ECE260B CSE241A Winter 2005 Logic Synthesis Website: / courses/ ece260bw05 ECE 260B CSE 241A Static Timing Analysis 1 Slides courtesy of Dr. Cho Moon
Introduction Why logic synthesis? Ubiquitous used almost everywhere VLSI is done Body of useful and general techniques same solutions can be used for different problems Foundation for many applications such as - Formal verification - ATPG - Timing analysis - Sequential optimization ECE 260B CSE 241A Static Timing Analysis 2
RTL Design Flow HDL RTL Synthesis Manual Design Module Generators Library netlist Logic Synthesis a b 0 1 s d clk q netlist a b 0 1 d q Physical Synthesis s clk layout ECE 260B CSE 241A Static Timing Analysis 3 Slide courtesy of Devadas, et. al
Logic Synthesis Problem Given Initial gate-level netlist Design constraints - Input arrival times, output required times, power consumption, noise immunity, etc Target technology libraries Produce Smaller, faster or cooler gate-level netlist that meets constraints Very hard optimization problem! ECE 260B CSE 241A Static Timing Analysis 4
Combinational Logic Synthesis netlist 2-level Logic opt Library Logic Synthesis tech independent tech dependent multilevel Logic opt netlist Library ECE 260B CSE 241A Static Timing Analysis 5 Slide courtesy of Devadas, et. al
Outline Introduction Two-level Logic Synthesis Multi-level Logic Synthesis Timing Optimization in Synthesis ECE 260B CSE 241A Static Timing Analysis 6
Two-level Logic Synthesis Problem Given an arbitrary logic function in two-level form, produce a smaller representation. For sum-of-products (SOP) implementation on PLAs, fewer product terms and fewer inputs to each product term mean smaller area. F = A B + A B C F = A B I1 I2 O1 O2 ECE 260B CSE 241A Static Timing Analysis 7
Boolean Functions f(x) : B n B B = {0, 1}, x = (x 1, x 2,, x n ) x 1, x 2, are variables x 1, x 1, x 2, x 2, are literals each vertex of B n is mapped to 0 or 1 the onset of f is a set of input values for which f(x) = 1 the offset of f is a set of input values for which f(x) = 0 ECE 260B CSE 241A Static Timing Analysis 8
Logic Functions: ECE 260B CSE 241A Static Timing Analysis 9 Slide courtesy of Devadas, et. al
Cube Representation ECE 260B CSE 241A Static Timing Analysis 10 Slide courtesy of Devadas, et. al
Sum-of-products (SOP) A function can be represented by a sum of cubes (products): f = ab + ac + bc Since each cube is a product of literals, this is a sum of products representation A SOP can be thought of as a set of cubes F F = {ab, ac, bc} = C A set of cubes that represents f is called a cover of f. F={ab, ac, bc} is a cover of f = ab + ac + bc. ECE 260B CSE 241A Static Timing Analysis 11
Prime Cover A cube is prime if there is no other cube that contains it (for example, b c is not a prime but b is) A cover is prime iff all of its cubes are prime c a b ECE 260B CSE 241A Static Timing Analysis 12
Irredundant Cube A cube of a cover C is irredundant if C fails to be a cover if c is dropped from C A cover is irredundant iff all its cubes are irredudant (for exmaple, F = a b + a c + b c) c a ECE 260B CSE 241A Static Timing Analysis 13 b Not covered
Quine-McCluskey Method We want to find a minimum prime and irredundant cover for a given function. Prime cover leads to min number of inputs to each product term. Min irredundant cover leads to min number of product terms. Quine-McCluskey (QM) method (1960 s) finds a minimum prime and irredundant cover. Step 1: List all minterms of on-set: O(2^n) n = #inputs Step 2: Find all primes: O(3^n) n = #inputs Step 3: Construct minterms vs primes table Step 4: Find a min set of primes that covers all the minterms: O (2^m) m = #primes ECE 260B CSE 241A Static Timing Analysis 14
QM Example (Step 1) F = a b c + a b c + a b c + a b c + a b c List all on-set minterms Minterms a b c a b c a b c a b c a b c ECE 260B CSE 241A Static Timing Analysis 15
QM Example (Step 2) F = a b c + a b c + a b c + a b c + a b c Find all primes. primes b c a b a c b c ECE 260B CSE 241A Static Timing Analysis 16
QM Example (Step 3) F = a b c + a b c + a b c + a b c + a b c Construct minterms vs primes table (prime implicant table) by determining which cube is contained in which prime. X at row i, colum j means that cube in row i is contained by prime in column j. b c a b a c b c a b c X a b c X X a b c X X a b c X X a b c X ECE 260B CSE 241A Static Timing Analysis 17
QM Example (Step 4) F = a b c + a b c + a b c + a b c + a b c Find a minimum set of primes that covers all the minterms Minimum column covering problem b c a b a c b c a b c X a b c X X a b c X X a b c X X a b c X Essential primes ECE 260B CSE 241A Static Timing Analysis 18
ESPRESSO Heuristic Minimizer Quine-McCluskey gives a minimum solution but is only good for functions with small number of inputs (< 10) ESPRESSO is a heuristic two-level minimizer that finds a minimal solution ESPRESSO(F) { } do { reduce(f); expand(f); irredundant(f); } while (fewer terms in F); verfiy(f); ECE 260B CSE 241A Static Timing Analysis 19
ESPRESSO ILLUSTRATED Reduce Expand Irredundant ECE 260B CSE 241A Static Timing Analysis 20
Outline Introduction Two-level Logic Synthesis Multi-level Logic Synthesis Timing optimization in Synthesis ECE 260B CSE 241A Static Timing Analysis 21
Multi-level Logic Synthesis Two-level logic synthesis is effective and mature Two-level logic synthesis is directly applicable to PLAs and PLDs But There are many functions that are too expensive to implement in two-level forms (too many product terms!) Two-level implementation constrains layout (AND-plane, OR-plane) Rule of thumb: Two-level logic is good for control logic Multi-level logic is good for datapath or random logic ECE 260B CSE 241A Static Timing Analysis 22
Two-Level (PLA) vs. Multi-Level PLA control logic constrained layout highly automatic technology independent multi-valued logic slower? input, output, state encoding ECE 260B CSE 241A Static Timing Analysis 23 Multi-level all logic general automatic partially technology independent coming can be high speed
Multi-level Logic Synthesis Problem Given Initial Boolean network Design constraints - Arrival times, required times, power consumption, noise immunity, etc Target technology libraries Produce a minimum area netlist consisting of the gates from the target libraries such that design constraints are satisfied ECE 260B CSE 241A Static Timing Analysis 24
Modern Approach to Logic Optimization Divide logic optimization into two subproblems: Technology independent optimization determine overall logic structure estimate costs (mostly) independent of technology simplified cost modeling Technology dependent optimization (technology mapping) binding onto the gates in the library detailed technology specific cost model Orchestration of various optimization/transformation techniques for each subproblem ECE 260B CSE 241A Static Timing Analysis 25 Slide courtesy of Keutzer
Optimization Cost Criteria The accepted optimization criteria for multi-level logic are to minimize some function of: 1. Area occupied by the logic gates and interconnect (approximated by literals = transistors in technology independent optimization) 2. Critical path delay of the longest path through the logic 3. Degree of testability of the circuit, measured in terms of the percentage of faults covered by a specified set of test vectors for an approximate fault model (e.g. single or multiple stuck-at faults) 4. Power consumed by the logic gates 5. Noise Immunity 6. Wireability while simultaneously satisfying upper or lower bound constraints placed on these physical quantities ECE 260B CSE 241A Static Timing Analysis 26
Representation: Boolean Network Boolean network: directed acyclic graph (DAG) node logic function representation f j (x,y) node variable y j : y j = f j (x,y) edge (i,j) if f j depends explicitly on y i Inputs x = (x 1, x 2,,x n ) Outputs z = (z 1, z 2,,z p ) ECE 260B CSE 241A Static Timing Analysis 27 Slide courtesy of Brayton
Network Representation Boolean network: ECE 260B CSE 241A Static Timing Analysis 28
Node Representation: Sum of Products (SOP) Example: abc +a bd+b d +b e f (sum of cubes) Advantages: easy to manipulate and minimize many algorithms available (e.g. AND, OR, TAUTOLOGY) two-level theory applies Disadvantages: Not representative of logic complexity. For example f=ad+ae+bd+be+cd+ce f =a b c +d e These differ in their implementation by an inverter. hence not easy to estimate logic; difficult to estimate progress during logic manipulation ECE 260B CSE 241A Static Timing Analysis 29
Factored Forms Example: (ad+ b c)(c+ d (e+ ac ))+(d+ e)fg Advantages good representative of logic complexity f=ad+ae+bd+be+cd+ce f=(a+b+c)(d+e) f =a b c +d e in many designs (e.g. complex gate CMOS) the implementation of a function corresponds directly to its factored form good estimator of logic implementation complexity doesn t blow up easily Disadvantages not as many algorithms available for manipulation hence usually just convert into SOP before manipulation ECE 260B CSE 241A Static Timing Analysis 30
Factored Forms Note: literal count transistor count area (however, area also depends on wiring) ECE 260B CSE 241A Static Timing Analysis 31
Factored Forms Definition : a factored form can be defined recursively by the following rules. A factored form is either a product or sum where: a product is either a single literal or product of factored forms a sum is either a single literal or sum of factored forms A factored form is a parenthesized algebraic expression. In effect a factored form is a product of sums of products or a sum of products of sums Any logic function can be represented by a factored form, and any factored form is a representation of some logic function. ECE 260B CSE 241A Static Timing Analysis 32
Factored Forms When measured in terms of number of inputs, there are functions whose size is exponential in sum of products representation, but polynomial in factored form. Example: Achilles heel function i = n / 2 i= 1 ( x 2 i 1 + x2i ) There are n literals in the factored form and (n/2) 2 n/2 literals in the SOP form. Factored forms are useful in estimating area and delay in a multi-level synthesis and optimization system. In many design styles (e.g. complex gate CMOS design) the implementation of a function corresponds directly to its factored form. ECE 260B CSE 241A Static Timing Analysis 33
Factored Forms Factored forms cam be graphically represented as labeled trees, called factoring trees, in which each internal node including the root is labeled with either + or, and each leaf has a label of either a variable or its complement. Example: factoring tree of ((a +b)cd+e)(a+b )+e ECE 260B CSE 241A Static Timing Analysis 34
Reduced Ordered BDDs like factored form, represents both function and complement like network of muxes, but restricted since controlled by primary input variables - not really a good estimator for implementation complexity given an ordering, reduced BDD is canonical, hence a good replacement for truth tables for a good ordering, BDDs remain reasonably small for complicated functions (e.g. not multipliers) manipulations are well defined and efficient true support (dependency) is displayed ECE 260B CSE 241A Static Timing Analysis 35
Technology-Independent Optimization Technology-independent optimization is a bag of tricks: Two-level minimization (also called simplify) Constant propagation (also called sweep) f = a b + c; b = 1 => f = a + c Decomposition (single function) f = abc+abd+a c d +b c d => f = xy + x y ; x = ab ; y = c+d Extraction (multiple functions) f = (az+bz )cd+e g = (az+bz )e h = cde f = xy+e g = xe h = ye x = az+bz y = cd ECE 260B CSE 241A Static Timing Analysis 36
More Technology-Independent Optimization More technology-independent optimization tricks: Substitution g = a+b f = a+bc f = g(a+c) Collapsing (also called elimination) f = ga+g b g = c+d f = ac+ad+bc d g = c+d Factoring (series-parallel decomposition) f = ac+ad+bc+bd+e => f = (a+b)(c+d)+e ECE 260B CSE 241A Static Timing Analysis 37
Summary of Typical Recipe for TI Optimization Propagate constants Simplify: two-level minimization at Boolean network node Decomposition Local Boolean optimizations Boolean techniques exploit Boolean identities (e.g., a a = 0) Consider f = a b + a c + b a + b c + c a + c b Algebraic factorization procedures f = a (b + c ) + a (b + c) + b c + c b Boolean factorization procedures f = (a + b + c) (a + b + c ) ECE 260B CSE 241A Static Timing Analysis 38 Slide courtesy of Keutzer
Technology-Dependent Optimization Technology-dependent optimization consists of Technology mapping: maps Boolean network to a set of gates from technology libraries Local transformations Discrete resizing Cloning Fanout optimization (buffering) Logic restructuring ECE 260B CSE 241A Static Timing Analysis 39 Slide courtesy of Keutzer
Technology Mapping Input Output Technology independent, optimized logic network Description of the gates in the library with their cost Netlist of gates (from library) which minimizes total cost General Approach Construct a subject DAG for the network Represent each gate in the target library by pattern DAG s Find an optimal-cost covering of subject DAG using the collection of pattern DAG s Canonical form: 2-input NAND gates and inverters ECE 260B CSE 241A Static Timing Analysis 40
DAG Covering DAG covering is an NP-hard problem Solve the sub-problem optimally Partition DAG into a forest of trees Solve each tree optimally using tree covering Stitch trees back together ECE 260B CSE 241A Static Timing Analysis 41 Slide courtesy of Keutzer
Tree Covering Algorithm Transform netlist and libraries into canonical forms 2-input NANDs and inverters Visit each node in BFS from inputs to outputs Find all candidate matches at each node N - Match is found by comparing topology only (no need to compare functions) Find the optimal match at N by computing the new cost - New cost = cost of match at node N + sum of costs for matches at children of N Store the optimal match at node N with cost Optimal solution is guaranteed if cost is area Complexity = O(n) where n is the number of nodes in netlist ECE 260B CSE 241A Static Timing Analysis 42
Tree Covering Example Find an òptimal (in area, delay, power) mapping of this circuit into the technology library (simple example below) ECE 260B CSE 241A Static Timing Analysis 43 Slide courtesy of Keutzer
Elements of a library - 1 Element/ Area Cost Tree Representation (normal form) INVERTER 2 NAND2 3 NAND3 4 NAND4 5 ECE 260B CSE 241A Static Timing Analysis 44 Slide courtesy of Keutzer
Trivial Covering subject DAG 7 NAND2 (3) = 21 5 INV (2) = 10 Area cost 31 Can we do better with tree covering? ECE 260B CSE 241A Static Timing Analysis 45 Slide courtesy of Keutzer
Optimal tree covering - 1 3 2 2 3 `subject tree ECE 260B CSE 241A Static Timing Analysis 46 Slide courtesy of Keutzer
Optimal tree covering - 2 3 8 2 2 3 5 `subject tree ECE 260B CSE 241A Static Timing Analysis 47 Slide courtesy of Keutzer
Optimal tree covering - 3 3 8 13 2 2 3 5 `subject tree Cover with ND2 or ND3? 1 NAND2 3 + subtree 5 Area cost 8 ECE 260B CSE 241A Static Timing Analysis 48 Slide courtesy of Keutzer 1 NAND3 = 4
Optimal tree covering 3b 3 8 13 2 2 3 5 4 `subject tree Label the root of the sub-tree with optimal match and cost ECE 260B CSE 241A Static Timing Analysis 49 Slide courtesy of Keutzer
Optimal tree covering - 4 3 Cover with INV or AO21? 8 13 2 2 2 5 4 `subject tree 1 Inverter 2 + subtree 13 Area cost 15 ECE 260B CSE 241A Static Timing Analysis 50 1 AO21 4 + subtree 1 3 + subtree 2 2 Slide courtesy of Keutzer Area cost 9
Optimal tree covering 4b 3 8 13 9 2 2 2 5 4 `subject tree Label the root of the sub-tree with optimal match and cost ECE 260B CSE 241A Static Timing Analysis 51 Slide courtesy of Keutzer
Optimal tree covering - 5 9 Cover with ND2 or ND3? 8 2 4 `subject tree NAND2 subtree 1 9 subtree 2 4 1 NAND2 3 Area cost 16 ECE 260B CSE 241A Static Timing Analysis 52 subtree 1 8 subtree 2 2 subtree 3 4 1 NAND3 4 Area cost 18 Slide courtesy of Keutzer NAND3
Optimal tree covering 5b 9 8 16 2 4 `subject tree Label the root of the sub-tree with optimal match and cost ECE 260B CSE 241A Static Timing Analysis 53 Slide courtesy of Keutzer
Optimal tree covering - 6 Cover with INV or AOI21? 13 16 5 `subject tree INV subtree 1 16 1 INV 2 AOI21 subtree 1 13 subtree 2 5 1 AOI21 4 Area cost 18 Area cost 22 ECE 260B CSE 241A Static Timing Analysis 54 Slide courtesy of Keutzer
Optimal tree covering 6b 13 18 16 5 `subject tree Label the root of the sub-tree with optimal match and cost ECE 260B CSE 241A Static Timing Analysis 55 Slide courtesy of Keutzer
Optimal tree covering - 7 Cover with ND2 or ND3 or ND4? `subject tree ECE 260B CSE 241A Static Timing Analysis 56 Slide courtesy of Keutzer
Cover 1 - NAND2 Cover with ND2? 9 18 16 4 `subject tree subtree 1 18 subtree 2 0 1 NAND2 3 ECE 260B CSE 241A Static Timing Analysis 57 Area cost 21 Slide courtesy of Keutzer
Cover 2 - NAND3 Cover with ND3? 9 4 `subject tree ECE 260B CSE 241A Static Timing Analysis 58 subtree 1 9 subtree 2 4 subtree 3 0 1 NAND3 4 Slide courtesy of Keutzer Area cost 17
Cover - 3 Cover with ND4? 8 2 4 `subject tree subtree 1 8 subtree 2 2 subtree 3 4 subtree 4 0 1 NAND4 5 Area cost 19 ECE 260B CSE 241A Static Timing Analysis 59 Slide courtesy of Keutzer
Optimal Cover was Cover 2 ND2 AOI21 ND3 INV ND3 `subject tree ECE 260B CSE 241A Static Timing Analysis 60 INV 2 ND2 3 2 ND3 8 AOI21 4 Slide courtesy of Keutzer Area cost 17
Summary of Technology Mapping DAG covering formulation Separated library issues from mapping algorithm (can t do this with rule-based systems) Tree covering approximation Very efficient (linear time) Applicable to wide range of libraries (std cells, gate arrays) and technologies (FPGAs, CPLDs) Weaknesses Problems with DAG patterns (Multiplexors, full adders, ) Large input gates lead to a large number of patterns ECE 260B CSE 241A Static Timing Analysis 61
Outline Introduction Two-level Logic Synthesis Multi-level Logic Synthesis Timing optimization in Synthesis ECE 260B CSE 241A Static Timing Analysis 62
Timing Optimization in Synthesis Factors determining delay of circuit: Underlying circuit technology Circuit type (e.g. domino, static CMOS, etc.) Gate type Gate size Logical structure of circuit Length of computation paths False paths Buffering Parasitics Wire loads Layout ECE 260B CSE 241A Static Timing Analysis 63
Problem Statement Given: Initial circuit function description Library of primitive functions Performance constraints (arrival/required times) Generate: an implementation of the circuit using the primitive functions, such that: 1. performance constraints are met 2. circuit area is minimized ECE 260B CSE 241A Static Timing Analysis 64
Current Design Process Gate library Perf. Constraints Delay models Behavioral description Logic and latches Logic equations ECE 260B CSE 241A Static Timing Analysis 65 Gate netlist Layout Behavior Optiization (scheduling) Partitioning (retiming) Logic synthesis Technology independent Technology mapping Timing driven place and route
Synthesis delay models Why are technology independent delay reductions hard? b e t t e r Lack of fast and accurate delay models s 1. # levels, fast but crude l 2. o # levels + correction term (fanout, wires, ): a little better, but still crude (what coefficients to use?) w 3. e Technology mapped: reasonable, but very slow r 4. Place and route: better but extremely slow 5. Silicon: best, but infeasibly slow (except for FPGAs) ECE 260B CSE 241A Static Timing Analysis 66
Clustering/ partial-collapse Traditional critical-path based methods require Well defined critical path Good delay/slack information Problems: Good delay information comes from mapper and layout Delay estimates and models are weak Possible solutions: Better delay modeling at technology independent level Make speedup, insensitive to actual critical paths and mapped delays ECE 260B CSE 241A Static Timing Analysis 67
Overview of Solutions for delay 1. Circuit re-structuring Rescheduling operations to reduce time of computation 2. Implementation of function trees (technology mapping) Selection of gates from library - Minimum delay (load independent model) - Minimize delay and area Implementation of buffer trees 3. Resizing Focus here on circuit re-structuring ECE 260B CSE 241A Static Timing Analysis 68
Circuit re-structuring Approaches: Local: Mimic optimization techniques in adders Carry lookahead (tree height reduction) Conditional sum (generalized select transformation) Carry bypass (generalized bypass transformation) Global: Reduce depth of entire circuit Partial collapsing Boolean simplification ECE 260B CSE 241A Static Timing Analysis 69
Re-structuring methods Performance measured by 1. levels, 2. sensitizable paths, 3. technology dependent delays Level based optimizations: Tree height reduction (Singh 88) Partial collapsing and simplification (Touati 91) Generalized select transform (Berman 90) Sensitizable paths Generalized bypass transform (Mcgeer 91) ECE 260B CSE 241A Static Timing Analysis 70
Re-structuring for delay: tree-height reduction i l 6 n 5 5 j m 1 4 3 h 1 Critical region k 1 i Collapsed Critical region n 0 0 2 1 j m 4 3 h 5 1 Duplicated logic k 0 0 0 0 2 0 0 a b c d e f g 0 0 a b 0 0 2 0 0 c d e f g ECE 260B CSE 241A Static Timing Analysis 71
Restructuring for delay: path reduction 1 i Collapsed Critical region n 0 0 2 1 j m 4 3 h 5 Duplicated logic 1 k 1 i 4 n 3 2 1 0 0 2 1 j New delay = 5 m 4 3 h 5 1 k 0 0 a b 0 0 2 0 0 c d e f g 0 0 a b 0 0 2 0 0 c d e f g ECE 260B CSE 241A Static Timing Analysis 72
Generalized select transform (GST) Late signal feeds multiplexor a b c d e f g out a=0 a=1 b b ECE 260B CSE 241A Static Timing Analysis 73 c d e f g c d e f g a 0 1 out
Generalized bypass transform (GBX) Make critical path false Speed up the circuit Bypass logic of critical path(s) f m =f f m+1 f n =g f m =f f m+1 f n =g 0 g 1 Boolean difference ECE 260B CSE 241A Static Timing Analysis 74 dg df s a 0 redundant
0/1 b Note: h a GBX Boolean a difference = = h h a a c a=0 a=1 g 0 g 1 dh da b b h GST vs GBX 0/1 c c d e f g c d e f g b a g GBX 0 g 1 h a=0 a=1 b b c d e f g c d e f g ECE 260B CSE 241A Static Timing Analysis 75 a 0 1 out GST
Thank you! ECE 260B CSE 241A Static Timing Analysis 76