Handling Complexity in FEV

Erik Seligman

CS 510, Lecture 6, January 2009

Outline

Complexity Intro & Basics Hierarchical Comparison Cut Points Case Splitting Cheating

Outline

Complexity Intro & Basics Hierarchical Comparison Cut Points Case Splitting Cheating

Complexity: Intro & Basics

Complexity = Time or Memory Blowup Two ways to see complexity

• Tool crash (hopefully won’t happen)

• Compare Points reported as ‘Abort’

What does this mean?• Logic was too complex to analyze

• Maybe bad options selected

Tool Solutions Some tool options can help resolve

• set compare effort high|ultra|complete– Causes LEC to work harder before giving up

• analyze datapath [-merge] | analyze multiplier– LEC looks for common datapath structures– Useful if lots of arithmetic operations– Specialized alg if you have a multiplier

• compare -single– Slow, but concentrates on each point standalone

analyze abort• New feature to look at abort point & try to ID good

tool options• Sloooow though (can run overnight, or abort too!)

Tool Capacity Issues

Monitor LEC process for memory blowup• May be root cause of abort or seg fault

Look for options for run on hi-mem server• Memory blowup is known FV hazard

• If you were close, this may be just enough

Also try new “compare –parallel”• Uses multiple machines on network

• May improve memory & runtime

Design Issues: Memories

Memories are complex• Usually bbox a memory, verify standalone

• Conformal supplies special library– LEC is easy with ‘verplex memory primitives’

– Otherwise extremely hard

How are memories FEVed?• Inherently transistor-based, not simple gates

• Need to define common structures for tool

Design Issues: Don’t Cares

Don’t-Care (DC) Space• Remember, a DC is an RTL ambiguity

– Synthesis has freedom to decide

• Large DC space makes verification hard– If DCs cause aborts, consider assigning all values

How to diagnose?• Look for this compile message

– F34: Convert X assignment(s) as don't care(s)– report messages -rule F34 –verbose for exact lines

• Experiment: set x conversion 0 –gold– Ambiguous (X) cases = 0 false negatives– But experiment useful to see if DCs did cause aborts

Outline

Complexity Intro & Basics Hierarchical Comparison Cut Points Case Splitting Cheating

Using Hierarchical Verification

Hierarchy Example

• Simple option: FEV full TOP design

• Best option if feasible

Hierarchy Example

• Hierarchical: 3 FEV comparisons• TOP.Green, Top.Yellow• TOP with Green and Yellow bboxed

• Are there problems with this approach?

Hierarchy Problem: Constraints

• Circuit topology constraints for bboxes• What constraints needed in this example?

Hierarchy Problem: Constraints

• Circuit topology constraints for bboxes• What constraints needed in this example?

• Green: add pin eq a b• Yellow: add pin constraint 0 c• TOP: add pin constraint 0 Yellow.d

Hierarchy In Conformal

write hier dofile –constraint• Odd but convenient command

• IDs common hierarchies

• Writes new dofile, verifying all pieces

• Generates summary report at end

Tricky when debugging!• Need to rerun on hierarchy with error

Generated dofile: excerpts

set root module topadd pin constraint 0 yellow.d -bothset sys mode leccomparereport hier_compare data …set root module greenadd pin eq a b –both…

Challenges of Hierarchical FEV Synthesis tools can flatten hierarchy

• May make hierarchical FEV impossible

• May need to request backend preserve some hierarchy

Mismatching submodule interfaces• Extra pins inserted in synthesis

• Pin name changes

• May need to use Conformal commands to ignore/map pins

Hierarchy Special Case: Cell Libraries Should use pre-FEVed library

• No sense in repeatedly verifying basic flops, latches, ANDs, ORs, etc

• Be sure lib team is doing low-level FEV!

Check that this is the case• Should have .v (or .lib) file defining each cell

• You should see simple behavioral RTL for cells, not full transistor-level logic

always @(posedge clk) q<=d

Outline

Complexity Intro & Basics Hierarchical Comparison Cut Points Case Splitting Cheating

Cut Points in FEV

What is a cut point?

Verifying big logic cone may be hard• Harder in low-frequency designs

Divide logic at points other than states?• Hard– synthesis only requires state matching

• But often some internals correspond too

• In extreme cases, recode RTL to enable

Cut point = non-state to treat as key point• Map & verify just like latches/flops

• Reduces logic cones being analyzed

Cut Point Example

add cut point p1 –goldadd cut point p2 –rev

add ren rule r1 p1 p2 -gold

Cut Point Problems?

Is p1 still a useful cut point?

Cut Point Problems?

Is p1 still a useful cut point? Maybe…

Cut Point Problems?

Is p1 still a useful cut point? Maybe…• Don’t forget about constraint issues too

Outline

Complexity Intro & Basics Hierarchical Comparison Cut Points Case Splitting Cheating

Case Splitting

What is Case Splitting?

FV is analyzes all cases together• Good tool engines may be smarter

What if small inputs activate/deactivate lots of logic?• Example: mode bits

Constrain appropriate pins to 1 or 0• Then compare twice

• Or constrain <n> bits, then 2^n compares

Case Splitting Example

Suppose compare of f2 & f4 Aborts, and we want to case-split on f1/f3.

Compare Case #1

First assign const of 0 to flop.

Compare Case #2

Then assign const value of 1.

After both cases pass, we are equivalent!

Case splitting in Conformal

Conformal terminology: “partition” Convenient commands

• add partition key_point

• write partition dofile

Be careful!• <n> partition points 2^n iterations

• Poorly chosen points worse than useless!

Outline

Complexity Intro & Basics Hierarchical Comparison Cut Points Case Splitting Cheating

Last Resorts: Cheating

Simulation

Obvious, defeats point of formal! But may be useful last resort

• If small handful of points defying FEV Be sure you exhausted FEV options!

• All tool compare/analyze options?

• Did you try hierarchical verify?

• Did you try cut points & case splitting?

• Did you consider recoding RTL to increase cut points and/or hierarchy, or reduce don’t-care space?

Simulation compare in LEC

compare –random <n>• Runs <n> simulation vectors

• Should only run on specific problem points

Not a good solution• But better than nothing

• Probably will get more vectors than GLS– Fast simulation only on problem cone

References / Further Reading

http://www.cdnusers.org/community/encounter/Resources/resources_design/equiv/Dtp_cdnliveemea2006_itayarom.pdf

http://www.cdnusers.org/Portals/0/cdnlive/na2006/2.4.1/2.4.1_paper.pdf

http://www.venusmultimedia.net/Aborts.htm