Use stackalloc in TypeMapHelper to reduce allocations

src/Xamarin.Android.Build.Tasks/Utilities/TypeMapHelper.cs

@@ -17,7 +17,7 @@ public static ulong HashJavaName (string name, bool is64Bit)
 
 		// Native code (EmbeddedAssemblies::typemap_java_to_managed in embedded-assemblies.cc) will operate on wchar_t cast to a byte array, we need to do
 		// the same
-		return HashBytes (Encoding.Unicode.GetBytes (name), is64Bit);
+		return HashString (name, Encoding.Unicode, is64Bit);
 	}
 
 	/// <summary>
@@ -29,10 +29,27 @@ public static ulong HashJavaNameForCLR (string name, bool is64Bit)
 			return UInt64.MaxValue;
 		}
 
-		return HashBytes (Encoding.UTF8.GetBytes (name), is64Bit);
+		return HashString (name, Encoding.UTF8, is64Bit);
 	}
 
-	static ulong HashBytes (byte[] bytes, bool is64Bit)
+	// Java type names are always ASCII and typically 20-100 characters,
+	// so the encoded byte count is well within stackalloc limits.
+	// The unsafe Encoding.GetBytes(char*, int, byte*, int) overload is
+	// used because the Span-based overload requires netstandard2.1+.
+	static unsafe ulong HashString (string name, Encoding encoding, bool is64Bit)
+	{
+		int byteCount = encoding.GetByteCount (name);
+		Span<byte> buffer = byteCount <= 256
+			? stackalloc byte [byteCount]
+			: new byte [byteCount];
+		fixed (char* pChars = name)
+		fixed (byte* pBuffer = buffer) {
+			encoding.GetBytes (pChars, name.Length, pBuffer, byteCount);
+		}
+		return HashBytes (buffer, is64Bit);
+	}
+
+	static ulong HashBytes (ReadOnlySpan<byte> bytes, bool is64Bit)
 	{
 		if (is64Bit) {
 			return XxHash3.HashToUInt64 (bytes);

tools/tmt/tmt.csproj

@@ -9,6 +9,7 @@
     <OutputPath>../../bin/$(Configuration)/bin/typemap-tool</OutputPath>
     <OutputType>Exe</OutputType>
     <LibZipSharpBundleAllNativeLibraries>true</LibZipSharpBundleAllNativeLibraries>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <Nullable>enable</Nullable>
     <RollForward>Major</RollForward>
   </PropertyGroup>