CONTRACTS: use alias analysis to infer loop assigns in synthesizer

regression/contracts/loop_assigns-fail/test.desc

@@ -1,10 +1,11 @@
 CORE
 main.c
 --apply-loop-contracts
-activate-multi-line-match
-Failed to infer variables being modified by the loop at file main.c line \d+ function main\.\nPlease specify an assigns clause\.\nReason:\nAlias analysis returned UNKNOWN!
-^EXIT=6$
+^EXIT=10$
 ^SIGNAL=0$
+^\[main.assigns.\d+\] .* Check that i is assignable: SUCCESS$
+^\[main.assigns.\d+\] .* Check that b->data\[(.*)i\] is assignable: FAILURE$
+^VERIFICATION FAILED$
 --
 --
 This test (taken from #6021) shows the need for assigns clauses on loops.

src/goto-instrument/loop_utils.cpp

@@ -52,7 +52,7 @@ void get_assigns_lhs(
       const typecast_exprt typed_mod{mod, ptr.pointer.type()};
       if(mod.id() == ID_unknown)
       {
-        throw analysis_exceptiont("Alias analysis returned UNKNOWN!");
+        continue;
       }
       if(ptr.offset.is_nil())
         assigns.insert(dereference_exprt{typed_mod});

src/goto-synthesizer/enumerative_loop_contracts_synthesizer.cpp

@@ -19,8 +19,12 @@ Author: Qinheping Hu
 #include <util/replace_symbol.h>
 #include <util/simplify_expr.h>
 
+#include <analyses/local_may_alias.h>
 #include <analyses/natural_loops.h>
+#include <goto-instrument/contracts/cfg_info.h>
+#include <goto-instrument/contracts/utils.h>
 #include <goto-instrument/havoc_utils.h>
+#include <goto-instrument/loop_utils.h>
 
 #include "cegis_verifier.h"
 #include "expr_enumerator.h"
@@ -82,17 +86,21 @@ void enumerative_loop_contracts_synthesizert::init_candidates()
 {
   for(auto &function_p : goto_model.goto_functions.function_map)
   {
-    natural_loopst natural_loops;
+    natural_loops_mutablet natural_loops;
     natural_loops(function_p.second.body);
 
+    // TODO: use global may alias instead.
+    local_may_aliast local_may_alias(function_p.second);
+
     // Initialize invariants for unannotated loops as true
     for(const auto &loop_head_and_content : natural_loops.loop_map)
     {
       goto_programt::const_targett loop_end =
-        get_loop_end_from_loop_head_and_content(
+        get_loop_end_from_loop_head_and_content_mutable(
           loop_head_and_content.first, loop_head_and_content.second);
 
       loop_idt new_id(function_p.first, loop_end->loop_number);
+      loop_cfg_infot cfg_info(function_p.second, loop_head_and_content.second);
 
       log.debug() << "Initialize candidates for the loop at "
                   << loop_end->source_location() << messaget::eom;
@@ -116,6 +124,17 @@ void enumerative_loop_contracts_synthesizert::init_candidates()
         if(loop_end->condition().find(ID_C_spec_assigns).is_nil())
         {
           assigns_map[new_id] = {};
+
+          // Infer loop assigns using alias analysis.
+          get_assigns(
+            local_may_alias, loop_head_and_content.second, assigns_map[new_id]);
+
+          // remove loop-local symbols from the inferred set
+          cfg_info.erase_locals(assigns_map[new_id]);
+
+          // If the set contains pairs (i, a[i]),
+          // we widen them to (i, __CPROVER_POINTER_OBJECT(a))
+          widen_assigns(assigns_map[new_id], ns);
         }
       }
     }
@@ -127,7 +146,6 @@ void enumerative_loop_contracts_synthesizert::synthesize_assigns(
   const exprt &checked_pointer,
   const std::list<loop_idt> cause_loop_ids)
 {
-  namespacet ns(goto_model.symbol_table);
   auto new_assign = checked_pointer;
 
   // Add the new assigns target to the most-inner loop that doesn't contain
@@ -242,7 +260,6 @@ exprt enumerative_loop_contracts_synthesizert::synthesize_strengthening_clause(
   //              | StartBool <= StartBool | StartBool < StartBool
   // Start -> Start + Start | terminal_symbols
   // where a0, and a1 are symbol expressions.
-  namespacet ns(goto_model.symbol_table);
   enumerator_factoryt factory = enumerator_factoryt(ns);
   recursive_enumerator_placeholdert start_bool_ph(factory, "StartBool", ns);
   recursive_enumerator_placeholdert start_ph(factory, "Start", ns);

src/goto-synthesizer/enumerative_loop_contracts_synthesizer.h

@@ -31,7 +31,8 @@ class enumerative_loop_contracts_synthesizert
   enumerative_loop_contracts_synthesizert(
     goto_modelt &goto_model,
     messaget &log)
-    : loop_contracts_synthesizer_baset(goto_model, log)
+    : loop_contracts_synthesizer_baset(goto_model, log),
+      ns(goto_model.symbol_table)
   {
   }
 
@@ -45,6 +46,8 @@ class enumerative_loop_contracts_synthesizert
     return assigns_map;
   }
 
+  namespacet ns;
+
 private:
   /// Initialize invariants as true for all unannotated loops.
   void init_candidates();