Reversible Logic Synthesis Benchmarks Page

Function rd32 (.pla file) has 3 inputs and 2 outputs. Its ouput is binary representation of the number of 1s in its input.
 

Picture	Machine-readable version	Model	Garbage	Gate count	Quantum cost	Author(s)	Date
here	here	NCT	2*	4*	8	Feynman (source)	1986

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* - the number was shown to be minimal