BurstCompile attribute

To improve the performance of Burst, you can change how it behaves when it compiles a job with the [BurstCompile] attribute. Use it do the following:

  • Use a different accuracy for math functions (for example, sin, cos).
  • Relax the order of math computations so that Burst can rearrange the floating point calculations.
  • Force a synchronous compilation of a job (only for just-in-time compilation).

For example, you can use the [BurstCompile] attribute to change the floating precision and float mode of Burst like so:

[BurstCompile(FloatPrecision.Med, FloatMode.Fast)]

FloatPrecision

Use the FloatPrecision enumeration to define Burst's floating precision accuracy.

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:

  • FloatPrecision.Standard: Default value, which is the same as FloatPrecision.Medium. This provides an accuracy of 3.5 ulp.
  • FloatPrecision.High: Provides an accuracy of 1.0 ulp.
  • FloatPrecision.Medium: Provides an accuracy of 3.5 ulp.
  • FloatPrecision.Low: Has an accuracy defined per function, and functions might specify a restricted range of valid inputs.

Note: In previous versions of the Burst API, the FloatPrecision enum was named Accuracy.

FloatPrecision.Low

If you use the FloatPrecision.Low mode, the following functions have a precision of 350.0 ulp. All other functions inherit the ulp from FloatPrecision.Medium.

  • Unity.Mathematics.math.sin(x)
  • Unity.Mathematics.math.cos(x)
  • Unity.Mathematics.math.exp(x)
  • Unity.Mathematics.math.exp2(x)
  • Unity.Mathematics.math.exp10(x)
  • Unity.Mathematics.math.log(x)
  • Unity.Mathematics.math.log2(x)
  • Unity.Mathematics.math.log10(x)
  • Unity.Mathematics.math.pow(x, y)
    • Negative x to the power of a fractional y aren't supported.
  • Unity.Mathematics.math.fmod(x, y)

FloatMode

Use the FloatMode enumeration to define Burst's floating point math mode. It provides the following modes:

  • FloatMode.Default: Defaults to FloatMode.Strict mode.
  • 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.
  • FloatMode.Fast: Burst can perform instruction re-arrangement and use dedicated or less precise hardware SIMD instructions.
  • FloatMode.Deterministic: Unsupported. Deterministic mode is reserved for a future iteration of Burst.

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.

Use FloatMode.Fast for scenarios where the exact order of the calculation and the uniform handling of NaN values aren't required.