Improved perf of deserializing integers from json (30%)

[jorgecarleitao]

The following bench,

use criterion::{criterion_group, criterion_main, Criterion};

fn add_benchmark(c: &mut Criterion) {
    let a = serde_json::json!(1);

    c.bench_function("json_f64", |b| {
        b.iter(|| {
            a.as_f64().and_then(num::cast::cast::<f64, i64>).unwrap();
        })
    });

    c.bench_function("json_i64", |b| {
        b.iter(|| {
            a.as_i64().and_then(num::cast::cast::<i64, i64>).unwrap();
        })
    });
}

criterion_group!(benches, add_benchmark);
criterion_main!(benches);

yields

json_f64                time:   [3.6079 ns 3.6164 ns 3.6274 ns]
json_i64                time:   [2.7989 ns 2.8087 ns 2.8214 ns]

showing a 30% difference between casting to float and then to integer vs casting to integer directly (and this is independent of i64 or i32.

The code change follows from this observation: all integer types use as_i64 instead of as_f64.

[codecov]

Codecov Report

Merging #340 (4b1b44f) into main (f79ae3e) will decrease coverage by 0.00%.
The diff coverage is 58.82%.

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   80.97%   80.97%   -0.01%     
==========================================
  Files         326      326              
  Lines       21167    21224      +57     
==========================================
+ Hits        17141    17186      +45     
- Misses       4026     4038      +12     

Impacted Files	Coverage Δ
src/io/json/read/deserialize.rs	75.75% <58.82%> (+0.95%)	⬆️
src/io/json/read/reader.rs	74.32% <0.00%> (-1.36%)	⬇️
tests/it/io/csv/read.rs	100.00% <0.00%> (ø)
src/array/equal/mod.rs	91.26% <0.00%> (+1.58%)	⬆️
src/io/csv/read/deserialize.rs	60.28% <0.00%> (+4.03%)	⬆️
src/io/json_integration/write.rs	6.25% <0.00%> (+6.25%)	⬆️

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