Introduction to static and dynamic branching

Static branching

When a shaderA program that runs on the GPU. More info
See in Glossary program includes conditionals that are evaluated at compile time, it uses static branching. The compiler excludes the code from the unused branch, so it does not appear in the compiled shader program.

Internally, Unity uses static branching when it creates shader variants. Static branching on its own does not have any of the performance disadvantages of shader variants.

Advantages and disadvantages of static branching

The main advantage of static branching is that it has no negative impact on runtime performance. The main disadvantage of static branching is that you can only use it at compile time.

Static branching means that the compiler excludes unneeded code from the shader program. It results in small, specialized shader programs that contain only the necessary code. There is no runtime performance cost to static branching; in fact, the smaller programs are likely to result in quicker load times and lower runtime memory usage.

To use static branching, the conditions must be constant at compile time. This means that you can’t use it to execute code for different conditions at runtime.

Dynamic branching

When a shader program includes conditionals that are evaluated at runtime, it uses dynamic branching.

There are two types of dynamic branching: dynamic branching based on uniform variables, and dynamic branching based on any other runtime value. Uniform-based branching is usually more efficient, because the uniform value is constant for the whole draw call.

You can use shader keywords for dynamic branching. This allows you to use C# scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary and the Material InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
See in Glossary to configure runtime branching behavior for your shaders. This results is uniform-based branching; when you do this, Unity compiles the shader keywords as uniforms.

Advantages and disadvantages of dynamic branching

The main advantage of dynamic branching is that it allows you to use conditionals at runtime without increasing the number of shader variants in your project. The main disadvantage of dynamic branching is that it impacts GPU performance.

The GPU performance impact varies by hardware, and by shader code. The reasons are:

• Branching based on non-uniform variables means that the GPU must either perform different operations at the same time (and therefore break parallelism), or “flatten the branch” and maintain parallelism by performing the operations for both branches and then discarding one result. Branching based on uniform variables means that the GPU must flatten the branch. Both of these approaches result in reduced GPU performance.
• For any type of dynamic branching, the GPU must allocate register space for the worst case. If one branch is much more costly than the other, this means that the GPU wastes register space. This can lead to fewer invocations of the shader program in parallel, which reduces performance.

In general, if your code branches on uniform values and both branches have roughly similar workloads, then the impact on GPU performance is likely to be small. However, you should always profile your application and consider the advantages and disadvantages case-by-case.

Note: Dynamic branching can also lead to large shader programs, because the code for all conditions is compiled into the same shader program. However, the effect of these larger files on load times and memory usage is usually less significant than the impact of shader variants.

For information about other ways of using conditionals in shaders, and how to decide which technique is right for your use case, see Conditionals in shaders.