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What is shader branching?
Since the GPU relies on many parallel calculations being executable at once, branches force the GPU to waste time executing the same shader code multiple times for a single vertex/fragment, instead of for other vertices/fragments.
Are IF statements bad in shaders?
Unless… it doesn’t. For example, if the condition is one that is taken by every invocation in the wavefront, then no runtime divergence is needed. As such, the cost of the if is just the cost of checking a condition.
What are compute shaders used for?
A Compute Shader is a Shader Stage that is used entirely for computing arbitrary information. While it can do rendering, it is generally used for tasks not directly related to drawing triangles and pixels.
What are different types of shaders?
There are three types of shaders in common use (pixel, vertex, and geometry shaders), with several more recently added. While older graphics cards utilize separate processing units for each shader type, newer cards feature unified shaders which are capable of executing any type of shader.
Do Gpus have branch predictions?
1 Predication. The simplest approach to implementing branching on the GPU is predication, as discussed earlier. With predication, the GPU effectively evaluates both sides of the branch and then discards one of the results, based on the value of the Boolean branch condition.
When is a branch in a shader is free?
If the condition is uniform (i.e. constant for the entire pass), then the branch is essentially free because the framework will essentially compile two versions of the shader (branch taken and not) and choose one of these for the entire pass based on your input variable.
When to use if statement in shader stack?
In this case, definitely go for the if statement as it will make your shader faster. If the condition varies per vertex/pixel, then it can indeed degrade performance and older shader models don’t even support dynamic branching.
Why does branch divergence slow down the shader?
Branch divergence in warp makes all parts of the branch execute in sequence which slows down the shader. Branching divergence between warps does not affect runtime. Branch execution on gpus is surprisingly deep, and there are many edge cases I didn’t address and I also didn’t touch gpu vendors other than Nvidia.
How many branches should a shader have per warp?
Here’s another shader which should have roughly four branches per warp. This shader will take at least 4n cycles. However, let’s look a second shader: This shader should run roughly twice as fast as the other shader even though roughly the same number of pixels take each branch.