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    <p>The languages have similar feature-sets. The performance difference comes from the fact that Fortran says aliasing is not allowed, unless an EQUIVALENCE statement is used. 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 allow them to generate more efficient code. Take a look at this little example in C:</p> <pre><code>void transform (float *output, float const * input, float const * matrix, int *n) { int i; for (i=0; i&lt;*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; } } </code></pre> <p>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 <code>int</code> pointer!). The C compiler is forced to reload the four matrix values from memory for all computations.</p> <p>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.</p> <p>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 <code>double*</code> will not overlap with an <code>int*</code> (with the specific exception that <code>char*</code> and <code>void*</code> can overlap with anything).</p> <p>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: <code>CALL TRANSFORM(A(1, 30), A(2, 31), A(3, 32), 30)</code>, 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.</p>
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    1. POIs Fortran easier to optimize than C for heavy calculations?
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    1. USMatt Wenham
    1. USNils Pipenbrinck
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    1. COAside from the restrict keyword, most compilers have had command line optimization flags to instruct the compiler to not worry about aliasing. Since doing actual aliasing is pretty rare, I consider that level of optimization the starting point default.
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      1. USTall Jeff
    2. COAdditionally, without the flag or the new keyword, in C just assign pointer based operations through an automatic variable. This hints to the compiler that aliasing can be ignored and while it looks like more code, you'll actually end up with the fastest possible result through the optimizer
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    3. CO"rare" may or may not be a good reason to have flaky nondeterministic bugs. I've seen it in cases where a function is used to process data either in-place or out-of-place.
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