> Is it the mod operator?
Seems to be very likely.
For
a1 = a1 % (((long long )1) << m2);
Run
it is easy to detect for the compiler that you are doing a modulo division by a
power of two, which is a plain bit masking operation. (Well, when signed
numbers are involved, an additional bit test may be necessary.)
For
a1 = a1 mod (1 shl m2)
Run
shl() is a proc call with unknown result, so no change for backend compiler to
discover it, so a full division is done, which is slow, very slow compared to
most other operations. See Agner Fogg.
So you may try a mask or shift op in Nim. Division by 1^x should be something
like and masking with (1^x - 1), so maybe
a1 = a1 and ((1 shl m2) - 1)
Run
Well that is more a guess that early in the morning, you may think about it
yourself, and be carefully when numbers are signed, maybe casts may be
necessary. Such masking ops are generally simpler with unsigned numbers.
