# NoAlias attribute Use the [`[NoAlias]`](xref:Unity.Burst.NoAliasAttribute) attribute to give Burst additional information on the aliasing of pointers and structs. In most use cases, you won't need to use the `[NoAlias]` attribute. You don't need to use it with [`[NativeContainer]`](https://docs.unity3d.com/ScriptReference/Unity.Collections.LowLevel.Unsafe.NativeContainerAttribute.html) attributed structs, or with fields in job structs. This is because the Burst compiler infers the no-alias information. The `[NoAlias]` attribute is exposed so that you can construct complex data structures where Burst can't infer the aliasing. If you use the `[NoAlias]` attribute on a pointer that could alias with another, it might result in undefined behavior and make it hard to track down bugs. You can use this attribute in the following ways: * On a function parameter it signifies that the parameter doesn't alias with any other parameter to the function. * On a struct field it signifies that the field doesn't alias with any other field of the struct. * On a struct it signifies that the address of the struct can't appear within the struct itself. * On a function return value it signifies that the returned pointer doesn't alias with any other pointer returned from the same function. ## NoAlias function parameter The following is an example of aliasing: ```c# int Foo(ref int a, ref int b) { b = 13; a = 42; return b; } ``` For this, Burst produces the following assembly: ```x86asm mov dword ptr [rdx], 13 mov dword ptr [rcx], 42 mov eax, dword ptr [rdx] ret ``` This means that Burst does the following: * Stores 13 into `b`. * Stores 42 into `a`. * Reloads the value from `b` to return it. Burst has to reload `b` because it doesn't know whether `a` and `b` are backed by the same memory or not. Add the `[NoAlias]` attribute to the code to change this: ```c# int Foo([NoAlias] ref int a, ref int b) { b = 13; a = 42; return b; } ``` For this, Burst produces the following assembly: ```x86asm mov dword ptr [rdx], 13 mov dword ptr [rcx], 42 mov eax, 13 ret ``` In this case, the load from `b` has been replaced with moving the constant 13 into the return register. ## NoAlias struct field The following example is the same as the previous, but applied to a struct: ```c# struct Bar { public NativeArray a; public NativeArray b; } int Foo(ref Bar bar) { bar.b[0] = 42.0f; bar.a[0] = 13; return (int)bar.b[0]; } ``` For this, Burst produces the following assembly: ```x86asm mov rax, qword ptr [rcx + 16] mov dword ptr [rax], 1109917696 mov rcx, qword ptr [rcx] mov dword ptr [rcx], 13 cvttss2si eax, dword ptr [rax] ret ``` In this case, Burst does the following: * Loads the address of the data in `b` into `rax`. * Stores 42 into it (`1109917696` is `0x42280000`, which is `42.0f`). * Loads the address of the data in `a` into `rcx`. * Stores 13 into it. * Reloads the data in `b` and converts it to an integer for returning. If you know that the two `NativeArrays` aren't backed by the same memory, you can change the code to the following: ```c# struct Bar { [NoAlias] public NativeArray a; [NoAlias] public NativeArray b; } int Foo(ref Bar bar) { bar.b[0] = 42.0f; bar.a[0] = 13; return (int)bar.b[0]; } ``` If you attribute both `a` and `b` with `[NoAlias]` it tells Burst that they don't alias with each other within the struct, which produces the following assembly: ```x86asm mov rax, qword ptr [rcx + 16] mov dword ptr [rax], 1109917696 mov rax, qword ptr [rcx] mov dword ptr [rax], 13 mov eax, 42 ret ``` This means that Burst can return the integer constant 42. ## NoAlias struct Burst assumes that the pointer to a struct doesn't appear within the struct itself. However, there are cases where this isn't true: ```c# unsafe struct CircularList { public CircularList* next; public CircularList() { // The 'empty' list just points to itself. next = this; } } ``` Lists are one of the few structures where it's normal to have the pointer to the struct accessible from somewhere within the struct itself. The following example indicates where `[NoAlias]` on a struct can help: ```c# unsafe struct Bar { public int i; public void* p; } float Foo(ref Bar bar) { *(int*)bar.p = 42; return ((float*)bar.p)[bar.i]; } ``` This produces the following assembly: ```x86asm mov rax, qword ptr [rcx + 8] mov dword ptr [rax], 42 mov rax, qword ptr [rcx + 8] mov ecx, dword ptr [rcx] movss xmm0, dword ptr [rax + 4*rcx] ret ``` In this case, Burst: * Loads `p` into `rax`. * Stores 42 into `p`. * Loads `p` into `rax` again. * Loads `i` into `ecx`. * Returns the index into `p` by `i`. In this situation, Burst loads `p` twice. This is because it doesn't know if `p` points to the address of the struct `bar`. Once it stores 42 into `p` it has to reload the address of `p` from `bar`, which is a costly operation. Add `[NoAlias]` to prevent this: ```c# [NoAlias] unsafe struct Bar { public int i; public void* p; } float Foo(ref Bar bar) { *(int*)bar.p = 42; return ((float*)bar.p)[bar.i]; } ``` This produces the following assembly: ```x86asm mov rax, qword ptr [rcx + 8] mov dword ptr [rax], 42 mov ecx, dword ptr [rcx] movss xmm0, dword ptr [rax + 4*rcx] ret ``` In this situation, Burst only loads the address of `p` once, because `[NoAlias]` tells it that `p` can't be the pointer to `bar`. ## NoAlias function return Some functions can only return a unique pointer. For instance, `malloc` only returns a unique pointer. In this case, `[return:NoAlias]` gives some useful information to Burst. >[!IMPORTANT] >Only use `[return: NoAlias]` on functions that are guaranteed to produce a unique pointer. For example, with bump-allocations, or with things like `malloc`. Burst aggressively inlines functions for performance considerations, so with small functions, Burst inlines them into their parents to produce the same result without the attribute. The following example uses a bump allocator backed with a stack allocation: ```c# // Only ever returns a unique address into the stackalloc'ed memory. // We've made this no-inline because Burst will always try and inline // small functions like these, which would defeat the purpose of this // example [MethodImpl(MethodImplOptions.NoInlining)] unsafe int* BumpAlloc(int* alloca) { int location = alloca[0]++; return alloca + location; } unsafe int Func() { int* alloca = stackalloc int[128]; // Store our size at the start of the alloca. alloca[0] = 1; int* ptr1 = BumpAlloc(alloca); int* ptr2 = BumpAlloc(alloca); *ptr1 = 42; *ptr2 = 13; return *ptr1; } ``` This produces the following assembly: ```x86asm push rsi push rdi push rbx sub rsp, 544 lea rcx, [rsp + 36] movabs rax, offset memset mov r8d, 508 xor edx, edx call rax mov dword ptr [rsp + 32], 1 movabs rbx, offset "BumpAlloc(int* alloca)" lea rsi, [rsp + 32] mov rcx, rsi call rbx mov rdi, rax mov rcx, rsi call rbx mov dword ptr [rdi], 42 mov dword ptr [rax], 13 mov eax, dword ptr [rdi] add rsp, 544 pop rbx pop rdi pop rsi ret ``` The key things that Burst does: * Has `ptr1` in `rdi`. * Has `ptr2` in `rax`. * Stores 42 into `ptr1`. * Stores 13 into `ptr2`. * Loads `ptr1` again to return it. If you add the `[return: NoAlias]` attribute: ```c# [MethodImpl(MethodImplOptions.NoInlining)] [return: NoAlias] unsafe int* BumpAlloc(int* alloca) { int location = alloca[0]++; return alloca + location; } unsafe int Func() { int* alloca = stackalloc int[128]; // Store our size at the start of the alloca. alloca[0] = 1; int* ptr1 = BumpAlloc(alloca); int* ptr2 = BumpAlloc(alloca); *ptr1 = 42; *ptr2 = 13; return *ptr1; } ``` It produces the following assembly: ```x86asm push rsi push rdi push rbx sub rsp, 544 lea rcx, [rsp + 36] movabs rax, offset memset mov r8d, 508 xor edx, edx call rax mov dword ptr [rsp + 32], 1 movabs rbx, offset "BumpAlloc(int* alloca)" lea rsi, [rsp + 32] mov rcx, rsi call rbx mov rdi, rax mov rcx, rsi call rbx mov dword ptr [rdi], 42 mov dword ptr [rax], 13 mov eax, 42 add rsp, 544 pop rbx pop rdi pop rsi ret ``` In this case, Burst doesn't reload `ptr2`, and moves 42 into the return register.