1. A Routing Technique for Structured Designs which Exploits Regularity Sabyasachi Das Intel Corporation Sunil P. Khatri Univ. of Colorado, Boulder

2. 2 Outline  Motivation  Our routing approach  Extracting Net-Clusters  Selecting a representative bit-slice  Routing same-bit and cross-bit nets  Route Propagation  Advantages of our approach  Experimental results  Conclusions & future work

3. 3 Motivation  Datapath designs  Characterized by regularity across bit-slices  One of the more critical parts of the design  Commonly found in microprocessors, DSP, graphic ICs.  Effective exploitation of regularity is crucial for efficient datapath design automation  Datapath placement techniques are available  Not many results are available on datapath routing

4. 4 Regularity in Datapath Circuits  Note vertical bit-slices  Goal:  Goal: exploit this regularity and thereby efficiently route the datapath design

5. 5 Overall Routing Flow

6. 6 Net-Cluster Extraction  A net-cluster is a collection of nets (spread over multiple bit-slices), in which all nets have similar connections.  net 1 and net 2 are in cluster N if foreach pin p net 1 and q net 2 :  y p = y q AND |x p – x q | = k * bit_pitch  Our algorithms to extract net-clusters:  Footprint-driven clustering  Instance-driven clustering  Cluster-merging

7. 7 Footprint-driven Clustering  Computationally inexpensive  Footprints (of a net) are of two types:  Global footprint contains the pin-names (of connecting instances) and master-cell names.  Detailed footprint contains the instance names and the net-name.  The FDC technique has two steps: 1. Groups are created such that each net in the group has the same global footprint. 2. Create net clusters from nets in a group with identical detailed footprints (modulo bit-slice index)

8. 8 Footprint-driven Clustering 1. Global footprint step: two groups created G1 = {LB[3:0], SB[3:0]} G2 = {SM[3:0]} 2. Detailed footprint step: three net-clusters are obtained:  From G1: NC1 = {LB[3:0]} NC2 = {SB[3:0]}  From G2: NC3 = {SM[3:0]}

9. 9 Instance-driven Clustering  For unclustered nets, first apply global footprint step  For each such group  Consider net 1 and net 2 (p-pin)  Sort the pins by y-coordinate  Insert these in a cluster if:  Sorted y-coordinates of pins all p pins match  Corresponding x-coordinates are k bit-pitches apart  IDC does not rely on uniform net names  In the example, two net- clusters are obtained:  NC1 = {AB, CD, EF, GH} and NC2 = {KL, MN, RS, TV}

10. 10 Cluster Merging  Net clusters obtained by FDC and IDC may be further merged  In the example:  FDC finds 3 clusters:  NC1 = {LB[3:0]}  NC2 = {SM[1:0]}  NC3 = {SB[3:0]}  IDC finds 1 cluster:  NC4 = {ABC, DEF}  Cluster merging merges NC2 and NC4  NC-New = {ABC, DEF, SM[1], SM[0]}

11. 11 Selecting the Representative Bit-Slice  We conceptually extend the datapath to have an infinite number of bit- slices.  Any bit-slice can be chosen as the representative bit- slice for routing.  While routing, we need to account for:  “Same-bit” nets  Backward “cross-bit” nets  Forward “cross-bit” nets

12. 12 Routing Same-bit Nets  Our routing approach is a combination of pattern-based routing and maze routing  We call our approach strap-based routing. A strap is defined as a straight segment, which can be vertical or horizontal.  Our router utilizes minimal memory, since it loads the design data of one (representative) bit-slice.  As a result, our method is significantly faster than traditional routing approaches in the datapath context.

13. 13 Routing Same-bit Nets.. 2  Our router routes nets in a sequential manner:  Nets routed in decreasing order of y-coordinate of their topmost pins.  Longer nets are given preference.  User may specify a different order  Router tries to find direct-routes (only vertical or only horizontal strap or VTH/HTV strap).  If a direct route is unavailable, more complex patterns are attempted.  Rip-up and re-route if there is a conflict.  If strap-based router cannot find a route, maze router is used.

14. 14 Routing Cross-Bit Nets  We model a single cross-bit net (spread over multiple bit-slices), as a combination of multiple smaller sub-nets  Each confined within a single bit-slice.  We virtually instantiate these sub-nets into the representative bit-slice.  If maximum backward/forward connectivity is k, then we need to find k different routes  This ensures that the k distinct routes do not conflict in the real design  In practice, the value of k is not very high.

15. 15 Propagating the Routes  After routing nets in the representative bit- slice, these routes are propagated to other bit-slices (for other nets in same net-cluster).  There are two cases:  For same-bit nets: The original route is propagated by simple coordinate translation  For cross-bit nets: If maximum degree of forward/backward connectivity is k:  For the n th net in the net cluster, we propagate the (n%k) th route (of the k routes we obtained in the previous step)  This ensures that the propagated routes do not conflict

16. 16 Advantages of Our Approach  Speed of Routing:  A small fraction of nets are routed, the rest are propagated.  Lower memory requirement:  Since only one bit-slice is routed  Predictable Routes:  Wiring parasitics are similar for different nets in a net-cluster.  Better Debuggability :  Easy to find and fix poorly routed nets since (similarly routed) nets would typically all fail timing  Easy Incremental Routing: Rip-up all the routes for a given net-cluster and again route them identically. This can be done multiple times.???

17. 17 Experimental Results (Circuits Utilized) Circuit# Instance# Connections# Bits Industry-11056550432 Industry-223681267232 Industry-346722918464 Industry-462084812864  We utilized industrial 32 and 64-bit datapath circuits

18. 18 Experimental Results (Run-time) CircuitInd. RouterOur RouterRatio Industry-170120.17 Industry-2145260.18 Industry-3264360.14 Industry-4394420.11 Average0.15 at least 7X faster  Our router is at least 7X faster

19. 19 Experimental Results (Wire-length) CircuitInd. RouterOur Router% Improvement Industry-147458467901.41 Industry-29745399476-2.07 Industry-3276456282569-2.21 Industry-45896795645684.25 Average0.35  Average wire-length reduction for our method is minimal

20. 20 Experimental Results (Via-count) CircuitInd. RouterOur Router% Improvement Industry-16856567817.18 Industry-2258762233613.68 Industry-339568394520.28 Industry-444568429763.57 Average8.68  Our router utilizes fewer vias since it is strap-based.

21. 21 Conclusions and Future Work  Our regular routing technique is a useful component in a total datapath design automation flow significant speed-up  Results in significant speed-up highly regular  Routes are highly regular  In future, we plan to focus on  Cross-talk aware datapath routing  Handling irregular net connectivity

22. 22 Thank You!