Optimized string.Replace(char, char)

src/libraries/Common/tests/Tests/System/StringTests.cs

@@ -4697,14 +4697,18 @@ public static void Remove_Invalid()
         [InlineData("Aaaaaaaa", 'A', 'a', "aaaaaaaa")] // Single iteration of vectorised path; no remainders through non-vectorised path
         // Three leading 'a's before a match (copyLength > 0), Single iteration of vectorised path; no remainders through non-vectorised path
         [InlineData("aaaAaaaaaaa", 'A', 'a', "aaaaaaaaaaa")]
-        // Single iteration of vectorised path; 3 remainders through non-vectorised path
+        // Single iteration of vectorised path; 3 remainders handled by vectorized path
         [InlineData("AaaaaaaaaAa", 'A', 'a', "aaaaaaaaaaa")]
+        // Eight chars before a match (copyLength > 0), single iteration of vectorized path for the remainder
+        [InlineData("12345678AAAAAAA", 'A', 'a', "12345678aaaaaaa")]
         // ------------------------- For Vector<ushort>.Count == 16 (AVX2) -------------------------
         [InlineData("AaaaaaaaAaaaaaaa", 'A', 'a', "aaaaaaaaaaaaaaaa")] // Single iteration of vectorised path; no remainders through non-vectorised path
         // Three leading 'a's before a match (copyLength > 0), Single iteration of vectorised path; no remainders through non-vectorised path
         [InlineData("aaaAaaaaaaaAaaaaaaa", 'A', 'a', "aaaaaaaaaaaaaaaaaaa")]
-        // Single iteration of vectorised path; 3 remainders through non-vectorised path
+        // Single iteration of vectorised path; 3 remainders handled by vectorized path
         [InlineData("AaaaaaaaAaaaaaaaaAa", 'A', 'a', "aaaaaaaaaaaaaaaaaaa")]
+        // Sixteen chars before a match (copyLength > 0), single iteration of vectorized path for the remainder
+        [InlineData("1234567890123456AAAAAAAAAAAAAAA", 'A', 'a', "1234567890123456aaaaaaaaaaaaaaa")]
         // ----------------------------------- General test data -----------------------------------
         [InlineData("Hello", 'l', '!', "He!!o")] // 2 match, non-vectorised path
         [InlineData("Hello", 'e', 'e', "Hello")] // oldChar and newChar are same; nothing to replace

src/libraries/System.Private.CoreLib/src/System/Numerics/Vector.cs

@@ -895,6 +895,14 @@ public static bool LessThanOrEqualAll<T>(Vector<T> left, Vector<T> right)
         public static bool LessThanOrEqualAny<T>(Vector<T> left, Vector<T> right)
             where T : struct => LessThanOrEqual(left, right).As<T, nuint>() != Vector<nuint>.Zero;
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static Vector<T> LoadUnsafe<T>(ref T source, nint elementOffset)
+            where T : struct
+        {
+            source = ref Unsafe.Add(ref source, elementOffset);
+            return Unsafe.ReadUnaligned<Vector<T>>(ref Unsafe.As<T, byte>(ref source));
+        }
+
         /// <summary>Computes the maximum of two vectors on a per-element basis.</summary>
         /// <param name="left">The vector to compare with <paramref name="right" />.</param>
         /// <param name="right">The vector to compare with <paramref name="left" />.</param>
@@ -1658,6 +1666,14 @@ public static Vector<T> SquareRoot<T>(Vector<T> value)
             return result;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static void StoreUnsafe<T>(this Vector<T> source, ref T destination, nint elementOffset)
+            where T : struct
+        {
+            destination = ref Unsafe.Add(ref destination, elementOffset);
+            Unsafe.WriteUnaligned(ref Unsafe.As<T, byte>(ref destination), source);
+        }
+
         /// <summary>Subtracts two vectors to compute their difference.</summary>
         /// <param name="left">The vector from which <paramref name="right" /> will be subtracted.</param>
         /// <param name="right">The vector to subtract from <paramref name="left" />.</param>

src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

@@ -994,7 +994,7 @@ public string Replace(char oldChar, char newChar)
             if (firstIndex < 0)
                 return this;
 
-            int remainingLength = Length - firstIndex;
+            nint remainingLength = (nint)(uint)(Length - firstIndex);
             string result = FastAllocateString(Length);
 
             int copyLength = firstIndex;
@@ -1006,35 +1006,56 @@ public string Replace(char oldChar, char newChar)
             }
 
             // Copy the remaining characters, doing the replacement as we go.
-            ref ushort pSrc = ref Unsafe.Add(ref Unsafe.As<char, ushort>(ref _firstChar), copyLength);
-            ref ushort pDst = ref Unsafe.Add(ref Unsafe.As<char, ushort>(ref result._firstChar), copyLength);
+            ref ushort pSrc = ref Unsafe.Add(ref GetRawStringDataAsUshort(), (nint)(uint)copyLength);
+            ref ushort pDst = ref Unsafe.Add(ref result.GetRawStringDataAsUshort(), (nint)(uint)copyLength);
+            nint i = 0;
 
-            if (Vector.IsHardwareAccelerated && remainingLength >= Vector<ushort>.Count)
+            if (Vector.IsHardwareAccelerated && Length >= Vector<ushort>.Count)
             {
-                Vector<ushort> oldChars = new Vector<ushort>(oldChar);
-                Vector<ushort> newChars = new Vector<ushort>(newChar);
+                Vector<ushort> oldChars = new(oldChar);
+                Vector<ushort> newChars = new(newChar);
 
-                do
+                Vector<ushort> original;
+                Vector<ushort> equals;
+                Vector<ushort> results;
+
+                nint lengthToExamine = remainingLength - Vector<ushort>.Count;
+
+                if (lengthToExamine > 0)
                 {
-                    Vector<ushort> original = Unsafe.ReadUnaligned<Vector<ushort>>(ref Unsafe.As<ushort, byte>(ref pSrc));
-                    Vector<ushort> equals = Vector.Equals(original, oldChars);
-                    Vector<ushort> results = Vector.ConditionalSelect(equals, newChars, original);
-                    Unsafe.WriteUnaligned(ref Unsafe.As<ushort, byte>(ref pDst), results);
-
-                    pSrc = ref Unsafe.Add(ref pSrc, Vector<ushort>.Count);
-                    pDst = ref Unsafe.Add(ref pDst, Vector<ushort>.Count);
-                    remainingLength -= Vector<ushort>.Count;
+                    do
+                    {
+                        original = Vector.LoadUnsafe(ref pSrc, i);
+                        equals = Vector.Equals(original, oldChars);
+                        results = Vector.ConditionalSelect(equals, newChars, original);
+                        results.StoreUnsafe(ref pDst, i);
+
+                        i += Vector<ushort>.Count;
+                    }
+                    while (i < lengthToExamine);
                 }
-                while (remainingLength >= Vector<ushort>.Count);
-            }
 
-            for (; remainingLength > 0; remainingLength--)
-            {
-                ushort currentChar = pSrc;
-                pDst = currentChar == oldChar ? newChar : currentChar;
+                // There are [0, Vector<ushort>.Count) elements remaining now.
+                // As the operation is idempotent, and we know that in total there are at least Vector<ushort>.Count
+                // elements available, we read a vector from the very end of the string, perform the replace
+                // and write to the destination at the very end.
+                // Thus we can eliminate the scalar processing of the remaining elements.
+                // We perform this operation even if there are 0 elements remaining, as it is cheaper than the
+                // additional check which would introduce a branch here.
 
-                pSrc = ref Unsafe.Add(ref pSrc, 1);
-                pDst = ref Unsafe.Add(ref pDst, 1);
+                i = (nint)(uint)Length - Vector<ushort>.Count;
+                original = Vector.LoadUnsafe(ref GetRawStringDataAsUshort(), i);
+                equals = Vector.Equals(original, oldChars);
+                results = Vector.ConditionalSelect(equals, newChars, original);
+                results.StoreUnsafe(ref result.GetRawStringDataAsUshort(), i);
+            }
+            else
+            {
+                for (; i < remainingLength; ++i)
+                {
+                    ushort currentChar = Unsafe.Add(ref pSrc, i);
+                    Unsafe.Add(ref pDst, i) = currentChar == oldChar ? newChar : currentChar;
+                }
             }
 
             return result;

src/libraries/System.Private.CoreLib/src/System/String.cs

@@ -508,6 +508,7 @@ public static bool IsNullOrWhiteSpace([NotNullWhen(false)] string? value)
         public ref readonly char GetPinnableReference() => ref _firstChar;
 
         internal ref char GetRawStringData() => ref _firstChar;
+        internal ref ushort GetRawStringDataAsUshort() => ref Unsafe.As<char, ushort>(ref _firstChar);
 
         // Helper for encodings so they can talk to our buffer directly
         // stringLength must be the exact size we'll expect