The hw implementation of xor is simpler than sub, so it should consume slightly less energy. Wondering how much energy was saved in the whole world by using xor instead of sub.

I doubt any of that is measurable, since all ALU operations are usually implemented with the same logic (e.g. see https://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-...)

For a 32 bit number you're looking at going from using 256 to ~1800 transistors in the operation itself. A modern core will have roughly 1,000,000,000 transistors. Some of those are for vector operations that aren't involved in a xor or sub, but most of them are for allowing the core to extract more parallelism from the instruction stream. It's really just a dust mote compared to the power reduction you could get by, e.g., targeting a 10 MHz lower clock rate.

I guess everything what was saved was burned by the first useless image created per AI