Architecture-Specific Packing for Virtex-5 FPGAs Taneem Ahmed, Paul Kundarewich, Jason Anderson, Brad Taylor, Rajat Aggarwal February 25th, 2008

Overview Virtex-5 6-LUT Packing Virtex-5 DSP and Block RAM Packing Results Summary

Simplified FPGA Logic Element 4-LUT A4 A3 A2 A1 O4 FF

Simplified FPGA Logic Block FF 4-LUT General Interconnect General Interconnect

Virtex-5 Logic Block CLB General Interconnect General Interconnect SLICE FF 6-LUT General Interconnect General Interconnect SLICE FF 6-LUT

Dual-Output 6-LUT 6-LUT A6 A5 A4 A3 A2 A1 O6 O5

Dual-Output 6-LUT Usage

Dual-Output Packing 6-LUT 6-LUT Number of 6-LUTs used: 2 VCC a b Y Logic A6 A5 A4 A3 A2 A1 O6 5-LUT O5 x y X Logic x y b a Y Logic X Number of 6-LUTs used: 1! Number of 6-LUTs used: 2

Virtex-5 LUT/FF Pair CY F7 F7 A O6 XOR AMUX 6-LUT O6 O5 CIN AX FF AQ

Dual-Output Packing Tradeoff AX 6-LUT F7 O5 O6 FF 6-LUT

Dual-Output Packing in Placer Goal: To reduce area without performance hit Can be done pre-placement Will be sub-optimal without delay estimates Use delay estimates available during placement to make good decisions on when to merge two LUTs Approach: Allow second 5-LUT to be used, when performance impact is small Incorporate LUT packing in placer’s cost function

Placer Cost Function Previous cost function: Cost = a * W + b * T W: wirelength cost T: timing performance cost Extend cost function with two new terms One based on 6-LUT utilization (L) One based on SLICE utilization (S) Cost = a * W + b * T + c * L + d * S

6-LUT Utilization Term L is computed based on all the used 6-LUT slots Where

SLICE Utilization Term S is computed based on all the available SLICEs Let: Ni = Number of used 5-LUTs in SLICE i (at most 8)  m S = Si i=0

Performance Recovery Helpful to prohibit pack in certain cases for performance reasons Other used elements in a SLICE may block the “good” path from the O5 output to external interconnect.

Performance Recovery: XOR AX LUT6 CY F7 O5 O6 CIN FF AQ AMUX A LUT6 FF

Performance Recovery: F7 XOR AX LUT6 CY F7 O5 O6 CIN FF AQ AMUX A LUT6 F7 FF

6-LUT Reduction 5.5% 6-LUT Reduction

SLICE Reduction 10.23% SLICE Reduction

Performance Results 3.3% Performance Degradation

Overview Virtex-5 6-LUT Packing Virtex-5 DSP and Block RAM Packing Summary

New Type of Packing Problem Traditionally, packing is considered to be a problem of just LUTs and flops However, Virtex-5 contains large IP blocks that present their own packing problem

Virtex-5 Block RAMs A 36 Kbit block RAM tile can store: 36Kb RAM A 36 Kbit block RAM tile can store: a) single 36 Kb RAM b) two independent 18 Kb RAMs Block RAM has configurable “aspect ratio” 18 Kb RAM can be configured as: 16K x 1, 8K x 2, 2K x 9, or 1K x 18 Tools decide which independent 18 Kb block RAMs to locate in which tile 18 Kb RAM 18 Kb RAM

Virtex-5 DSP48E Block A multiply-accumulate operation, pervasive in DSP circuits, can be realized in a single DSP48E. Multiple DSP48Es can be chained together to form more complex functions through the PCIN and PCOUT ports PCOUT 48-bit 25x18 B (18-bit) X ALU A (25-bit) Optional pipeline register/ routing logic Routing logic Optional pipeline register/ routing logic P C (48-bit) = Pattern detect PCIN

Block RAM and DSP Floorplan Block RAM and DSP48E tiles are organized in columns Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile Virtex-5 DSP tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E

Block RAM/DSP Packing Problem: Placer algorithms are heuristic and sometimes do not find an optimal block RAM packing Goal: Leverage preferred block RAM packing patterns to achieve high performance Target area: DSP designs DSP designs make heavy use of block RAMs and DSP blocks

DSP Block RAM Designs Most common DSP application is the Finite Impulse Response Filter or FIR filter FIR filters have multiple instances of a “tap” which involve DSP and block RAMs

FIR Filter A Finite Impulse Response or FIR filter is a digital filter that takes a weighted average of the signals in a delay line An N-tap filter can be expressed as: y[n] = c0*x[n] + c1*x[n-1]+…+cn*[n-N+1] Where: y[n] is the output of the filter at time n x[n] is the data input “signal” at time n Ci is the coefficient Each coefficient/data product in sum is referred to as a “tap” DSP units used for the multiply and accumulate Block RAMs used to store the data and coefficients

FIR Designs – Use Case 1 2-tap FIR filter involving small block RAMs RAMD1 RAMC1 Data RAM 18 Kb block RAM RAMD0 RAMC0 Coefficient RAM DSP0 Tap 0 DSP1 Tap 1 PCOUT PCIN A B data input data output 36 Kb block RAM Tile

Packing for Use Case 1 Packing both 18k Block RAMs into a Block RAM tile permits a natural alignment between the DSP and Block RAMs Operates as two independent 18 Kb block RAMs Block RAM tile DSP48E Virtex-5 DSP tile DSP48E Block RAM tile DSP48E DSP48E High Performance! Block RAM tile DSP48E DSP48E Block RAM tile DSP48E DSP48E

FIR Designs – Use Case 2 2-tap FIR filter involving larger block RAMs DSP0 DSP1 PCOUT PCIN RAMD0 RAMD1 A B 18 Kb block RAM 36 Kb block RAM RAMC0 RAMC1 Data RAM Coefficient RAM Tap 1 Tap 0

Packing for Use Case 2 Two Block RAM columns feed one DSP column Again provides a natural alignment between the DSP and Block RAMs Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Block RAM tile DSP48E Virtex-5 DSP tile

Block RAM Chains Use Case: 18k Block RAM’s data input and output pins connected together (e.g. FIFO) Algorithm: Look for such chains and pack them together into single block RAM tile Special Case: 18k block RAMs separated by registers in RAM0 dia doa addra RAM1 dib dob addrb out 18 Kb block RAM

Block RAM/DSP Packing Results Circuit Perf RAM Packing (MHz) Perf. Baseline (MHz) Percent Improvement Circuit 1 500 400 25% Circuit 2 450 365 23% Circuit 3 470 6% Circuit 4 425 435 -2% Circuit 5 215 200 8% Geomean 359 11%

Summary Described two architecture specific packing approaches for a 65nm commercial FPGA: Xilinx Virtex-5 Dual-output LUT packing in placement: Achieves 10.2% SLICE reduction and 5.5% LUT reduction Packing for DSPs and block RAMs: Achieves 11% performance improvement

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