358 lines
8.4 KiB
Markdown
358 lines
8.4 KiB
Markdown
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# NoAlias attribute
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Use the [`[NoAlias]`](xref:Unity.Burst.NoAliasAttribute) attribute to give Burst additional information on the aliasing of pointers and structs.
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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.
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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.
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You can use this attribute in the following ways:
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* On a function parameter it signifies that the parameter doesn't alias with any other parameter to the function.
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* On a struct field it signifies that the field doesn't alias with any other field of the struct.
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* On a struct it signifies that the address of the struct can't appear within the struct itself.
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* On a function return value it signifies that the returned pointer doesn't alias with any other pointer returned from the same function.
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## NoAlias function parameter
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The following is an example of aliasing:
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```c#
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int Foo(ref int a, ref int b)
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{
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b = 13;
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a = 42;
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return b;
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}
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```
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For this, Burst produces the following assembly:
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```x86asm
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mov dword ptr [rdx], 13
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mov dword ptr [rcx], 42
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mov eax, dword ptr [rdx]
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ret
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```
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This means that Burst does the following:
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* Stores 13 into `b`.
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* Stores 42 into `a`.
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* Reloads the value from `b` to return it.
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Burst has to reload `b` because it doesn't know whether `a` and `b` are backed by the same memory or not.
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Add the `[NoAlias]` attribute to the code to change this:
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```c#
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int Foo([NoAlias] ref int a, ref int b)
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{
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b = 13;
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a = 42;
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return b;
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}
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```
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For this, Burst produces the following assembly:
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```x86asm
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mov dword ptr [rdx], 13
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mov dword ptr [rcx], 42
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mov eax, 13
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ret
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```
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In this case, the load from `b` has been replaced with moving the constant 13 into the return register.
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## NoAlias struct field
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The following example is the same as the previous, but applied to a struct:
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```c#
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struct Bar
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{
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public NativeArray<int> a;
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public NativeArray<float> b;
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}
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int Foo(ref Bar bar)
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{
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bar.b[0] = 42.0f;
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bar.a[0] = 13;
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return (int)bar.b[0];
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}
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```
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For this, Burst produces the following assembly:
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```x86asm
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mov rax, qword ptr [rcx + 16]
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mov dword ptr [rax], 1109917696
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mov rcx, qword ptr [rcx]
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mov dword ptr [rcx], 13
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cvttss2si eax, dword ptr [rax]
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ret
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```
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In this case, Burst does the following:
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* Loads the address of the data in `b` into `rax`.
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* Stores 42 into it (`1109917696` is `0x42280000`, which is `42.0f`).
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* Loads the address of the data in `a` into `rcx`.
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* Stores 13 into it.
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* Reloads the data in `b` and converts it to an integer for returning.
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If you know that the two `NativeArrays` aren't backed by the same memory, you can change the code to the following:
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```c#
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struct Bar
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{
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[NoAlias]
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public NativeArray<int> a;
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[NoAlias]
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public NativeArray<float> b;
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}
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int Foo(ref Bar bar)
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{
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bar.b[0] = 42.0f;
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bar.a[0] = 13;
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return (int)bar.b[0];
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}
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```
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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:
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```x86asm
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mov rax, qword ptr [rcx + 16]
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mov dword ptr [rax], 1109917696
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mov rax, qword ptr [rcx]
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mov dword ptr [rax], 13
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mov eax, 42
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ret
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```
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This means that Burst can return the integer constant 42.
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## NoAlias struct
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Burst assumes that the pointer to a struct doesn't appear within the struct itself. However, there are cases where this isn't true:
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```c#
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unsafe struct CircularList
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{
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public CircularList* next;
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public CircularList()
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{
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// The 'empty' list just points to itself.
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next = this;
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}
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}
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```
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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.
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The following example indicates where `[NoAlias]` on a struct can help:
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```c#
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unsafe struct Bar
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{
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public int i;
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public void* p;
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}
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float Foo(ref Bar bar)
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{
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*(int*)bar.p = 42;
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return ((float*)bar.p)[bar.i];
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}
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```
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This produces the following assembly:
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```x86asm
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mov rax, qword ptr [rcx + 8]
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mov dword ptr [rax], 42
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mov rax, qword ptr [rcx + 8]
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mov ecx, dword ptr [rcx]
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movss xmm0, dword ptr [rax + 4*rcx]
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ret
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```
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In this case, Burst:
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* Loads `p` into `rax`.
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* Stores 42 into `p`.
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* Loads `p` into `rax` again.
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* Loads `i` into `ecx`.
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* Returns the index into `p` by `i`.
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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.
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Add `[NoAlias]` to prevent this:
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```c#
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[NoAlias]
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unsafe struct Bar
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{
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public int i;
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public void* p;
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}
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float Foo(ref Bar bar)
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{
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*(int*)bar.p = 42;
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return ((float*)bar.p)[bar.i];
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}
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```
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This produces the following assembly:
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```x86asm
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mov rax, qword ptr [rcx + 8]
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mov dword ptr [rax], 42
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mov ecx, dword ptr [rcx]
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movss xmm0, dword ptr [rax + 4*rcx]
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ret
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```
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In this situation, Burst only loads the address of `p` once, because `[NoAlias]` tells it that `p` can't be the pointer to `bar`.
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## NoAlias function return
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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.
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>[!IMPORTANT]
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>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.
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The following example uses a bump allocator backed with a stack allocation:
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```c#
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// Only ever returns a unique address into the stackalloc'ed memory.
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// We've made this no-inline because Burst will always try and inline
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// small functions like these, which would defeat the purpose of this
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// example
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[MethodImpl(MethodImplOptions.NoInlining)]
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unsafe int* BumpAlloc(int* alloca)
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{
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int location = alloca[0]++;
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return alloca + location;
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}
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unsafe int Func()
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{
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int* alloca = stackalloc int[128];
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// Store our size at the start of the alloca.
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alloca[0] = 1;
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int* ptr1 = BumpAlloc(alloca);
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int* ptr2 = BumpAlloc(alloca);
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*ptr1 = 42;
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*ptr2 = 13;
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return *ptr1;
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}
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```
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This produces the following assembly:
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```x86asm
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push rsi
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push rdi
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push rbx
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sub rsp, 544
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lea rcx, [rsp + 36]
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movabs rax, offset memset
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mov r8d, 508
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xor edx, edx
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call rax
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mov dword ptr [rsp + 32], 1
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movabs rbx, offset "BumpAlloc(int* alloca)"
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lea rsi, [rsp + 32]
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mov rcx, rsi
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call rbx
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mov rdi, rax
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mov rcx, rsi
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call rbx
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mov dword ptr [rdi], 42
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mov dword ptr [rax], 13
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mov eax, dword ptr [rdi]
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add rsp, 544
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pop rbx
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pop rdi
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pop rsi
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ret
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```
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The key things that Burst does:
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* Has `ptr1` in `rdi`.
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* Has `ptr2` in `rax`.
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* Stores 42 into `ptr1`.
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* Stores 13 into `ptr2`.
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* Loads `ptr1` again to return it.
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If you add the `[return: NoAlias]` attribute:
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```c#
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[MethodImpl(MethodImplOptions.NoInlining)]
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[return: NoAlias]
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unsafe int* BumpAlloc(int* alloca)
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{
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int location = alloca[0]++;
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return alloca + location;
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}
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unsafe int Func()
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{
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int* alloca = stackalloc int[128];
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// Store our size at the start of the alloca.
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alloca[0] = 1;
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int* ptr1 = BumpAlloc(alloca);
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int* ptr2 = BumpAlloc(alloca);
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*ptr1 = 42;
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*ptr2 = 13;
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return *ptr1;
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}
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```
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It produces the following assembly:
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```x86asm
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push rsi
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push rdi
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push rbx
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sub rsp, 544
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lea rcx, [rsp + 36]
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movabs rax, offset memset
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mov r8d, 508
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xor edx, edx
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call rax
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mov dword ptr [rsp + 32], 1
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movabs rbx, offset "BumpAlloc(int* alloca)"
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lea rsi, [rsp + 32]
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mov rcx, rsi
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call rbx
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mov rdi, rax
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mov rcx, rsi
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call rbx
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mov dword ptr [rdi], 42
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mov dword ptr [rax], 13
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mov eax, 42
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add rsp, 544
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pop rbx
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pop rdi
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pop rsi
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ret
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```
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In this case, Burst doesn't reload `ptr2`, and moves 42 into the return register.
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