https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77689
--- Comment #6 from morandidodo at gmail dot com --- (In reply to Richard Biener from comment #1) > First of all you need to make sure to have recent glibc installed to benefit > from > the vectorized math routines therein. Otherwise we cannot vectorize > std::log. > > Second, I think the if (i == 0) needs loop splitting (patch pending, author > CCed). > If you re-write the loop to > > if (s != 0) > a[i] = b[i] * c[i]; > for (unsigned i = 1; i < s; ++i) > a[i] = (b[i] + c[i]) * c[i-1] * std::log(i); > > then GCC will be able to vectorize it. Confirmed because of that issue. I can confirm the glibc is the last one (I use Arch Linux, it generally has the last version of different software). I know that, in this case, the condition inside the loop can be easily avoided, but it is not the case when the checked value depends on an array. However, I just made some tests, and it seems that the performance lost is particularly related to std::log. If something else is being used, like std::sqrt for example, the code compiled with gcc is still slower, but less.
