Satisfiability modulo the Theory of Bit Vectors

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Satisfiability modulothe Theory of Bit Vectors

• Alessandro Cimatti
• IRST, Trento, Italy
• cimatti_at_irst.itc.it

Joint work with R. Bruttomesso, A. Franzen, A.
Griggio, R. Sebastiani
We gratefully acknowledge support from the
Academic Research Program of Intel
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Index of the talk

• Satisfiability Modulo Theory
• The theory of Bit Vectors
• Satisfiability Modulo BV
• Bit blasting
• Eager encoding into Linear Integer Arithmetic
• A lazy approach
• Conclusions
• ( A preview of QF_UFBV32 at SMT-COMP )

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SMT in a nutshell

• Satisfiability Modulo Theory
• or beyond boolean SAT
• Decide the satisfiability of a first order
  formula with respect to a background theory
• Examples of relevant theories
• uninterpreted functions xy f(x) ! f(y)
• difference logic x y lt 7
• linear arithmetic 3x 2y lt 12
• arrays read(write(M, a0, v0) a1)
• their combinations
• bit vectors

4
Why SMT

• From SAT-based to SMT-based verification
• Representation of interesting problems
• timed automata
• hybrid automata
• pipelines
• software
• Efficient solving
• leverage availability of structural information
• hopefully retaining efficiency of boolean SAT

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Satisfiability Modulo Theory

• Satisfiability
• is there a truth-assignment to boolean variables
• and a valuation to individual variables
• such that formula evaluates to true?
• Standard semantics for FOL
• Assignment to individual variables
• Induces truth values to atoms
• Truth assignment to boolean atoms
• Induced value to whole formula

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Propositionalstructure
-
-
-
-

-

-
TA
TA
TA
TA
P P P
x y z w x
x y z w x
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Two Main Approaches to SMT

• the eager approach
• the lazy approach
• theory independent view
• theory specific view

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Eager Approach to SMT

• Main idea compilation to SAT
• STEP1 Theory part compiled to equisatisfiable
  pure SAT problem
• STEP2 run propositional SAT solver

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Lifted theory
Propositionalstructure
P P P
TA TA TA TA
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The Lazy approach

• Ingredients
• a boolean SAT solver
• a theory solver
• The boolean solver is modified to enumerate
  boolean (partial) models
• The theory solver is used to Check for theory
  consistency

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Propositionalstructure
TA
TA
TA
TA
P P P
TA TA TA TA
x y z w x
x y z w x
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MathSAT intuitions

• Two ingredients boolean search and theory
  reasoning
• find boolean model
• theory atoms treated as boolean atoms
• truth values to boolean and theory atoms
• model propositionally satisfies the formula
• check consistency wrt theory
• set of constraints induced by truth values to
  theory atoms
• existence of values to theory variables
• Example (P v (x 3)) (Q v (x y lt 1)) (y lt
  2) (P xor Q)
• Boolean model
• !P, (x 3), Q, (x y lt 1), (y lt 2)
• Check (x 3), (x y lt 1), (y lt 2)
• Theory contradiction!
• Another boolean model
• P , !(x 3) , !Q, (x y lt 1), (y lt 2)
• Check !(x 3), (x y lt 1), (y lt 2)
• Consistent e.g. x 0, y 0

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Boolean SAT search space
P
Q
Q
R
S
S
T
S
T
R
R
?
?
?
T SAT!
?
?

• The DPLL procedure
• Incremental construction of satisfying assignment
• Backtrack/backjump on conflict
• Learn reason for conflict
• Splitting heuristics

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MathSAT approach

• DPLL-based enumeration of boolean models
• Retain all propositional optimizations
• Conflict-directed backjumping, learning
• No overhead if no theory reasoning
• Tight integration between
• boolean reasoning and
• theory reasoning

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MathSAT search space
P
Q
Q
R
S
S
T
S
T
R
R
Bool ?
Bool T Math ?
Bool ?
Bool T Math T SAT!
Bool T Math ?
Bool ?

• Many boolean models are not theory consistent!

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Early pruning

• Check theory consistency of partial assignments

P
EPMath ?
EPMath T
Q
EPMath T
S
Pruned away in the EP step
EPMath T
T
EPMath T
R
Bool ?
Bool T Math T SAT!
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THEORY OF FIXED-WIDTH BIT VECTORS
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Bit Vectors Example
input a, b, c, d regN

• LTmp0 a
• LTmp1 2 b
• LTmp2 LTmp0 LTmp1
• LTmp3 4 c
• LTmp4 LTmp2 LTmp3
• LTmp5 8 d
• LOut LTmp4 LTmp5
• Are they equivalent?
• ((a 2b) 4c) 8d

• RTmp0 d
• RTmp1 RTmp0 ltlt 1
• RTmp2 c RTmp1
• RTmp3 RTmp2 ltlt 1
• RTmp4 b RTmp3
• RTmp5 RTmp4 ltlt 1
• ROut a RTmp5
• a ((b ((c (dltlt1)) ltlt1)) ltlt1)

I.e. LOut ROut ?
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Fixed Width Bit Vectors

• Constants
• 0b00001111, 0xFFFF,
• Variables
• valued over BitVectors of corresponding width
• implicit restriction to finite domain
• Function symbols
• selection x150
• concatenation y z
• bitwise operators x y, z w,
• arithmetic operators x y, z w,
• shifting x ltlt 2, y gtgt 3
• Predicate symbols
• comparators , ? , gt , lt , ,

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Fragments of BV theory

• Core
• selection
• concatenation
• Bitwise operators
• x y, x y, x y
• Arithmetic operators
• x y, x y, c x
• Core Bitwise Arithmetic
• Complexity of equality between BV terms
• Core is in P
• Core B A in NP
• Variable width bit vectors not covered here
• core is in NP
• small additions yield undecidability

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Decision procedures for BV

• Many approaches
• Cyrluk, Moeller, Ruess
• Moeller, Ruess
• Bjørner, Pichora
• Barrett, Dill, Levitt
• Focus on deciding conjunctions of literals
• Emphasis on proof obligations in ITP
• some emphasis on variable width, generic wrt N
• Shostak-style integration
• canonization
• solving

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SATISFIABILITY MODULO THEORY OF BIT VECTORS
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Satisfiability modulo Bit Vectors

• Applications of interest
• RTL hardware descriptions essentially bit vectors
• assembly-level programs
• software with finite precision arithmetic
• Key feature
• combination of control flow and data flow
• In principle, boolean logic can be encoded into
  BV
• control (boolean logic) encoded into width 1 BVs.
• Likely inefficient in comparison to SAT
• More natural to keep them separate at modeling
• structural info can be exploited for verification

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Approaches to SMT(BV)

• Bit blasting
• Eager Encoding into LA
• Lazy approach

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SMT(BV) via Bit Blasting
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SMT(BV) via Bit Blasting

• Boolean variables untouched
• Bit vector variables as collections of
  (unrelated) boolean variables
• x0, x1, , x63
• Selection/concatenations are trivial
• static detection
• Equalities / Assignments x y
• (x0 lt-gt y0) (x1 lt-gt y1) (x63 lt-gt y63)
• Bitwise operators x y
• x0 y0, x1 y1, , x63 y63
• Arithmetic operators x y
• BVADD(x0, , x63, y0, , y63)

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Comparison of Data Paths
input a, b, c, d regN

• LTmp0 a
• LTmp1 2 b
• LTmp2 LTmp0 LTmp1
• LTmp3 4 c
• LTmp4 LTmp2 LTmp3
• LTmp5 8 d
• LOut LTmp4 LTmp5
• Are they equivalent?
• ((a 2b) 4c) 8d

• RTmp0 d
• RTmp1 RTmp0 ltlt 1
• RTmp2 c RTmp1
• RTmp3 RTmp2 ltlt 1
• RTmp4 b RTmp3
• RTmp5 RTmp4 ltlt 1
• ROut a RTmp5
• a ((b ((c (dltlt1)) ltlt1)) ltlt1)

I.e. LOut ROut ?
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Bit Blasting Words

• a,b,c,d,
• blasted to a1,aN, b1,bN, c1,cN,
  d1,dN,
• LTmp6 ! RTmp6
• (LOut.1 ! ROut.1) or or (LOut.N ! ROut.N)
• LTmp1 2 b
• formula in 2N vars, conjunction of N iffs
• LTmp2 LTmp0 LTmp1
• formula relating 3N vars
• possibly additional vars required (e.g. carries)
• N 16 bits?
• 13 secs
• N 32 bits?
• 170 secs
• But obviously N 64 bits!
• stopped after 2h CPU time

Scalabilitywith respect to N???
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Bit-Blasting Pros and Conses

• Bottlenecks
• dependency on word width
• wrong level of abstraction
• boolean synthesis of arithmetic circuits
• assignments are pervasive
• conflicts are very fine grained
• e.g. discover x lt y
• Advantages
• let the SAT solver do all the work
• and nowadays SAT solvers are tough nuts to crack
• amalgamation of the decision process
• no distinction between control and data
• conflicts can be as fine grained as possible
• built-in capability to generate new atoms

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Enhancements to BitBlasting

• Tuning SAT solver on structural information
• e.g. splitting heuristic for adders
• Preprocessing SAT GBD05
• rewrite and normalize bit vector terms
• bit blasting to SAT

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SMT(BV) via reduction to SMT(LA)
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From BV to LIA

• RTL-Datapath Verification using Integer Linear
  Programming BD01
• BV constants as integers
• 0b32_1111 as 15
• BV variables as integer valued variables, with
  range constraints
• reg x 310 as x in range 0, 232)
• Assignments treated as equality, e.g. x y
• Arithmetic, e.g. z x y
• Linear arithmetic? not quite! BV Arithmetic is
  modulo 2N
• z x y - 2N s, with z in 0, 2N)
• Concatenation x y as 2n x y
• Selection relational encoding (based on
  integrity)
• x2316 as xm, where
• x 224 xh 216 xm xl, xl in 0, 216), xm
  in 0, 28), xl in 0, 28)
• Bitwise operators
• based on selection of individual bits
• SOLVER
• the omega test

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From SMT(BV) into SMT(LIA)

• Generalizes BD01 to deal with boolean structure
• Eager encoding into SMT(LIA)
• Unfortunately, not very efficient
• More precisely, a failure

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Retrospective Analysis

• Crazy approach?
• Arithmetic
• Linear arithmetic? not quite! BV Arithmetic is
  modulo 2N
• Selection and Concatenation
• an easy problem becomes expensive!
• Bitwise operators
• HARD!!!
• Available solvers not adequate
• integers with infinite precision
• reasoning with integers may be hard (e.g. BnB
  within real relaxation)
• Functional dependencies are lost!
• A clear culprit static encoding
• depending on control flow, same signal is split
  in different parts
• z if P then x70 y30 else x52
  y103
• x, y and also z are split more than needed
• the notion of maximal chunk depends on P !!!

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SMT(BV) via online BV reasoning
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A lazy approach

• Based on standard MathSAT schema
• DPLL-based model enumeation
• Dedicated Solver for Bit vectors
• The encoding leverages information resulting from
  decisions
• Given values to control variables, the data path
  is easier to deal with (e.g. maximal chunks are
  bigger)
• Layering in the theory solver
• equality reasoning
• limited simplification rules
• full blown bit vector solver only at the end

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The architecture
Boolean enumeration
BV solver
EUF reasoning
LIAencoding
BV rewriter
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Rewriting rules

• evaluation of constant terms
• 0b8_0101010142 becomes 0b3_101
• rules for equality
• x y and Phi(x) becomes Phi(y)
• based on congruence closure
• splitting concatenations
• (x y) z becomes x zh_n y zl_n

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Rewriting rules

• pushing selections
• (x y)70 becomes (x70 y70)
• (x y)238 becomes (x70 y158)
• pigeon-hole rules
• from (x ! 0 x ! 1 x ! 2 x lt 3) derive
  false

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BV rewriter

• Rules are applied until
• fix point reached
• contradiction found
• Implementation based on EUF reasoner
• rules as merges between eq classes
• Open issues
• incrementality/backtrackability
• selective rule activation
• conflic set reconstruction
• When it fails

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LIA encoding (the last hope)

• LIA encoding
• idenfication of maximal slices
• purification separating out arithmetic and BW
  by introduction of additional variables
• NB on resulting problems
• LIA encoding always superior to bit blasting!!!
• cfr DB01

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Status of Implementation

• Implementation still in prototypical state
• Does a lot of stupid things
• conflict minimization by deletion filtering
• checking that conflict are in fact minimal
• unnecessary calls to LA for SAT clusters
• calling LA solver implemented as dump on file,
  and run external MathSAT
• huge conflict sets

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A very very preliminary evaluation
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Competitors

• Run against MiniSAT 1.14
• winner of SAT competition in 2005
• KEY REMARK
• boolean methods are very mature
• A good reason for giving up?

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Test benches

• 74 benchmarks from industrial partner
• would have been ideal for SMT-COMP
• QF_UFBV32
• Unfortunately
• can not be disclosed
• will have to be destroyed after the
  collaboration
• hopefully our lives will be spared ?

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Conclusions

• A market need for SMT(BV) solvers
• Bit Blasting tough competitors
• After a failure,
• Preliminary results are encouraging
• Future challenges
• optimize BV solver
• better conflict sets
• tackle some RTL verification cases
• extension to memories

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A small digression on QF_UFBV32 at SMT-COMP
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QF_UFBV32 at SMT-COMP

• the MathSAT you will see there IS NOT the one I
  described
• We currently have no results for QF_UFBV
• Easy benchmarks
• QF_UFBV32 not particularly SMT
• the boolean component is nearly missing
• the BV part is easily solvable by bit blasting
• We entered SMT-COMP QF_UFBV32
• MathSAT based on BIT BLASTING to SAT
• NuSMV based on bit blasting to BDDs

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QF_UFBV Bit Blasting to SAT

• Preprocessing based on
• Ackermans elimination of function symbols
• rewriting simplification
• bit blasting
• Core call SAT solver underlying MathSAT
• every SAT problem in lt 0.3 secs
• most UNSAT within seconds
• a handful of hard ones between 300 and 500 secs

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BDDs (???) on SMT-COMP tests

• Even NuSMV entered SMT-COMP
• Ackermans elimination of functional symbols
• Rewriting preprocessor
• Core solver
• based on BDDs
• conjunctively partitioned problem
• structural BDD-based ordering (bit interleaving)
• (almost) no dynamic reordering
• affinity-based clustering, threshold 100 nodes
• early quantification
• Seems to work well both on SAT and UNSAT instances

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RESULTS

• first STP
• then YICES
• then NuSMV
• then CVC3 (but no results on two samples)
• then MathSAT BITBLASTING
• 3rd on SAT
• last on UNSAT

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SAT instances
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UNSAT instances