The Myth of the Free ALUs

One of the myths which arised with the new generation of consoles was that arithmetic operations (aka ALUs) were from now on literally free. If long ago we used cube map LUTs to normalize vectors, on GCN it costs nothing. Sure, they do have some cost, but with 6-7 ALU cycles per byte read from VRAM recommended at the top memory bandwidth (which you even almost never get), they should be all hidden beyond the memory fetches latency.

Well, it turns out, they are not. And here are some of the reasons why.

Some ALUs are more expensive than others. 

If an extra mad would unlikely (I won quite significantly one day by optimizing out a redundant matrix multiplication from a tight loop) make a change, divisions or even trigonometry would. Even in an SSAO shader could benefit up to 13% performance increase when arithmetic is optimized.

There are no free lunches now. 

Everything you could have relied hardware to do before (cube map texcoord calculation, attribute interpolation etc.) is now done with ALUs. You can use nointerpolation to avoid, if needed.

The scalar/vector ALUs are unbalanced.

In case your shader utilizes too many SALUs, not only you may become scalar register bound, but also your VALU units might stall while waiting for a scalar op result. An example here is excessive lane swizzling: when I tried to use it as a "cheap shared memory replacement" for blur filter, I got a ridiculous SALU/VALU ratio.

Triangles got to go... or not?

Few months ago I had an argue with a colleague of mine of whether we still will be using triangles in 10 (or 15) years. While I naturally opposed the idea, I feel there is something into it. So why are we using triangles, really?

Saving by calculating in VS and interpolating in PS

In fact, this is less and less is a viable argument. Nowadays characters already feature near-to-one-pixel triangles. We are actually LOOSING in this case: we get 4-fold PS cost and even rasterizer choking on some architectures. Besides, one modern architectures VS parameter cache is a bottleneck, so it is even less expensive to recompute a view position in a PS, than calculate it in VS and interpolate.

To match DCC tools

It is actually quite awkward to model your characters with triangles - a lot of artists use tools like ZBrush where they sculpt with spheres instead.

Clipping/culling

Once again - it is not really optimal when you triangles become very tiny. Besides, one might use bounding volumes for other representations to clip invisible surface.

I probably see how one can gain benefit for fx/billboards - but that's quite a specific case. Again, overdraw due to unused space on those is a big issue.

All in all, to support triangles we now have a lot of overhead - VS, sometimes tessellation, rasterization - while sometimes they are nearly equivalent to final pixels (and this trend seem to continue).

I am curious to hear your opinions and rationales guys. Will triangles live forever or will be replaced with something else (voxels? splats?) in a decade or two?