Re: [bloom] Benchmarks with Knuth multiplier-based hash production

From: Ivan Matek (libbooze_at_[hidden])
Date: 2025-06-06 20:06:01

On Fri, Jun 6, 2025 at 9:02 PM Joaquin M López Muñoz via Boost <
boost_at_[hidden]> 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.