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Well, at least division isn't. On reasonable recent Intel Broadwell, signed 64-bit division (IDIV) takes 59 cycles, while signed 32-bit division takes just 9 cycles.

Granted, divisions are rare, but that's still nearly an order of magnitude difference in that corner case.

32-bit and 64-bit addition, multiplication, comparison, logical operations etc. are in almost all cases just as fast.

You'll still need more storage on the stack for 64-bit variables. So more cache misses (and perhaps TLB misses and page faults) when corresponding boundary is crossed.