Loop unrolling threshold was too small

it prevented unrolling on this simple example:

    #pythran export conv(float[][], float[][])
    import numpy as np
    def conv(image, weights):
        result = np.zeros_like(image)
        for i in range(1, image.shape[0]-2):
            for j in range(1, image.shape[1]-2):
                for ii in range(-1,2,1):
                    for jj in range(-1,2,1):
                        coef = weights[ii+1, jj+1]
                        result[i,j] += image[ii+i, jj+j] * coef
        return result