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c++gcclanguage-lawyerstrict-aliasing
PeterSW
PeterSW

Reputation: 5261

g++'s strict-aliasing warning accuracy

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?

Upvotes: 1

Views: 67

Answers (1)

Passer By
Passer By

Reputation: 21160

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

Upvotes: 3

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