Generate synthesisable code for sequential processes

Whilst adding `wait until ...' at the end of every
process is a valid translation of the input, it is not
actually synthesisable in at least one commercial
synthesiser (XST). According to the XST manual the
correct template is to use `wait until ...' at the
start of sequential processes and `wait on ...'
(equivalent to `wait until ...' with 'Event on all
the signals) at the end of combinatorial processes.
This patch implements that.

tgt-vhdl/stmt.cc

@@ -387,7 +387,10 @@ static int draw_delay(vhdl_procedural *proc, stmt_container *container,
 
 /*
  * A wait statement waits for a level change on a @(..) list of
- * signals.
+ * signals. Purely combinatorial processes (i.e. no posedge/negedge
+ * events) produce a `wait on' statement at the end of the process.
+ * Sequential processes produce a `wait until' statement at the
+ * start of the process.
  */
 static int draw_wait(vhdl_procedural *_proc, stmt_container *container,
                      ivl_statement_t stmt)
@@ -400,6 +403,34 @@ static int draw_wait(vhdl_procedural *_proc, stmt_container *container,
       new vhdl_binop_expr(VHDL_BINOP_OR, vhdl_type::boolean());
    
    int nevents = ivl_stmt_nevent(stmt);
+   
+   bool combinatorial = true;  // True if no negedge/posedge events
+   for (int i = 0; i < nevents; i++) {
+      ivl_event_t event = ivl_stmt_events(stmt, i);
+      if (ivl_event_npos(event) > 0 || ivl_event_nneg(event) > 0)
+         combinatorial = false;
+   }
+
+   if (combinatorial) {
+      vhdl_wait_stmt *wait = new vhdl_wait_stmt(VHDL_WAIT_ON); 
+      
+      for (int i = 0; i < nevents; i++) {
+         ivl_event_t event = ivl_stmt_events(stmt, i);
+         
+         int nany = ivl_event_nany(event);
+         for (int i = 0; i < nany; i++) {
+            ivl_nexus_t nexus = ivl_event_any(event, i);
+            vhdl_var_ref *ref = nexus_to_var_ref(proc->get_scope(), nexus);
+
+            wait->add_sensitivity(ref->get_name());
+            delete ref;
+         }
+      }
+
+      draw_stmt(proc, container, ivl_stmt_sub_stmt(stmt));
+      container->add_stmt(wait);
+   }
+   else {
       for (int i = 0; i < nevents; i++) {
          ivl_event_t event = ivl_stmt_events(stmt, i);
          
@@ -437,6 +468,7 @@ static int draw_wait(vhdl_procedural *_proc, stmt_container *container,
       
       container->add_stmt(new vhdl_wait_stmt(VHDL_WAIT_UNTIL, test));
       draw_stmt(proc, container, ivl_stmt_sub_stmt(stmt));
+   }
    
    return 0;
 }

tgt-vhdl/vhdl_syntax.cc

@@ -307,6 +307,7 @@ void vhdl_wait_stmt::emit(std::ostream &of, int level) const
       break;
    case VHDL_WAIT_ON:
       {
+         of << " on ";
          string_list_t::const_iterator it = sensitivity_.begin();
          while (it != sensitivity_.end()) {
             of << *it;

tgt-vhdl/vhdl_syntax.hh

@@ -323,6 +323,7 @@ public:
    ~vhdl_wait_stmt();
    
    void emit(std::ostream &of, int level) const;
+   void add_sensitivity(const std::string &s) { sensitivity_.push_back(s); }
 private:
    vhdl_wait_type_t type_;
    vhdl_expr *expr_;