  CHC-COMP-21 is the fourth competition of solvers for Constrained Horn
Clauses. In this year, 7 solvers participated at the competition, and were
evaluated in 7 separate tracks on problems in linear integer arithmetic, linear
real arithmetic, arrays, and algebraic data-types. The competition was run in
March 2021 using the StarExec computing cluster. This report gives an overview
of the competition design, explains the organisation of the competition, and
presents the competition results.
