What to do about wrong “control reaches end of non-void function” gcc warning

$ cat t.cpp
int sign(int i) {
    if(i > 0) return 1;
    if(i == 0) return 0;
    if(i < 0) return -1;
}
$ g++ -c t.cpp -Wall
t.cpp: In function ‘int sign(int)’:
t.cpp:5: warning: control reaches end of non-void function
$

int sign(int i) {
    if(i > 0) return 1;
    if(i == 0) return 0;
    return -1; // i<0
}

int sign(int i) {
    return (i>0) ? 1 : ((i<0)?-1:0);
}

int sign(int i) {
    return (i > 0) - (i < 0);
}

int sign(int i)
{
    if (i > 0)
        return 1;
    else if (i == 0)
        return 0;
    else
        return -1; // i<0
}

Unoptimized     Optimized (-O5)
 if      166             110
 ?:      150              98
 >-<     122              98