chenziteng: TInt64 is a class where the value is represented as two TUints. See e32std.h.
yaront: ARM does not natively support 64-bit arithmetic so 64-bit arithmetic functions have to be constructed from more basic building blocks. Symbian's TInt64 implementation is relatively efficient. So unless you can refactor your loop to work on e.g. 32-bit integers, TInt64 is probably your best shot.
After checking, I noticed that ARM doesn't work with 64 bit at all. I know that on other platforms where the CPU doesn't suport it, the compiler itself (through the "long long" type) uses 2 32 bits integers to simulate a 64 int type.
However the Symbian compiler doesn't recongnize the "long long" as a 64 bit value, and the only option is the TInt64.
I'll believe you if you say it's an efficient implementation, however it's still a class, and there is the overhead of class construction which in low-level loops is a problem.
The best solution to this problem is a built-in support by the compiler for the "long long" type.