From 1ec71f8cfd5e43e2e890ecb33edee7d662f1c283 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Lema=C5=84czyk?= <marek@lemanczyk.eu>
Date: Sat, 20 Sep 2025 19:48:36 +0200
Subject: [PATCH] Align GPU prime sieve with CPU GCD heuristic

---
 PerfectNumbers.Core/PrimeTester.cs | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/PerfectNumbers.Core/PrimeTester.cs b/PerfectNumbers.Core/PrimeTester.cs
index 5d83d5a..1d7c02a 100644
--- a/PerfectNumbers.Core/PrimeTester.cs
+++ b/PerfectNumbers.Core/PrimeTester.cs
@@ -255,12 +255,15 @@ public sealed class PrimeTester(bool useInternal = false)
             return;
         }
 
-        // Early reject special gcd heuristic with floor(log2 n)
-        ulong m = 63UL - (ulong)ILGPU.Algorithms.XMath.LeadingZeroCount(n);
-        if (BinaryGcdGpu(n, m) != 1UL)
+        if ((n % 10UL) == 1UL)
         {
-            results[index] = 0;
-            return;
+            // Early reject special GCD heuristic with floor(log2 n)
+            ulong m = 63UL - (ulong)ILGPU.Algorithms.XMath.LeadingZeroCount(n);
+            if (BinaryGcdGpu(n, m) != 1UL)
+            {
+                results[index] = 0;
+                return;
+            }
         }
 
         long len = smallPrimes.Length;