g++'s strict-aliasing warning accuracy

Question

GCC's documentation says that -Wstrict-aliasing=3 is the most accurate level and that lower levels are more likely to give false positives.

I believe the following examples all violate the strict aliasing rule:

float violate1(float a_float)
{
    float * f_data(&a_float);
    int * i_data((int *)f_data);
    int value(*i_data);
    return value + a_float;
}

float violate2(float a_float)
{
    int * i_data((int *)&a_float);
    int value(*i_data);
    return value + a_float;
}

float violate3(float *f_data)
{
    int * i_data((int *)f_data);
    int value(*i_data);
    return value + *f_data;
}

Yet g++ only gives warnings for them all when -Wstrict-aliasing=1 is used. With -Wstrict-aliasing=3 no warnings are issued: https://godbolt.org/g/aox2S1

Are the examples in fact not violations or is it that GCC's warnings are not a reliable indication of violations?

fwiw warnings cannot be 100% reliable, if they were they would probably be errors — 463035818_is_not_an_ai, May 17 '18 at 16:15
Those are strict aliasing violation. 3 is more accurate: less false positives, but miss some true positives, 1 got more true positives, but also more false positives. — Jarod42, May 17 '18 at 16:30
Related [strict aliasing in C](https://stackoverflow.com/q/25117826/1708801) — Shafik Yaghmour, May 17 '18 at 16:31

score 3 · Answer 1 · edited Jun 20 '20 at 09:12

The strict-aliasing rule is stated in very clear terms from [basic.lval]

If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined:

the dynamic type of the object [...]

This would mean violate1 and violate2 is the same thing and is necessarily a violation.

An object pointer can be casted to and from another arbitrary object pointer type, the result is the original pointer. In violate3, if f_data is an int* previously casted to float*, there would be no violation with *i_data, but then it would be a violation at *f_data

g++'s strict-aliasing warning accuracy

1 Answers1