Initializing portions of arrays

[matthewlai]

The value of loop unrolling in modern CPUs is probably mostly only when combined with auto-vectorization - when unrolling loops allows the use of SIMD instructions.

This reminds me of a (funny?) story, I tried to iterate over a simple vector of ints and doing a sum or something like that.
The compiler (msc v11) vectorized the loop and it ran pretty fast.
Then I tried to use range-based for loop of C++11: for( type x:arr ) { ... } and to my surprise the compiler failed to vectorize in this case,
because range-based loop uses iterators (=pointers for vector) and somehow it failed to optimize this case.
The same of course applies to iterating the standard way using iterators.
I wonder if the most recent version of their compiler still has problems with this.

That is very interesting. I haven't used MSVC in many many years, only GCC and LLVM. I didn't notice any performance loss converting a bunch of loops to range-based.