Everyone is saying "nothing", but I reviewed the patches (I did /not/ bother watching the video, however, as it was long and I was in a place where I felt uncomfortable playing sound; so maybe the video says "we didn't do anything" ;P), and what they did is to remove all of the non-strict pointer aliasing (one of C's optimization weaknesses) and thereby were able to kick the optimizer up to -O3, which is helping some already tightly-coded algorithms actually avoid touching memory.