Dynamic FPGA Routing for Just-in-Time Compilation

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Dynamic FPGA Routing for Just-in-Time Compilation

• Roman Lyseckya, Frank Vahida, Sheldon X.-D. Tanb
• aDepartment of Computer Science and Engineering
• bDepartment of Electrical Engineering
• University of California, Riverside
• Also with the Center for Embedded Computer
  Systems at UC Irvine
• This work was supported in part by the National
  Science Foundation, the Semiconductor Research
  Corporation, and a Department of Education GAANN
  fellowship

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IntroductionJust-in-Time Compilation has Become
Commonplace

• Just-in-Time Compilation
• Modern Pentium processors
• Dynamically translate instructions onto
  underlying RISC architecture
• Transmeta Crusoe Efficeon
• Dynamic code morphing
• Translate x86 instructions to underlying VLIW
  processor
• Interpreted languages
• Distribute SW as processor independent
  bytecode/source
• SW typically executed on a virtual machine
• JIT compile bytecode to processors native
  instructions
• Java, Python, etc.

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IntroductionJust-in-Time Compilation also
Performs Optimization

• Dynamic optimizations are increasingly common
• Dynamically recompile binary during execution
• Dynamo Bala, et al., 2000 - Dynamic software
  optimizations
• Identify frequently executed code segments
  (hotpaths)
• Recompile with higher optimization
• BOA Gschwind, et al., 2000 - Dynamic optimizer
  for Power PC
• Advantages
• Transparent optimizations
• No designer effort
• No tool restrictions
• Adapts to actual usage
• Speedups of up 20-30 -- 1.3X
• JIT compilation operates on software binaries

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IntroductionBut Todays Binaries are More than
just Software
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IntroductionJust-in-Time FPGA Compilation?

• JIT FPGA compilation
• Idea standard binary for FPGA
• Similar benefits as standard binary for
  microprocessor
• Portability, transparency, standard tools
• Embedded JIT compilation tools optimized for each
  FPGA

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IntroductionOne Use of JIT FPGA Compilation
CableTV Company
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IntroductionOne Use of JIT FPGA Compilation
CableTV Company
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IntroductionOne Use of JIT FPGA Compilation
CableTV Company
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IntroductionAnother Use - Warp Processors
(Dynamic HW/SW Partitioning)
Profiler
µP
I
D
Warp Config. Logic Architecture
Dynamic Part. Module (DPM)
Lysecky/Vahid, DATE04 Stitt/Lysecky/Vahid
DAC03 Stitt/Vahid, ICCAD02
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IntroductionAnother Use - Warp Processors
(Dynamic HW/SW Partitioning)
Profiler
ARM
I
D
WCLA
DPM
Lysecky/Vahid, DATE04 Stitt/Lysecky/Vahid,
DAC03 Stitt/Vahid, ICCAD02
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IntroductionAll that CAD on-chip?

• CAD people may first think Just-in-Time FPGA
  compilation is absurd
• CAD tools are extremely complex
• Require long execution times on power desktop
  workstations
• Require very large memory resources
• Usually require GBytes of hard drive space
• Costs of complete CAD tools package can exceed 1
  million
• All that CAD on-chip?

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Simultaneous FPGA/CAD Design

• Careful simultaneous design of configurable logic
  fabric and CAD tools
• Analyze architectural features as to their
  impacts on on-chip Just-in-Time CAD tools
• Fast execution time
• Very low data memory
• Produce reasonable (good) hardware circuits

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Simultaneous FPGA/CAD Design Configurable Logic
Fabric

• Array of configurable logic blocks (CLBs)
  surrounded by switch matrices (SMs)
• Each CLB is directly connected to a SM
• Switch matrix connections
• Four short wires connect adjacent SMs
• Four long wires connect every other SM together

SM
SM
SM
CLB
CLB
SM
SM
SM
Lysecky/Vahid, DATE04
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Simultaneous FPGA/CAD Design Combinational Logic
Block Design

• Incorporate two 3-input 2-output LUTs
• Corresponds to four 3-input LUTs
• Allows for good quality circuit while reducing
  on-chip CAD tools complexity
• Provide routing resources between adjacent CLBs
  to support carry chains

Lysecky/Vahid, DATE04
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Simultaneous FPGA/CAD Design Switch Matrix

• Switch Matrix
• SM connected using eight channels per side
• Four short channels
• Four long channels
• Routes wires from different side using the same
  channel
• Each short channel is associated with single long
  channel
• Wires are routed using a single pair of channels
  through configurable logic fabric

Lysecky/Vahid, DATE04
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FPGA Routing

• FPGA Routing
• Find a path within FPGA to connect source and
  sinks of each net within our hardware circuit
• Typically use a form of maze routing Lee, 1961
• Routes each net using Dijkstras shortest path
  algorithm

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FPGA Routing

• Pathfinder Ebeling, et al., 1995
• Introduced negotiated congestion
• During each routing iteration, route nets using
  shortest path
• Allows overuse (congestion) of routing resources
• If congestion exists (illegal routing)
• Update cost of congested resources based on the
  amount of overuse
• Rip-up all routes and reroute all nets

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FPGA Routing

• VPR Versatile Place and Route Betz, et al.,
  1997
• Uses modified Pathfinder algorithm
• Increase performance over original Pathfinder
  algorithm
• Routability-driven routing
• Goal Use fewest tracks possible
• Timing-driven routing
• Goal Optimize circuit speed

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JIT FPGA Routing

• Riverside On-Chip Router (ROCR)
• Represent routing nets between CLBs as routing
  between SMs
• Resource Graph
• Nodes correspond to SMs
• Edges correspond to channels between SMs
• Capacity of edge equal to the number of wires
  within the channel
• Requires much less memory than VPR as resource
  graph is much smaller

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JIT FPGA Routing

• Riverside On-Chip Router (ROCR) - Global Routing
• Based on VPRs routability-driven router
• Utilizes similar cost model consisting of base,
  historical congestion, and current congestion
  costs
• Routes nets between SMs using greedy, depth-first
  routing algorithm
• Faster than traditional VPRs breadth-first
  routing method
• Requires addition of adjustment cost to direct
  ROCR to re-route illegal nets using different
  initial routing path
• Ignores illegal routing within SMs
• If congestion exists, rip-up and re-route only
  the illegal routes
• Reduces computation time during successive
  routing iterations

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JIT FPGA Routing

• Riverside On-Chip Router (ROCR) - Detailed
  Routing
• Assign specific channels to each route
• Construct routing conflict graph
• Routes conflict if assigning same channel results
  in an illegal routing within any SM
• Use Brelazs greedy vertex coloring algorithm
  Brelaz, 1979
• If illegal routes exist, rip-up illegal routes
  and repeat global routing

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Experiments Memory Usage
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Experiments Algorithm Performance
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Experiments Critical Path Results
But 10 shorter critical path than VPR (RD)
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Experiments Wire Segments
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Conclusions

• Developed Riverside On-Chip Router (ROCR)
• Fast, lean on-chip router for JIT FPGA
  compilation
• Order of magnitude less memory required
• On average 10X faster than VPRs faster routing
  algorithm
• Produces acceptable circuit quality
• Uses only 10 more routing resources
• Critical path 10 shorter than VPRs
  routability-driven router
• JIT FPGA Compilation
• Enables development of a standard HW binary
• Brings portability of SW design to HW designers
• Presently requires custom FPGA fabric
• Future work - Overhead of mapping simple fabric
  onto commercial fabric?