quinmars quinmars - 29 days ago 15
C Question

Is Fortran faster than C?

From time to time I read that Fortran is or can be faster then C for heavy calculations. Is that really true? I must admit that I hardly know Fortran, but the Fortran code I have seen so far did not show that the language has features that C doesn't have.

If it is true, please tell me why. Please don't tell me what languages or libs are good for number crunching, I don't intend to write an app or lib to do that, I'm just curious.

Answer

The languages have similar feature-set. The performance difference comes from the fact that Fortran says aliasing is not allowed. Any code that has aliasing is not valid Fortran but it is up to the programmer and not the compiler to detect these errors. Thus Fortran compilers ignore possible aliasing of memory pointers and allows them to generate more efficient code. Take a look at this little example in C:

void transform (float *output, float const * input, float const * matrix, int *n)
{
    int i;
    for (i=0; i<*n; i++)
    {
        float x = input[i*2+0];
        float y = input[i*2+1];
        output[i*2+0] = matrix[0] * x + matrix[1] * y;
        output[i*2+1] = matrix[2] * x + matrix[3] * y;
    }
}

This function would run slower than the Fortran counterpart after optimization. Why so? If you write values into the output array you may change the values of matrix. After all the pointers could overlap and point to the same chunk of memory (including the int pointer!). The C compiler is forced to reload the four matrix values from memory for all computations.

In Fortran the compiler can load the matrix values once and store them in registers. It can do so because the Fortran compiler assumes pointers/arrays do not overlap in memory.

Fortunately the restrict keyword and strict-aliasing have been introduced to the C99 standard to address this problem. It's well supported in most C++ compilers these days as well. The keyword allows you to give the compiler a hint that the programmer promises that a pointer does not alias with any other pointer. The strict-aliasing means that the programmer promises that pointers of different type will never overlap, for example a double* will not overlap with an int* (with the specific exception that char* and void* can overlap with anything).

If you use them you will get the same speed from C and Fortran. However, the ability to use the restrict keyword only with performance critical functions means that C (and C++) programs are much safer and easier to write. For example, consider the invalid Fortran code: CALL TRANSFORM(A(1, 30), A(2, 31), A(3, 32), 30), which most Fortran compilers will happily compile without any warning but introduces a bug that only shows up on some compilers, on some hardware and with some optimization options.