1. A method of a graphics-processing unit (GPU) for tile-based rendering of a display area, comprising: computing vertex positions of a plurality of vertexes, wherein a first vertex of the plurality of vertexes corresponds to a first thread and a second vertex of the plurality of vertexes corresponds to a second thread; determining whether a thread merge condition is satisfied; merging the first thread and the second thread to a thread group when determining that the thread merge condition is satisfied; computing vertex varyings of the plurality of vertexes, wherein when the first thread and the second thread are merged to the thread group, a varying of the first vertex and a varying of the second vertex are computed with respect to a program counter, wherein the thread merge condition is satisfied if: the first thread and the second thread belong to the same draw call; or a shader code assembly of the first thread is identical to a shader code assembly of the second thread and hardware settings of the first thread are identical to hardware settings of the second thread.