使用非类型模板参数的C ++优化

Question

I am not sure is there any sense in this case bits to be a non-type template:

template< int bits > 
inline bool FitsInBits(int x )
{
#if bits >= 32
    #error 'bits' should be less than 32
#endif
    return x >= -((int)1<<(bits-1)) && x < ((int)1<<(bits-1));
}

Rather then:

inline bool FitsInBits(int x, int bits )
{
    return x >= -((int)1<<(bits-1)) && x < ((int)1<<(bits-1));
}

As I understand the compiler will create many variants of FitsInBits in the first case at compile time? But I don't see how this will optimize the calculation.

Answer 1

The template isn't necessarily more efficient, but it assures you that the values of: -((int)1<<(bits-1)) and ((int)1<<(bits-1)) can be computed at compile time, so all that happens at run-time is basically:

return x >= constant_a && x < constant_b;

This, in turn, is sufficiently trivial that there's a pretty good chance the compiler can/will generate inline-code for it.

Assuming that bits is a value known at compile time, the non-template version can (and probably will) do the same--but since bits is a normal parameter to a normal function, you could (perhaps accidentally) pass some value for bits that wasn't known until run time (eg, based on data entered by the user).

If you do so, the compiler can't/won't give you a warning about having done so--and in this case, the expressions probably won't reduce to constants as above, so you'd very likely end up with code to load 1 into one register, load bits into another, decrement the second, shift the first left by the number of places specified in the second, compare to x , conditionally jump to a label, negate the first, compare again, do another conditional jump, etc. This is still only around a dozen instructions (or so), but undoubtedly slower than when the values are constants.