"basically free" is very different then actually "free". Adding a ton of extra branches comes at a cost. Even though it is cheap it still takes an instructions

"basically free" is very different then actually "free". Adding a ton of extra branches comes at a cost. Even though it is cheap it still takes an instructions that could be used for something else. You fill your branch prediction target cache must faster, ejecting branches you actually care about. It also makes it harder for the compiler to move code around and harder for the prefetcher to break data dependencies. This all adds up to non-trival overhead. You can tell your C compiler to always check arithmetic, but most don't because of the cost.