RELEASE-NOTES

Release Notes for CVC4 1.6, June 2018

** Getting started

If you run CVC4 without arguments, you will find yourself in an interactive
CVC4 session, which expects commands in CVC4's native language (the so-called
"presentation" language).  To use SMT-LIB, use the "--lang smt" option on the
command line.  For stricter adherence to the standard, use "--smtlib-strict"
(see below regarding SMT-LIB compliance).

When a filename is given on the command line, the file's extension determines
the language parser that's used (e.g., file.smt is SMT-LIB 1.2, file.smt2
is SMT-LIB 2.6, and file.cvc is the presentation language).  To override
this, you can use the --lang option.

** Type correctness

The CVC family of systems relies on Type Correctness Conditions (TCCs) when
mixing two types that have a compatible base type.  TCCs, and the checking of
such, are not supported by CVC4 1.6.  This is an issue when mixing integers and
reals.  A function defined only on integers can be applied to REAL (as INT is a
subtype of REAL), and CVC4 will not complain.  It is up to the user to ensure 
that the REAL expression must be an integer.  If the REAL expression is not
an integer and is used where an INT is expected, CVC4 may produce strange
results.

For example: 

  f : INT -> INT;
  ASSERT f(1/3) = 0;
  ASSERT f(2/3) = 1;
  CHECKSAT;
  % sat
  COUNTEREXAMPLE;
  % f : (INT) -> INT = LAMBDA(x1:INT) : 0;

This kind of problem can be identified by checking TCCs.  Though CVC4 does not
(yet) support TCCs, CVC3 can be used to produce TCCs for this input (with the
+dump-tcc option). The TCC can then be checked by CVC4 or another solver.
(CVC3 can also check TCCs at the same time it creates them, with +tcc.)

** Changes in CVC's Presentation Language

The native language of all solvers in the CVC family, referred to as the
"presentation language," has undergone some revisions for CVC4.  The
most notable is that CVC4 does _not_ add counterexample assertions to
the current assertion set after a SAT/INVALID result.  For example:

  x, y : INT;
  ASSERT x = 1 OR x = 2;
  ASSERT y = 1 OR y = 2;
  ASSERT x /= y;
  CHECKSAT;
  % sat
  QUERY x = 1;
  % invalid
  QUERY x = 2;
  % invalid

Here, CVC4 responds "invalid" to the second and third queries, because
each has a counterexample (x=2 is a counterexample to the first, and
x=1 is a counterexample to the second).  However, CVC3 will respond
with "valid" to one of these two, as the first query (the CHECKSAT)
had the side-effect of locking CVC3 into one of the two cases; the
later queries are effectively querying the counterexample that was
found by the first.  CVC4 removes this side-effect of the CHECKSAT and
QUERY commands.

CVC4 supports rational literals (of type REAL) in decimal; CVC3 did not
support decimals.

CVC4 does not have support for predicate subtypes, although these are
planned for future releases.

** SMT-LIB compliance

Every effort has been made to make CVC4 compliant with the SMT-LIB 2.6
standard (http://smtlib.org/).  However, when parsing SMT-LIB input,
certain default settings don't match what is stated in the official
standard.  To make CVC4 adhere more strictly to the standard, use the
"--smtlib-strict" command-line option.  Even with this setting, CVC4 is
somewhat lenient; some non-conforming input may still be parsed and
processed.

For the latest news on SMT-LIB compliance, please check:

  http://cvc4.cs.stanford.edu/wiki/SMT-LIB_Compliance

** Getting statistics

Statistics can be dumped on exit (both normal and abnormal exits) with
the --stats command line option.

** Time and resource limits

CVC4 can be made to self-timeout after a given number of milliseconds.
Use the --tlimit command line option to limit the entire run of
CVC4, or use --tlimit-per to limit each individual query separately.
Occasionally, you may encounter a problem for which --tlimit does not work very
well.  If you suspect this might be the case, please report it as a bug.  You
can also use "-vv" (double verbosity) to see what CVC4 is doing.

Time-limited runs are not deterministic; two consecutive runs with the
same time limit might produce different results (i.e., one may time out
and respond with "unknown", while the other completes and provides an
answer).  To ensure that results are reproducible, use --rlimit or
--rlimit-per.  These options take a "resource count" (presently, based on
the number of SAT conflicts) that limits the search time.  A word of
caution, though: there is no guarantee that runs of different versions of
CVC4 or of different builds of CVC4 (e.g., two CVC4 binaries with different
features enabled, or for different architectures) will interpret the resource
count in the same manner.

CVC4 does not presently have a way to limit its memory use; you may opt
to run it from a shell after using "ulimit" to limit the size of the
heap.

** Proof support

CVC4 1.6 has support for proofs when using uninterpreted functions, arrays,
bitvectors, or their combinations, and proofs are enabled by default.
(Run the configure script with --disable-proof to disable proofs).  Proofs
are exported in LFSC format.

** Nonlinear arithmetic

CVC4 1.6 has a state-of-the-art linear arithmetic solver as well as some
heuristic support for non-linear arithmetic.

** Portfolio solving

If enabled at configure-time (./configure --with-portfolio), a second
CVC4 binary will be produced ("pcvc4").  This binary has support for
running multiple instances of CVC4 in different threads.  Use --threads=N
to specify the number of threads, and use --thread0="options for thread 0"
--thread1="options for thread 1", etc., to specify a configuration for the
threads.  Lemmas are *not* shared between the threads by default; to adjust
this, use the --filter-lemma-length=N option to share lemmas of N literals
(or smaller).  (Some lemmas are ineligible for sharing because they include
literals that are "local" to one thread.)

Currently, the portfolio **does not work** with the theory of inductive
datatypes. This limitation will be addressed in a future release.

** Questions ??

CVC4 and its predecessors have an active user base.  You might want to
subscribe to the mailing list (http://cvc4.stanford.edu/#Technical_Support)
and post a question there.

** Reporting bugs and making feature requests

CVC4 is under active development.  Should you find a bug in CVC4's
documentation, behavior, API, or SMT-LIB compliance, or if you have
a feature request, please let us know on our bugtracker at

  https://github.com/CVC4/CVC4/issues

or send an email to cvc-bugs@cs.stanford.edu.