56 lines
3.2 KiB
Markdown
56 lines
3.2 KiB
Markdown
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# BurstCompile attribute
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To improve the performance of Burst, you can change how it behaves when it compiles a job with the [`[BurstCompile]`](xref:Unity.Burst.BurstCompileAttribute) attribute. Use it do the following:
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* Use a different accuracy for math functions (for example, sin, cos).
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* Relax the order of math computations so that Burst can rearrange the floating point calculations.
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* Force a synchronous compilation of a job (only for [just-in-time compilation](compilation.md)).
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For example, you can use the `[BurstCompile]` attribute to change the [floating precision](xref:Unity.Burst.FloatPrecision) and [float mode](xref:Unity.Burst.FloatMode) of Burst like so:
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[BurstCompile(FloatPrecision.Med, FloatMode.Fast)]
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## FloatPrecision
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Use the [`FloatPrecision`](xref:Unity.Burst.FloatPrecision) enumeration to define Burst's floating precision accuracy.
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Float precision is measured in ulp (unit in the last place or unit of least precision). This is the space between floating-point numbers: the value the least significant digit represents if it's 1. `Unity.Burst.FloatPrecision` provides the following accuracy:
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* `FloatPrecision.Standard`: Default value, which is the same as `FloatPrecision.Medium`. This provides an accuracy of 3.5 ulp.
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* `FloatPrecision.High`: Provides an accuracy of 1.0 ulp.
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* `FloatPrecision.Medium`: Provides an accuracy of 3.5 ulp.
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* `FloatPrecision.Low`: Has an accuracy defined per function, and functions might specify a restricted range of valid inputs.
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**Note:** In previous versions of the Burst API, the `FloatPrecision` enum was named `Accuracy`.
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### FloatPrecision.Low
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If you use the [`FloatPrecision.Low`](xref:Unity.Burst.FloatPrecision) mode, the following functions have a precision of 350.0 ulp. All other functions inherit the ulp from `FloatPrecision.Medium`.
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* `Unity.Mathematics.math.sin(x)`
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* `Unity.Mathematics.math.cos(x)`
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* `Unity.Mathematics.math.exp(x)`
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* `Unity.Mathematics.math.exp2(x)`
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* `Unity.Mathematics.math.exp10(x)`
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* `Unity.Mathematics.math.log(x)`
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* `Unity.Mathematics.math.log2(x)`
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* `Unity.Mathematics.math.log10(x)`
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* `Unity.Mathematics.math.pow(x, y)`
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* Negative `x` to the power of a fractional `y` aren't supported.
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* `Unity.Mathematics.math.fmod(x, y)`
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## FloatMode
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Use the [`FloatMode`](xref:Unity.Burst.FloatMode) enumeration to define Burst's floating point math mode. It provides the following modes:
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* `FloatMode.Default`: Defaults to `FloatMode.Strict` mode.
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* `FloatMode.Strict`: Burst doesn't perform any re-arrangement of the calculation and respects special floating point values (such as denormals, NaN). This is the default value.
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* `FloatMode.Fast`: Burst can perform instruction re-arrangement and use dedicated or less precise hardware SIMD instructions.
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* `FloatMode.Deterministic`: Unsupported. Deterministic mode is reserved for a future iteration of Burst.
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For hardware that can support Multiply and Add (e.g mad `a * b + c`) into a single instruction, you can use `FloatMode.Fast` to enable this optimization. However, the reordering of these instructions might lead to a lower accuracy.
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Use `FloatMode.Fast` for scenarios where the exact order of the calculation and the uniform handling of NaN values aren't required.
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