Fix ILogB for Half and BFloat16 subnormal values

src/libraries/System.Private.CoreLib/src/System/Half.cs

@@ -1532,7 +1532,7 @@ public static int ILogB(Half x)
                 }
 
                 Debug.Assert(IsSubnormal(x));
-                return MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
+                return MinExponent - (ushort.LeadingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
             }
 
             return x.Exponent;

src/libraries/System.Private.CoreLib/src/System/Numerics/BFloat16.cs

@@ -1189,7 +1189,7 @@ public static int ILogB(BFloat16 x)
                 }
 
                 Debug.Assert(IsSubnormal(x));
-                return MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
+                return MinExponent - (ushort.LeadingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
             }
 
             return x.Exponent;

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/HalfTests.cs

@@ -2244,5 +2244,30 @@ public static void RadiansToDegreesTest(Half value, Half expectedResult, Half al
             AssertExtensions.Equal(-expectedResult, Half.RadiansToDegrees(-value), allowedVariance);
             AssertExtensions.Equal(+expectedResult, Half.RadiansToDegrees(+value), allowedVariance);
         }
+
+        [Theory]
+        [InlineData(float.PositiveInfinity, int.MaxValue)]
+        [InlineData(float.NaN, int.MaxValue)]
+        [InlineData(0.0f, int.MinValue)]
+        [InlineData(1.0f, 0)]
+        [InlineData(2.0f, 1)]
+        [InlineData(4.0f, 2)]
+        [InlineData(0.5f, -1)]
+        public static void ILogB(float value, int expectedResult)
+        {
+            Assert.Equal(expectedResult, Half.ILogB((Half)value));
+        }
+
+        [Fact]
+        public static void ILogB_Subnormal()
+        {
+            // Half.Epsilon is the smallest positive subnormal value
+            // Its ILogB should be -24 (floor(log2(5.9604645E-08)))
+            Assert.Equal(-24, Half.ILogB(Half.Epsilon));
+
+            // Test another subnormal value: 0x0200 (half of max subnormal)
+            Half subnormal = BitConverter.UInt16BitsToHalf(0x0200);
+            Assert.Equal(-15, Half.ILogB(subnormal));
+        }
     }
 }

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Numerics/BFloat16Tests.cs

@@ -2271,6 +2271,31 @@ public static void RadiansToDegreesTest(BFloat16 value, BFloat16 expectedResult,
             AssertEqual(+expectedResult, BFloat16.RadiansToDegrees(+value), allowedVariance);
         }
 
+        [Theory]
+        [InlineData(float.PositiveInfinity, int.MaxValue)]
+        [InlineData(float.NaN, int.MaxValue)]
+        [InlineData(0.0f, int.MinValue)]
+        [InlineData(1.0f, 0)]
+        [InlineData(2.0f, 1)]
+        [InlineData(4.0f, 2)]
+        [InlineData(0.5f, -1)]
+        public static void ILogB(float value, int expectedResult)
+        {
+            Assert.Equal(expectedResult, BFloat16.ILogB((BFloat16)value));
+        }
+
+        [Fact]
+        public static void ILogB_Subnormal()
+        {
+            // BFloat16.Epsilon is the smallest positive subnormal value
+            // Its ILogB should be -133
+            Assert.Equal(-133, BFloat16.ILogB(BFloat16.Epsilon));
+
+            // Test another subnormal value: 0x0040 (half of max subnormal)
+            BFloat16 subnormal = BitConverter.UInt16BitsToBFloat16(0x0040);
+            Assert.Equal(-127, BFloat16.ILogB(subnormal));
+        }
+
         #region AssertExtentions
         static bool IsNegativeZero(BFloat16 value)
         {