[DarkShikari]

It is an extraordinarily difficult problem to transform scalar code into vector instructions. The only way to get even passable output from a vectorizing compiler is to write the code as vectors to begin with, such as with cross-platform assembly tools like Orc.

And even then you'll often end up significantly worse off than if you wrote the assembly by hand.

A run of Intel's compiler on the C versions of our DSP functions resulted in a grand total of one vectorization, which was done terribly, too.

[jrockway]

The problem is that you used C, which doesn't have any syntax to represent meta-information about the problem you're trying to solve. When you write out C code to, say, add a list of numbers, it's hard for the compiler to optimize that. But it's very easy for the compiler when you tell it "sum this list of numbers".