It's a digital alu_xor having two copies of ALUs, namely: alu1 and alu2. The outputs generated by both the ALUs are XORed to yield any discrepancies between the designs. Any difference in the result turns out to be latch-ups or conditions relating to faults.
References
Refer to README for a quick start of how to use caravel_user_project
Refer to README for this sample project documentation.