Letter to the Editor | Open Access
Comment on “High Efficiency Generalized Parallel Counters for Look-Up Table Based FPGAs”
This brief points out some problems when mapping the optimized GPCs using the heuristic of the paper above. A thorough analysis revealed that a significant number of additional LUTs are required to route the signals when mapping the optimized designs on current FPGAs. Taking these resources into account, the optimized GPCs require at least the same resources as previous state of the art.
In a recent paper from Khurshid and Mir , a heuristic is presented to optimize the mapping of generalized parallel counters (GPCs), as used in compressor trees, to the look-up tables (LUTs) of field programmable gate arrays (FPGAs). The authors claim in their results that the optimized GPCs provide a significant reduction in LUTs compared to previously proposed GPC mappings of our group [2, 3] and other groups [4, 5]. However, as pointed out in this brief, their optimized designs require a significant number of additional LUTs to route the signals to the used resources.
2. Problems When Mapping to Xilinx FPGAs
To illustrate the mapping problems, a (1,4,1,5;5) GPC is used, which was also used in  as a detailed example. Their optimization result for a Xilinx Virtex 5 FPGA is shown in Figure 1(a). The FPGA mapping of our GPC  is shown in Figure 1(b). In Figure 1(b), a simplified Slice with all relevant routing multiplexers is used. The authors in  claim to reduce the LUT resources from four LUTs to two LUTs. However, they did not consider the fact that the LUTs and the fast carry chain cannot be connected arbitrarily. They are organized in a Slice which provides a limited set of multiplexers for internal signal routing . Hence, the only way to route all four outputs of the two LUTs in Figure 1(a) to the carry chain is by using additional LUTs. These are listed as “route-thru LUTs” in the Xilinx reports and cannot be ignored when comparing complexity as they cannot be used for any other logic. Figure 1(c) shows the best mapping to a Xilinx Virtex 5 Slice we found for the GPC of Figure 1(a). It can be seen that the leftmost and the rightmost LUTs are required to route the results of the two LUTs in the middle to the corresponding inputs of the carry chain. Even if only a fraction of these LUTs is used, they cannot implement any additional logic as either both LUT outputs or the corresponding Slice outputs are occupied, leading to an overall LUT cost of four, the same as previously reported .
(a) GPC from , claimed to use two LUTs
(b) Slice mapping of previous GPC  using four LUTs
(c) Corrected Slice mapping of GPC from  requiring four LUTs
Looking at the delay, the GPC delay can be broken down into the delays of LUTs, carry chain sections, and local routing, which are denoted as , , and , respectively [1, 2]. The critical path of the GPC in Figure 1(c) starts at the second right LUT and runs along a local routing through the rightmost LUT and three sections of the carry chain. This leads to a delay of . Assuming , this is about three times slower than the delay of the GPC in  which is . Table 1 lists the results of , the corrected results when considering the routing restrictions as well as the best design from the literature. The results of GPCs (3;2), (6;3), and (5,3;4) are not listed as they were correct in  but did not show any improvement compared to previous designs. For GPGs (7;3), (5,0,6;5), and (2,0,4,5;5), no mapping for Xilinx FPGAs was provided in , so they could not be reproduced. It can be concluded for the Xilinx designs that none of the proposed GPCs could be improved in terms of resources but most of them require a larger delay than previous GPCs.
3. Problems When Mapping to Altera FPGAs
Similar issues occur for the proposed Altera mappings. First, unlike Xilinx Virtex 5 FPGAs, the carry inputs of Altera’s adaptive logic module (ALM) used in Stratix IV cannot be fed from the global routing. The carry chain can only start from the first or fifth ALM of a logic array block (LAB) . Figure 2 shows the solution of the (1,4,1,5;5) GPC as provided in . It is claimed that two LUTs are sufficient to realize the GPC. Note that a LUT in  refers to a two-output function or half of an ALM. Hence, to route inputs and to the inputs of the first full adder (FA), two additional LUTs are required. Next, the signals and are directly connected with an FA. Again, this can only be realized by routing through a LUT or by bypassing the LUT which makes the output of the LUT inaccessible. Finally, the carry output from an FA also cannot be routed to the output (unlike Xilinx Virtex 5 FPGAs). Thus, an addition with zero is necessary to access the carry output. Figure 3 shows the best mapping to Stratix IV Altera ALMs we found. It can be seen that three ALMs are required to implement the GPC which corresponds to six LUTs. The same problems occur for the other GPC mappings provided in . As there were no previous results reported for Altera FPGAs, a detailed evaluation of Altera GPCs is omitted.
The authors declare that they have no competing interests.
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Copyright © 2016 Martin Kumm and Peter Zipf. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.