Fix Int128 checked-convert to signed IntX (dotnet#100342)

* Fix Int128 checked-convert to signed IntX

* Simplify implementation

Author: skyoxZ

src/libraries/System.Private.CoreLib/src/System/Int128.cs

@@ -262,17 +262,12 @@ public static explicit operator Half(Int128 value)
         /// <exception cref="OverflowException"><paramref name="value" /> is not representable by <see cref="Int128" />.</exception>
         public static explicit operator checked short(Int128 value)
         {
-            if (~value._upper == 0)
-            {
-                long lower = (long)value._lower;
-                return checked((short)lower);
-            }
-
-            if (value._upper != 0)
+            long lower = (long)value._lower;
+            if ((long)value._upper != lower >> 63)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return checked((short)value._lower);
+            return checked((short)lower);
         }
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="int" /> value.</summary>
@@ -286,17 +281,12 @@ public static explicit operator checked short(Int128 value)
         /// <exception cref="OverflowException"><paramref name="value" /> is not representable by <see cref="Int128" />.</exception>
         public static explicit operator checked int(Int128 value)
         {
-            if (~value._upper == 0)
-            {
-                long lower = (long)value._lower;
-                return checked((int)lower);
-            }
-
-            if (value._upper != 0)
+            long lower = (long)value._lower;
+            if ((long)value._upper != lower >> 63)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return checked((int)value._lower);
+            return checked((int)lower);
         }
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="long" /> value.</summary>
@@ -310,17 +300,12 @@ public static explicit operator checked int(Int128 value)
         /// <exception cref="OverflowException"><paramref name="value" /> is not representable by <see cref="Int128" />.</exception>
         public static explicit operator checked long(Int128 value)
         {
-            if (~value._upper == 0)
-            {
-                long lower = (long)value._lower;
-                return lower;
-            }
-
-            if (value._upper != 0)
+            long lower = (long)value._lower;
+            if ((long)value._upper != lower >> 63)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return checked((long)value._lower);
+            return lower;
         }
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="IntPtr" /> value.</summary>
@@ -334,17 +319,12 @@ public static explicit operator checked long(Int128 value)
         /// <exception cref="OverflowException"><paramref name="value" /> is not representable by <see cref="Int128" />.</exception>
         public static explicit operator checked nint(Int128 value)
         {
-            if (~value._upper == 0)
-            {
-                long lower = (long)value._lower;
-                return checked((nint)lower);
-            }
-
-            if (value._upper != 0)
+            long lower = (long)value._lower;
+            if ((long)value._upper != lower >> 63)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return checked((nint)value._lower);
+            return checked((nint)lower);
         }
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="sbyte" /> value.</summary>
@@ -360,17 +340,12 @@ public static explicit operator checked nint(Int128 value)
         [CLSCompliant(false)]
         public static explicit operator checked sbyte(Int128 value)
         {
-            if (~value._upper == 0)
-            {
-                long lower = (long)value._lower;
-                return checked((sbyte)lower);
-            }
-
-            if (value._upper != 0)
+            long lower = (long)value._lower;
+            if ((long)value._upper != lower >> 63)
             {
                 ThrowHelper.ThrowOverflowException();
             }
-            return checked((sbyte)value._lower);
+            return checked((sbyte)lower);
         }
 
         /// <summary>Explicitly converts a 128-bit signed integer to a <see cref="float" /> value.</summary>

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Int128Tests.cs

@@ -107,6 +107,51 @@ public static void EqualsTest(Int128 i1, object obj, bool expected)
             Assert.Equal(expected, i1.Equals(obj));
         }
 
+        [Fact]
+        public static void CheckedConvertToInt64()
+        {
+            Assert.Equal(123L, checked((long)new Int128(0, 123)));
+            Assert.Equal(-123L, checked((long)(Int128)(-123)));
+            Assert.Throws<OverflowException>(() => checked((long)new Int128(1, 1)));
+            Assert.Throws<OverflowException>(() => checked((long)new Int128(ulong.MaxValue, 42)));
+        }
+
+        [Fact]
+        public static void CheckedConvertToInt32()
+        {
+            Assert.Equal(123, checked((int)new Int128(0, 123)));
+            Assert.Equal(-123, checked((int)(Int128)(-123)));
+            Assert.Throws<OverflowException>(() => checked((int)new Int128(1, 1)));
+            Assert.Throws<OverflowException>(() => checked((int)new Int128(ulong.MaxValue, 42)));
+        }
+
+        [Fact]
+        public static void CheckedConvertToInt16()
+        {
+            Assert.Equal((short)123, checked((short)new Int128(0, 123)));
+            Assert.Equal((short)(-123), checked((short)(Int128)(-123)));
+            Assert.Throws<OverflowException>(() => checked((short)new Int128(1, 1)));
+            Assert.Throws<OverflowException>(() => checked((short)new Int128(ulong.MaxValue, 42)));
+        }
+
+        [Fact]
+        public static void CheckedConvertToSByte()
+        {
+            Assert.Equal((sbyte)123, checked((sbyte)new Int128(0, 123)));
+            Assert.Equal((sbyte)(-123), checked((sbyte)(Int128)(-123)));
+            Assert.Throws<OverflowException>(() => checked((sbyte)new Int128(1, 1)));
+            Assert.Throws<OverflowException>(() => checked((sbyte)new Int128(ulong.MaxValue, 42)));
+        }
+
+        [Fact]
+        public static void CheckedConvertToIntPtr()
+        {
+            Assert.Equal((nint)123, checked((nint)new Int128(0, 123)));
+            Assert.Equal((nint)(-123), checked((nint)(Int128)(-123)));
+            Assert.Throws<OverflowException>(() => checked((nint)new Int128(1, 1)));
+            Assert.Throws<OverflowException>(() => checked((nint)new Int128(ulong.MaxValue, 42)));
+        }
+
         public static IEnumerable<object[]> ToString_TestData()
         {
             foreach (NumberFormatInfo defaultFormat in new[] { null, NumberFormatInfo.CurrentInfo })