From 2d2cad2c5a4fbae0d2f008b4164ffb1a49ba3a88 Mon Sep 17 00:00:00 2001
From: Richard Thier <magosit@outlook.hu>
Date: Thu, 11 Apr 2024 19:45:30 +0200
Subject: [PATCH] mormord sort more branchless plus extra edge-case handling
 for empty sized calls

---
 ypsu.cpp | 46 ++++++++++++++++++++++++++++------------------
 1 file changed, 28 insertions(+), 18 deletions(-)

diff --git a/ypsu.cpp b/ypsu.cpp
index 81d579f..0f778d7 100644
--- a/ypsu.cpp
+++ b/ypsu.cpp
@@ -150,6 +150,9 @@ static inline uint32_t morgrab(uint32_t elem) noexcept {
 }
 template<int j>
 static inline void mormord_sort_impl(uint32_t *a, int n) noexcept {
+	/* Edge-case */
+	if(n == 0) return;
+
 	/* Preparation */
 	uint32_t radics[256] = {0};
 	uint32_t radics2[256] = {0};
@@ -184,34 +187,41 @@ static inline void mormord_sort_impl(uint32_t *a, int n) noexcept {
 	uint32_t reali = 0;
 	#pragma GCC unroll 16
 	for(int i = 0; i < 256; ++i) {
+		radics[i] += prev;
 		if(radics[i] != 0) {
-			radics[i] += prev;
 			real_radics[reali] = prev;
 			real_radics[reali + 1] = radics[i];
-			prev = radics[i];
 			reali += 2;
-		} else {
-			radics[i] += prev;
-			prev = radics[i];
 		}
+		prev = radics[i];
 	}
 
-	// Inplace swap
+
+	// Inplace swap - own ideas + some ideas based-on "famous" ct-swap (for branchless / more ILP):
+	//	void ct-swap(bool secret, uint64_t a[], uint64_t b[], size_t len) {
+	//		uint64_t mask = ~((uint64_t)secret - 1); // 1->111....111; 0->000....000
+	//		for (size_t i = 0; i < len; i++) {
+	//			uint64_t delta = (a[i] ^ b[i]) & mask;
+	//			a[i] = a[i] ^ delta; // b[i], ha secret - amúgy a[i]
+	//			b[i] = b[i] ^ delta; // fordítva
+	//		}
+	//	}
 	uint32_t pivoti = 0;
 	while(pivoti < n) {
 		uint32_t radixval = morgrab<j>(a[pivoti]);
-		uint32_t targeti = radics[radixval] - 1;
-		if(targeti > pivoti) {
-			// swap
-			uint32_t tmp = a[pivoti];
-			a[pivoti] = a[targeti];
-			a[targeti] = tmp;
-			// dec index
-			--radics[radixval];
-		} else {
-			// progress pivot
-			++pivoti;
-		}
+		uint32_t targeti = --radics[radixval]; // dec index (!)
+
+		// Bitmask: true -> 11.....1; false -> 00.....0
+		uint32_t mask = ~((targeti > pivoti) - 1);
+
+		// Branchless swap (using bitmask)
+		uint32_t delta = (a[pivoti] ^ a[targeti]) & mask;
+		a[pivoti] = a[pivoti] ^ delta;
+		a[targeti] = a[targeti] ^ delta;
+
+		// "else" branch
+		pivoti += !mask;
+		radics[radixval] += !mask; // undec index (!)
 	}
 
 	// Ends recursion