Remove some uneccessary zero-time waits from VHDL outputs

This patch optimises away straight line sequences like:

wait for 0 ns;
wait for X ns;

to:

wait for X ns;

This tidies up the output a bit.

It also has the effect of removing all code from initial
processes where the assignments have been extracted as
VHDL signal intialisers. (c.f. pr2391337)

tgt-vhdl/process.cc

@@ -56,10 +56,15 @@ static int generate_vhdl_process(vhdl_entity *ent, ivl_process_t proc)
    // However, if no statements were added to the container
    // by draw_stmt, don't bother adding a wait as `emit'
    // will optimise the process out of the output
-   if (ivl_process_type(proc) == IVL_PR_INITIAL
-       && !vhdl_proc->get_container()->empty()) {
-      vhdl_wait_stmt *wait = new vhdl_wait_stmt();
-      vhdl_proc->get_container()->add_stmt(wait);
+   if (ivl_process_type(proc) == IVL_PR_INITIAL) {
+      // Get rid of any useless `wait for 0 ns's at the end of the process
+      prune_wait_for_0(vhdl_proc->get_container());
+
+      // The above pruning might have removed all logic from the process 
+      if (!vhdl_proc->get_container()->empty()) {
+         vhdl_wait_stmt *wait = new vhdl_wait_stmt();
+         vhdl_proc->get_container()->add_stmt(wait);
+      }
    }
    
    // Add a comment indicating where it came from

tgt-vhdl/stmt.cc

@@ -396,7 +396,7 @@ static int draw_assign(vhdl_procedural *proc, stmt_container *container,
       // the process
       if (!is_last)
          container->add_stmt
-            (new vhdl_wait_stmt(VHDL_WAIT_FOR, new vhdl_const_time(0)));
+            (new vhdl_wait_stmt(VHDL_WAIT_FOR0));
    }
    else
       make_assignment(proc, container, stmt, true);
@@ -404,6 +404,27 @@ static int draw_assign(vhdl_procedural *proc, stmt_container *container,
    return 0;
 }
 
+/*
+ * The VHDL code generator inserts `wait for 0 ns' after each
+ * not-last-in-block blocking assignment.
+ * If this is immediately followed by another `wait for ...' then
+ * we might as well not emit the first zero-time wait.
+ */
+void prune_wait_for_0(stmt_container *container)
+{   
+   vhdl_wait_stmt *wait0;
+   stmt_container::stmt_list_t &stmts = container->get_stmts();
+   while (stmts.size() > 0
+          && (wait0 = dynamic_cast<vhdl_wait_stmt*>(stmts.back()))) {
+      if (wait0->get_type() == VHDL_WAIT_FOR0) {
+         delete wait0;
+         stmts.pop_back();
+      }
+      else
+         break;
+   }
+}
+
 /*
  * Delay statements are equivalent to the `wait for' form of the
  * VHDL wait statement.
@@ -429,6 +450,8 @@ static int draw_delay(vhdl_procedural *proc, stmt_container *container,
          return 1;
    }
 
+   prune_wait_for_0(container);
+
    ivl_statement_t sub_stmt = ivl_stmt_sub_stmt(stmt);
    vhdl_wait_stmt *wait =
       new vhdl_wait_stmt(VHDL_WAIT_FOR, time);

tgt-vhdl/vhdl_syntax.cc

@@ -316,6 +316,9 @@ void vhdl_wait_stmt::emit(std::ostream &of, int level) const
       of << " for ";
       expr_->emit(of, level);
       break;
+   case VHDL_WAIT_FOR0:
+      of << " for 0 ns";
+      break;
    case VHDL_WAIT_UNTIL:
       assert(expr_);
       of << " until ";

tgt-vhdl/vhdl_syntax.hh

@@ -335,8 +335,11 @@ public:
    void add_stmt(vhdl_seq_stmt *stmt);
    void emit(std::ostream &of, int level, bool newline=true) const;
    bool empty() const { return stmts_.empty(); }
+
+   typedef std::list<vhdl_seq_stmt*> stmt_list_t;
+   stmt_list_t &get_stmts() { return stmts_; }
 private:
-   std::list<vhdl_seq_stmt*> stmts_;
+   stmt_list_t stmts_;
 };
 
 
@@ -381,6 +384,7 @@ public:
 enum vhdl_wait_type_t {
    VHDL_WAIT_INDEF,  // Suspend indefinitely
    VHDL_WAIT_FOR,    // Wait for a constant amount of time
+   VHDL_WAIT_FOR0,   // Special wait for zero time
    VHDL_WAIT_UNTIL,  // Wait on an expression
    VHDL_WAIT_ON,     // Wait on a sensitivity list
 };
@@ -398,6 +402,7 @@ public:
    
    void emit(std::ostream &of, int level) const;
    void add_sensitivity(const std::string &s) { sensitivity_.push_back(s); }
+   vhdl_wait_type_t get_type() const { return type_; }
 private:
    vhdl_wait_type_t type_;
    vhdl_expr *expr_;

tgt-vhdl/vhdl_target.h

@@ -44,7 +44,7 @@ ivl_signal_t find_signal_named(const string &name, const vhdl_scope *scope);
 
 int draw_stask_display(vhdl_procedural *proc, stmt_container *container,
                        ivl_statement_t stmt, bool newline = true);
-
+void prune_wait_for_0(stmt_container *container);   
 void require_support_function(support_function_t f);
 
 #endif /* #ifndef INC_VHDL_TARGET_H */