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So with the following code, changing the type of the parameter x from const ull to const ull& (with typedef unsigned long long ull ) results in a roughly 25% speedup when compiled with gcc 4.7.2 and flags -O3 -std=c++11 -g , and I can't figure out why this would make such a big difference. static void inline single_mult(const std::vector<ull>::iterator& data, const std::vector<ull>::const_iterator& rbegin, const std::vector<ull>::const_iterator& rend, const ull x) { ull tmp=0, carry=0, i=0; for (auto rhs_it = rbegin; rhs_it != rend; ++rhs_it) { tmp = x*(*rhs_it) + data[i] + carry; if (tmp >= imax) { carry = tmp >> numbits; tmp &= imax - 1; } else { carry = 0; } data[i++] = tmp; } data[i] += carry; } It is called in the following function (for doing schoolbook long multiplication) static void naive(std::vector<ull>::iterator data, std::vector<ull>::const_iterator cbegin, std::vector<ull>::const_iterator cend , std::vector<ull>::const_iterator rbegin, std::vector<ull>::const_iterator rend) { for (auto data_it = cbegin; data_it != cend; ++data_it) { if (*data_it != 0) { single_mult(data, rbegin, rend, *data_it); } ++data; } } The timing is done by calling clock() around a loop to measure how long it takes. Not the most accurate/precise way, but I figured a consistent 25% difference meant the difference was statistically significant. Full working code block: #include <vector>

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