Apparatus and method for tracing activities of a shader program executed on shader circuitry of a data processing apparatus

A method of generating execution path information when executing a shader program on shader circuitry, the method comprising the steps of:

• responsive to input data for a pixel, executing the shader program to generate a colour value for said pixel, the shader program having multiple execution paths via which the colour value may be generated, and which execution path of said plurality is taken being dependent on said input data;
• storing the colour value generated by said shader circuitry in an image buffer;
• in a trace mode of operation, the execution of said shader program by said shader circuitry causing a trace vector to be generated containing a plurality of items of execution path information indicative of the execution path taken, the trace vector comprising a plurality of fields, each field being used to store one item of execution path information;
• the trace vector as output being of the same size as the colour value, and in said trace mode of operation the method further comprising the steps of storing said trace vector in a storage location of the image buffer in place of the colour value generated by the shader program.

A graphics processing unit comprising:

• shader circuitry, responsive to input data for a pixel, to execute a shader program to generate a colour value for said pixel, the shader program having multiple execution paths via which the colour value may be generated, and which execution path of said plurality is taken being dependent on said input data;
• the shader circuitry being arranged to output the colour value for storage in an image buffer having a plurality of storage locations, each storage location for storing the colour value generated by said shader circuitry for an associated pixel;
• in a trace mode of operation, execution of said shader program by said shader circuitry causing a trace vector to be generated containing a plurality of items of execution path information indicative of the execution path taken, the trace vector comprising a plurality of fields, each field being used to store one item of execution path information;
• the trace vector as output being of the same size as the colour value, and in said trace mode of operation said trace vector being output for storing in one of said storage locations of the image buffer in place of the colour value generated by the shader program.