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if the corresponding TSD entry is empty. Cuts down lock/unlock pairs for this operation from 256 to the number of active TSD entries; sicne this is done when every thread exits, it saves many total lock/unlock pairs.
if the corresponding TSD entry is empty. Cuts down lock/unlock pairs for this operation from 256 to the number of active TSD entries; sicne this is done when every thread exits, it saves many total lock/unlock pairs.