Reduce allocated strings in CodeWriter (#12366)

For nearly all cases, the result of GetIndentString calls can be returned without allocation, but slicing into a string of spaces or tabs. ComputeIndent accounts for 11.0 MB (0.4%) of total allocations and 27 ms in in the ScrollingAndTypingInCohosting speedometer test. With this change, those numbers go to 0.0 MB (0.0%) and 13 ms respectively.

Test insertion: https://dev.azure.com/devdiv/DevDiv/_git/VS/pullrequest/680751

Author: ToddGrun

src/Compiler/Microsoft.AspNetCore.Razor.Language/test/CodeGeneration/IndentCacheTests.cs

@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+using IndentCache = Microsoft.AspNetCore.Razor.Language.CodeGeneration.CodeWriter.IndentCache;
+
+namespace Microsoft.AspNetCore.Razor.Language.Test.CodeGeneration;
+
+public class IndentCacheTest
+{
+    [Theory]
+    [InlineData(0, false, 4, "")]
+    [InlineData(4, false, 4, "    ")]
+    [InlineData(8, false, 4, "        ")]
+    [InlineData(0, true, 4, "")]
+    [InlineData(4, true, 4, "\t")]
+    [InlineData(8, true, 4, "\t\t")]
+    [InlineData(6, true, 4, "\t  ")]
+    [InlineData(5, true, 4, "\t ")]
+    [InlineData(3, true, 4, "   ")]
+    public void GetIndentString_ReturnsExpectedString(int size, bool useTabs, int tabSize, string expected)
+    {
+        var result = IndentCache.GetIndentString(size, useTabs, tabSize);
+        Assert.Equal(expected, result.ToString());
+    }
+
+    [Fact]
+    public void GetIndentString_TabSizeOne_UsesOnlyTabs()
+    {
+        var result = IndentCache.GetIndentString(size: 5, useTabs: true, tabSize: 1);
+        Assert.Equal(new string('\t', 5), result.ToString());
+    }
+
+    [Fact]
+    public void GetIndentString_TabSizeGreaterThanSize_UsesSpaces()
+    {
+        var result = IndentCache.GetIndentString(size: 3, useTabs: true, tabSize: 10);
+        Assert.Equal("   ", result.ToString());
+    }
+
+    [Fact]
+    public void GetIndentString_TabsAndSpacesInResultExceedCachedSizes()
+    {
+        var spaceCount = IndentCache.MaxSpaceCount + 1;
+        var tabCount = IndentCache.MaxTabCount + 1;
+        var tabSize = spaceCount + 1;
+
+        var size = tabSize * tabCount + spaceCount;
+        var result = IndentCache.GetIndentString(size, useTabs: true, tabSize);
+
+        var expected = new string('\t', tabCount) + new string(' ', spaceCount);
+        Assert.Equal(expected, result.ToString());
+    }
+}

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/CodeGeneration/CodeWriter.IndentCache.cs

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+
+namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration;
+
+public sealed partial class CodeWriter
+{
+    internal static class IndentCache
+    {
+        internal const int MaxTabCount = 64;
+        internal const int MaxSpaceCount = 128;
+
+        private static readonly ReadOnlyMemory<char> s_tabs = new string('\t', MaxTabCount).AsMemory();
+        private static readonly ReadOnlyMemory<char> s_spaces = new string(' ', MaxSpaceCount).AsMemory();
+
+        public static ReadOnlyMemory<char> GetIndentString(int size, bool useTabs, int tabSize)
+        {
+            if (!useTabs)
+            {
+                return SliceOrCreate(size, s_spaces);
+            }
+
+            var tabCount = size / tabSize;
+            var spaceCount = size % tabSize;
+
+            if (spaceCount == 0)
+            {
+                return SliceOrCreate(tabCount, s_tabs);
+            }
+
+            return string.Create(length: tabCount + spaceCount, state: tabCount, static (destination, tabCount) =>
+            {
+                destination[..tabCount].Fill('\t');
+                destination[tabCount..].Fill(' ');
+            }).AsMemory();
+        }
+
+        private static ReadOnlyMemory<char> SliceOrCreate(int length, ReadOnlyMemory<char> chars)
+        {
+            return (length <= chars.Length)
+                ? chars[..length]
+                : new string(chars.Span[0], length).AsMemory();
+        }
+    }
+}

src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/CodeGeneration/CodeWriter.cs

@@ -9,7 +9,6 @@
 using System.IO;
 using System.Runtime.CompilerServices;
 using System.Text;
-using Microsoft.AspNetCore.Razor.PooledObjects;
 using Microsoft.CodeAnalysis.Text;
 
 namespace Microsoft.AspNetCore.Razor.Language.CodeGeneration;
@@ -118,7 +117,7 @@ public int CurrentIndent
             if (_indentSize != value)
             {
                 _indentSize = value;
-                _indentString = ComputeIndent(value, IndentWithTabs, TabSize);
+                _indentString = IndentCache.GetIndentString(value, IndentWithTabs, TabSize);
             }
         }
     }
@@ -189,7 +188,7 @@ public CodeWriter Indent(int size)
 
         var indentString = size == _indentSize
             ? _indentString
-            : ComputeIndent(size, IndentWithTabs, TabSize);
+            : IndentCache.GetIndentString(size, IndentWithTabs, TabSize);
 
         AddTextChunk(indentString);
 
@@ -200,30 +199,6 @@ public CodeWriter Indent(int size)
         return this;
     }
 
-    private static ReadOnlyMemory<char> ComputeIndent(int size, bool useTabs, int tabSize)
-    {
-        if (size == 0)
-        {
-            return ReadOnlyMemory<char>.Empty;
-        }
-
-        if (useTabs)
-        {
-            var tabCount = size / tabSize;
-            var spaceCount = size % tabSize;
-
-            using var _ = StringBuilderPool.GetPooledObject(out var builder);
-            builder.SetCapacityIfLarger(tabCount + spaceCount);
-
-            builder.Append('\t', tabCount);
-            builder.Append(' ', spaceCount);
-
-            return builder.ToString().AsMemory();
-        }
-
-        return new string(' ', size).AsMemory();
-    }
-
     public CodeWriter Write(string value)
     {
         ArgHelper.ThrowIfNull(value);