Fastor | A lightweight high performance tensor algebra framework

The reason the Numpy implementation is much faster is that it does not compute the same thing as the two others.

Indeed, the python version does not read z in the expression np.sin(x) * np.cos(x). As a result, the Numba JIT is clever enough to execute the loop only once justifying a factor of 100 between Fastor and Numba. You can check that by replacing range(100) by range(10000000000) and observing the same timings.

Finally, XTensor is faster than Fastor in this benchmark as it seems to use its own fast SIMD implementation of exp/sin/cos while Fastor seems to use a scalar implementation from libm justifying the factor of 2 between XTensor and Fastor.

Answer to the update:

Fastor/Xtensor performs really bad in exp, sin, cos, which was surprising.

No. We cannot conclude that from the benchmark. What you are comparing is the ability of compilers to optimize your code. In this case, Numba is better than plain C++ compilers as it deals with a high-level SIMD-aware code while C++ compilers have to deals with a huge low-level template-based code coming from the Fastor/Xtensor libraries. Theoretically, I think that it should be possible for a C++ compiler to apply the same kind of high-level optimization than Numba, but it is just harder. Moreover, note that Numpy tends to create/allocate temporary arrays while Fastor/Xtensor should not.

In practice, Numba is faster because u is a constant and so is exp(u), sin(u) and cos(u). Thus, Numba precompute the expression (computed only once) and still perform the sum in the loop. The following code give the same timing: