Optimize NaiveDate::add_days for small values

bench/benches/chrono.rs

@@ -6,7 +6,7 @@ use chrono::format::StrftimeItems;
 use chrono::prelude::*;
 #[cfg(feature = "unstable-locales")]
 use chrono::Locale;
-use chrono::{DateTime, FixedOffset, Local, Utc, __BenchYearFlags};
+use chrono::{DateTime, Duration, FixedOffset, Local, Utc, __BenchYearFlags};
 
 fn bench_datetime_parse_from_rfc2822(c: &mut Criterion) {
     c.bench_function("bench_datetime_parse_from_rfc2822", |b| {
@@ -195,6 +195,15 @@ fn bench_format_manual(c: &mut Criterion) {
         })
     });
 }
+
+fn bench_naivedate_add_signed(c: &mut Criterion) {
+    let date = NaiveDate::from_ymd_opt(2023, 7, 29).unwrap();
+    let extra = Duration::days(25);
+    c.bench_function("bench_naivedate_add_signed", |b| {
+        b.iter(|| black_box(date).checked_add_signed(extra).unwrap())
+    });
+}
+
 criterion_group!(
     benches,
     bench_datetime_parse_from_rfc2822,
@@ -210,6 +219,7 @@ criterion_group!(
     bench_format,
     bench_format_with_items,
     bench_format_manual,
+    bench_naivedate_add_signed,
 );
 
 #[cfg(feature = "unstable-locales")]

src/naive/date.rs

@@ -782,9 +782,15 @@ impl NaiveDate {
 
     /// Add a duration of `i32` days to the date.
     pub(crate) const fn add_days(self, days: i32) -> Option<Self> {
-        if days == 0 {
-            return Some(self);
+        // fast path if the result is within the same year
+        const ORDINAL_MASK: i32 = 0b1_1111_1111_0000;
+        if let Some(ordinal) = ((self.ymdf & ORDINAL_MASK) >> 4).checked_add(days) {
+            if ordinal > 0 && ordinal <= 365 {
+                let year_and_flags = self.ymdf & !ORDINAL_MASK;
+                return Some(NaiveDate { ymdf: year_and_flags | (ordinal << 4) });
+            }
         }
+        // do the full check
         let year = self.year();
         let (mut year_div_400, year_mod_400) = div_mod_floor(year, 400);
         let cycle = internals::yo_to_cycle(year_mod_400 as u32, self.of().ordinal());