The last turtle is probably assembly, or at least intrinsics, in most good BLAS implementations. Probably called from C, because that’s easier in C.
The nice thing about Fortran is that you get something almost as good, but a normal engineer can write it. There are more C programmers than Fortran programmers, but there are more Fortran programmers than there are C programmers who can write really good assembly kernels. And all the C programmers in that last group are already working on vendor BLAS implementations.
The nice thing about Fortran is that you get something almost as good, but a normal engineer can write it. There are more C programmers than Fortran programmers, but there are more Fortran programmers than there are C programmers who can write really good assembly kernels. And all the C programmers in that last group are already working on vendor BLAS implementations.