Use multi-reg load/store for EncodeToUtf8 (dotnet#95513)

* Use multi-reg load/store for EncodeToUtf8

* Use the fixed version of multi-reg store

* Update variable naming

src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Encoder.cs

@@ -85,6 +85,15 @@ public static unsafe OperationStatus EncodeToUtf8(ReadOnlySpan<byte> bytes, Span
                             goto DoneExit;
                     }
 
+                    end = srcMax - 48;
+                    if (AdvSimd.Arm64.IsSupported && (end >= src))
+                    {
+                        AdvSimdEncode(ref src, ref dest, end, maxSrcLength, destLength, srcBytes, destBytes);
+
+                        if (src == srcEnd)
+                            goto DoneExit;
+                    }
+
                     end = srcMax - 16;
                     if ((Ssse3.IsSupported || AdvSimd.Arm64.IsSupported) && BitConverter.IsLittleEndian && (end >= src))
                     {
@@ -480,6 +489,64 @@ private static unsafe void Avx2Encode(ref byte* srcBytes, ref byte* destBytes, b
             destBytes = dest;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        [CompExactlyDependsOn(typeof(AdvSimd.Arm64))]
+        private static unsafe void AdvSimdEncode(ref byte* srcBytes, ref byte* destBytes, byte* srcEnd, int sourceLength, int destLength, byte* srcStart, byte* destStart)
+        {
+            // C# implementatino of https://github.com/aklomp/base64/blob/3a5add8652076612a8407627a42c768736a4263f/lib/arch/neon64/enc_loop.c
+            Vector128<byte> str1;
+            Vector128<byte> str2;
+            Vector128<byte> str3;
+            Vector128<byte> res1;
+            Vector128<byte> res2;
+            Vector128<byte> res3;
+            Vector128<byte> res4;
+            Vector128<byte> tblEnc1 = Vector128.Create("ABCDEFGHIJKLMNOP"u8).AsByte();
+            Vector128<byte> tblEnc2 = Vector128.Create("QRSTUVWXYZabcdef"u8).AsByte();
+            Vector128<byte> tblEnc3 = Vector128.Create("ghijklmnopqrstuv"u8).AsByte();
+            Vector128<byte> tblEnc4 = Vector128.Create("wxyz0123456789+/"u8).AsByte();
+            byte* src = srcBytes;
+            byte* dest = destBytes;
+
+            // If we have Neon support, pick off 48 bytes at a time for as long as we can.
+            do
+            {
+                // Load 48 bytes and deinterleave:
+                AssertRead<Vector128<byte>>(src, srcStart, sourceLength);
+                (str1, str2, str3) = AdvSimd.Arm64.LoadVector128x3AndUnzip(src);
+
+                // Divide bits of three input bytes over four output bytes:
+                res1 = AdvSimd.ShiftRightLogical(str1, 2);
+                res2 = AdvSimd.ShiftRightLogical(str2, 4);
+                res3 = AdvSimd.ShiftRightLogical(str3, 6);
+                res2 = AdvSimd.ShiftLeftAndInsert(res2, str1, 4);
+                res3 = AdvSimd.ShiftLeftAndInsert(res3, str2, 2);
+
+                // Clear top two bits:
+                res2 &= AdvSimd.DuplicateToVector128((byte)0x3F);
+                res3 &= AdvSimd.DuplicateToVector128((byte)0x3F);
+                res4 = str3 & AdvSimd.DuplicateToVector128((byte)0x3F);
+
+                // The bits have now been shifted to the right locations;
+                // translate their values 0..63 to the Base64 alphabet.
+                // Use a 64-byte table lookup:
+                res1 = AdvSimd.Arm64.VectorTableLookup((tblEnc1, tblEnc2, tblEnc3, tblEnc4), res1);
+                res2 = AdvSimd.Arm64.VectorTableLookup((tblEnc1, tblEnc2, tblEnc3, tblEnc4), res2);
+                res3 = AdvSimd.Arm64.VectorTableLookup((tblEnc1, tblEnc2, tblEnc3, tblEnc4), res3);
+                res4 = AdvSimd.Arm64.VectorTableLookup((tblEnc1, tblEnc2, tblEnc3, tblEnc4), res4);
+
+                // Interleave and store result:
+                AssertWrite<Vector128<byte>>(dest, destStart, destLength);
+                AdvSimd.Arm64.StoreVector128x4AndZip(dest, (res1, res2, res3, res4));
+
+                src += 48;
+                dest += 64;
+            } while (src <= srcEnd);
+
+            srcBytes = src;
+            destBytes = dest;
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         [CompExactlyDependsOn(typeof(Ssse3))]
         [CompExactlyDependsOn(typeof(AdvSimd.Arm64))]