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Fix ReadOnlySpanGetReferenceAndReadInteger on BigEndian #2 #86323

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May 16, 2023
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Fix ReadOnlySpanGetReferenceAndReadInteger on BigEndian #2 #86323

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6 changes: 3 additions & 3 deletions src/libraries/System.Memory/tests/MemoryMarshal/GetReference.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -107,13 +107,13 @@ public static void ReadOnlySpanGetReferenceAndReadInteger()
Assert.Equal(BitConverter.IsLittleEndian ?
0x65_00_68 :
0x68_00_65,
Unsafe.As<byte, int>(ref Unsafe.Add(ref Unsafe.As<char, byte>(
Unsafe.ReadUnaligned<int>(ref Unsafe.Add(ref Unsafe.As<char, byte>(
ref MemoryMarshal.GetReference("hello world 1".AsSpan())), 0)));

Assert.Equal(BitConverter.IsLittleEndian ?
0x6F_00_6C_00_6C_00_65_00 :
0x00_65_00_6C_00_6C_00_6F,
Unsafe.As<byte, long>(ref Unsafe.Add(ref Unsafe.As<char, byte>(
0x68_00_65_00_6C_00_6C_00,
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I'm not clear on why this is the right answer. Can you explain?

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@EgorBo EgorBo May 16, 2023 •
edited
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My understanding is this:

// "hello world 2"
// LE: 68_00_65_00_6C_00_6C_00_6F_00_20_00_77_00_6F_00_72_00_6C_00_64_00_20_00_32_00
//        6F_00_6C_00_6C_00_65_00 (we read 8 bytes with offset = 1 bytes)
// 
//
// BE: 00_68_00_65_00_6C_00_6C_00_6F_00_20_00_77_00_6F_00_72_00_6C_00_64_00_20_00_32
//        68_00_65_00_6C_00_6C_00 (we read 8 bytes with offset = 1 bytes)
//

Console.WriteLine(
    BitConverter.ToString(
        Encoding.Unicode.GetBytes("hello world 2")).Replace("-", "_"));

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@EgorBo EgorBo May 16, 2023 •
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Same picture for the int case (that worked):

// "hello world 1"
// LE: 68_00_65_00_6C_00_6C_00_6F_00_20_00_77_00_6F_00_72_00_6C_00_64_00_20_00_31_00
//     00_65_00_68 (read 4 bytes)
// 
//
// BE: 00_68_00_65_00_6C_00_6C_00_6F_00_20_00_77_00_6F_00_72_00_6C_00_64_00_20_00_31
//     00_68_00_65 (read 4 bytes)

so chars here have swapped order ('h' is 68_00 on LE and 00_68 on BE)

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I see. Thanks.

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@stephentoub can you approve then? 🙂

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GitHub wasn't letting me. Now it is. Done.

Unsafe.ReadUnaligned<long>(ref Unsafe.Add(ref Unsafe.As<char, byte>(
ref MemoryMarshal.GetReference("hello world 2".AsSpan())), 1)));
}
}
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