*33*3`.
Godbolt: https://godbolt.org/z/o1c6bWosM
</details>

[fuhsnn]

The two ++i are unspecified evaluation order, not undefined, I don't think it's a factor. Anyway, can still observe it without side-effects:

#include <stdio.h>

int main() {
    const int j = 2;

    int arr[11][j + 20][33][j + 30];

    printf("%lu\t", _Countof arr);
    printf("%lu\t", _Countof *arr);
    printf("%lu\t", _Countof **arr);
    printf("%lu\t", _Countof ***arr);
    printf("\n");
    printf("%lu\t",  sizeof arr / sizeof *arr);
    printf("%lu\t",  sizeof *arr / sizeof **arr);
    printf("%lu\t",  sizeof **arr / sizeof ***arr);
    printf("%lu\t",  sizeof ***arr / sizeof ****arr);
    printf("\n\t^ here\n");
}

Got:

11	23232	33	32	
11	22	33	32	
	^ here

[cor3ntin]

@AaronBallman

[AaronBallman]

Huh, this seems to be some sort of odd codegen issue perhaps. Changing to using a constexpr variable (so everything is constant-sized array) rather than via a VLA gives the correct results. The IR we generate is definitely nonsense:

  %call = call i32 (ptr, ...) @printf(ptr noundef @.str, i64 noundef 11)
  %call1 = call i32 (ptr, ...) @printf(ptr noundef @.str, i64 noundef 23232)  <---- say what?
  %call2 = call i32 (ptr, ...) @printf(ptr noundef @.str, i64 noundef 33)
  %call3 = call i32 (ptr, ...) @printf(ptr noundef @.str, i64 noundef 32)