Ensure that GetContentHash produces the same value on different endian machines

src/Compilers/Core/Portable/Text/SourceText.cs

@@ -3,7 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
-using System.Buffers;
+using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.ComponentModel;
@@ -642,7 +642,26 @@ ImmutableArray<byte> computeContentHash()
                             sourceIndex: index, destination: charBuffer,
                             destinationIndex: 0, count: charsToCopy);
 
-                        hash.Append(MemoryMarshal.AsBytes(charBuffer.AsSpan(0, charsToCopy)));
+                        var charSpan = charBuffer.AsSpan(0, charsToCopy);
+
+                        // Ensure everything is always little endian, so we get the same results across all platforms.
+                        // This will be entirely elided by the jit on a little endian machine.
+                        if (!BitConverter.IsLittleEndian)
+                        {
+                            var shortSpan = MemoryMarshal.Cast<char, short>(charSpan);
+
+#if NET8_0_OR_GREATER
+                            // Defer to the platform to do the reversal.  It ships with a vectorized
+                            // implementation for this on .NET 8 and above.
+                            BinaryPrimitives.ReverseEndianness(source: shortSpan, destination: shortSpan);
+#else
+                            // Otherwise, fallback to the simple approach of reversing each pair of bytes.
+                            for (var i = 0; i < shortSpan.Length; i++)
+                                shortSpan[i] = BinaryPrimitives.ReverseEndianness(shortSpan[i]);
+#endif
+                        }
+
+                        hash.Append(MemoryMarshal.AsBytes(charSpan));
                     }
 
                     // Switch this to ImmutableCollectionsMarshal.AsImmutableArray(hash.GetHashAndReset()) when we move to S.C.I v8.