From d606311e5508ffccb0fb7e88537c8fe899f702ac Mon Sep 17 00:00:00 2001
From: Shawn Xu <xu107288696@gmail.com>
Date: Sun, 19 Jan 2025 17:55:53 -0800
Subject: [PATCH] Fix undefined behavior

From cppreference: "It is undefined behavior to read from the member of the
union that wasn't most recently written. Many compilers implement, as a
non-standard language extension, the ability to read inactive members of a
union."

closes https://github.com/official-stockfish/Stockfish/pull/5811

no functional change
---
 src/bitboard.h | 10 +++++-----
 src/misc.h     |  7 ++-----
 2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/src/bitboard.h b/src/bitboard.h
index 6f9cca0b..df15113b 100644
--- a/src/bitboard.h
+++ b/src/bitboard.h
@@ -22,6 +22,7 @@
 #include <algorithm>
 #include <cassert>
 #include <cmath>
+#include <cstring>
 #include <cstdint>
 #include <cstdlib>
 #include <string>
@@ -268,11 +269,10 @@ inline int popcount(Bitboard b) {
 
 #ifndef USE_POPCNT
 
-    union {
-        Bitboard bb;
-        uint16_t u[4];
-    } v = {b};
-    return PopCnt16[v.u[0]] + PopCnt16[v.u[1]] + PopCnt16[v.u[2]] + PopCnt16[v.u[3]];
+    std::uint16_t indices[4];
+    std::memcpy(indices, &b, sizeof(b));
+    return PopCnt16[indices[0]] + PopCnt16[indices[1]] + PopCnt16[indices[2]]
+         + PopCnt16[indices[3]];
 
 #elif defined(_MSC_VER)
 
diff --git a/src/misc.h b/src/misc.h
index 81c7b17f..8adbac68 100644
--- a/src/misc.h
+++ b/src/misc.h
@@ -120,11 +120,8 @@ void sync_cout_start();
 void sync_cout_end();
 
 // True if and only if the binary is compiled on a little-endian machine
-static inline const union {
-    uint32_t i;
-    char     c[4];
-} Le                                    = {0x01020304};
-static inline const bool IsLittleEndian = (Le.c[0] == 4);
+static inline const std::uint16_t Le             = 1;
+static inline const bool          IsLittleEndian = *reinterpret_cast<const char*>(&Le) == 1;
 
 
 template<typename T, std::size_t MaxSize>