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Ensure BitIncrement/BitDecrement for Half works correctly (#73631)
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@tannergooding
tannergooding authored Aug 11, 2022
1 parent 50fec86 commit 3e0a5ad
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50 changes: 48 additions & 2 deletions src/libraries/System.Private.CoreLib/src/System/Half.cs
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Expand Up @@ -1261,10 +1261,56 @@ bool IFloatingPoint<Half>.TryWriteSignificandLittleEndian(Span<byte> destination
public static Half Atan2Pi(Half y, Half x) => (Half)float.Atan2Pi((float)y, (float)x);

/// <inheritdoc cref="IFloatingPointIeee754{TSelf}.BitDecrement(TSelf)" />
public static Half BitDecrement(Half x) => (Half)MathF.BitDecrement((float)x);
public static Half BitDecrement(Half x)
{
ushort bits = x._value;

if ((bits & PositiveInfinityBits) >= PositiveInfinityBits)
{
// NaN returns NaN
// -Infinity returns -Infinity
// +Infinity returns MaxValue
return (bits == PositiveInfinityBits) ? MaxValue : x;
}

if (bits == PositiveZeroBits)
{
// +0.0 returns -Epsilon
return -Epsilon;
}

// Negative values need to be incremented
// Positive values need to be decremented

bits += (ushort)(((short)bits < 0) ? +1 : -1);
return new Half(bits);
}

/// <inheritdoc cref="IFloatingPointIeee754{TSelf}.BitIncrement(TSelf)" />
public static Half BitIncrement(Half x) => (Half)MathF.BitIncrement((float)x);
public static Half BitIncrement(Half x)
{
ushort bits = x._value;

if ((bits & PositiveInfinityBits) >= PositiveInfinityBits)
{
// NaN returns NaN
// -Infinity returns MinValue
// +Infinity returns +Infinity
return (bits == NegativeInfinityBits) ? MinValue : x;
}

if (bits == NegativeZeroBits)
{
// -0.0 returns Epsilon
return Epsilon;
}

// Negative values need to be decremented
// Positive values need to be incremented

bits += (ushort)(((short)bits < 0) ? -1 : +1);
return new Half(bits);
}

/// <inheritdoc cref="IFloatingPointIeee754{TSelf}.FusedMultiplyAdd(TSelf, TSelf, TSelf)" />
public static Half FusedMultiplyAdd(Half left, Half right, Half addend) => (Half)MathF.FusedMultiplyAdd((float)left, (float)right, (float)addend);
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89 changes: 89 additions & 0 deletions src/libraries/System.Runtime/tests/System/HalfTests.cs
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Expand Up @@ -2,6 +2,7 @@
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections.Generic;
using System.Dynamic;
using System.Globalization;
using Xunit;

Expand Down Expand Up @@ -1986,5 +1987,93 @@ public static void TanPiTest(Half value, Half expectedResult, Half allowedVarian
AssertExtensions.Equal(-expectedResult, Half.TanPi(-value), allowedVariance);
AssertExtensions.Equal(+expectedResult, Half.TanPi(+value), allowedVariance);
}

public static IEnumerable<object[]> BitDecrement_TestData()
{
yield return new object[] { Half.NegativeInfinity, Half.NegativeInfinity };
yield return new object[] { BitConverter.UInt16BitsToHalf(0xC248), BitConverter.UInt16BitsToHalf(0xC249) }; // value: -(pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xC170), BitConverter.UInt16BitsToHalf(0xC171) }; // value: -(e)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xC09B), BitConverter.UInt16BitsToHalf(0xC09C) }; // value: -(ln(10))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBE48), BitConverter.UInt16BitsToHalf(0xBE49) }; // value: -(pi / 2)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBDC5), BitConverter.UInt16BitsToHalf(0xBDC6) }; // value: -(log2(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBDA8), BitConverter.UInt16BitsToHalf(0xBDA9) }; // value: -(sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBC83), BitConverter.UInt16BitsToHalf(0xBC84) }; // value: -(2 / sqrt(pi))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBC00), BitConverter.UInt16BitsToHalf(0xBC01) };
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBA48), BitConverter.UInt16BitsToHalf(0xBA49) }; // value: -(pi / 4)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB9A8), BitConverter.UInt16BitsToHalf(0xB9A9) }; // value: -(1 / sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB98C), BitConverter.UInt16BitsToHalf(0xB98D) }; // value: -(ln(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB918), BitConverter.UInt16BitsToHalf(0xB919) }; // value: -(2 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB6F3), BitConverter.UInt16BitsToHalf(0xB6F4) }; // value: -(log10(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB518), BitConverter.UInt16BitsToHalf(0xB519) }; // value: -(1 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x8000), -Half.Epsilon };
yield return new object[] { Half.NaN, Half.NaN };
yield return new object[] { BitConverter.UInt16BitsToHalf(0x0000), -Half.Epsilon };
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3518), BitConverter.UInt16BitsToHalf(0x3517) }; // value: (1 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x36F3), BitConverter.UInt16BitsToHalf(0x36F2) }; // value: (log10(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3918), BitConverter.UInt16BitsToHalf(0x3917) }; // value: (2 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x398C), BitConverter.UInt16BitsToHalf(0x398B) }; // value: (ln(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x39A8), BitConverter.UInt16BitsToHalf(0x39A7) }; // value: (1 / sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3A48), BitConverter.UInt16BitsToHalf(0x3A47) }; // value: (pi / 4)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3C00), BitConverter.UInt16BitsToHalf(0x3BFF) };
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3C83), BitConverter.UInt16BitsToHalf(0x3C82) }; // value: (2 / sqrt(pi))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3DA8), BitConverter.UInt16BitsToHalf(0x3DA7) }; // value: (sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3DC5), BitConverter.UInt16BitsToHalf(0x3DC4) }; // value: (log2(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3E48), BitConverter.UInt16BitsToHalf(0x3E47) }; // value: (pi / 2)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x409B), BitConverter.UInt16BitsToHalf(0x409A) }; // value: (ln(10))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x4170), BitConverter.UInt16BitsToHalf(0x416F) }; // value: (e)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x4248), BitConverter.UInt16BitsToHalf(0x4247) }; // value: (pi)
yield return new object[] { Half.PositiveInfinity, Half.MaxValue };
}

[Theory]
[MemberData(nameof(BitDecrement_TestData))]
public static void BitDecrement(Half value, Half expectedResult)
{
AssertExtensions.Equal(expectedResult, Half.BitDecrement(value), Half.Zero);
}

public static IEnumerable<object[]> BitIncrement_TestData()
{
yield return new object[] { Half.NegativeInfinity, Half.MinValue };
yield return new object[] { BitConverter.UInt16BitsToHalf(0xC248), BitConverter.UInt16BitsToHalf(0xC247) }; // value: -(pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xC170), BitConverter.UInt16BitsToHalf(0xC16F) }; // value: -(e)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xC09B), BitConverter.UInt16BitsToHalf(0xC09A) }; // value: -(ln(10))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBE48), BitConverter.UInt16BitsToHalf(0xBE47) }; // value: -(pi / 2)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBDC5), BitConverter.UInt16BitsToHalf(0xBDC4) }; // value: -(log2(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBDA8), BitConverter.UInt16BitsToHalf(0xBDA7) }; // value: -(sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBC83), BitConverter.UInt16BitsToHalf(0xBC82) }; // value: -(2 / sqrt(pi))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBC00), BitConverter.UInt16BitsToHalf(0xBBFF) };
yield return new object[] { BitConverter.UInt16BitsToHalf(0xBA48), BitConverter.UInt16BitsToHalf(0xBA47) }; // value: -(pi / 4)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB9A8), BitConverter.UInt16BitsToHalf(0xB9A7) }; // value: -(1 / sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB98C), BitConverter.UInt16BitsToHalf(0xB98B) }; // value: -(ln(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB918), BitConverter.UInt16BitsToHalf(0xB917) }; // value: -(2 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB6F3), BitConverter.UInt16BitsToHalf(0xB6F2) }; // value: -(log10(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0xB518), BitConverter.UInt16BitsToHalf(0xB517) }; // value: -(1 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x8000), Half.Epsilon };
yield return new object[] { Half.NaN, Half.NaN };
yield return new object[] { BitConverter.UInt16BitsToHalf(0x0000), Half.Epsilon };
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3518), BitConverter.UInt16BitsToHalf(0x3519) }; // value: (1 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x36F3), BitConverter.UInt16BitsToHalf(0x36F4) }; // value: (log10(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3918), BitConverter.UInt16BitsToHalf(0x3919) }; // value: (2 / pi)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x398C), BitConverter.UInt16BitsToHalf(0x398D) }; // value: (ln(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x39A8), BitConverter.UInt16BitsToHalf(0x39A9) }; // value: (1 / sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3A48), BitConverter.UInt16BitsToHalf(0x3A49) }; // value: (pi / 4)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3C00), BitConverter.UInt16BitsToHalf(0x3C01) };
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3C83), BitConverter.UInt16BitsToHalf(0x3C84) }; // value: (2 / sqrt(pi))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3DA8), BitConverter.UInt16BitsToHalf(0x3DA9) }; // value: (sqrt(2))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3DC5), BitConverter.UInt16BitsToHalf(0x3DC6) }; // value: (log2(e))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x3E48), BitConverter.UInt16BitsToHalf(0x3E49) }; // value: (pi / 2)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x409B), BitConverter.UInt16BitsToHalf(0x409C) }; // value: (ln(10))
yield return new object[] { BitConverter.UInt16BitsToHalf(0x4170), BitConverter.UInt16BitsToHalf(0x4171) }; // value: (e)
yield return new object[] { BitConverter.UInt16BitsToHalf(0x4248), BitConverter.UInt16BitsToHalf(0x4249) }; // value: (pi)
yield return new object[] { Half.PositiveInfinity, Half.PositiveInfinity };
}

[Theory]
[MemberData(nameof(BitIncrement_TestData))]
public static void BitIncrement(Half value, Half expectedResult)
{
AssertExtensions.Equal(expectedResult, Half.BitIncrement(value), Half.Zero);
}
}
}

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