Natarajan Viswanathan

1
FastPlace An Analytical Placer for Mixed-Mode
Designs

• Natarajan Viswanathan
• Min Pan
• Chris Chu
• Iowa State University
• International Symposium on Physical Design
• April 6, 2005

2
Overview

• Stage 1 Global Placement
• Hybrid Net Model
• Cell Shifting
• Contour based Iterative Local Refinement
• Stage 2 Legalization
• Legalize movable macros
• Legalize standard cells
• Stage 3 Detailed Placement
• Global Swap and Global Move
• Local Re-ordering
• Segment-based Shifting
• Local Shifting

3
Global Placement Approach

• Framework
• repeat
• Solve the convex quadratic program ?
• Reduce wirelength by iterative heuristic ?
• Spread the cells ?
• until the cells are evenly distributed ?
• Special features of FastPlace
• Iterative Local Refinement
• Minimize wirelength based on linear objective ?
• Hybrid Net Model
• Speed up solving of convex QP ?
• Cell Shifting
• Easy-to-compute technique ?
• Enable fast convergence ?

• Framework
• repeat
• Solve the convex quadratic program
• Reduce wirelength by iterative heuristic
• Spread the cells
• until the cells are evenly distributed

4
GP Contour Map for Fixed Macros
Circuit adaptec2
Before
After
5
Global Placement Whats New

• Contour Based Iterative Local Refinement
• Generate a contour map of the placement region
  based on the fixed macros
• Use the contour map to guide the standard cells
  and movable macros out of the fixed macros
• Consider moving an object in 8 directions during
  Iterative Local Refinement
• Compute a score for each direction based on
• Half-perimeter wirelength (HPWL) reduction
• Cell density at the source and destination
  regions
• Contour height at the source and destination
  regions
• Move in the direction with highest positive score
  
• (Do not move if no positive score)

6
Legalization Flow

• Legalize movable macros
• Move movable macros to the nearest possible legal
  positions considering only the fixed macros
• Fix movable macros
• Legalize standard cells
• Divide the placement region into bins and ripple
  standard cells among bins to even out the current
  placement
• Determine the placeable segments and move
  standard cells among segments to satisfy segment
  capacities
• Legalize standard cells within segments

7
Detailed Placement Techniques (1)
Optimal region of cell i
j
i

• Global Swap / Global Move
• For every candidate cell j in the optimal region
  of cell i
• - compute the benefit to swap i with j
• - compute the benefit to move i to the space
  around j
• Greedily pick the cell / space with best benefit
  to perform the swap / move

8
Detailed Placement Techniques (2)

• Local re-ordering
• For 2 to 3 consecutive cells in a segment, get
  the best order in terms of wirelength
• Segment-based shifting
• Consider one segment at a time
• Shift cells in the segment simultaneously to
  minimize wirelength
• Local shifting
• Consider one cell at a time
• Shift the cell within its local space to minimize
  wirelength

9
Detailed Placement Flow

• 1. Perform initial segment-based shifting
• 2. Repeat
• Perform global swapping and moving on all the
  cells
• Perform local re-ordering with 2 cells
• Until no significant improvement in wirelength
• 3. Perform local re-ordering with 3 cells
• 4. Perform final segment-based shifting
• 5. Perform local shifting within each segment

10
Not There Yet

• A change in the objective
• Original FastPlace -- speed
• Placement contest -- HPWL
• FastPlace has the potential to obtain good WL
• For standard cell placement, 9 reduction in WL
  with 47 increase in runtime over ISPD-04 version
• For mixed size placement, we still have a lot to
  do
• Proper cell shifting in the presence of macros
• Proper legalization technique considering
  fixed/movable macros
• Proper tuning of parameters
• Stay tuned!