GAP Message (Diagnostic ID 30759)

Message

Remove unused field(s) '%s' from the struct '%s'. The fields: '%s' were conservatively assumed by the compiler as referenced since their address is taken.

This message is emitted only with whole-program recognition.

Advice

Some unused fields were seen in a class or structure type. If the unused fields can be removed from the structure definition, it will lead to reduced memory usage and better cache utilization since the cache will no longer be filled with unused data.

You must verify that the fields that are reported as unused are not accessed elsewhere in the application. You also need to be careful when removing unused fields if the code relies on the structure fields to be laid out in a specific order. For example, if the application code uses the address of a field to access other fields, it may stop working once unused fields are removed. Note that such code is not considered valid in the first place.

The unused field analysis considers address taken fields as used. It will report address taken fields also when reporting any unused fields.

Example

Consider the following:

//unused_field_3.c 
struct str {
    int a1, b1, c1, d1, e1, f1; 
};
 
struct str sp[1000000];
 
int hot_func1() {
    int i, ret = 0;
 
    for (i = 0; i < 1000000; i++) {
        ret += sp[i].a1;
        ret += sp[i].b1;
    }
    return ret; 
}
 
int hot_func2() {
    int ret = 0, i;
    for (i = 0; i < 1000000; i++) {
        ret += sp[i].a1;
        ret -= sp[i].c1;
    }
    return ret; 
}
 
int *gip;
 
int hot_func3() {
    int ret = 0, i;
    for (i = 0; i < 1000000; i++) {
        ret += sp[i].d1;
    }
    gip = &(sp->f1);
    return ret; 
}
 
int main() {
    hot_func1();
    hot_func2();
    hot_func3(); 
}

Verify

Make sure that the restructured code satisfies the original program semantics.