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04/06/03 1
ECE 551: Digital System Design & Synthesis
Lecture Set 99.1: Constraints and Timing
(In separate file)9.2: Optimization - Part 1
9.3: Optimization - Part 2
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ECE 551 - Digital System Design & Synthesis Lecture 9.2 - Optimization and Timing Analysis
Overview Optimization Phases Compilation Types Optimization Flow Compilation Strategies Controlling Specific Optimization Steps Controlling Logic-Level and Gate-Level
Optimization
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References
Design Compiler Reference Manual: Optimization and Timing Analysis
Design Compiler User Guide
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Optimization Process
Optimization modifies the initial netlist resulting from elaboration. Uses cells from the technology library Attempts to meet all specified constraints
The process is divided into major phases All or some selection of the major phases
may be performed during optimization Phase selection can be controlled by the
user
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Major Optimization Phases
Combinational optimization Technology-independent Technology-dependent
Sequential optimization Initial Final
I/O pad optimization Local optimizations
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Combinational Optimization Converts a logic-level description to a gate-level
netlist Example: See next slide Subphases: See slide after next
Technology-independent • Operates at the Boolean level with interconnected gates
represented by Boolean equations.• Applies algebraic and Boolean techniques• Flattening and Structuring (covered later!)
Technology-specific (mapping)• Operates at the gate level • Selects components from library to approach defined area
and delay goals (covered later!)
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Combinational Optimization
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Combinational Optimization
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Initial Sequential Optimization
Maps sequential logic to cells in the library First phase of technology-specific
optimization Cannot select optimum sequential cell due to
lack of combinational delay info Defines sequential cells producing “islands”
of combinational logic Sets timing constraints on combinational
“islands” required to meet setup and hold constraints on the sequential cells
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Final Sequential Optimization
Optimizes timing critical sequential logic Later phase of technology-dependent optimization Has accurate values for delays through I/Os and through
combinational logic Effects:
• Improve delay by using higher-performance sequential cells
• Reduce delay and area by use of complex sequential cells
• Improve area by remapping sequential elements (due to setting compile_sequential_area_recovery to true)
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I/O Pad Optimization
Insertion and mapping of I/O pads Follows mapping of combinational logic
and initial mapping of sequential logic Adds input and output buffers to each
port in top-level design Sizes buffers to meet timing constraints
on paths involving inputs and outputs Uses smallest buffer that meets timing
specs to reduce current and power consumption
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Local Optimization Incremental modifications to locally
optimize gate level logic Final phase of gate level optimization Example: See next slide Effects
• Improve delay• Improve area
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Local Optimization
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Compilation Types
Full - optimizes a complete RTL design Can include all of the various phases
both technology-independent and technology-dependent
Removes and rebuilds logic/gate structure across entire design
Invoked by compile
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Compilation Types (continued)
Test-Ready - integrates optimization and scan insertion Accounts for timing impact of scan
design throughout the synthesis process Eliminates recompile after scan
insertion Invoked by compile -scan
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Compilation Types (continued)
Incremental - focuses on areas of design where constraints not met Structures where constraints met are
preserved Incremental mapping begins with existing
gates from earlier compilation Restructuring only if area or speed
improved Invoked with compile -incremental_mapping
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Compilation Types (continued)
Top Level - fixes constraint violations at top level as subblocks in the design are assembled. Assumes subblocks compiled and
meeting timing constraints Design rule violations fixed even if in
subblocks Invoked with compile -top
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Compilation Types (continued)
Critical Path Resynthesis Finds critical path and attempts full
compilation on logic along that path Repeats on new critical path Requires DC Expert version Invoked with compile -map_effort_high
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Compilation Types (continued) Logic Duplication and Mapping to Wide-
Fanin Gates (DC Ultra only) Evaluates cells along critical path Determines if timing or area can be improved
by replacing cell groups with complex, wide-fanin cells
Also improves timing of high-fanout or heavily-loaded nets by duplicating and restructuring large sections of logic
Improves timing at expense of area
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Compilation Types (continued) Optimizing Once for Best- and Worst-
Case Conditions Optimizing with Multiple Libraries
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Optimization Flow FSM Optimization Subdesign Ungrouping (set_ungroup) High-Level Optimization Synthetic Library Implementation Selection Sequential Inference Flattening Structuring Initial Combinational Mapping I/O Pad Insertion and Optimization
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Optimization Flow (continued) Mapping Optimization
Phase 1: Delay Optimization Phase 2: Design Rule Fixing, Part 1 (no
delay cost increase) Phase 3: Design Rule Fixing, Part 2 (delay
cost increase; not set_cost_priority delay) Phase 4: Area Optimization
Verifying Design Functionality
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Compilation Strategies
StrategiesTop-down hierarchicalCompile-characterize-write script-recompile
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Top-Down Hierarchical Compile (TDHC) Hierarchy not collapsed Constraints at top level Continues to optimize circuit until all
constraints met after each module compiled
Requires recompile of subdesigns on critical paths
Stops when no further improvement or goals achieved
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TDHC (continued) Note that entire design is being
manipulated at once Default strategy Intermodule dependencies handled
automatically Multiple instances of same design must be
handled using: uniquify set_dont_touch ungroup
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TDHC (continued) Procedure:
Read in entire design Make top level the current design Resolve multiple references: dcug 8-18 to 8-26 Apply constraints and attributes at the top level Compile
Preserves hierarchy and optimizes individual levels based on constraints at top level
Example: dcrmo pp. 2-5 to 2-7 Acrobat Document
Acrobat Document
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Compile-Characterize- Write Script-Recompile (CCWSR) Optimize (nonunique) designs using
context information or time budgets Mark lower level blocks that have been
optimized don’t_touch Optimize higher level blocks in succession Useful for medium/large designs without
good interblock specifications Facilitates large designs since only single
block in optimization at a time
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CCWSR (continued)
Requires iteration until design is stable Requires manual revision control Procedure:
1. Compile subblocks independently using a default script for drive and load estimates
2. Read in entire compiled design3. Characterize one subblock4. Use write_script to save characterization
info
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CCWSR (continued)
5. Clear memory; read in previously characterized subblock and recompile using saved script
6. Read in the entire compiled design again without the old subblock; use recompiled subblock
7. Choose another subblock and repeat steps 3 through 7 until all subblocks are recompiled, using their actual environments.
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CCWSR (continued)
Comments Compile automatically goes down
hierarchy to submodules - to restrict to current design, remove or omit submodule from database or use set_dont_touch
Compile is bottom-up Characterize is top-down
Example: dcrmo pp. 2-9 and 2-10Acrobat Document
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CCWSR (Continued)
What is really being done if pure CCWSR used? Compile all subblocks bottom up using rough
estimates of drive, load and timing constraints. Note that some constraints such as clock period are known.
Read in the entire design and apply the top level constraints. Then the attributes and constraints of any subblock can be determined based on the prior compile of all subblocks.
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CCWSR (Continued)
The above is precisely what characterize does.
The script written captures the attributes and constraints of the subblock chosen (one with worst violations is a good choice)
The subblock can then be recompiled based on this current “state” of the optimization process.
The new subblock replaces the old and process repeated for a different subblock
See Chapter 7 of User Guide for Script!
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Mixing Strategies
See figure on next slide Detailed time budget for A, B, C, and D
means they can be compiled separately.
Procedure• Compile A• Use CCWSR for the ascending the hierarchy at B• Compile C• Use TDHC for the hierarchy at D
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Mixing Strategies