On Fri, Jun 6, 2025 at 9:02 PM Joaquin M López Muñoz via Boost < boost@lists.boost.org> wrote:
Feedback welcome,
Just a general question about benchmarks, not this one in particular. If you are doing something in a loop(e.g. this one <https://github.com/joaquintides/bloom/blob/1f0f953196c8e16e1c5fd8f62e18f4883dbcd44e/benchmark/comparison_table.cpp#L161>) did you check compiler did not unroll it in one case and not in another. You might say this is "natural" in a sense you did not tell compiler to unroll or not unroll the loop, but I would disagree partially since in real code it is unlikely that people will always iterate over an array of contiguous data, in real code they might get some data parse it, then do one lookup, get more data, parse it, do one lookup... I am not saying this is likely to happen, but may be worth checking. because long time ago I was playing around with benchmarking some hash map code and "weird" results that did not made sense were caused because compiler unrolled loop in one case, and not in another. After I did #pragma <https://releases.llvm.org/4.0.0/tools/clang/docs/AttributeReference.html#pragma-unroll-pragma-nounroll> to disable loop unrolling results made sense.