[cfe-dev] Addresses of referenced objects in clang 3.5

[Richtarsky, Martin]

Hi,

since clang 3.5 code as below is optimized away with -O1, assuming that the address of the object
can never be NULL:

class A { };

int func(A& a)
{
    if (&a)
    {
        return 1;
    }
    return 0;
}

int main()
{
    A &a = *reinterpret_cast<A *>(0);
    return func(a);
}

clang++ -Wall -O1  ptrnull.cpp -o ptrnull

ptrnull.cpp:5:10: warning: reference cannot be bound to dereferenced null pointer
in well-defined C++ code; pointer may be assumed to always convert to true [-Wundefined-bool-conversion]
    if (&a)
    ~~   ^
1 warning generated.


./ptrnull
Return code: 1

This is as described in the release notes.

When changing func() as follows the warning is not shown, but clang still performs
the same optimization. Shouldn't the warning be shown here aswell?

int func(A& a)
{
    A *aptr = &a;
    if (aptr)
    {
        return 1;
    }
    return 0;
}

Is it possible to disable this optimization specifically (without resorting to -O0)
to gradually fix codebases that rely on such checks? I saw that I can use
-fsanitize=null, which helps, but disabling the optimization would still
be the best temporary workaround.

Thanks and Best regards,
Martin