Vampire 4.9

Michael Rawson
TU Wien, Austria

There have been a number of improvements since Vampire 4.8, although it is still the same beast. For the first time this year, Vampire's schedules were constructed mostly using the Snake strategy selection tool, although a return of the traditional Spider is still possible in future. Improvements from the past year include:

A runtime-specialised version of unidirectional term ordering checks
Improvements to unification with abstraction
Surprising improvements to Vampire's basic routines such as renaming and unification
A simple interactive mode
Revitalisation of code trees
Experimental features not yet fully understood, mostly aimed at unit-equational reasoning.
Portability: Vampire is much more standards-compliant and portable than previously, with much-reduced dependence on platform-specific APIs and hardware architectures, aided by C++17

Vampire's higher-order support remains very similar to last year, although a re-implementation intended for mainline Vampire is already underway.

Architecture

Vampire [KV13] is an automatic theorem prover for first-order logic with extensions to theory-reasoning and higher-order logic. Vampire implements the calculi of ordered binary resolution, and superposition for handling equality. It also implements a MACE-style finite model builder for finding finite counter-examples [RSV16]. Splitting in resolution-based proof search is controlled by the AVATAR architecture which uses a SAT or SMT solver to make splitting decisions [Vor14, RB+16]. A number of standard redundancy criteria and simplification techniques are used for pruning the search space: subsumption, tautology deletion, subsumption resolution and rewriting by ordered unit equalities. The reduction ordering is the Knuth-Bendix Ordering. Substitution tree and code tree indexes are used to implement all major operations on sets of terms, literals and clauses. Internally, Vampire works only with clausal normal form. Problems in the full first-order logic syntax are clausified during preprocessing [RSV16]. Vampire implements many useful preprocessing transformations including the SInE axiom selection algorithm. When a theorem is proved, the system produces a verifiable proof, which validates both the clausification phase and the refutation of the CNF.

Strategies

Vampire 4.8 provides a very large number of options for strategy selection. The most important ones are:

Choices of saturation algorithm:
- Limited Resource Strategy [RV03]
- DISCOUNT loop
- Otter loop
- MACE-style finite model building with sort inference
Splitting via AVATAR [Vor14]
A variety of optional simplifications.
Parameterized reduction orderings.
A number of built-in literal selection functions and different modes of comparing literals [HR+16].
Age-weight ratio that specifies how strongly lighter clauses are preferred for inference selection. This has been extended with a layered clause selection approach [GS20].
Set-of-support strategy with extensions for theory reasoning.
For theory-reasoning:
- Ground equational reasoning via congruence closure.
- Addition of theory axioms and evaluation of interpreted functions [RSV21].
- Use of Z3 with AVATAR to restrict search to ground-theory-consistent splitting branches [RB+16].
- Specialised theory instantiation and unification [RSV18].
- Extensionality resolution with detection of extensionality axioms

Implementation

Vampire 4.9 is implemented in C++. It makes use of fixed versions of Minisat and Z3. See the GitHub repository and associated wiki for more information.

Expected Competition Performance

Vampire 4.9 should be an improvement on the previous version. A reasonably strong performance across all divisions is therefore expected. In the higher-order divisions, performance should be the same as last year.

References

Sud22: Martin Suda (2022), Vampire Getting Noisy: Will Random Bits Help Conquer Chaos? (System Description), IJCAR LNCS 13385, pp.659-667, Springer.