37 lines
1.3 KiB
Markdown
37 lines
1.3 KiB
Markdown
# 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. |