Fast-track ASCII/UTF8 conversion in wasm

src/libraries/System.Private.CoreLib/src/System/Text/Unicode/Utf8Utility.SizeOpt.cs

@@ -19,8 +19,10 @@ public static OperationStatus TranscodeToUtf16(byte* pInputBuffer, int inputLeng
  Debug.Assert(outputCharsRemaining >= 0, "Destination length must not be negative.");
  Debug.Assert(pOutputBuffer != null || outputCharsRemaining == 0, "Destination length must be zero if destination buffer pointer is null.");
 
- var input = new ReadOnlySpan<byte>(pInputBuffer, inputLength);
- var output = new Span<char>(pOutputBuffer, outputCharsRemaining);
+ // try fast-tracking ASCII first before falling back to the standard loop
+ int numAsciiBytesTranscoded = (int)ASCIIUtility.WidenAsciiToUtf16(pInputBuffer, pOutputBuffer, (uint)Math.Min(inputLength, outputCharsRemaining));
+ var input = new ReadOnlySpan<byte>(pInputBuffer, inputLength).Slice(numAsciiBytesTranscoded);
+ var output = new Span<char>(pOutputBuffer, outputCharsRemaining).Slice(numAsciiBytesTranscoded);
 
  OperationStatus opStatus = OperationStatus.Done;
  while (!input.IsEmpty)
@@ -49,9 +51,10 @@ public static OperationStatus TranscodeToUtf8(char* pInputBuffer, int inputLengt
  Debug.Assert(outputBytesRemaining >= 0, "Destination length must not be negative.");
  Debug.Assert(pOutputBuffer != null || outputBytesRemaining == 0, "Destination length must be zero if destination buffer pointer is null.");
 
-
- var input = new ReadOnlySpan<char>(pInputBuffer, inputLength);
- var output = new Span<byte>(pOutputBuffer, outputBytesRemaining);
+ // try fast-tracking ASCII first before falling back to the standard loop
+ int numAsciiCharsTranscoded = (int)ASCIIUtility.NarrowUtf16ToAscii(pInputBuffer, pOutputBuffer, (uint)Math.Min(inputLength, outputBytesRemaining));
+ var input = new ReadOnlySpan<char>(pInputBuffer, inputLength).Slice(numAsciiCharsTranscoded);
+ var output = new Span<byte>(pOutputBuffer, outputBytesRemaining).Slice(numAsciiCharsTranscoded);
 
  OperationStatus opStatus = OperationStatus.Done;
  while (!input.IsEmpty)
@@ -86,9 +89,12 @@ public static OperationStatus TranscodeToUtf8(char* pInputBuffer, int inputLengt
  Debug.Assert(inputLength >= 0, "Input length must not be negative.");
  Debug.Assert(pInputBuffer != null || inputLength == 0, "Input length must be zero if input buffer pointer is null.");
 
- var input = new ReadOnlySpan<byte>(pInputBuffer, inputLength);
- int cumulativeUtf16CodeUnitCount = 0;
- int cumulativeScalarValueCount = 0;
+ // try fast-tracking ASCII first before falling back to the standard loop
+ int numLeadingAsciiBytes = (int)ASCIIUtility.GetIndexOfFirstNonAsciiByte(pInputBuffer, (uint)inputLength);
+ var input = new ReadOnlySpan<byte>(pInputBuffer, inputLength).Slice(numLeadingAsciiBytes);
+ int cumulativeUtf16CodeUnitCount = numLeadingAsciiBytes;
+ int cumulativeScalarValueCount = numLeadingAsciiBytes;
+
  while (!input.IsEmpty)
  {
  if (Rune.DecodeFromUtf8(input, out Rune rune, out int bytesConsumed) != OperationStatus.Done)