[Tinycc-devel] Miscompiled code (when building GCC)

[Andrius Štikonas]

Hi,

When trying to compile gcc, we stumbled upon strange segfault that seem to be 
coming from tcc.

This is the reproducer:

struct thing {
    unsigned index;
    char mem[1];
};

struct function {
    struct thing* t;
};

static inline struct thing* obtain (const struct function *f) {
    return f->t;
}

int main () {
    char *ptr;
    struct thing u = {.index = 0, .mem = {'a'}};
    struct function g = {.t = &u};
    struct function *f = &g;
    ptr = &(obtain(f)->mem[obtain(f)->index]);
    return *ptr;
}

When tcc (mob) is used to build it we get a segfault. On the other hand, GCC 
returns 97 ('a').

Some curious observation. Replacing the ptr line with
    struct thing * x = obtain(f);
    ptr = &(x->mem[x->index]);
or with
    int x = obtain(f)->index;
    ptr = &(obtain(f)->mem[x]);

Seems to workaround the crash on tcc-riscv64 but not on tcc-amd64.

Kind regards,
Andrius