1#include <alloca.h>
2
3int square(int num) {
4    char b1[num];
5    char *b2 = (char *)alloca(num * sizeof(char));
6}

Gets compiled to

1square(int):
2        push    rbp
3        mov     rbp, rsp
4        sub     rsp, 48
5        mov     DWORD PTR [rbp-36], edi
6        // VLA
7        mov     eax, DWORD PTR [rbp-36]
8        movsx   rdx, eax
9        sub     rdx, 1
10        mov     QWORD PTR [rbp-8], rdx
11        cdqe
12        mov     edx, 16
13        sub     rdx, 1
14        add     rax, rdx
15        mov     ecx, 16
16        mov     edx, 0
17        div     rcx
18        imul    rax, rax, 16
19        sub     rsp, rax
20        mov     rax, rsp
21        mov     QWORD PTR [rbp-16], rax
22        // Alloca
23        mov     eax, DWORD PTR [rbp-36]
24        cdqe
25        lea     rdx, [rax+8]
26        mov     eax, 16
27        sub     rax, 1
28        add     rax, rdx
29        mov     esi, 16
30        mov     edx, 0
31        div     rsi
32        imul    rax, rax, 16
33        sub     rsp, rax
34        mov     rax, rsp
35        add     rax, 15
36        shr     rax, 4
37        sal     rax, 4
38        mov     QWORD PTR [rbp-24], rax
39        ud2

GCC 15.2 x86-64 with -O0