From c30af8f3db1722a687fb8452f0038f56682636d4 Mon Sep 17 00:00:00 2001
From: Charles Schlosser <cs.schlosser@gmail.com>
Date: Wed, 31 Dec 2025 03:57:04 +0000
Subject: [PATCH] fix UB in random implementation and tests

libeigen/eigen!2102
---
 Eigen/src/Core/RandomImpl.h        | 19 +++++++++++--------
 Eigen/src/Core/arch/Default/Half.h |  2 +-
 test/rand.cpp                      |  4 +++-
 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/Eigen/src/Core/RandomImpl.h b/Eigen/src/Core/RandomImpl.h
index 1a82e6253..4a622fc37 100644
--- a/Eigen/src/Core/RandomImpl.h
+++ b/Eigen/src/Core/RandomImpl.h
@@ -56,19 +56,21 @@ struct random_bits_impl {
   EIGEN_STATIC_ASSERT(std::is_unsigned<Scalar>::value, SCALAR MUST BE A BUILT - IN UNSIGNED INTEGER)
   using RandomDevice = eigen_random_device;
   using RandomReturnType = typename RandomDevice::ReturnType;
-  static constexpr int kEntropy = RandomDevice::Entropy;
   static constexpr int kTotalBits = sizeof(Scalar) * CHAR_BIT;
+  static constexpr int kEntropy = plain_enum_min(kTotalBits, RandomDevice::Entropy);
   // return a Scalar filled with numRandomBits beginning from the least significant bit
   static EIGEN_DEVICE_FUNC inline Scalar run(int numRandomBits) {
     eigen_assert((numRandomBits >= 0) && (numRandomBits <= kTotalBits));
-    const Scalar mask = Scalar(-1) >> ((kTotalBits - numRandomBits) & (kTotalBits - 1));
     Scalar randomBits = 0;
-    for (int shift = 0; shift < numRandomBits; shift += kEntropy) {
-      RandomReturnType r = RandomDevice::run();
-      randomBits |= static_cast<Scalar>(r) << shift;
+    for (int filledBits = 0; filledBits < numRandomBits; filledBits += kEntropy) {
+      Scalar r = static_cast<Scalar>(RandomDevice::run());
+      int remainingBits = numRandomBits - filledBits;
+      if (remainingBits < kEntropy) {
+        // clear the excess bits to avoid UB and rounding bias
+        r >>= kEntropy - remainingBits;
+      }
+      randomBits |= r << filledBits;
     }
-    // clear the excess bits
-    randomBits &= mask;
     return randomBits;
   }
 };
@@ -204,7 +206,8 @@ struct random_int_impl<Scalar, false, true> {
 template <typename Scalar>
 struct random_int_impl<Scalar, true, true> {
   static constexpr int kTotalBits = sizeof(Scalar) * CHAR_BIT;
-  using BitsType = typename make_unsigned<Scalar>::type;
+  // avoid implicit integral promotion to `int`
+  using BitsType = std::conditional_t<(sizeof(Scalar) < sizeof(int)), unsigned int, std::make_unsigned_t<Scalar> >;
   static EIGEN_DEVICE_FUNC inline Scalar run(const Scalar& x, const Scalar& y) {
     if (y <= x) return x;
     // Avoid overflow by representing `range` as an unsigned type
diff --git a/Eigen/src/Core/arch/Default/Half.h b/Eigen/src/Core/arch/Default/Half.h
index 210dfff1e..47832a4df 100644
--- a/Eigen/src/Core/arch/Default/Half.h
+++ b/Eigen/src/Core/arch/Default/Half.h
@@ -721,7 +721,7 @@ EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC float half_to_float(__half_raw h) {
     o_bits = Eigen::numext::bit_cast<uint32_t>(Eigen::numext::bit_cast<float>(o_bits) - magic);
   }
 
-  o_bits |= (h.x & 0x8000) << 16;  // sign bit
+  o_bits |= (h.x & 0x8000u) << 16;  // sign bit
   return Eigen::numext::bit_cast<float>(o_bits);
 #endif
 }
diff --git a/test/rand.cpp b/test/rand.cpp
index 69e7cf93e..0145299a7 100644
--- a/test/rand.cpp
+++ b/test/rand.cpp
@@ -83,7 +83,9 @@ class HistogramHelper {
 
 // helper class to avoid extending std:: namespace
 template <typename T>
-struct get_range_type : internal::make_unsigned<T> {};
+struct get_range_type {
+  using type = std::conditional_t<(sizeof(T) < sizeof(int)), unsigned int, std::make_unsigned_t<T>>;
+};
 template <typename T>
 struct get_range_type<SafeScalar<T>> : internal::make_unsigned<T> {};