We have an application that is available in both IA-32 and x64 flavors. Engineering code with lots of math so we optimize as much as possible: using /O3 in both IA-32 and x64 compiles. The x64 bit version behaves differently on Win7 than on Win8. The Win8 results are incorrect. The IA-32 version runs fine in both Win7 and Win8. How differently is the compiler behaving (regarding optimizations, say) when generating x64 code?