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Ensure that IsPow2 for floating-point numbers correctly take subnormal values into account #88839

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Jul 14, 2023
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Ensure that IsPow2 for floating-point numbers correctly take subnormal values into account #88839

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24 changes: 20 additions & 4 deletions src/libraries/System.Private.CoreLib/src/System/Double.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -533,12 +533,28 @@ public static bool IsPow2(double value)
{
ulong bits = BitConverter.DoubleToUInt64Bits(value);

ushort biasedExponent = ExtractBiasedExponentFromBits(bits);;
if ((long)bits <= 0)
{
// Zero and negative values cannot be powers of 2
return false;
}

ushort biasedExponent = ExtractBiasedExponentFromBits(bits);
ulong trailingSignificand = ExtractTrailingSignificandFromBits(bits);

return (value > 0)
&& (biasedExponent != MinBiasedExponent) && (biasedExponent != MaxBiasedExponent)
&& (trailingSignificand == MinTrailingSignificand);
if (biasedExponent == MinBiasedExponent)
{
// Subnormal values have 1 bit set when they're powers of 2
return ulong.PopCount(trailingSignificand) == 1;
}
else if (biasedExponent == MaxBiasedExponent)
{
// NaN and Infinite values cannot be powers of 2
return false;
}

// Normal values have 0 bits set when they're powers of 2
return trailingSignificand == MinTrailingSignificand;
}

/// <inheritdoc cref="IBinaryNumber{TSelf}.Log2(TSelf)" />
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22 changes: 19 additions & 3 deletions src/libraries/System.Private.CoreLib/src/System/Half.cs
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Expand Up @@ -1194,12 +1194,28 @@ public static bool IsPow2(Half value)
{
ushort bits = BitConverter.HalfToUInt16Bits(value);

if ((short)bits <= 0)
{
// Zero and negative values cannot be powers of 2
return false;
}

byte biasedExponent = ExtractBiasedExponentFromBits(bits);
ushort trailingSignificand = ExtractTrailingSignificandFromBits(bits);

return (value > Zero)
&& (biasedExponent != MinBiasedExponent) && (biasedExponent != MaxBiasedExponent)
&& (trailingSignificand == MinTrailingSignificand);
if (biasedExponent == MinBiasedExponent)
{
// Subnormal values have 1 bit set when they're powers of 2
return ushort.PopCount(trailingSignificand) == 1;
}
else if (biasedExponent == MaxBiasedExponent)
{
// NaN and Infinite values cannot be powers of 2
return false;
}

// Normal values have 0 bits set when they're powers of 2
return trailingSignificand == MinTrailingSignificand;
}

/// <inheritdoc cref="IBinaryNumber{TSelf}.Log2(TSelf)" />
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22 changes: 19 additions & 3 deletions src/libraries/System.Private.CoreLib/src/System/Single.cs
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Expand Up @@ -529,12 +529,28 @@ public static bool IsPow2(float value)
{
uint bits = BitConverter.SingleToUInt32Bits(value);

if ((int)bits <= 0)
{
// Zero and negative values cannot be powers of 2
return false;
}

byte biasedExponent = ExtractBiasedExponentFromBits(bits);
uint trailingSignificand = ExtractTrailingSignificandFromBits(bits);

return (value > 0)
&& (biasedExponent != MinBiasedExponent) && (biasedExponent != MaxBiasedExponent)
&& (trailingSignificand == MinTrailingSignificand);
if (biasedExponent == MinBiasedExponent)
{
// Subnormal values have 1 bit set when they're powers of 2
return uint.PopCount(trailingSignificand) == 1;
}
else if (biasedExponent == MaxBiasedExponent)
{
// NaN and Infinite values cannot be powers of 2
return false;
}

// Normal values have 0 bits set when they're powers of 2
return trailingSignificand == MinTrailingSignificand;
}

/// <inheritdoc cref="IBinaryNumber{TSelf}.Log2(TSelf)" />
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2 changes: 1 addition & 1 deletion ...ntime.InteropServices.UnitTests/System/Runtime/InteropServices/NFloatTests.GenericMath.cs
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Expand Up @@ -244,7 +244,7 @@ public static void IsPow2Test()
Assert.False(BinaryNumberHelper<NFloat>.IsPow2(NegativeZero));
Assert.False(BinaryNumberHelper<NFloat>.IsPow2(NFloat.NaN));
Assert.False(BinaryNumberHelper<NFloat>.IsPow2(Zero));
Assert.False(BinaryNumberHelper<NFloat>.IsPow2(NFloat.Epsilon));
Assert.True(BinaryNumberHelper<NFloat>.IsPow2(NFloat.Epsilon));
Assert.False(BinaryNumberHelper<NFloat>.IsPow2(MaxSubnormal));
Assert.True(BinaryNumberHelper<NFloat>.IsPow2(MinNormal));
Assert.True(BinaryNumberHelper<NFloat>.IsPow2(One));
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2 changes: 1 addition & 1 deletion src/libraries/System.Runtime/tests/System/DoubleTests.GenericMath.cs
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Expand Up @@ -108,7 +108,7 @@ public static void IsPow2Test()
Assert.False(BinaryNumberHelper<double>.IsPow2(-0.0));
Assert.False(BinaryNumberHelper<double>.IsPow2(double.NaN));
Assert.False(BinaryNumberHelper<double>.IsPow2(0.0));
Assert.False(BinaryNumberHelper<double>.IsPow2(double.Epsilon));
Assert.True(BinaryNumberHelper<double>.IsPow2(double.Epsilon));
Assert.False(BinaryNumberHelper<double>.IsPow2(MaxSubnormal));
Assert.True(BinaryNumberHelper<double>.IsPow2(MinNormal));
Assert.True(BinaryNumberHelper<double>.IsPow2(1.0));
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2 changes: 1 addition & 1 deletion src/libraries/System.Runtime/tests/System/HalfTests.GenericMath.cs
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Expand Up @@ -120,7 +120,7 @@ public static void IsPow2Test()
Assert.False(BinaryNumberHelper<Half>.IsPow2(NegativeZero));
Assert.False(BinaryNumberHelper<Half>.IsPow2(Half.NaN));
Assert.False(BinaryNumberHelper<Half>.IsPow2(Zero));
Assert.False(BinaryNumberHelper<Half>.IsPow2(Half.Epsilon));
Assert.True(BinaryNumberHelper<Half>.IsPow2(Half.Epsilon));
Assert.False(BinaryNumberHelper<Half>.IsPow2(MaxSubnormal));
Assert.True(BinaryNumberHelper<Half>.IsPow2(MinNormal));
Assert.True(BinaryNumberHelper<Half>.IsPow2(One));
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2 changes: 1 addition & 1 deletion src/libraries/System.Runtime/tests/System/SingleTests.GenericMath.cs
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Expand Up @@ -108,7 +108,7 @@ public static void IsPow2Test()
Assert.False(BinaryNumberHelper<float>.IsPow2(-0.0f));
Assert.False(BinaryNumberHelper<float>.IsPow2(float.NaN));
Assert.False(BinaryNumberHelper<float>.IsPow2(0.0f));
Assert.False(BinaryNumberHelper<float>.IsPow2(float.Epsilon));
Assert.True(BinaryNumberHelper<float>.IsPow2(float.Epsilon));
Assert.False(BinaryNumberHelper<float>.IsPow2(MaxSubnormal));
Assert.True(BinaryNumberHelper<float>.IsPow2(MinNormal));
Assert.True(BinaryNumberHelper<float>.IsPow2(1.0f));
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