1. A parallel-code optimization system comprising: a Procedural Concurrency Graph (PCG) generator executed by a computer system, the PCG generator producing an initial PCG of a computer program including parallel code, and determining a refined PCG from the initial PCG by applying a concurrency-type refinement and an interference-type refinement to the initial PCG, wherein the initial PCG and the refined PCG include nodes and edges connecting pairs of the nodes, wherein the nodes represent procedures in the parallel code, and each edge represents a may-happen-in-parallel relation, and is associated with an immediate interference set of lvalues that represents immediate interference between the corresponding pair of nodes; and a storage device storing the refined PCG.