I was looking at the things that were said for XE2 in Lunar Lake and it appears that the slides suggest that they had special handling to emulate SIMD32 using SIMD16 in hardware, so you might be right.
> So this is a situation where wider lanes actually need more hardware to run at full speed and not having it causes a penalty. I see your point here, but I will note that you can add that criss-cross hardware for 32-wide operations while still having 16-wide be your default.
To go from SIMD8 to SIMD16, Intel halved the number of units while making them double the width. They could have done that again to avoid the need for additional hardware.
I have not seen the Xe2 instruction set to have any hints about how they are doing these operations in their hardware. I am going to leave it at that since I have spent far too much time analyzing the technical marketing for a GPU architecture that I am not likely to use. No matter how well they made it, it just was not scaled up enough to make it interesting to me as a developer that owns a RTX 3090 Ti. I only looked into it as much as I did since I am excited to see Intel moving forward here. That said, if they launched a 48GB variant, I would buy it in a heartbeat and start writing code to run on it.
From a hardware design perspective, it saves you some die size in the scheduler.
From a performance perspective, as long as the hardware designer kept 32 in mind, it can schedule 32 lanes and duplicate the signals to the 16 or 8 wide lanes with no loss of performance.
> That documentation talks about writing to a temporary location and reading form a temporary location in order to do cross lane operations.
> If games’ shaders are written with an assumption that SIMD32 is used, then native SIMD32 is going to be more performant than native SIMD16 because of faster cross lane operations.
So this is a situation where wider lanes actually need more hardware to run at full speed and not having it causes a penalty. I see your point here, but I will note that you can add that criss-cross hardware for 32-wide operations while still having 16-wide be your default.