Modifying a constant in code might make sense because it saves several precious bytes of variable storage.

No, it might make sense because it saves precious memory bandwidth as well as uop count. This code wasn't called that often, generally once per drawing operation, but I don't like being uselessly wasteful - that mindset is how software got so bloated.

Your program is either front-end stalled by uop count or instruction cache latency, or back-end bound by memory bandwidth or ALU throughput or a serial dependency chain. It doesn't matter which is the bottleneck for this case, because inlining constants improves most of the above!