A programmable execution unit of a graphics processor that executes program instructions to perform graphics shading operations can use at least two different register file mapping configurations for mapping registers to execution threads.When a shader program is to be executed, how the shader program will use the registers is considered and the register file mapping configuration to use for the shader program is then selected based on the assessment of the register use by the shader program.Appropriate state information is then set to cause the threads being executed by the programmable execution unit to use the registers according to the selected register file mapping configuration when executing the shader program.