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Just in case you are using Xilinx tools, here are some words from the XST User Guide:
The goal of resource sharing (also known as folding) is to minimize the number of operators and the subsequent logic in the synthesized design. This optimization is based on the principle that two similar arithmetic resources may be implemented as one single arithmetic operator if they are never used at the same time. XST performs both resource sharing and, if required, reduces the number of multiplexers that are created in the process. XST supports resource sharing for adders, subtractors, adders/subtractors and multipliers.
If the optimization goal is SPEED, then the disabling of resource sharing may lead to better results. XST advises you to try to deactivate resource sharing at the Advance HDL Synthesis step in order to improve clock frequency.