Singularity/Library/PackageCache/com.unity.burst@1.8.4/Documentation~/optimization-constant.md

Constant intrinsic

Use the IsConstantExpression intrinsic to check if a given expression is constant at compile-time:

using static Unity.Burst.CompilerServices.Constant;

var somethingWhichWillBeConstantFolded = math.pow(42.0f, 42.0f);

if (IsConstantExpression(somethingWhichWillBeConstantFolded))
{
    // Burst knows that somethingWhichWillBeConstantFolded is a compile-time constant
}

This is useful to check if a complex expression is always constant folded. You can use it for optimizations for a known constant value. For example, if you want to implement a pow-like function for integer powers:

using static Unity.Burst.CompilerServices.Constant;

public static float MyAwesomePow(float f, int i)
{
    if (IsConstantExpression(i) && (2 == i))
    {
        return f * f;
    }
    else
    {
        return math.pow(f, (float)i);
    }
}

The IsConstantExpression check means that Burst always removes the branch if i isn't constant, because the if condition is false. This means that if i is constant and is equal to 2, you can use a more optimal simple multiply instead.

Note

Constant folding only takes place during optimizations. If you've disabled optimizations, the intrinsic returns false.