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Address testing guide nits #19888

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Jan 6, 2015
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Address testing guide nits #19888

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20fa7cb
Add header to optimizations section
steveklabnik Dec 15, 2014
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restore paragraph
steveklabnik Dec 15, 2014
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13 changes: 11 additions & 2 deletions src/doc/guide-testing.md
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Original file line number Diff line number Diff line change
Expand Up @@ -503,6 +503,8 @@ Advice on writing benchmarks:
* Make the code in the `iter` loop do something simple, to assist in pinpointing
performance improvements (or regressions)

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This benchmark is still incorrect: the text implies that this new benchmark is then fixed (i.e. accurately benchmarking the time it takes to xor 1000 uints), but it's not. It looks like we may need a new benchmark, or stronger black-boxing:

let mut n = 1000;
// `n` is unchanged, butthe optimiser is forced to assume that `n` may be mutated
test::black_box(&mut n);

b.iter(|| {
    test::black_box(range(0u, n).fold(...));
})

(or the version that does the range.fold without a semicolon.)

I haven't tested that, but one should see numbers on the order of the 375 ns/iter that it was before (the exact values isn't too important and varies based on the specific computer, but it should certainly be hundreds of nanoseconds, not ones).

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I'd like to finish this PR up, did you ever come up with a good replacement?

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Oh, whoops, forgot to write it down:

    b.iter(|| {
        let mut n = 1000_u32;
        test::black_box(&mut n); // pretend to modify `n`
        range(0, n).fold(0, |a, b| a ^ b)
    })

## Gotcha: optimizations

There's another tricky part to writing benchmarks: benchmarks compiled with
optimizations activated can be dramatically changed by the optimizer so that
the benchmark is no longer benchmarking what one expects. For example, the
Expand Down Expand Up @@ -554,8 +556,12 @@ extern crate test;
# fn main() {
# struct X; impl X { fn iter<T>(&self, _: || -> T) {} } let b = X;
b.iter(|| {
test::black_box(range(0u, 1000).fold(0, |old, new| old ^ new));
});
let mut n = 1000_u32;

test::black_box(&mut n); // pretend to modify `n`

range(0, n).fold(0, |a, b| a ^ b)
})
# }
```

Expand All @@ -571,3 +577,6 @@ test bench_xor_1000_ints ... bench: 1 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
```

However, the optimizer can still modify a testcase in an undesirable manner
even when using either of the above.