STL frontend: expr2stl conversion

src/statement-list/converters/expr2statement_list.cpp

@@ -10,12 +10,105 @@ Author: Matthias Weiss, [email protected]
 
 #include <ansi-c/expr2c.h>
 
+#include <util/suffix.h>
+#include <util/symbol_table.h>
+
+#include <algorithm>
+#include <cstring>
+#include <iostream>
+
+/// STL code for an AND instruction.
+#define AND 'A'
+
+/// STL code for an OR instruction.
+#define OR 'O'
+
+/// STL code for a XOR instruction.
+#define XOR 'X'
+
+/// Postfix for any negated boolean instruction.
+#define NOT_POSTFIX 'N'
+
+/// STL code for a NOT instruction.
+#define NOT "NOT"
+
+/// Separator between the instruction code and it's operand.
+#define OPERAND_SEPARATOR ' '
+
+/// Separator between the end of an instruction and the next one.
+#define LINE_SEPARATOR ";\n"
+
+/// Separator for the end of an instruction that introduces a new layer of
+/// nesting.
+#define NESTING_OPEN_LINE_SEPARATOR "(;\n"
+
+/// Separator for the end of an instruction that closes a nesting layer. Also
+/// known as the NESTING CLOSED instruction.
+#define NESTING_CLOSED_LINE_SEPARATOR ");\n"
+
+/// Prefix for the reference to any variable.
+#define REFERENCE_FLAG '#'
+
+/// CBMC-internal separator for variable scopes.
+#define SCOPE_SEPARATOR "::"
+
+/// Modifies the parameters of a binary equal expression with the help of its
+/// symmetry properties. This function basically converts the operands to
+/// operands of a XOR expression so that the whole expression can be treated as
+/// such. This can reduce complexity in some cases.
+/// \param lhs: Left side of the binary expression.
+/// \param rhs: Right side of the binary expression.
+/// \return: List of instrumented operands.
+static std::vector<exprt>
+instrument_equal_operands(const exprt &lhs, const exprt &rhs)
+{
+  std::vector<exprt> result;
+  if(ID_not != lhs.id() && ID_not != rhs.id())
+  {
+    // lhs == rhs is equivalent to X lhs; XN rhs;
+    result.push_back(lhs);
+    result.push_back(not_exprt{rhs});
+  }
+  else if(ID_not != lhs.id() && ID_not == rhs.id())
+  {
+    // lhs == !rhs is equivalent to X lhs; X rhs;
+    result.push_back(lhs);
+    result.push_back(rhs.op0());
+  }
+  else if(ID_not == lhs.id() && ID_not != rhs.id())
+  {
+    // !lhs == rhs is equivalent to X lhs; X rhs;
+    result.push_back(lhs.op0());
+    result.push_back(rhs);
+  }
+  else // ID_not == lhs.id() && ID_not == rhs.id()
+  {
+    // !lhs == !rhs is equivalent to X lhs; XN rhs;
+    result.push_back(lhs.op0());
+    result.push_back(rhs);
+  }
+  return result;
+}
+
+/// Checks if the expression or one of its parameters is not of type bool.
+/// \param expr: Expression to verify.
+/// \return: True if the expression and its operands are not or only partially
+///   of type bool, false otherwise.
+static bool is_not_bool(const exprt &expr)
+{
+  if(!expr.is_boolean())
+    return true;
+  for(const exprt &op : expr.operands())
+    if(!op.is_boolean())
+      return true;
+  return false;
+}
+
 std::string expr2stl(const exprt &expr, const namespacet &ns)
 {
-  // TODO: Implement expr2stl.
-  // Expand this section so that it is able to properly convert from
-  // CBMC expressions to STL code.
-  return expr2c(expr, ns);
+  expr2stlt expr2stl{ns};
+
+  return expr2stl.convert(expr);
 }
 
 std::string type2stl(const typet &type, const namespacet &ns)
@@ -25,3 +118,187 @@ std::string type2stl(const typet &type, const namespacet &ns)
   // CBMC types to STL code.
   return type2c(type, ns);
 }
+
+expr2stlt::expr2stlt(const namespacet &ns)
+  : inside_bit_string(false), is_reference(false), ns(ns)
+{
+}
+
+std::string expr2stlt::convert(const exprt &expr)
+{
+  // Redirect to expr2c if no boolean expression.
+  if(is_not_bool(expr))
+    return expr2c(expr, ns);
+
+  if(ID_and == expr.id())
+    convert(to_and_expr(expr));
+  else if(ID_or == expr.id())
+    convert(to_or_expr(expr));
+  else if(ID_xor == expr.id())
+    convert(to_xor_expr(expr));
+  else if(ID_notequal == expr.id())
+    convert(to_notequal_expr(expr));
+  else if(ID_equal == expr.id())
+    convert(to_equal_expr(expr));
+  else if(ID_symbol == expr.id())
+    convert(to_symbol_expr(expr));
+  else if(ID_not == expr.id() && expr.op0().type().id() == ID_bool)
+    convert(to_not_expr(expr));
+  else // TODO: support more instructions in expr2stl.
+    return expr2c(expr, ns);
+
+  return result.str();
+}
+
+void expr2stlt::convert(const and_exprt &expr)
+{
+  std::vector<exprt> operands = expr.operands();
+  convert_multiary_bool(operands, AND);
+}
+
+void expr2stlt::convert(const or_exprt &expr)
+{
+  std::vector<exprt> operands = expr.operands();
+  convert_multiary_bool(operands, OR);
+}
+
+void expr2stlt::convert(const xor_exprt &expr)
+{
+  std::vector<exprt> operands = expr.operands();
+  convert_multiary_bool(operands, XOR);
+}
+
+void expr2stlt::convert(const notequal_exprt &expr)
+{
+  std::vector<exprt> operands = expr.operands();
+  convert_multiary_bool(operands, XOR);
+}
+
+void expr2stlt::convert(const equal_exprt &expr)
+{
+  std::vector<exprt> operands =
+    instrument_equal_operands(expr.lhs(), expr.rhs());
+  convert_multiary_bool(operands, XOR);
+}
+
+void expr2stlt::convert(const not_exprt &expr)
+{
+  // If a NOT expression is being handled here it must always mark the
+  // beginning of a new bit string.
+  PRECONDITION(!inside_bit_string);
+
+  if(ID_symbol == expr.op().id())
+  {
+    // Use AN to load the symbol.
+    is_reference = true;
+    result << AND << NOT_POSTFIX << OPERAND_SEPARATOR;
+    convert(to_symbol_expr(expr.op()));
+    result << LINE_SEPARATOR;
+  }
+  else
+  {
+    // Use NOT to negate the RLO after the wrapped expression was loaded.
+    convert(expr.op());
+    result << NOT LINE_SEPARATOR;
+  }
+}
+
+void expr2stlt::convert(const symbol_exprt &expr)
+{
+  if(is_reference)
+  {
+    result << REFERENCE_FLAG;
+    is_reference = false;
+  }
+  result << id2string(id_shorthand(expr.get_identifier()));
+}
+
+void expr2stlt::convert_multiary_bool(
+  std::vector<exprt> &operands,
+  const char operation)
+{
+  if(inside_bit_string)
+    convert_multiary_bool_operands(operands, operation);
+  else
+  {
+    convert_first_non_trivial_operand(operands);
+    convert_multiary_bool_operands(operands, operation);
+  }
+}
+
+void expr2stlt::convert_multiary_bool_operands(
+  const std::vector<exprt> &operands,
+  const char operation)
+{
+  for(const exprt &op : operands)
+  {
+    if(ID_not == op.id())
+    {
+      result << operation << NOT_POSTFIX;
+      convert_bool_operand(op.op0());
+    }
+    else
+    {
+      result << operation;
+      convert_bool_operand(op);
+    }
+  }
+}
+
+void expr2stlt::convert_bool_operand(const exprt &op)
+{
+  if(op.id() == ID_symbol)
+  {
+    is_reference = true;
+    result << OPERAND_SEPARATOR;
+    convert(to_symbol_expr(op));
+    result << LINE_SEPARATOR;
+  }
+  else
+  {
+    inside_bit_string = false;
+    result << NESTING_OPEN_LINE_SEPARATOR;
+    convert(op);
+    result << NESTING_CLOSED_LINE_SEPARATOR;
+  }
+}
+
+void expr2stlt::convert_first_non_trivial_operand(std::vector<exprt> &operands)
+{
+  exprt non_trivial_op;
+  for(auto it{begin(operands)}; it != end(operands); ++it)
+  {
+    if(
+      (ID_symbol == it->id()) ||
+      (ID_not == it->id() && ID_symbol == it->op0().id()))
+      continue;
+    else
+    {
+      non_trivial_op = *it;
+      operands.erase(it);
+      break;
+    }
+  }
+  // Important for some scenarios: Convert complex op first, set bit string
+  // flag to true afterwards.
+  if(non_trivial_op.is_not_nil())
+    convert(non_trivial_op);
+
+  inside_bit_string = true;
+}
+
+irep_idt expr2stlt::id_shorthand(const irep_idt &identifier)
+{
+  const symbolt *symbol;
+  std::string shorthand = id2string(identifier);
+  if(
+    !ns.lookup(identifier, symbol) && !symbol->base_name.empty() &&
+    has_suffix(shorthand, id2string(symbol->base_name)))
+    return symbol->base_name;
+
+  const std::string::size_type pos = shorthand.rfind(SCOPE_SEPARATOR);
+  if(pos != std::string::npos)
+    shorthand.erase(0, pos + std::strlen(SCOPE_SEPARATOR));
+
+  return shorthand;
+}