From 929bc0e191d0927b1735b9a1ddc0e8b77e3a25ec Mon Sep 17 00:00:00 2001
From: Alexander Grund <alexander.grund@tu-dresden.de>
Date: Mon, 20 Sep 2021 10:37:50 +0200
Subject: [PATCH] Fix alias violation in BFloat16

reinterpret_cast between unrelated types is undefined behavior and leads
to misoptimizations on some platforms.
Use the safer (and faster) version via bit_cast


(cherry picked from commit b5eaa4269503f77d0aa58d2f8ed9419e1ba7784d)
---
 Eigen/src/Core/arch/Default/BFloat16.h | 16 ++--------------
 1 file changed, 2 insertions(+), 14 deletions(-)

diff --git a/Eigen/src/Core/arch/Default/BFloat16.h b/Eigen/src/Core/arch/Default/BFloat16.h
index 1c28f4f95..f21d1a0a3 100644
--- a/Eigen/src/Core/arch/Default/BFloat16.h
+++ b/Eigen/src/Core/arch/Default/BFloat16.h
@@ -251,12 +251,7 @@ EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw truncate_to_bfloat16(const
     output.value = std::signbit(v) ? 0xFFC0: 0x7FC0;
     return output;
   }
-  const uint16_t* p = reinterpret_cast<const uint16_t*>(&v);
-#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-  output.value = p[0];
-#else
-  output.value = p[1];
-#endif
+  output.value = static_cast<numext::uint16_t>(numext::bit_cast<numext::uint32_t>(v) >> 16);
   return output;
 }
 
@@ -462,14 +457,7 @@ EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC __bfloat16_raw float_to_bfloat16_rtne<true
 }
 
 EIGEN_STRONG_INLINE EIGEN_DEVICE_FUNC float bfloat16_to_float(__bfloat16_raw h) {
-    float result = 0;
-    unsigned short* q = reinterpret_cast<unsigned short*>(&result);
-#if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
-    q[0] = h.value;
-#else
-    q[1] = h.value;
-#endif
-    return result;
+    return numext::bit_cast<float>(static_cast<numext::uint32_t>(h.value) << 16);
 }
 // --- standard functions ---