https://git.ruby-lang.org/ruby.git/commit/?id=e7ea6e078f

From e7ea6e078fecb70fbc91b04878b69f696749afac Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Wed, 13 Nov 2019 16:53:12 +0900
Subject: Check more likely condition first [Feature #16335]


diff --git a/time.c b/time.c
index a7a95b6..8a573c0 100644
--- a/time.c
+++ b/time.c
@@ -3067,7 +3067,16 @@ time_arg(int argc, const VALUE *argv, struct vtm *vtm) https://github.com/ruby/ruby/blob/trunk/time.c#L3067
 static int
 leap_year_p(long y)
 {
-    return ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0);
+    /* TODO:
+     *  ensure about negative years in proleptic Gregorian calendar.
+     */
+    unsigned long uy = (unsigned long)(LIKELY(y >= 0) ? y : -y);
+
+    if (LIKELY(uy % 4 != 0)) return 0;
+
+    unsigned long century = uy / 100;
+    if (LIKELY(uy != century * 100)) return 1;
+    return century % 4 == 0;
 }
 
 static time_t
-- 
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