GAP Message - remark #30758: (DTRANS) Remove unused field(s) "e1" from the struct "str"...

Message


Remove unused field(s) "e1" from the struct "str". [VERIFY] The suggestion is based on the field references in the current compilation. Please make sure that there are no references to these fields across the entire application.

This message is emitted if "level of analysis" = extreme or -[Q]guide=4
This message is emitted even without whole-program recognition in advanced mode
Applies to C/C++ only

Description

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. The advice is based on the analysis of the source code that is seen. 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. As an 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.

Example

//unused_field_2.c 
struct str {
    int a1, b1, c1, d1, e1;
};

extern struct str sp[];

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 hot_func3() {
    int ret = 0, i;
    for (i = 0; i < 1000000; i++) {
        ret += sp[i].d1;
    }
    return ret;
} 

For the above example, the compiler generates the following advice with
'icl -Qguide=4 -c unused_field_2.c '

 

unused_field_2.c(1): remark #30758: (DTRANS) Remove unused field(s) "e1" from the struct "str". [VERIFY] The suggestion is based on the field references in the current compilation. Please make sure that there are no references to these fields across the entire application.

 

For the above example, if the unused fields can be removed, the only source change needed would be the following:

//unused_field_2.c
struct str {
    int a1, b1, c1, d1;
};

...
...

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