Merge pull request #8442 from diffblue/zero_extend

zero extension expression

Author: kroening

src/solvers/flattening/boolbv.cpp

@@ -165,6 +165,8 @@ bvt boolbvt::convert_bitvector(const exprt &expr)
     return convert_replication(to_replication_expr(expr));
   else if(expr.id()==ID_extractbits)
     return convert_extractbits(to_extractbits_expr(expr));
+  else if(expr.id() == ID_zero_extend)
+    return convert_bitvector(to_zero_extend_expr(expr).lower());
   else if(expr.id()==ID_bitnot || expr.id()==ID_bitand ||
           expr.id()==ID_bitor || expr.id()==ID_bitxor ||
           expr.id()==ID_bitxnor || expr.id()==ID_bitnor ||

src/solvers/floatbv/float_bv.cpp

@@ -692,8 +692,10 @@ exprt float_bvt::mul(
 
   // zero-extend the fractions (unpacked fraction has the hidden bit)
   typet new_fraction_type=unsignedbv_typet((spec.f+1)*2);
-  const exprt fraction1=typecast_exprt(unpacked1.fraction, new_fraction_type);
-  const exprt fraction2=typecast_exprt(unpacked2.fraction, new_fraction_type);
+  const exprt fraction1 =
+    zero_extend_exprt{unpacked1.fraction, new_fraction_type};
+  const exprt fraction2 =
+    zero_extend_exprt{unpacked2.fraction, new_fraction_type};
 
   // multiply the fractions
   unbiased_floatt result;
@@ -750,7 +752,7 @@ exprt float_bvt::div(
     unsignedbv_typet(div_width));
 
   // zero-extend fraction2 to match fraction1
-  const typecast_exprt fraction2(unpacked2.fraction, fraction1.type());
+  const zero_extend_exprt fraction2{unpacked2.fraction, fraction1.type()};
 
   // divide fractions
   unbiased_floatt result;

src/solvers/smt2/smt2_conv.cpp

@@ -2456,6 +2456,10 @@ void smt2_convt::convert_expr(const exprt &expr)
   {
     convert_expr(simplify_expr(to_bitreverse_expr(expr).lower(), ns));
   }
+  else if(expr.id() == ID_zero_extend)
+  {
+    convert_expr(to_zero_extend_expr(expr).lower());
+  }
   else if(expr.id() == ID_function_application)
   {
     const auto &function_application_expr = to_function_application_expr(expr);

src/solvers/smt2_incremental/convert_expr_to_smt.cpp

@@ -1469,6 +1469,15 @@ static smt_termt convert_expr_to_smt(
     count_trailing_zeros.pretty());
 }
 
+static smt_termt convert_expr_to_smt(
+  const zero_extend_exprt &zero_extend,
+  const sub_expression_mapt &converted)
+{
+  UNREACHABLE_BECAUSE(
+    "zero_extend expression should have been lowered by the decision "
+    "procedure before conversion to smt terms");
+}
+
 static smt_termt convert_expr_to_smt(
   const prophecy_r_or_w_ok_exprt &prophecy_r_or_w_ok,
   const sub_expression_mapt &converted)
@@ -1822,6 +1831,10 @@ static smt_termt dispatch_expr_to_smt_conversion(
   {
     return convert_expr_to_smt(*count_trailing_zeros, converted);
   }
+  if(const auto zero_extend = expr_try_dynamic_cast<zero_extend_exprt>(expr))
+  {
+    return convert_expr_to_smt(*zero_extend, converted);
+  }
   if(
     const auto prophecy_r_or_w_ok =
       expr_try_dynamic_cast<prophecy_r_or_w_ok_exprt>(expr))

src/solvers/smt2_incremental/smt2_incremental_decision_procedure.cpp

@@ -3,6 +3,7 @@
 #include "smt2_incremental_decision_procedure.h"
 
 #include <util/arith_tools.h>
+#include <util/bitvector_expr.h>
 #include <util/byte_operators.h>
 #include <util/c_types.h>
 #include <util/range.h>
@@ -296,6 +297,17 @@ static exprt lower_rw_ok_pointer_in_range(exprt expr, const namespacet &ns)
   return expr;
 }
 
+static exprt lower_zero_extend(exprt expr, const namespacet &ns)
+{
+  expr.visit_pre([](exprt &expr) {
+    if(auto zero_extend = expr_try_dynamic_cast<zero_extend_exprt>(expr))
+    {
+      expr = zero_extend->lower();
+    }
+  });
+  return expr;
+}
+
 void smt2_incremental_decision_proceduret::ensure_handle_for_expr_defined(
   const exprt &in_expr)
 {
@@ -677,8 +689,10 @@ void smt2_incremental_decision_proceduret::define_object_properties()
 
 exprt smt2_incremental_decision_proceduret::lower(exprt expression) const
 {
-  const exprt lowered = struct_encoding.encode(lower_enum(
-    lower_byte_operators(lower_rw_ok_pointer_in_range(expression, ns), ns),
+  const exprt lowered = struct_encoding.encode(lower_zero_extend(
+    lower_enum(
+      lower_byte_operators(lower_rw_ok_pointer_in_range(expression, ns), ns),
+      ns),
     ns));
   log.conditional_output(log.debug(), [&](messaget::mstreamt &debug) {
     if(lowered != expression)

src/util/bitvector_expr.cpp

@@ -54,8 +54,7 @@ exprt update_bit_exprt::lower() const
     typecast_exprt(src(), src_bv_type), bitnot_exprt(mask_shifted));
 
   // zero-extend the replacement bit to match src
-  auto new_value_casted = typecast_exprt(
-    typecast_exprt(new_value(), unsignedbv_typet(width)), src_bv_type);
+  auto new_value_casted = zero_extend_exprt{new_value(), src_bv_type};
 
   // shift the replacement bits
   auto new_value_shifted = shl_exprt(new_value_casted, index());
@@ -85,7 +84,7 @@ exprt update_bits_exprt::lower() const
     bitand_exprt(typecast_exprt(src(), src_bv_type), mask_shifted);
 
   // zero-extend or shrink the replacement bits to match src
-  auto new_value_casted = typecast_exprt(new_value(), src_bv_type);
+  auto new_value_casted = zero_extend_exprt{new_value(), src_bv_type};
 
   // shift the replacement bits
   auto new_value_shifted = shl_exprt(new_value_casted, index());
@@ -279,3 +278,19 @@ exprt find_first_set_exprt::lower() const
 
   return typecast_exprt::conditional_cast(result, type());
 }
+
+exprt zero_extend_exprt::lower() const
+{
+  const auto old_width = to_bitvector_type(op().type()).get_width();
+  const auto new_width = to_bitvector_type(type()).get_width();
+
+  if(new_width > old_width)
+  {
+    return concatenation_exprt{
+      bv_typet{new_width - old_width}.all_zeros_expr(), op(), type()};
+  }
+  else // new_width <= old_width
+  {
+    return extractbits_exprt{op(), 0, type()};
+  }
+}

src/util/bitvector_expr.h

@@ -1663,4 +1663,48 @@ inline find_first_set_exprt &to_find_first_set_expr(exprt &expr)
   return ret;
 }
 
+/// \brief zero extension
+/// The operand is converted to the given type by either
+/// a) truncating if the new type is shorter, or
+/// b) padding with most-significant zero bits if the new type is larger, or
+/// c) reinterprets the operand as the given type if their widths match.
+class zero_extend_exprt : public unary_exprt
+{
+public:
+  zero_extend_exprt(exprt _op, typet _type)
+    : unary_exprt(ID_zero_extend, std::move(_op), std::move(_type))
+  {
+  }
+
+  // a lowering to extraction or concatenation
+  exprt lower() const;
+};
+
+template <>
+inline bool can_cast_expr<zero_extend_exprt>(const exprt &base)
+{
+  return base.id() == ID_zero_extend;
+}
+
+/// \brief Cast an exprt to a \ref zero_extend_exprt
+///
+/// \a expr must be known to be \ref zero_extend_exprt.
+///
+/// \param expr: Source expression
+/// \return Object of type \ref zero_extend_exprt
+inline const zero_extend_exprt &to_zero_extend_expr(const exprt &expr)
+{
+  PRECONDITION(expr.id() == ID_zero_extend);
+  zero_extend_exprt::check(expr);
+  return static_cast<const zero_extend_exprt &>(expr);
+}
+
+/// \copydoc to_zero_extend_expr(const exprt &)
+inline zero_extend_exprt &to_zero_extend_expr(exprt &expr)
+{
+  PRECONDITION(expr.id() == ID_zero_extend);
+  zero_extend_exprt::check(expr);
+  return static_cast<zero_extend_exprt &>(expr);
+}
+
 #endif // CPROVER_UTIL_BITVECTOR_EXPR_H

src/util/format_expr.cpp

@@ -376,6 +376,12 @@ void format_expr_configt::setup()
               << format(expr.type()) << ')';
   };
 
+  expr_map[ID_zero_extend] =
+    [](std::ostream &os, const exprt &expr) -> std::ostream & {
+    return os << "zero_extend(" << format(to_zero_extend_expr(expr).op())
+              << ", " << format(expr.type()) << ')';
+  };
+
   expr_map[ID_floatbv_typecast] =
     [](std::ostream &os, const exprt &expr) -> std::ostream & {
     const auto &floatbv_typecast_expr = to_floatbv_typecast_expr(expr);

src/util/irep_ids.def

@@ -188,6 +188,7 @@ IREP_ID_ONE(extractbit)
 IREP_ID_ONE(extractbits)
 IREP_ID_ONE(update_bit)
 IREP_ID_ONE(update_bits)
+IREP_ID_ONE(zero_extend)
 IREP_ID_TWO(C_reference, #reference)
 IREP_ID_TWO(C_rvalue_reference, #rvalue_reference)
 IREP_ID_ONE(true)

src/util/lower_byte_operators.cpp

@@ -2491,15 +2491,16 @@ static exprt lower_byte_update(
     exprt zero_extended;
     if(bit_width > update_size_bits)
     {
-      zero_extended = concatenation_exprt{
-        bv_typet{bit_width - update_size_bits}.all_zeros_expr(),
-        value,
-        bv_typet{bit_width}};
-
-      if(!is_little_endian)
-        to_concatenation_expr(zero_extended)
-          .op0()
-          .swap(to_concatenation_expr(zero_extended).op1());
+      if(is_little_endian)
+        zero_extended = zero_extend_exprt{value, bv_typet{bit_width}};
+      else
+      {
+        // Big endian -- the zero is added as LSB.
+        zero_extended = concatenation_exprt{
+          value,
+          bv_typet{bit_width - update_size_bits}.all_zeros_expr(),
+          bv_typet{bit_width}};
+      }
     }
     else
       zero_extended = value;