SMT2: support onehot and onehot0

This adds support for the onehot and onehot0 predicates to the SMT2 backend.

Author: kroening

src/solvers/flattening/boolbv.cpp

@@ -412,8 +412,10 @@ literalt boolbvt::convert_rest(const exprt &expr)
           expr.id()==ID_reduction_nor || expr.id()==ID_reduction_nand ||
           expr.id()==ID_reduction_xor || expr.id()==ID_reduction_xnor)
     return convert_reduction(to_unary_expr(expr));
-  else if(expr.id()==ID_onehot || expr.id()==ID_onehot0)
-    return convert_onehot(to_unary_expr(expr));
+  else if(expr.id() == ID_onehot)
+    return convert_onehot(to_onehot_expr(expr));
+  else if(expr.id() == ID_onehot0)
+    return convert_onehot(to_onehot0_expr(expr));
   else if(
     const auto binary_overflow =
       expr_try_dynamic_cast<binary_overflow_exprt>(expr))

src/solvers/smt2/smt2_conv.cpp

@@ -1831,6 +1831,10 @@ void smt2_convt::convert_expr(const exprt &expr)
       out << ")) #b1)"; // bvlshr, extract, =
     }
   }
+  else if(expr.id() == ID_onehot)
+  {
+    convert_expr(to_onehot_expr(expr).lower());
+  }
   else if(expr.id()==ID_extractbits)
   {
     const extractbits_exprt &extractbits_expr = to_extractbits_expr(expr);

src/util/bitvector_expr.cpp

@@ -306,3 +306,38 @@ exprt zero_extend_exprt::lower() const
     return extractbits_exprt{op(), 0, type()};
   }
 }
+
+static std::pair<exprt, exprt> onehot_lowering(const exprt &expr)
+{
+  exprt one_seen = false_exprt{};
+  const auto width = to_bitvector_type(expr.type()).get_width();
+  exprt::operandst more_than_one_seen_disjuncts;
+
+  for(std::size_t i = 0; i < width; i++)
+  {
+    auto bit = extractbit_exprt{expr, i};
+    more_than_one_seen_disjuncts.push_back(and_exprt{bit, one_seen});
+    one_seen = or_exprt{one_seen, bit};
+  }
+
+  auto more_than_one_seen = disjunction(more_than_one_seen_disjuncts);
+
+  return {one_seen, more_than_one_seen};
+}
+
+exprt onehot_exprt::lower() const
+{
+  auto symbol = symbol_exprt{"onehot-op", op().type()};
+  auto onehot_lowering = ::onehot_lowering(symbol);
+
+  return let_exprt{
+    symbol,
+    op(),
+    and_exprt{onehot_lowering.first, not_exprt{onehot_lowering.second}}};
+}
+
+exprt onehot0_exprt::lower() const
+{
+  auto symbol = symbol_exprt{"onehot-op", op().type()};
+  return let_exprt{symbol, op(), not_exprt{::onehot_lowering(symbol).second}};
+}

src/util/bitvector_expr.h

@@ -1742,4 +1742,74 @@ inline zero_extend_exprt &to_zero_extend_expr(exprt &expr)
   return static_cast<zero_extend_exprt &>(expr);
 }
 
+/// \brief A Boolean expression returning true iff the given
+/// operand consists of exactly one '1' and '0' otherwise.
+class onehot_exprt : public unary_predicate_exprt
+{
+public:
+  explicit onehot_exprt(exprt _op)
+    : unary_predicate_exprt(ID_onehot, std::move(_op))
+  {
+  }
+
+  /// lowering to extractbit
+  exprt lower() const;
+};
+
+/// \brief Cast an exprt to a \ref onehot_exprt
+///
+/// \a expr must be known to be \ref onehot_exprt.
+///
+/// \param expr: Source expression
+/// \return Object of type \ref onehot_exprt
+inline const onehot_exprt &to_onehot_expr(const exprt &expr)
+{
+  PRECONDITION(expr.id() == ID_onehot);
+  onehot_exprt::check(expr);
+  return static_cast<const onehot_exprt &>(expr);
+}
+
+/// \copydoc to_onehot_expr(const exprt &)
+inline onehot_exprt &to_onehot_expr(exprt &expr)
+{
+  PRECONDITION(expr.id() == ID_onehot);
+  onehot_exprt::check(expr);
+  return static_cast<onehot_exprt &>(expr);
+}
+
+/// \brief A Boolean expression returning true iff the given
+/// operand consists of exactly one '0' and '1' otherwise.
+class onehot0_exprt : public unary_predicate_exprt
+{
+public:
+  explicit onehot0_exprt(exprt _op)
+    : unary_predicate_exprt(ID_onehot0, std::move(_op))
+  {
+  }
+
+  /// lowering to extractbit
+  exprt lower() const;
+};
+
+/// \brief Cast an exprt to a \ref onehot0_exprt
+///
+/// \a expr must be known to be \ref onehot0_exprt.
+///
+/// \param expr: Source expression
+/// \return Object of type \ref onehot0_exprt
+inline const onehot0_exprt &to_onehot0_expr(const exprt &expr)
+{
+  PRECONDITION(expr.id() == ID_onehot0);
+  onehot0_exprt::check(expr);
+  return static_cast<const onehot0_exprt &>(expr);
+}
+
+/// \copydoc to_onehot0_expr(const exprt &)
+inline onehot0_exprt &to_onehot0_expr(exprt &expr)
+{
+  PRECONDITION(expr.id() == ID_onehot0);
+  onehot0_exprt::check(expr);
+  return static_cast<onehot0_exprt &>(expr);
+}
+
 #endif // CPROVER_UTIL_BITVECTOR_EXPR_H

unit/util/bitvector_expr.cpp

@@ -1,8 +1,15 @@
 // Author: Diffblue Ltd.
 
+#include <util/arith_tools.h>
 #include <util/bitvector_expr.h>
 #include <util/bitvector_types.h>
+#include <util/cout_message.h>
+#include <util/namespace.h>
+#include <util/std_expr.h>
+#include <util/symbol_table.h>
 
+#include <solvers/flattening/boolbv.h>
+#include <solvers/sat/satcheck.h>
 #include <testing-utils/use_catch.h>
 
 TEST_CASE(
@@ -64,3 +71,34 @@ TEMPLATE_TEST_CASE(
     }
   }
 }
+
+TEST_CASE("onehot expression lowering", "[core][util][expr]")
+{
+  console_message_handlert message_handler;
+  message_handler.set_verbosity(0);
+  satcheckt satcheck{message_handler};
+  symbol_tablet symbol_table;
+  namespacet ns{symbol_table};
+  boolbvt boolbv{ns, satcheck, message_handler};
+  unsignedbv_typet u32{32};
+
+  GIVEN("A bit-vector that is one-hot")
+  {
+    boolbv << onehot_exprt{from_integer(64, u32)}.lower();
+
+    THEN("the lowering of onehot is true")
+    {
+      REQUIRE(boolbv() == decision_proceduret::resultt::D_SATISFIABLE);
+    }
+  }
+
+  GIVEN("A bit-vector that is not one-hot")
+  {
+    boolbv << onehot_exprt{from_integer(5, u32)}.lower();
+
+    THEN("the lowering of onehot is false")
+    {
+      REQUIRE(boolbv() == decision_proceduret::resultt::D_UNSATISFIABLE);
+    }
+  }
+}