65 lines
1.8 KiB
Markdown
65 lines
1.8 KiB
Markdown
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# AssumeRange attribute
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Use the [`AssumeRange`](xref:Unity.Burst.CompilerServices.AssumeRangeAttribute) attribute to tell Burst that a given scalar-integer lies within a certain constrained range. If Burst has this information, it can improve the performance of your application. The following code is an example of this:
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```c#
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[return:AssumeRange(0u, 13u)]
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static uint WithConstrainedRange([AssumeRange(0, 26)] int x)
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{
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return (uint)x / 2u;
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}
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```
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This example tells Burst the following:
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* The variable `x` is in the closed-interval range `[0..26]`, or more plainly that `x >= 0 && x <= 26`.
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* The return value from `WithConstrainedRange` is in the closed-interval range `[0..13]`, or more plainly that `x >= 0 && x <= 13`.
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Burst uses these assumptions to create better code generation. However, there are some restrictions:
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* You can only place these on scalar-integer (signed or unsigned) types.
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* The type of the range arguments must match the type being attributed.
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Burst has deductions for the `.Length` property of `NativeArray` and `NativeSlice` which indicates that these always return non-negative integers:
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```c#
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static bool IsLengthNegative(NativeArray<float> na)
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{
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// Burst always replaces this with the constant false
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return na.Length < 0;
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}
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```
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For example, if you have a container like the following:
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```c#
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struct MyContainer
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{
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public int Length;
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// Some other data...
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}
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```
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The following example shows how to tell Burst that `Length` is always a positive integer:
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```c#
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struct MyContainer
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{
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private int _length;
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[return: AssumeRange(0, int.MaxValue)]
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private int LengthGetter()
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{
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return _length;
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}
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public int Length
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{
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get => LengthGetter();
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set => _length = value;
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}
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// Some other data...
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}
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```
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