One general prediction I can make is that every x86 processor I’ve timed seems to have a huge latency when shuffling data between x87 float and general-purpose registers.

Assume the following simplified scenario:

You have a base class for objects that exist in the game world called CEntity that is maybe a couple of hundred bytes or so of data members that describe all its necessary state data, the position, a pair of angles or maybe a transformation matrix, the velocity, whether it has a collision mesh, whether it is static and so on.

From this class you derive a bunch of CRockets, CPlayers, CPhysicsObject etc. To improve locality of reference you store all your CEntities in a simple array rather than spreading them out in memory. You might just allocate a big enough array that it’s not going to reach capacity under any but pathological cases or you might use some dynamic array that you reallocate to double the size if it’s filled to capacity or reallocate to half the size if emptied to one quarter or something like that.

In order to give the processor the best possible chance to precache necessary data your update loop calls the Update() method on the CEntity at index zero, then index one and so on in the order they are stored in memory. Each Update() does one normalization among various other stuff.

Under these conditions, which are about as predictable as possible, will the processor look at the memory access pattern and correctly identify that there’s a 224(or whatever) byte stride and that it is supposed to cache this data ahead of time? If the data is not precached the processor is going to sit idle and wait on RAM for at least several hundred clock cycles in which case it totally swamps the performance of your arithmetic to the point that it barely matters.

Soylent: What you’re suggesting indeed makes sense when one has long lists of packed vectors to transform, but that’s not what I’m timing here. I’m looking at cases where you have to perform a single vector normalization as part of the logic in some larger function — like performing an angle comparison when an AI selects which enemy to target next. Mike Acton might say that even game logic should be packed structures of arrays so that instead of having one entity per AI, you have a big structure of all the positions for all AIs, all velocities for all AIs, all nav state for all AIs, and so on; but games just aren’t built like that yet. We’re still in a world where each rocket is represented by an instance of a CRocket class, and each CRocket has its own Update() function, and each CRocket does its own logic one at a time.