prepare for #6160

The idea is to set _concurrent_dec_ref from the API
(function not yet provided externally, but you can experiment with it by setting the default of m_concurrent_dec_ref to true).
It then provides concurrency support for dec_ref operations.

src/api/api_context.cpp

@@ -35,11 +35,12 @@ namespace api {
 
     object::object(context& c): m_ref_count(0), m_context(c) { this->m_id = m_context.add_object(this); }
 
-    void object::inc_ref() { m_ref_count++; }
+    void object::inc_ref() { ++m_ref_count; }
 
-    void object::dec_ref() { SASSERT(m_ref_count > 0); m_ref_count--; if (m_ref_count == 0) m_context.del_object(this); }
+    void object::dec_ref() { SASSERT(m_ref_count > 0); if (--m_ref_count == 0) m_context.del_object(this); }
     
     unsigned context::add_object(api::object* o) {
+        flush_objects();
         unsigned id = m_allocated_objects.size();
         if (!m_free_object_ids.empty()) {
             id = m_free_object_ids.back();
@@ -50,9 +51,48 @@ namespace api {
     }
 
     void context::del_object(api::object* o) {
-        m_free_object_ids.push_back(o->id());
-        m_allocated_objects.remove(o->id());
-        dealloc(o);
+        if (m_concurrent_dec_ref) {
+            lock_guard lock(m_mux);
+            m_objects_to_flush.push_back(o);
+        }
+        else {
+            m_free_object_ids.push_back(o->id());
+            m_allocated_objects.remove(o->id());
+            dealloc(o);
+        }
+    }
+
+    void context::dec_ref(ast* a) {
+        if (m_concurrent_dec_ref) {
+            lock_guard lock(m_mux);
+            m_asts_to_flush.push_back(a);
+        }
+        else 
+            m().dec_ref(a);
+    }
+
+    void context::flush_objects() {
+        if (!m_concurrent_dec_ref)
+            return;        
+        {
+            lock_guard lock(m_mux);
+            if (m_asts_to_flush.empty() && m_objects_to_flush.empty())
+                return;
+            m_asts_to_flush2.append(m_asts_to_flush);
+            m_asts_to_flush.reset();
+            m_objects_to_flush2.append(m_objects_to_flush);
+            m_objects_to_flush.reset();
+        }
+        for (ast* a : m_asts_to_flush2)
+            m().dec_ref(a);
+        for (auto* o : m_objects_to_flush2) {
+            m_free_object_ids.push_back(o->id());
+            m_allocated_objects.remove(o->id());
+            dealloc(o);
+        }
+        m_objects_to_flush2.reset();
+        m_asts_to_flush2.reset();
+            
     }
 
     static void default_error_handler(Z3_context ctx, Z3_error_code c) {
@@ -106,6 +146,7 @@ namespace api {
 
     context::~context() {
         m_last_obj = nullptr;
+        flush_objects();
         for (auto& kv : m_allocated_objects) {
             api::object* val = kv.m_value;
             DEBUG_CODE(warning_msg("Uncollected memory: %d: %s", kv.m_key, typeid(*val).name()););
@@ -365,6 +406,7 @@ extern "C" {
         Z3_TRY;
         LOG_Z3_inc_ref(c, a);
         RESET_ERROR_CODE();
+        mk_c(c)->flush_objects();
         mk_c(c)->m().inc_ref(to_ast(a));
         Z3_CATCH;
     }
@@ -379,7 +421,7 @@ extern "C" {
             return;
         }
         if (a) {
-            mk_c(c)->m().dec_ref(to_ast(a));
+            mk_c(c)->dec_ref(to_ast(a));
         }
         Z3_CATCH;
     }

src/api/api_context.h

@@ -91,8 +91,11 @@ namespace api {
         smt_params                 m_fparams;
         // -------------------------------
 
-        ast_ref_vector             m_ast_trail;
+        bool                       m_concurrent_dec_ref = false;        
+        ptr_vector<ast>            m_asts_to_flush, m_asts_to_flush2;
+        ptr_vector<api::object>    m_objects_to_flush, m_objects_to_flush2;
 
+        ast_ref_vector             m_ast_trail;        
         ref<api::object>           m_last_obj; //!< reference to the last API object returned by the APIs
         u_map<api::object*>        m_allocated_objects; // !< table containing current set of allocated API objects
         unsigned_vector            m_free_object_ids;   // !< free list of identifiers available for allocated objects.
@@ -170,8 +173,11 @@ namespace api {
         void set_error_code(Z3_error_code err, std::string &&opt_msg);
         void set_error_handler(Z3_error_handler h) { m_error_handler = h; }
 
+        void set_concurrent_dec_ref() { m_concurrent_dec_ref = true; }
         unsigned add_object(api::object* o);
         void del_object(api::object* o);
+        void dec_ref(ast* a);
+        void flush_objects();
 
         Z3_ast_print_mode get_print_mode() const { return m_print_mode; }
         void set_print_mode(Z3_ast_print_mode m) { m_print_mode = m; }

src/api/api_util.h

@@ -20,6 +20,7 @@ Revision History:
 #include "util/params.h"
 #include "util/lbool.h"
 #include "ast/ast.h"
+#include <atomic>
 
 #define Z3_TRY try {
 #define Z3_CATCH_CORE(CODE) } catch (z3_exception & ex) { mk_c(c)->handle_exception(ex); CODE }
@@ -34,7 +35,7 @@ namespace api {
 
     // Generic wrapper for ref-count objects exposed by the API
     class object {
-        unsigned m_ref_count;
+        atomic<unsigned> m_ref_count;
         unsigned m_id;
         context& m_context;
     public: