[cfe-users] ABI change with unused function argument

[Gael Guennebaud]

Hi,

I've observed that clang changes the ABI when a function argument is not used.

This is a nice optimization, but the downside is that this introduces
segfaults when an argument is conditionally used with respect to
compilation options (e.g., -DNDEBUG) and that both the application and
a library instantiate this function, but only one is compiled with
-DNDEBUG.

Here is an example of how could look like such a function:

template<int A, typename T>
void foo(int a, T &t) {
  assert(A!=0 || a==0);
  if(A==0) { /* code making use of both a and t */ }
  else { /* code making use of t and A only */ }
}

and you can see a real-world example with a very ugly workaround there:

https://bitbucket.org/eigen/eigen/commits/1e19a9cb83f0/

Does clang proposes a cleaner and more general workaround for such an issue?

Thanks,

Gael