Some time ago I wrote versions of a (small) number of compositing functions (from libnr/nr-compose.cpp) using SSE2 (with intrinsics), but I'm not sure how I should integrate them with Inkscape code (separate file? compile settings? ...?).
Also, if there is anyone who has some experience with SSE2 I wouldn't mind if they took a look (I don't have very much experience with writing SSE2 and it's quite possible that I'm not achieving optimal performance).
BTW, I have code for: nr_R8G8B8A8_P_R8G8B8A8_P_R8G8B8A8_N_A8 nr_R8G8B8A8_P_EMPTY_R8G8B8A8_N_A8 nr_R8G8B8A8_P_R8G8B8A8_P_R8G8B8A8_P And it could probably be adapted to variants of those functions. The SSE2 versions are approximately 2 times as fast as the normal versions (on my machine, a P4).