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Application-Specific Instruction set Processor (ASIP) has become an increasingly popular platform for embedded systems because of its high performance and flexibility. Energy efficiency is critical for portable and embedded devices, and should be addressed separately from performance consideration. The hardware extension in ASIPs can speed-up program execution, but also incurs area overhead and static energy consumption of the processors. Traditional data path merging techniques reduce circuit overhead by reusing hardware resources for executing multiple custom instructions. However, they introduce structural hazard for custom instructions on extended processors, and hence reduce the performance improvement. In this paper, we introduce a pipelined configurable hardware structure for the hardware extension in ASIPs, so that structural hazards can be remedied. With multiple subgraphs of operations selected for custom hardware realization, we devise a novel operation-to-hardware mapping algorithm based on Integer Linear Programming (ILP) to automatically construct a resource-efficient pipelined configurable hardware extension. We demonstrate that different resource sharing schemes would affect both the hardware overhead and datapath delay of the custom instructions. We analyze the design tradeoffs between resource efficiency and performance improvement, and present the design space exploration results.