Port System.Memory performance improvements to Convert.TryFromBase64Chars (#17033)

* Copy over Base64.DecodeFromUtf8 verbatim

* Don't need the InPlace api

* Change input from Utf8 to Utf16

* isFinalBlock will always be true, so constant fold it through

* I only need a true/false result, so get rid of OperationStatus

* Rename variables to reflect Utf8->Utf16 change

* Update comments and some minor cleanup

* Make Convert use Base64 as a fast-path

* Incorporate PR feedback.

* Delete FromBase64_Decode

* Use a single safe stackalloc

* Rename Base64.cs to Convert.Base64.cs

and make its contents a part of the Convert class.

We should eventually move all the Base64-related
stuff into this new file but not as part of this PR.
We want to be able to diff the implementation change
separately from the code movement.

* Thanks VS for the unasked for indent

3 files changed, 327 insertions, 225 deletions

diff --git a/src/mscorlib/shared/System.Private.CoreLib.Shared.projitems b/src/mscorlib/shared/System.Private.CoreLib.Shared.projitems
index 2bc6464e97..270d06d3c4 100644
--- a/src/mscorlib/shared/System.Private.CoreLib.Shared.projitems
+++ b/src/mscorlib/shared/System.Private.CoreLib.Shared.projitems
@@ -94,6 +94,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Configuration\Assemblies\AssemblyHashAlgorithm.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Configuration\Assemblies\AssemblyVersionCompatibility.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Convert.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\Convert.Base64.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\CurrentSystemTimeZone.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DataMisalignedException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\DateTime.cs" />
diff --git a/src/mscorlib/shared/System/Convert.Base64.cs b/src/mscorlib/shared/System/Convert.Base64.cs
new file mode 100644
index 0000000000..7e2aee31b2
--- /dev/null
+++ b/src/mscorlib/shared/System/Convert.Base64.cs
@@ -0,0 +1,217 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using Internal.Runtime.CompilerServices;
+
+namespace System
+{
+    public static partial class Convert
+    {
+        /// <summary>
+        /// Decode the span of UTF-16 encoded text represented as base 64 into binary data.
+        /// If the input is not a multiple of 4, or contains illegal characters, it will decode as much as it can, to the largest possible multiple of 4.
+        /// This invariant allows continuation of the parse with a slower, whitespace-tolerant algorithm.
+        ///
+        /// <param name="utf16">The input span which contains UTF-16 encoded text in base 64 that needs to be decoded.</param>
+        /// <param name="bytes">The output span which contains the result of the operation, i.e. the decoded binary data.</param>
+        /// <param name="consumed">The number of input bytes consumed during the operation. This can be used to slice the input for subsequent calls, if necessary.</param>
+        /// <param name="written">The number of bytes written into the output span. This can be used to slice the output for subsequent calls, if necessary.</param>
+        /// <returns>Returns:
+        /// - true  - The entire input span was successfully parsed.
+        /// - false - Only a part of the input span was successfully parsed. Failure causes may include embedded or trailing whitespace, 
+        ///           other illegal Base64 characters, trailing characters after an encoding pad ('='), an input span whose length is not divisible by 4
+        ///           or a destination span that's too small. <paramref name="consumed"/> and <paramref name="written"/> are set so that 
+        ///           parsing can continue with a slower whitespace-tolerant algorithm.
+        ///           
+        /// Note: This is a cut down version of the implementation of Base64.DecodeFromUtf8(), modified the accept UTF16 chars and act as a fast-path
+        /// helper for the Convert routines when the input string contains no whitespace.
+        ///           
+        /// </summary> 
+        private static bool TryDecodeFromUtf16(ReadOnlySpan<char> utf16, Span<byte> bytes, out int consumed, out int written)
+        {
+            ref char srcChars = ref MemoryMarshal.GetReference(utf16);
+            ref byte destBytes = ref MemoryMarshal.GetReference(bytes);
+
+            int srcLength = utf16.Length & ~0x3;  // only decode input up to the closest multiple of 4.
+            int destLength = bytes.Length;
+
+            int sourceIndex = 0;
+            int destIndex = 0;
+
+            if (utf16.Length == 0)
+                goto DoneExit;
+
+            ref sbyte decodingMap = ref s_decodingMap[0];
+
+            // Last bytes could have padding characters, so process them separately and treat them as valid.
+            const int skipLastChunk = 4;
+
+            int maxSrcLength;
+            if (destLength >= (srcLength >> 2) * 3)
+            {
+                maxSrcLength = srcLength - skipLastChunk;
+            }
+            else
+            {
+                // This should never overflow since destLength here is less than int.MaxValue / 4 * 3 (i.e. 1610612733)
+                // Therefore, (destLength / 3) * 4 will always be less than 2147483641
+                maxSrcLength = (destLength / 3) * 4;
+            }
+
+            while (sourceIndex < maxSrcLength)
+            {
+                int result = Decode(ref Unsafe.Add(ref srcChars, sourceIndex), ref decodingMap);
+                if (result < 0)
+                    goto InvalidExit;
+                WriteThreeLowOrderBytes(ref Unsafe.Add(ref destBytes, destIndex), result);
+                destIndex += 3;
+                sourceIndex += 4;
+            }
+
+            if (maxSrcLength != srcLength - skipLastChunk)
+                goto InvalidExit;
+
+            // If input is less than 4 bytes, srcLength == sourceIndex == 0
+            // If input is not a multiple of 4, sourceIndex == srcLength != 0
+            if (sourceIndex == srcLength)
+            {
+                goto InvalidExit;
+            }
+
+            int i0 = Unsafe.Add(ref srcChars, srcLength - 4);
+            int i1 = Unsafe.Add(ref srcChars, srcLength - 3);
+            int i2 = Unsafe.Add(ref srcChars, srcLength - 2);
+            int i3 = Unsafe.Add(ref srcChars, srcLength - 1);
+            if (((i0 | i1 | i2 | i3) & 0xffffff00) != 0)
+                goto InvalidExit;
+
+            i0 = Unsafe.Add(ref decodingMap, i0);
+            i1 = Unsafe.Add(ref decodingMap, i1);
+
+            i0 <<= 18;
+            i1 <<= 12;
+
+            i0 |= i1;
+
+            if (i3 != EncodingPad)
+            {
+                i2 = Unsafe.Add(ref decodingMap, i2);
+                i3 = Unsafe.Add(ref decodingMap, i3);
+
+                i2 <<= 6;
+
+                i0 |= i3;
+                i0 |= i2;
+
+                if (i0 < 0)
+                    goto InvalidExit;
+                if (destIndex > destLength - 3)
+                    goto InvalidExit;
+                WriteThreeLowOrderBytes(ref Unsafe.Add(ref destBytes, destIndex), i0);
+                destIndex += 3;
+            }
+            else if (i2 != EncodingPad)
+            {
+                i2 = Unsafe.Add(ref decodingMap, i2);
+
+                i2 <<= 6;
+
+                i0 |= i2;
+
+                if (i0 < 0)
+                    goto InvalidExit;
+                if (destIndex > destLength - 2)
+                    goto InvalidExit;
+                Unsafe.Add(ref destBytes, destIndex) = (byte)(i0 >> 16);
+                Unsafe.Add(ref destBytes, destIndex + 1) = (byte)(i0 >> 8);
+                destIndex += 2;
+            }
+            else
+            {
+                if (i0 < 0)
+                    goto InvalidExit;
+                if (destIndex > destLength - 1)
+                    goto InvalidExit;
+                Unsafe.Add(ref destBytes, destIndex) = (byte)(i0 >> 16);
+                destIndex += 1;
+            }
+
+            sourceIndex += 4;
+
+            if (srcLength != utf16.Length)
+                goto InvalidExit;
+
+        DoneExit:
+            consumed = sourceIndex;
+            written = destIndex;
+            return true;
+
+        InvalidExit:
+            consumed = sourceIndex;
+            written = destIndex;
+            Debug.Assert((consumed % 4) == 0);
+            return false;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static int Decode(ref char encodedChars, ref sbyte decodingMap)
+        {
+            int i0 = encodedChars;
+            int i1 = Unsafe.Add(ref encodedChars, 1);
+            int i2 = Unsafe.Add(ref encodedChars, 2);
+            int i3 = Unsafe.Add(ref encodedChars, 3);
+
+            if (((i0 | i1 | i2 | i3) & 0xffffff00) != 0)
+                return -1; // One or more chars falls outside the 00..ff range. This cannot be a valid Base64 character.
+
+            i0 = Unsafe.Add(ref decodingMap, i0);
+            i1 = Unsafe.Add(ref decodingMap, i1);
+            i2 = Unsafe.Add(ref decodingMap, i2);
+            i3 = Unsafe.Add(ref decodingMap, i3);
+
+            i0 <<= 18;
+            i1 <<= 12;
+            i2 <<= 6;
+
+            i0 |= i3;
+            i1 |= i2;
+
+            i0 |= i1;
+            return i0;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static void WriteThreeLowOrderBytes(ref byte destination, int value)
+        {
+            destination = (byte)(value >> 16);
+            Unsafe.Add(ref destination, 1) = (byte)(value >> 8);
+            Unsafe.Add(ref destination, 2) = (byte)value;
+        }
+
+        // Pre-computing this table using a custom string(s_characters) and GenerateDecodingMapAndVerify (found in tests)
+        private static readonly sbyte[] s_decodingMap = {
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,         //62 is placed at index 43 (for +), 63 at index 47 (for /)
+            52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1,         //52-61 are placed at index 48-57 (for 0-9), 64 at index 61 (for =)
+            -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
+            15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,         //0-25 are placed at index 65-90 (for A-Z)
+            -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+            41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1,         //26-51 are placed at index 97-122 (for a-z)
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,         // Bytes over 122 ('z') are invalid and cannot be decoded
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,         // Hence, padding the map with 255, which indicates invalid input
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+            -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+        };
+
+        private const byte EncodingPad = (byte)'='; // '=', for padding
+    }
+} 
diff --git a/src/mscorlib/shared/System/Convert.cs b/src/mscorlib/shared/System/Convert.cs
index 756bf17fc5..63342ad000 100644
--- a/src/mscorlib/shared/System/Convert.cs
+++ b/src/mscorlib/shared/System/Convert.cs
@@ -96,7 +96,7 @@ namespace System
     // When passed Value.DBNull, the Value.ToXXX() methods all throw an
     // InvalidCastException.
 
-    public static class Convert
+    public static partial class Convert
     {
         //A typeof operation is fairly expensive (does a system call), so we'll cache these here
         //statically.  These are exactly lined up with the TypeCode, eg. ConvertType[TypeCode.Int16]
@@ -2656,44 +2656,124 @@ namespace System
             return TryFromBase64Chars(s.AsSpan(), bytes, out bytesWritten);
         }
 
-        public static unsafe bool TryFromBase64Chars(ReadOnlySpan<char> chars, Span<byte> bytes, out int bytesWritten)
+        public static bool TryFromBase64Chars(ReadOnlySpan<char> chars, Span<byte> bytes, out int bytesWritten)
         {
-            if (chars.Length == 0)
-            {
-                bytesWritten = 0;
-                return true;
-            }
+            // This is actually local to one of the nested blocks but is being declared at the top as we don't want multiple stackallocs
+            // for each iteraton of the loop. 
+            Span<char> tempBuffer = stackalloc char[4];  // Note: The tempBuffer size could be made larger than 4 but the size must be a multiple of 4.
 
-            // We need to get rid of any trailing white spaces.
-            // Otherwise we would be rejecting input such as "abc= ":
-            while (chars.Length > 0)
+            bytesWritten = 0;
+
+            while (chars.Length != 0)
             {
-                char lastChar = chars[chars.Length - 1];
-                if (lastChar != ' ' && lastChar != '\n' && lastChar != '\r' && lastChar != '\t')
+                // Attempt to decode a segment that doesn't contain whitespace.
+                bool complete = TryDecodeFromUtf16(chars, bytes, out int consumedInThisIteration, out int bytesWrittenInThisIteration);
+                bytesWritten += bytesWrittenInThisIteration;
+                if (complete)
+                    return true;
+
+                chars = chars.Slice(consumedInThisIteration);
+                bytes = bytes.Slice(bytesWrittenInThisIteration);
+
+                Debug.Assert(chars.Length != 0); // If TryDecodeFromUtf16() consumed the entire buffer, it could not have returned false.
+                if (chars[0].IsSpace())
                 {
-                    break;
-                }
-                chars = chars.Slice(0, chars.Length - 1);
-            }
+                    // If we got here, the very first character not consumed was a whitespace. We can skip past any consecutive whitespace, then continue decoding.
 
-            fixed (char* charsPtr = &MemoryMarshal.GetReference(chars))
-            {
-                int resultLength = FromBase64_ComputeResultLength(charsPtr, chars.Length);
-                Debug.Assert(resultLength >= 0);
-                if (resultLength > bytes.Length)
+                    int indexOfFirstNonSpace = 1;
+                    for (; ; )
+                    {
+                        if (indexOfFirstNonSpace == chars.Length)
+                            break;
+                        if (!chars[indexOfFirstNonSpace].IsSpace())
+                            break;
+                        indexOfFirstNonSpace++;
+                    }
+
+                    chars = chars.Slice(indexOfFirstNonSpace);
+
+                    if ((bytesWrittenInThisIteration % 3) != 0 && chars.Length != 0)
+                    {
+                        // If we got here, the last successfully decoded block encountered an end-marker, yet we have trailing non-whitespace characters.
+                        // That is not allowed.
+                        bytesWritten = default;
+                        return false;
+                    }
+
+                    // We now loop again to decode the next run of non-space characters. 
+                }
+                else
                 {
-                    bytesWritten = 0;
-                    return false;
+                    Debug.Assert(chars.Length != 0 && !chars[0].IsSpace());
+
+                    // If we got here, it is possible that there is whitespace that occurred in the middle of a 4-byte chunk. That is, we still have
+                    // up to three Base64 characters that were left undecoded by the fast-path helper because they didn't form a complete 4-byte chunk.
+                    // This is hopefully the rare case (multiline-formatted base64 message with a non-space character width that's not a multiple of 4.)
+                    // We'll filter out whitespace and copy the remaining characters into a temporary buffer.
+                    CopyToTempBufferWithoutWhiteSpace(chars, tempBuffer, out int consumedFromChars, out int charsWritten);
+                    if ((charsWritten & 0x3) != 0)
+                    {
+                        // Even after stripping out whitespace, the number of characters is not divisible by 4. This cannot be a legal Base64 string.
+                        bytesWritten = default;
+                        return false;
+                    }
+
+                    tempBuffer = tempBuffer.Slice(0, charsWritten);
+                    if (!TryDecodeFromUtf16(tempBuffer, bytes, out int consumedFromTempBuffer, out int bytesWrittenFromTempBuffer))
+                    {
+                        bytesWritten = default;
+                        return false;
+                    }
+                    bytesWritten += bytesWrittenFromTempBuffer;
+                    chars = chars.Slice(consumedFromChars);
+                    bytes = bytes.Slice(bytesWrittenFromTempBuffer);
+
+                    if ((bytesWrittenFromTempBuffer % 3) != 0)
+                    {
+                        // If we got here, this decode contained one or more padding characters ('='). We can accept trailing whitespace after this
+                        // but nothing else.
+                        for (int i = 0; i < chars.Length; i++)
+                        {
+                            if (!chars[i].IsSpace())
+                            {
+                                bytesWritten = default;
+                                return false;
+                            }
+                        }
+                        return true;
+                    }
+
+                    // We now loop again to decode the next run of non-space characters. 
                 }
+            }
 
-                fixed (byte* bytesPtr = &MemoryMarshal.GetReference(bytes))
+            return true;
+        }
+
+        private static void CopyToTempBufferWithoutWhiteSpace(ReadOnlySpan<char> chars, Span<char> tempBuffer, out int consumed, out int charsWritten)
+        {
+            Debug.Assert(tempBuffer.Length != 0); // We only bound-check after writing a character to the tempBuffer.
+
+            charsWritten = 0;
+            for (int i = 0; i < chars.Length; i++)
+            {
+                char c = chars[i];
+                if (!c.IsSpace())
                 {
-                    bytesWritten = FromBase64_Decode(charsPtr, chars.Length, bytesPtr, bytes.Length);
-                    return true;
+                    tempBuffer[charsWritten++] = c;
+                    if (charsWritten == tempBuffer.Length)
+                    {
+                        consumed = i + 1;
+                        return;
+                    }
                 }
             }
+            consumed = chars.Length;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static bool IsSpace(this char c) => c == ' ' || c == '\t' || c == '\r' || c == '\n';
+
         /// <summary>
         /// Converts the specified range of a Char array, which encodes binary data as Base64 digits, to the equivalent byte array.     
         /// </summary>
@@ -2730,8 +2810,6 @@ namespace System
             }
         }
 
-
-
         /// <summary>
         /// Convert Base64 encoding characters to bytes:
         ///  - Compute result length exactly by actually walking the input;
@@ -2769,11 +2847,10 @@ namespace System
             Byte[] decodedBytes = new Byte[resultLength];
 
             // Convert Base64 chars into bytes:
-            Int32 actualResultLength;
-            fixed (Byte* decodedBytesPtr = decodedBytes)
-                actualResultLength = FromBase64_Decode(inputPtr, inputLength, decodedBytesPtr, resultLength);
+            if (!TryFromBase64Chars(new ReadOnlySpan<char>(inputPtr, inputLength), decodedBytes, out int _))
+                throw new FormatException(SR.Format_BadBase64Char);
 
-            // Note that actualResultLength can differ from resultLength if the caller is modifying the array
+            // Note that the number of bytes written can differ from resultLength if the caller is modifying the array
             // as it is being converted. Silently ignore the failure.
             // Consider throwing exception in an non in-place release.
 
@@ -2781,199 +2858,6 @@ namespace System
             return decodedBytes;
         }
 
-
-        /// <summary>
-        /// Decode characters representing a Base64 encoding into bytes:
-        /// Walk the input. Every time 4 chars are read, convert them to the 3 corresponding output bytes.
-        /// This method is a bit lengthy on purpose. We are trying to avoid jumps to helpers in the loop
-        /// to aid performance.
-        /// </summary>
-        /// <param name="inputPtr">Pointer to first input char</param>
-        /// <param name="inputLength">Number of input chars</param>
-        /// <param name="destPtr">Pointer to location for the first result byte</param>
-        /// <param name="destLength">Max length of the preallocated result buffer</param>
-        /// <returns>If the result buffer was not large enough to write all result bytes, return -1;
-        /// Otherwise return the number of result bytes actually produced.</returns>
-        private static unsafe Int32 FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)
-        {
-            // You may find this method weird to look at. It's written for performance, not aesthetics.
-            // You will find unrolled loops label jumps and bit manipulations.
-
-            const UInt32 intA = (UInt32)'A';
-            const UInt32 inta = (UInt32)'a';
-            const UInt32 int0 = (UInt32)'0';
-            const UInt32 intEq = (UInt32)'=';
-            const UInt32 intPlus = (UInt32)'+';
-            const UInt32 intSlash = (UInt32)'/';
-            const UInt32 intSpace = (UInt32)' ';
-            const UInt32 intTab = (UInt32)'\t';
-            const UInt32 intNLn = (UInt32)'\n';
-            const UInt32 intCRt = (UInt32)'\r';
-            const UInt32 intAtoZ = (UInt32)('Z' - 'A');  // = ('z' - 'a')
-            const UInt32 int0to9 = (UInt32)('9' - '0');
-
-            Char* inputPtr = startInputPtr;
-            Byte* destPtr = startDestPtr;
-
-            // Pointers to the end of input and output:
-            Char* endInputPtr = inputPtr + inputLength;
-            Byte* endDestPtr = destPtr + destLength;
-
-            // Current char code/value:
-            UInt32 currCode;
-
-            // This 4-byte integer will contain the 4 codes of the current 4-char group.
-            // Eeach char codes for 6 bits = 24 bits.
-            // The remaining byte will be FF, we use it as a marker when 4 chars have been processed.            
-            UInt32 currBlockCodes = 0x000000FFu;
-
-            unchecked
-            {
-                while (true)
-                {
-                    // break when done:
-                    if (inputPtr >= endInputPtr)
-                        goto _AllInputConsumed;
-
-                    // Get current char:
-                    currCode = (UInt32)(*inputPtr);
-                    inputPtr++;
-
-                    // Determine current char code:
-
-                    if (currCode - intA <= intAtoZ)
-                        currCode -= intA;
-
-                    else if (currCode - inta <= intAtoZ)
-                        currCode -= (inta - 26u);
-
-                    else if (currCode - int0 <= int0to9)
-                        currCode -= (int0 - 52u);
-
-                    else
-                    {
-                        // Use the slower switch for less common cases:
-                        switch (currCode)
-                        {
-                            // Significant chars:
-                            case intPlus:
-                                currCode = 62u;
-                                break;
-
-                            case intSlash:
-                                currCode = 63u;
-                                break;
-
-                            // Legal no-value chars (we ignore these):
-                            case intCRt:
-                            case intNLn:
-                            case intSpace:
-                            case intTab:
-                                continue;
-
-                            // The equality char is only legal at the end of the input.
-                            // Jump after the loop to make it easier for the JIT register predictor to do a good job for the loop itself:
-                            case intEq:
-                                goto _EqualityCharEncountered;
-
-                            // Other chars are illegal:
-                            default:
-                                throw new FormatException(SR.Format_BadBase64Char);
-                        }
-                    }
-
-                    // Ok, we got the code. Save it:
-                    currBlockCodes = (currBlockCodes << 6) | currCode;
-
-                    // Last bit in currBlockCodes will be on after in shifted right 4 times:
-                    if ((currBlockCodes & 0x80000000u) != 0u)
-                    {
-                        if ((Int32)(endDestPtr - destPtr) < 3)
-                            return -1;
-
-                        *(destPtr) = (Byte)(currBlockCodes >> 16);
-                        *(destPtr + 1) = (Byte)(currBlockCodes >> 8);
-                        *(destPtr + 2) = (Byte)(currBlockCodes);
-                        destPtr += 3;
-
-                        currBlockCodes = 0x000000FFu;
-                    }
-                }
-            }  // unchecked while
-
-        // 'd be nice to have an assert that we never get here, but CS0162: Unreachable code detected.
-        // Debug.Fail("We only leave the above loop by jumping; should never get here.");
-
-        // We jump here out of the loop if we hit an '=':
-        _EqualityCharEncountered:
-
-            Debug.Assert(currCode == intEq);
-
-            // Recall that inputPtr is now one position past where '=' was read.
-            // '=' can only be at the last input pos:
-            if (inputPtr == endInputPtr)
-            {
-                // Code is zero for trailing '=':
-                currBlockCodes <<= 6;
-
-                // The '=' did not complete a 4-group. The input must be bad:
-                if ((currBlockCodes & 0x80000000u) == 0u)
-                    throw new FormatException(SR.Format_BadBase64CharArrayLength);
-
-                if ((int)(endDestPtr - destPtr) < 2)  // Autch! We underestimated the output length!
-                    return -1;
-
-                // We are good, store bytes form this past group. We had a single "=", so we take two bytes:
-                *(destPtr++) = (Byte)(currBlockCodes >> 16);
-                *(destPtr++) = (Byte)(currBlockCodes >> 8);
-
-                currBlockCodes = 0x000000FFu;
-            }
-            else
-            { // '=' can also be at the pre-last position iff the last is also a '=' excluding the white spaces:
-                // We need to get rid of any intermediate white spaces.
-                // Otherwise we would be rejecting input such as "abc= =":
-                while (inputPtr < (endInputPtr - 1))
-                {
-                    Int32 lastChar = *(inputPtr);
-                    if (lastChar != (Int32)' ' && lastChar != (Int32)'\n' && lastChar != (Int32)'\r' && lastChar != (Int32)'\t')
-                        break;
-                    inputPtr++;
-                }
-
-                if (inputPtr == (endInputPtr - 1) && *(inputPtr) == '=')
-                {
-                    // Code is zero for each of the two '=':
-                    currBlockCodes <<= 12;
-
-                    // The '=' did not complete a 4-group. The input must be bad:
-                    if ((currBlockCodes & 0x80000000u) == 0u)
-                        throw new FormatException(SR.Format_BadBase64CharArrayLength);
-
-                    if ((Int32)(endDestPtr - destPtr) < 1)  // Autch! We underestimated the output length!
-                        return -1;
-
-                    // We are good, store bytes form this past group. We had a "==", so we take only one byte:
-                    *(destPtr++) = (Byte)(currBlockCodes >> 16);
-
-                    currBlockCodes = 0x000000FFu;
-                }
-                else  // '=' is not ok at places other than the end:
-                    throw new FormatException(SR.Format_BadBase64Char);
-            }
-
-        // We get here either from above or by jumping out of the loop:
-        _AllInputConsumed:
-
-            // The last block of chars has less than 4 items
-            if (currBlockCodes != 0x000000FFu)
-                throw new FormatException(SR.Format_BadBase64CharArrayLength);
-
-            // Return how many bytes were actually recovered:
-            return (Int32)(destPtr - startDestPtr);
-        } // Int32 FromBase64_Decode(...)
-
-
         /// <summary>
         /// Compute the number of bytes encoded in the specified Base 64 char array:
         /// Walk the entire input counting white spaces and padding chars, then compute result length