StringBuilder: use Span.Fill in Append repeating char (#86287)

src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs

@@ -659,36 +659,57 @@ public StringBuilder Append(char value, int repeatCount)
                 return this;
             }
 
-            // this is where we can check if the repeatCount will put us over m_MaxCapacity
-            // We are doing the check here to prevent the corruption of the StringBuilder.
-            int newLength = Length + repeatCount;
-            if (newLength > m_MaxCapacity || newLength < repeatCount)
+            char[] chunkChars = m_ChunkChars;
+            int chunkLength = m_ChunkLength;
+
+            // Try to fit the whole repeatCount in the current chunk
+            // Use the same check as Span<T>.Slice for 64-bit so it can be folded
+            // Since repeatCount can't be negative, there's no risk for it to overflow on 32 bit
+            if (((nuint)(uint)chunkLength + (nuint)(uint)repeatCount) <= (nuint)(uint)chunkChars.Length)
             {
-                throw new ArgumentOutOfRangeException(nameof(repeatCount), SR.ArgumentOutOfRange_LengthGreaterThanCapacity);
+                chunkChars.AsSpan(chunkLength, repeatCount).Fill(value);
+                m_ChunkLength += repeatCount;
             }
-
-            int index = m_ChunkLength;
-            while (repeatCount > 0)
+            else
             {
-                if (index < m_ChunkChars.Length)
-                {
-                    m_ChunkChars[index++] = value;
-                    --repeatCount;
-                }
-                else
-                {
-                    m_ChunkLength = index;
-                    ExpandByABlock(repeatCount);
-                    Debug.Assert(m_ChunkLength == 0);
-                    index = 0;
-                }
+                AppendWithExpansion(value, repeatCount);
             }
 
-            m_ChunkLength = index;
             AssertInvariants();
             return this;
         }
 
+        private void AppendWithExpansion(char value, int repeatCount)
+        {
+            Debug.Assert(repeatCount > 0, "Invalid length; should have been validated by caller.");
+
+            // Check if the repeatCount will put us over m_MaxCapacity
+            if ((uint)(repeatCount + Length) > (uint)m_MaxCapacity)
+            {
+                throw new ArgumentOutOfRangeException(nameof(repeatCount), SR.ArgumentOutOfRange_LengthGreaterThanCapacity);
+            }
+
+            char[] chunkChars = m_ChunkChars;
+            int chunkLength = m_ChunkLength;
+
+            // Fill the rest of the current chunk
+            int firstLength = chunkChars.Length - chunkLength;
+            if (firstLength > 0)
+            {
+                chunkChars.AsSpan(chunkLength, firstLength).Fill(value);
+                m_ChunkLength = chunkChars.Length;
+            }
+
+            // Expand the builder to add another chunk
+            int restLength = repeatCount - firstLength;
+            ExpandByABlock(restLength);
+            Debug.Assert(m_ChunkLength == 0, "A new block was not created.");
+
+            // Fill the new chunk with the remaining part of repeatCount
+            m_ChunkChars.AsSpan(0, restLength).Fill(value);
+            m_ChunkLength = restLength;
+        }
+
         /// <summary>
         /// Appends a range of characters to the end of this builder.
         /// </summary>
@@ -990,12 +1011,21 @@ public StringBuilder Append(char value)
             }
             else
             {
-                Append(value, 1);
+                AppendWithExpansion(value);
             }
 
             return this;
         }
 
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void AppendWithExpansion(char value)
+        {
+            ExpandByABlock(1);
+            Debug.Assert(m_ChunkLength == 0, "A new block was not created.");
+            m_ChunkChars[0] = value;
+            m_ChunkLength++;
+        }
+
         [CLSCompliant(false)]
         public StringBuilder Append(sbyte value) => AppendSpanFormattable(value);
 

src/libraries/System.Runtime/tests/System/Text/StringBuilderTests.cs

@@ -550,7 +550,7 @@ public static void Append_Char_NoSpareCapacity_ThrowsArgumentOutOfRangeException
             var builder = new StringBuilder(0, 5);
             builder.Append("Hello");
 
-            AssertExtensions.Throws<ArgumentOutOfRangeException>("repeatCount", "requiredLength", () => builder.Append('a'));
+            AssertExtensions.Throws<ArgumentOutOfRangeException>("requiredLength", () => builder.Append('a'));
             AssertExtensions.Throws<ArgumentOutOfRangeException>("repeatCount", "requiredLength", () => builder.Append('a', 1));
         }