[Bug c/125887] New: VLAs inside statement expression leaks stack space

[superroman652 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125887

            Bug ID: 125887
           Summary: VLAs inside statement expression leaks stack space
           Product: gcc
           Version: 16.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: superroman652 at gmail dot com
  Target Milestone: ---

This bug occurs when size is not known statically or with optimizations
disabled. Works as intended when _ret is not inside stmt expr.

Discovered on archlinux x64, but also reproducible on gcc 15.2.0 on alpine
aarch64.

Not a duplicate of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113579 "GCC
leaks memory inside when using statement expressions" because that one is about
return inside stmt exprs rather than VLAs.

Code:

#include <stdio.h>

char *g_a;
char *g_b;

void poc_blex(int size) {
        char *a = ({
                char _ret[size];        
                _ret;
        });
        char b[size];
        g_a = a;
        g_b = b;
}

int main() {
        poc_blex(12); // Maybe a runtime value.
        printf("%llx %llx %d\n", g_a, g_b, g_a == g_b);
}