You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.

37 lines
1.3 KiB

2 years ago
# Constant intrinsic
Use the [`IsConstantExpression`](xref:Unity.Burst.CompilerServices.Constant.IsConstantExpression*) intrinsic to check if a given expression is constant at compile-time:
```c#
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:
```c#
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.