[cfe-users] ABI change with unused function argument

[David Blaikie]

Clang should only be doing this to internal symbols (or it should be making
the symbol internal so it can perform such an optimization). Do you have a
standalone (at least complete, ideally minimal) example of this issue?


On Mon, Feb 24, 2014 at 8:58 AM, Gael Guennebaud
<gael.guennebaud at gmail.com>wrote:

> 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
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