Avoiding heap allocations in generic serialization code

After.cs

// #NOTE: be mindful of how large T actually is and whether or not your runtime does or doesn't
// optimize passing large structs by reference instead of copying bits to other parts of the stack
public static T Read<T>(this BinaryReader reader, T item = default(T))
	where T : IStreamSerializable, new()
{
	if (item == null) // value types are never null, so this helps us avoid heap allocations
		item = new T();

	item.Load(reader);
	return item;
}

After.il

.method public hidebysig static 
	!!T Read<.ctor (IStreamSerializable) T> (
		class [mscorlib]System.IO.BinaryReader reader,
		[opt] !!T item
	) cil managed 
{
	.custom instance void [System.Core]System.Runtime.CompilerServices.ExtensionAttribute::.ctor() = (
		01 00 00 00
	)
	// Method begins at RVA 0x1d9f0
	// Code size 64 (0x40)
	.maxstack 5
	.locals init (
		[0] !!T
	)

	IL_0000: ldarg.1
	IL_0001: box !!T
	IL_0006: brtrue IL_0030

	IL_000b: ldloca.s 0
	IL_000d: initobj !!T
	IL_0013: ldloc.0
	IL_0014: box !!T
	IL_0019: brfalse IL_0029

	IL_001e: ldloca.s 0
	IL_0020: initobj !!T
	IL_0026: ldloc.0
	IL_0027: br.s IL_002e

	IL_0029: call !!0 [mscorlib]System.Activator::CreateInstance<!!T>()

	IL_002e: starg.s item

	IL_0030: ldarga.s item
	IL_0032: ldarg.0
	IL_0033: constrained. !!T
	IL_0039: callvirt instance void IStreamSerializable::Load(class [mscorlib]System.IO.BinaryReader)
	IL_003e: ldarg.1
	IL_003f: ret
} // end of method SerializationExtensions::Read

After.il.decompiled.cs

public static T Read<T>(this BinaryReader reader, T item = null) where T : IStreamSerializable, new()
{
	if (item == null)
	{
		item = ((default(T) == null) ? Activator.CreateInstance<T>() : default(T));
	}
	item.Load(reader);
	return item;
}

Before.cs

public static T Read<T>(this BinaryReader reader)
	where T : IStreamSerializable, new()
{
	T item = new T(); // when T is a value type, this causes a heap allocation in Unity!
	item.Load(reader);
	return item;
}

BeforeExcerptFromProfilerReport.txt

**** Type: MY_STRUCT // names have been changed to protect the innocent
	Unique backtraces 22
	Avg Alloc 238.500000
	Avg Free 2.000000
	Avg Garbage 0.838574
	Frame Count 2
	Allocs 477
	Frees 4
	Skipped Alloc 0
	Skipped Free 0
		91 (%19.077568) // number of allocations from this specific backtrace
		Sample address 000000002620D0F0
			(wrapper managed-to-native) object:__icall_wrapper_mono_object_new_specific
			(wrapper managed-to-native) object:__icall_wrapper_mono_object_new_specific
			SerializationExtensions:Read<MY_STRUCT>	C:\[REDACTED]\Serialization.cs(330)
			[REDACTED...]
			(wrapper runtime-invoke) object:runtime_invoke_void__this__