[overgard]

> Isn't the GPU just a computer that happens to support more parallelism than the CPU?

Not really. It's like quantum mechanics compared to classical physics.

For instance, "branches" don't work like you'd expect. On a cpu you execute one branch or the other. On a GPU, you get things like both branches execute, but then it just throws away the half that shouldn't have run, but that means you're bottlenecked by whichever branch takes the longest (Or something like that -- the details escape me but I do remember something about CUDA's branching doing weird things). Point being, GPU's are weird. It's nothing like programming a CPU at all.

[geon]

> On a GPU, you get things like both branches execute, but then it just throws away the half that shouldn't have run

It's not that weird. You don't really have thousands of parallell processors, but a single processor, operating on thousands of values. (Like SIMD on steroids.)

Since all operations must be done identically on all values, a "branch" is really doing both branches and recombining them with a mask of equally many booleans - as you say "throwing away" the unwanted branch.