This disclosure describes techniques for executing shader programs in a graphics processing unit (GPU). In some examples, the techniques for executing shader programs may include executing, with a shader unit of a graphics processor, a shader program that performs vertex shader processing and that generates multiple output vertices for each input vertex that is received by the shader program. In further examples, the techniques for executing shader programs may include executing a merged vertex/geometry shader program using a non-replicated mode of execution. The non-replicated mode of execution may involve assigning each of a plurality of primitives to one merged vertex/geometry shader program instance per primitive and causing each of the instances to output a plurality of vertices. In additional examples, the techniques for executing shader programs may include techniques for selecting one of a non-replicated mode and a replicated mode for executing a merged vertex/geometry shader program.