A user encountered a texture quality problem, in our code that uses the Quality_Fastest mode, when compiled with GCC 4.4.4 on Ubuntu 10.10. I reproduced it myself with 4.4.4 on Gentoo; the problem didn't occur with 4.3.4 or 4.5.0.

Eventually I narrowed it down to QuickCompressDXT.cpp selectDiagonal() returning non-deterministic results (which was surprising). It uses Vector3::xy(), which is defined as:

inline const Vector2 & Vector3::xy() const { return *(Vector2 *)this; }

which is a clear violation of aliasing rules.

(Unfortunately NVTT seems to compile with no warnings enabled, so GCC wasn't complaining "Vector.h:153: warning: dereferencing pointer ‘<anonymous>’ does break strict-aliasing rules" like it normally would with "-Wall".)

Fixing this to return a proper non-reference Vector2(m_x, m_y) results in correct behaviour. Compilers are good at inlining so this won't be less efficient. The other functions that do similar tricks ought to be fixed too.

This issue was closed by revision r1158.

Thanks for pointing this out. I've fixed this in trunk and will merge the fix in the 2.0 branch shortly.

This issue was closed by revision r1167.