1. A method comprising: identifying a subgraph of instructions for a shader encompassing a first instruction and one or more ancestor instructions on which the first instruction depends; for at least a subset of the instructions of the shader, calculating a metric indicative of whether a respective instruction reduces a vector register load utilized by the shader, wherein the calculating the metric comprises at least one or more of the following: determining a number of reads made from within the subgraph, determining a number of values produced within the subgraph and read from outside the subgraph, or determining a number of instructions in the subgraph; and with a computer, reordering the instructions based at least in part on the metric by moving at least one of the subset of instructions to reduce an active vector register count of the shader.