[SRM] Optimize MetadataBuilder.GetOrAddConstantBlob.

src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/BlobWriterImpl.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Buffers.Binary;
 using System.Diagnostics;
 
 namespace System.Reflection.Metadata
@@ -12,6 +13,7 @@ internal static class BlobWriterImpl
         internal const int MaxCompressedIntegerValue = 0x1fffffff;
         internal const int MinSignedCompressedIntegerValue = unchecked((int)0xF0000000);
         internal const int MaxSignedCompressedIntegerValue = 0x0FFFFFFF;
+        internal const int MaxScalarConstantSize = sizeof(ulong);
 
         internal static int GetCompressedIntegerSize(int value)
         {
@@ -144,17 +146,120 @@ internal static void WriteCompressedSignedInteger(BlobBuilder writer, int value)
             }
         }
 
+        /// <summary>
+        /// Writes a scalar (non-string) constant to a span.
+        /// </summary>
+        /// <param name="bytes">The span where the content will be encoded.</param>
+        /// <param name="value">The constant value.</param>
+        /// <returns>The number of bytes that was written.</returns>
+        internal static int WriteScalarConstant(Span<byte> bytes, object? value)
+        {
+            if (value == null)
+            {
+                // The encoding of Type for the nullref value for FieldInit is ELEMENT_TYPE_CLASS with a Value of a 4-byte zero.
+                BinaryPrimitives.WriteUInt32LittleEndian(bytes, 0);
+                return sizeof(uint);
+            }
+
+            var type = value.GetType();
+            if (type.IsEnum)
+            {
+                type = Enum.GetUnderlyingType(type);
+            }
+
+            if (type == typeof(bool))
+            {
+                bytes[0] = (byte)((bool)value ? 1 : 0);
+                return sizeof(bool);
+            }
+            else if (type == typeof(int))
+            {
+                BinaryPrimitives.WriteInt32LittleEndian(bytes, (int)value);
+                return sizeof(int);
+            }
+            else if (type == typeof(byte))
+            {
+                bytes[0] = (byte)value;
+                return sizeof(byte);
+            }
+            else if (type == typeof(char))
+            {
+                BinaryPrimitives.WriteUInt16LittleEndian(bytes, (char)value);
+                return sizeof(char);
+            }
+            else if (type == typeof(double))
+            {
+#if NET
+                BinaryPrimitives.WriteDoubleLittleEndian(bytes, (double)value);
+#else
+                double v = (double)value;
+                unsafe
+                {
+                    BinaryPrimitives.WriteUInt64LittleEndian(bytes, *(ulong*)(&v));
+                }
+#endif
+                return sizeof(double);
+            }
+            else if (type == typeof(short))
+            {
+                BinaryPrimitives.WriteInt16LittleEndian(bytes, (short)value);
+                return sizeof(short);
+            }
+            else if (type == typeof(long))
+            {
+                BinaryPrimitives.WriteInt64LittleEndian(bytes, (long)value);
+                return sizeof(long);
+            }
+            else if (type == typeof(sbyte))
+            {
+                bytes[0] = (byte)(sbyte)value;
+                return sizeof(sbyte);
+            }
+            else if (type == typeof(float))
+            {
+#if NET
+                BinaryPrimitives.WriteSingleLittleEndian(bytes, (float)value);
+#else
+                float v = (float)value;
+                unsafe
+                {
+                    BinaryPrimitives.WriteUInt32LittleEndian(bytes, *(uint*)(&v));
+                }
+#endif
+                return sizeof(float);
+            }
+            else if (type == typeof(ushort))
+            {
+                BinaryPrimitives.WriteUInt16LittleEndian(bytes, (ushort)value);
+                return sizeof(ushort);
+            }
+            else if (type == typeof(uint))
+            {
+                BinaryPrimitives.WriteUInt32LittleEndian(bytes, (uint)value);
+                return sizeof(uint);
+            }
+            else if (type == typeof(ulong))
+            {
+                BinaryPrimitives.WriteUInt64LittleEndian(bytes, (ulong)value);
+                return sizeof(ulong);
+            }
+            else
+            {
+                throw new ArgumentException(SR.Format(SR.InvalidConstantValueOfType, type));
+            }
+        }
+
         internal static void WriteConstant(ref BlobWriter writer, object? value)
         {
             if (value == null)
             {
-                // The encoding of Type for the nullref value for FieldInit is ELEMENT_TYPE_CLASS with a Value of a 32-bit.
+                // The encoding of Type for the nullref value for FieldInit is ELEMENT_TYPE_CLASS with a Value of a 4-byte zero.
                 writer.WriteUInt32(0);
                 return;
             }
 
             var type = value.GetType();
-            if (type.GetTypeInfo().IsEnum)
+            if (type.IsEnum)
             {
                 type = Enum.GetUnderlyingType(type);
             }
@@ -221,13 +326,13 @@ internal static void WriteConstant(BlobBuilder writer, object? value)
         {
             if (value == null)
             {
-                // The encoding of Type for the nullref value for FieldInit is ELEMENT_TYPE_CLASS with a Value of a 32-bit.
+                // The encoding of Type for the nullref value for FieldInit is ELEMENT_TYPE_CLASS with a Value of a 4-byte zero.
                 writer.WriteUInt32(0);
                 return;
             }
 
             var type = value.GetType();
-            if (type.GetTypeInfo().IsEnum)
+            if (type.IsEnum)
             {
                 type = Enum.GetUnderlyingType(type);
             }

src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/Ecma335/MetadataBuilder.Heaps.cs

@@ -256,11 +256,9 @@ public BlobHandle GetOrAddConstantBlob(object? value)
                 return GetOrAddBlobUTF16(str);
             }
 
-            var builder = PooledBlobBuilder.GetInstance();
-            builder.WriteConstant(value);
-            var result = GetOrAddBlob(builder);
-            builder.Free();
-            return result;
+            Span<byte> buffer = stackalloc byte[BlobWriterImpl.MaxScalarConstantSize];
+            int length = BlobWriterImpl.WriteScalarConstant(buffer, value);
+            return GetOrAddBlob(buffer.Slice(0, length));
         }
 
         /// <summary>