Fast Inverse Square Root benchmark

This repository contains a benchmark between different implementations of the Fast Inverse Square Root in Rust. The methods compared are using std::mem::compute and pointer casts. An implementation using f64::sqrt is also shown for reference.

This uses benchmark tests which are at the time of writing not stable so to run them you need a nightly build of rust. Installing and using nightly builds is covered in the rustup docs;

Running

With a nightly rust activated run the following

cargo bench

By default this will calculate 100 million inverse square roots per iteration. This value can be overridden with the environment variable NUM_ITERATIONS.

NUM_ITERATIONS=100 cargo bench # Using 100 calculations per iteration

Results

Macbook pro 13" mid 2012 2.5Ghz i5 2 cores

Using rustc 1.17.0-nightly (49c67bd63 2017-03-24)

100 million iterations

running 3 tests
test tests::bench_isqrt_no_transmute ... bench: 282,948,645 ns/iter (+/- 49,056,853)
test tests::bench_isqrt_sqrt         ... bench: 965,164,677 ns/iter (+/- 20,426,067)
test tests::bench_isqrt_transmute    ... bench: 273,885,037 ns/iter (+/- 15,464,136)

test result: ok. 0 passed; 0 failed; 0 ignored; 3 measured

10 million iterations

running 3 tests
test tests::bench_isqrt_no_transmute ... bench:  28,101,823 ns/iter (+/- 3,276,136)
test tests::bench_isqrt_sqrt         ... bench:  96,460,162 ns/iter (+/- 2,735,941)
test tests::bench_isqrt_transmute    ... bench:  27,624,626 ns/iter (+/- 2,690,027)

test result: ok. 0 passed; 0 failed; 0 ignored; 3 measured