45 lines
2.0 KiB
Markdown
45 lines
2.0 KiB
Markdown
# Constant intrinsic
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Use the [`IsConstantExpression`](xref:Unity.Burst.CompilerServices.Constant.IsConstantExpression*) intrinsic to check if a given expression is constant at compile-time:
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```c#
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using static Unity.Burst.CompilerServices.Constant;
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var somethingWhichWillBeConstantFolded = math.pow(42.0f, 42.0f);
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if (IsConstantExpression(somethingWhichWillBeConstantFolded))
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{
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// Burst knows that somethingWhichWillBeConstantFolded is a compile-time constant
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}
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```
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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:
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```c#
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using static Unity.Burst.CompilerServices.Constant;
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public static float MyAwesomePow(float f, int i)
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{
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if (IsConstantExpression(i) && (2 == i))
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{
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return f * f;
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}
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else
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{
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return math.pow(f, (float)i);
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}
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}
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```
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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 efficient simple multiply instead.
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The result of `IsConstantExpression` intentionally depends on the result of the optimizations being run. Therefore the result can change based on whether a function gets inlined or not. For example, in the previous case: `IsConstantExpression(i)` is false on its own, because `i` is a function
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argument which is not constant. However, if `MyAwesomePow` gets inlined with a constant value for `i`, then it evaluates true. If `MyAwesomePow` ends up not being inlined, then `IsConstantExpression(i)` remains false.
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>[!NOTE]
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> Constant folding only takes place during optimizations. If you've disabled optimizations, the intrinsic returns false.
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## Additional resources
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* [`[IsConstantExpression]` API reference](xref:Unity.Burst.CompilerServices.Constant.IsConstantExpression*)
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* [Burst intrinsics overview](csharp-burst-intrinsics.md) |