![]() ![]() ![]() Integer described by a pair of bits into an integer describing the number Instead use the literals: a 4-entry table of 2-bit values, converting the Keep the code concise, I'll break the rule of always naming constants and This part of the code never changes regardless of input word width, so to Don't set at instantiation (see above) Operation, which might cause difficulties when porting to other HDLs, andĪlso means you should read it carefully as some lines are long and haveĪ lot of bits being moved around between vectors, but this design choiceĪlso makes the code easily parameterizable. This implementation depends heavily on Verilog's vector part select I left the code as-is, and hardwired the POPCOUNT_WIDTH parameter instead. Log 2(N)+1 output width and eliminates the redundant logic. Optimization of the successive additions automatically infers the This simplifies code and the post-elaboration diagram, and the logic It is easier to leave the output width at WORD_WIDTH as That the output width should be log 2(N)+1 and not However, it turns out that it's error-prone to expect the user to remember If the value is smaller than that, then the Know the width of the output, so it should calculate log 2(N)+1Īnd pass it to this module. Rather than being calculated internally, as the enclosing module needs to When I originally wrote this, the POPCOUNT_WIDTH was a module parameter Leaning on Logic Optimization to Reduce User Errors Think of it as mappingīitmasks to the integers, allowing us to use arithmetic operations toĭetermine properties. ![]() There are many applications of population count. Generate a tree of LUTs and adders with a carry-chain log 2(N)+1īits long, which is the length of the expected final adder were this a tree ![]() However, the CAD toolĬan reduce the logic (most of the added bits are constant zeros) and Which would require more difficult, recursive code. The other, which expresses a chain of (N/2)-1 small adders (e.g.: 15 6-bitĪdders for a 32-bit input word) rather than the obvious tree of adders, The algorithm is straightforward: split the input word into bit pairs andĬount the number of bits set in each pair using a lookup table, then extendĮach count to log 2(N)+1 bits, and sum all the counts one after Word has a 6-bit popcount, so you can represent the integer "32") Log 2(N)+1 bits wide given an N-bit input word. Returns the number of bits set in the input word. ![]()
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