362 lines
8.5 KiB
Markdown
362 lines
8.5 KiB
Markdown
|
# 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 apply it to a struct definition that already has a [`[NativeContainer]`](xref:Unity.Collections.LowLevel.Unsafe.NativeContainerAttribute) attribute or to fields in job structs because in these cases Burst 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 `[NoAlias]` 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<int> a;
|
||
|
|
public NativeArray<float> 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<int> a;
|
||
|
|
|
||
|
|
[NoAlias]
|
||
|
|
public NativeArray<float> 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.
|
||
|
|
|
||
|
|
## Additional resources
|
||
|
|
|
||
|
|
* [Memory aliasing](aliasing.md)
|
||
|
|
* [Aliasing and the job system](aliasing-job-system.md)
|