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Replace Unsafe.As with Unsafe.BitCast in SpanHelpers.Fill #116221

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Merged
merged 3 commits into from
Jun 3, 2025
Merged

Replace Unsafe.As with Unsafe.BitCast in SpanHelpers.Fill #116221

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a9c8807
Replace Unsafe.As with Unsafe.BitCast in SpanHelpers.Fill
jkotas Jun 2, 2025
18674a4
Add test
jkotas Jun 3, 2025
d1def73
FB
jkotas Jun 3, 2025
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1 change: 1 addition & 0 deletions src/libraries/System.Memory/tests/Span/Fill.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -177,6 +177,7 @@ public static void FillWithRecognizedType()
0x1111111111111111, 0x2222222222222222, 0x3333333333333333, 0x4444444444444444,
0x5555555555555555, 0x6666666666666666, 0x7777777777777777, 0x8888888888888888)); // 512-bit struct, no SIMD
RunTest<MyRefContainingStruct>(new("Hello world!")); // struct contains refs, no SIMD
RunTest<int?>(42); // nullable type

static void RunTest<T>(T value)
{
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45 changes: 13 additions & 32 deletions src/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,74 +29,55 @@ public static unsafe void Fill<T>(ref T refData, nuint numElements, T value)
{
// We have enough data for at least one vectorized write.

T tmp = value; // Avoid taking address of the "value" argument. It would regress performance of the loops below.
Vector<byte> vector;

if (sizeof(T) == 1)
{
vector = new Vector<byte>(Unsafe.As<T, byte>(ref tmp));
vector = new Vector<byte>(Unsafe.BitCast<T, byte>(value));
}
else if (sizeof(T) == 2)
{
vector = (Vector<byte>)(new Vector<ushort>(Unsafe.As<T, ushort>(ref tmp)));
vector = (Vector<byte>)new Vector<ushort>(Unsafe.BitCast<T, ushort>(value));
}
else if (sizeof(T) == 4)
{
// special-case float since it's already passed in a SIMD reg
vector = (typeof(T) == typeof(float))
? (Vector<byte>)(new Vector<float>((float)(object)tmp!))
: (Vector<byte>)(new Vector<uint>(Unsafe.As<T, uint>(ref tmp)));
? (Vector<byte>)new Vector<float>(Unsafe.BitCast<T, float>(value))
: (Vector<byte>)new Vector<uint>(Unsafe.BitCast<T, uint>(value));
}
else if (sizeof(T) == 8)
{
// special-case double since it's already passed in a SIMD reg
vector = (typeof(T) == typeof(double))
? (Vector<byte>)(new Vector<double>((double)(object)tmp!))
: (Vector<byte>)(new Vector<ulong>(Unsafe.As<T, ulong>(ref tmp)));
? (Vector<byte>)new Vector<double>(Unsafe.BitCast<T, double>(value))
: (Vector<byte>)new Vector<ulong>(Unsafe.BitCast<T, ulong>(value));
}
else if (sizeof(T) == 16)
else if (sizeof(T) == Vector<byte>.Count)
{
Vector128<byte> vec128 = Unsafe.As<T, Vector128<byte>>(ref tmp);
if (Vector<byte>.Count == 16)
{
vector = vec128.AsVector();
}
else if (Vector<byte>.Count == 32)
{
vector = Vector256.Create(vec128).AsVector();
}
else if (Vector<byte>.Count == 64)
{
vector = Vector512.Create(vec128).AsVector();
}
else
{
Debug.Fail("Vector<T> is unexpected size.");
goto CannotVectorize;
}
vector = Unsafe.BitCast<T, Vector<byte>>(value);
}
else if (sizeof(T) == 32)
else if (sizeof(T) == 16)
{
Vector256<byte> vec256 = Unsafe.As<T, Vector256<byte>>(ref tmp);
if (Vector<byte>.Count == 32)
{
vector = vec256.AsVector();
vector = Vector256.Create(Unsafe.BitCast<T, Vector128<byte>>(value)).AsVector();
}
else if (Vector<byte>.Count == 64)
{
vector = Vector512.Create(vec256).AsVector();
vector = Vector512.Create(Unsafe.BitCast<T, Vector128<byte>>(value)).AsVector();
}
else
{
Debug.Fail("Vector<T> is unexpected size.");
goto CannotVectorize;
}
}
else if (sizeof(T) == 64)
else if (sizeof(T) == 32)
{
if (Vector<byte>.Count == 64)
{
vector = Unsafe.As<T, Vector512<byte>>(ref tmp).AsVector();
vector = Vector512.Create(Unsafe.BitCast<T, Vector256<byte>>(value)).AsVector();
}
else
{
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