The IL2CPP scripting backend converts IL code from scripts and assemblies in a Unity project to C++ code which is then compiled using platform native compilers. For the most part, the scripting backend should not matter. However, IL2CPP does provide a few useful options which can be controlled.
When using the IL2CPP scripting backend, it is possible to control how il2cpp.exe generates C++ code. Specifically, C# attributes can be used to enable or disable the following runtime checks:
| Option | Description | Default |
|---|---|---|
| Null checks | If this option is enabled, C++ code generated by IL2CPP will contain null checks and throw managed NullReferenceException exceptions as necessary. If this option is disabled, the null checks will not be emitted into the generated C++ code. For some projects, disabling this option may improve runtime performance. However, any access to null values in the generated code will not be protected, and may lead to incorrect behavior. Usually the game will crash soon after the null value is dereferenced, but we cannot guarantee this. Disable this option with caution. | Enabled |
| Array bounds checks | If this option is enabled, C++ code generated by IL2CPP will contain array bounds checks and throw managed IndexOutOfRangeException exceptions as necessary. If this option is disabled, the array bounds checks will not be emitted into the generated C++ code. For some projects, disabling this option may improve runtime performance. However, any access to an array with invalid indices in the generated code will not be protected, and may lead to incorrect behavior, including reading from or writing to arbitrary memory locations. In most cases, these memory accesses will occur without any immediate side effects, and may silently corrupt the state of the game. Disable this option with extreme caution. | Enabled |
| Divide by zero checks | If this option is enabled, C++ code generated by IL2CPP will contain divide by zero checks for integer division and throw managed DivideByZeroException exceptions as necessary. If this option is disabled, the divide by zero checks on integer division will not be emitted into the generated C++ code. For most projects this option should be disabled. Enable it only if divide by zero checks are required, as these checks have a runtime cost. | Disable |
The runtime checks can be enabled or disabled in C# code using the Il2CppSetOptions attribute. To use this attribute, find the Il2CppSetOptionsAttribute.cs source file in the IL2CPP directory in the Unity Editor installation (Data\il2cpp on Windows, Contents/Frameworks/il2cpp on OS X). Copy this source file into the Assets folder in your project. Then the attribute can be used like this:
[Il2CppSetOption(Option.NullChecks, false)]
public static string MethodWithNullChecksDisabled()
{
var tmp = new object();
return tmp.ToString();
}
The Il2CppSetOptions attribute can be applied to types, methods, and properties. The attribute from the most local scope will be used.
[Il2CppSetOption(Option.NullChecks, false)]
public class TypeWithNullChecksDisabled
{
public static string AnyMethod()
{
// Null checks will be disabled in this method.
var tmp = new object();
return tmp.ToString();
}
[Il2CppSetOption(Option.NullChecks, true)]
public static string MethodWithNullChecksEnabled()
{
// Null checks will be enabled in this method.
var tmp = new object();
return tmp.ToString();
}
}
public class SomeType
{
[Il2CppSetOption(Option.NullChecks, false)]
public string PropertyWithNullChecksDisabled
{
get
{
// Null checks will be disabled here.
var tmp = new object();
return tmp.ToString();
}
set
{
// Null checks will be disabled here.
value.ToString();
}
}
public string PropertyWithNullChecksDisabledOnGetterOnly
{
[Il2CppSetOption(Option.NullChecks, false)]
get
{
// Null checks will be disabled here.
var tmp = new object();
return tmp.ToString();
}
set
{
// Null checks will be enabled here.
value.ToString();
}
}
}
The C++ code generated by the IL2CPP scripting backend can be updated incrementally, allowing incremental C++ build systems to compile only the changes source files. This can signifigantly lower iteration times with the IL2CPP scripting backend. Incremental builds with IL2CPP are supported on the iOS platform only now.
To use incremental builds, choose the “Append” option after selecting “Build” from the “Build Settings” dialog. The “Replace” option will perform a clean build.