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Modern GPPs implement specialized instructions in the form of ISA extensions aiming to increase the performance of emerging applications. These extensions impose a significant overhead in the area and power of the processor because of their specific datapaths (e.g. hardware for SIMD and FP instructions may represent more than half of the core area). Considering that some devices (e.g., edge computing), must be as energy- and area-efficient as possible, and the sporadic usage of specialized instructions in many applications, we propose PHISA multicores. PHISA is composed of heterogeneous cores of the same single base ISA, but asymmetric functionality: some of the cores do not fully implement the costly instruction extensions, making room for the designers to add more efficient cores. We show that PHISA increases performance in (32%) and reduces energy consumption in (82%) compared to full-ISA systems with the same power budget, in multi-workload environments. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brasil (CAPES) - Finance Code 001, the Fundação de Amparo à Pesquisa do Estado do RS (FAPERGS), Brazil and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil.