At a guess, constant rematerialision failing to cross basic block boundaries. Feels like a plausible thing for a heuristic to miss. E.g. sink the constant into

At a guess, constant rematerialision failing to cross basic block boundaries. Feels like a plausible thing for a heuristic to miss. E.g. sink the constant into the loop so it's available when optimising that block, then fail to hoist it back out afterwards because constant materialisation is cheap.