On Jun 3, 2013, at 00:23 , Carter Schonwald
Int is "native register sized integer"
Actually it's not. Read the definition. Int is only guaranteed to be 29 bits. Here's *one* _actual_ data point (from a 2.8 GHz i7, 64-bit code): time ./fib fib(43) = 701408733 3.27 real 3.27 user 0.00 sys time ./fib-safe fib(43) = 701408733 3.45 real 3.45 user 0.00 sys (NB: I do not check the n-1 and n-2 as it's trivial to see from a data flow analysis that the proceeding conditional guarantees that those can't overflow. The empty asm() is necessary to get GCC to generate comparable code). Obviously, for some examples this will be much worse, for others, much better, but without this implemented in GHC it will be difficult to measure. Tommy