I like this as an example of how compilers cause bugs. This code looks reasonable and concise, but it shouldn’t. If you were writing microcode for a non-superscalar processor, you would do two separate memory to register loads and then three xors and then two register to memory stores. But the compiler teaches us to use shorthand and it will fill in the details. In this case the resulting code is ridiculous with loads and stores on each xor because the compiler can’t guarantee that i!=j, and it would normally perform terribly, but we’ve architected CPUs around compilers to use caches that make up for these problems, and they are so prevalent that it’s worth it to dedicate most of the CPU die area to branch prediction based on conventions of the C compiler, and most of the compiler code to conventions of the CPU branch predictor. It’s getting quite ridiculous, and maybe it’s time to move to architectures and languages that make everything explicit.

Edit: also, there is an xchg instruction that you have to pull out of intrinsics to use in C (good luck in most other HLL). It seems like a language should be more about how to use vocabulary than restricting it.