[release/9.0-staging] Fix ILogB for subnormal values (#116973)

* Fix LeadingZeroCount in ILogB

* Add test

---------

Co-authored-by: Huo Yaoyuan <[email protected]>

Author: github-actions[bot]

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

@@ -1568,7 +1568,7 @@ public static int ILogB(Half x)
                 }
 
                 Debug.Assert(IsSubnormal(x));
-                return MinExponent - (BitOperations.TrailingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
+                return MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
             }
 
             return x.Exponent;

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

@@ -880,7 +880,7 @@ public static int ILogB(double x)
                 }
 
                 Debug.Assert(double.IsSubnormal(x));
-                return double.MinExponent - (BitOperations.TrailingZeroCount(x.TrailingSignificand) - double.BiasedExponentLength);
+                return double.MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - double.BiasedExponentLength);
             }
 
             return x.Exponent;

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

@@ -210,7 +210,7 @@ public static int ILogB(float x)
                 }
 
                 Debug.Assert(float.IsSubnormal(x));
-                return float.MinExponent - (BitOperations.TrailingZeroCount(x.TrailingSignificand) - float.BiasedExponentLength);
+                return float.MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - float.BiasedExponentLength);
             }
 
             return x.Exponent;

src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Math.cs

@@ -2287,6 +2287,7 @@ public static void FusedMultiplyAdd(double x, double y, double z, double expecte
         [InlineData( 0.561760,                -1)]
         [InlineData( 0.774152,                -1)]
         [InlineData( -0.678764,               -1)]
+        [InlineData( 1e-308,               -1024)]
         public static void ILogB(double value, int expectedResult)
         {
             Assert.Equal(expectedResult, Math.ILogB(value));

src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/MathF.cs

@@ -742,6 +742,7 @@ public static void IEEERemainder()
         [InlineData(0.561760f, -1)]
         [InlineData(0.774152f, -1)]
         [InlineData(-0.678764f, -1)]
+        [InlineData(1e-44f, -147)]
         public static void ILogB(float value, int expectedResult)
         {
             Assert.Equal(expectedResult, MathF.ILogB(value));