[LAD] GCC Vector extensions

I am playing around with GCC and Clang vector extensions, on Linux and Mac OS X, and i am getting some strange behaviour. I am working on jMax Phoenix, and its dsp engine, in its current state, is very memory bound; it is based on the aggregation of very small granularity operations, like vector sum or multiply, each of them executed independently from and to memory. I tried to implements all this 'primitive' operations using the vector types. On clang/MacOSX i get an impressive improvement in performance, around 4x on the operations, even just using the vector types for copying data; my impression is that the compiler use some kind of vector load/store instruction that properly use the available memory bandwidth, but unfortunately i do not know more about the x86 architecture. On gcc/Linux, (gcc 4.5.2) the same code produce a *slow down* of around 2.5x. Well, anybody have an idea of why ? I am actually running linux (Ubuntu 11.04) under a VMWare virtual machine, i do not know is this may have any implications. Thanks, Maurizio De Cecco