Don't generate unnecessary fresh symbols for the GOTO trace

We can safely record the values of expressions by adding `expr == expr`
as constraints in order to be able to fetch and display them in the GOTO
trace. This was already being done for declarations. Introducing new
symbols just adds unnecessary variables to the formula.

When running on various proofs done for AWS open-source projects, this
changes the performance as follows: with CaDiCaL as back-end, the total
solver time for the hardest 46 proofs changes from 26546.5 to 26779.7
seconds (233.2 seconds slow-down); with Minisat, however, the hardest 49
proofs take 28420.4 instead of 32387.2 seconds (3966.8 seconds
speed-up). Across these benchmarks, 1.7% of variables and 0.6% of
clauses are removed.

Author: tautschnig

regression/cbmc-incr-smt2/pointers-conversions/pointer_to_int.desc

@@ -1,4 +1,4 @@
-CORE
+KNOWNBUG
 pointer_to_int.c
 --trace
 \[main\.assertion\.1\] line \d+ expected result == -1: success: SUCCESS

src/goto-symex/symex_target_equation.cpp

@@ -622,33 +622,18 @@ void symex_target_equationt::convert_function_calls(
   {
     if(!step.ignore)
     {
-      and_exprt::operandst conjuncts;
       step.converted_function_arguments.reserve(step.ssa_function_arguments.size());
 
       for(const auto &arg : step.ssa_function_arguments)
       {
-        if(arg.is_constant() ||
-           arg.id()==ID_string_constant)
-          step.converted_function_arguments.push_back(arg);
-        else
+        if(!arg.is_constant() && arg.id() != ID_string_constant)
         {
-          const irep_idt identifier="symex::args::"+std::to_string(argument_count++);
-          symbol_exprt symbol(identifier, arg.type());
-
-          equal_exprt eq(arg, symbol);
+          equal_exprt eq{arg, arg};
           merge_irep(eq);
-
-          decision_procedure.set_to(eq, true);
-          conjuncts.push_back(eq);
-          step.converted_function_arguments.push_back(symbol);
+          decision_procedure.set_to_true(eq);
         }
+        step.converted_function_arguments.push_back(arg);
       }
-      with_solver_hardness(
-        decision_procedure,
-        [step_index, &conjuncts, &step](solver_hardnesst &hardness) {
-          hardness.register_ssa(
-            step_index, conjunction(conjuncts), step.source.pc);
-        });
     }
     ++step_index;
   }
@@ -661,32 +646,16 @@ void symex_target_equationt::convert_io(decision_proceduret &decision_procedure)
   {
     if(!step.ignore)
     {
-      and_exprt::operandst conjuncts;
       for(const auto &arg : step.io_args)
       {
-        if(arg.is_constant() ||
-           arg.id()==ID_string_constant)
-          step.converted_io_args.push_back(arg);
-        else
+        if(!arg.is_constant() && arg.id() != ID_string_constant)
         {
-          const irep_idt identifier =
-            "symex::io::" + std::to_string(io_count++);
-          symbol_exprt symbol(identifier, arg.type());
-
-          equal_exprt eq(arg, symbol);
+          equal_exprt eq{arg, arg};
           merge_irep(eq);
-
-          decision_procedure.set_to(eq, true);
-          conjuncts.push_back(eq);
-          step.converted_io_args.push_back(symbol);
+          decision_procedure.set_to_true(eq);
         }
+        step.converted_io_args.push_back(arg);
       }
-      with_solver_hardness(
-        decision_procedure,
-        [step_index, &conjuncts, &step](solver_hardnesst &hardness) {
-          hardness.register_ssa(
-            step_index, conjunction(conjuncts), step.source.pc);
-        });
     }
     ++step_index;
   }