Support for loops with no loop condition.

SystemVerilog makes all of the initialisation, condition, and step
components of a for loop optional. We already support this for the
initialisation and step components.

elaborate.cc

@@ -5613,7 +5613,7 @@ NetProc* PForeach::elaborate_static_array_(Design*des, NetScope*scope,
  *
  *  - index variable (name1_)  (optional)
  *  - initial value (expr1_)   (only if name1_ is present)
- *  - condition expression (cond_)
+ *  - condition expression (cond_) (optional)
  *  - step statement (step_)   (optional)
  *  - sub-statement (statement_)
  *
@@ -5693,13 +5693,16 @@ NetProc* PForStatement::elaborate(Design*des, NetScope*scope) const
       // Elaborate the condition expression. Try to evaluate it too,
       // in case it is a constant. This is an interesting case
       // worthy of a warning.
-      NetExpr*ce = elab_and_eval(des, scope, cond_, -1);
+      NetExpr*ce = nullptr;
+      if (cond_) {
+	    ce = elab_and_eval(des, scope, cond_, -1);
 	    if (!ce)
 		  error_flag = true;
 	    if (dynamic_cast<NetEConst*>(ce)) {
 		  cerr << get_fileline() << ": warning: condition expression "
 			"of for-loop is constant." << endl;
 	    }
+      }
 
       // Error recovery - if we failed to elaborate any of the loop
       // expressions, give up now. Error counts where handled elsewhere.

parse.y

@@ -728,7 +728,7 @@ Module::port_t *module_declare_port(const YYLTYPE&loc, char *id,
 %type <spec_optional_args> timeskew_fullskew_opt_notifier timeskew_fullskew_opt_event_based_flag
 %type <spec_optional_args> timeskew_fullskew_opt_remain_active_flag
 
-%type <expr>  assignment_pattern expression expr_mintypmax
+%type <expr>  assignment_pattern expression expression_opt expr_mintypmax
 %type <expr>  expr_primary_or_typename expr_primary
 %type <expr>  class_new dynamic_array_new
 %type <expr>  var_decl_initializer_opt initializer_opt
@@ -1754,7 +1754,7 @@ lifetime_opt /* IEEE1800-2005: A.2.1.3 */
   /* Loop statements are kinds of statements. */
 
 loop_statement /* IEEE1800-2005: A.6.8 */
-  : K_for '(' lpvalue '=' expression ';' expression ';' for_step_opt ')'
+  : K_for '(' lpvalue '=' expression ';' expression_opt ';' for_step_opt ')'
     statement_or_null
       { PForStatement*tmp = new PForStatement($3, $5, $7, $9, $11);
 	FILE_NAME(tmp, @1);
@@ -1762,7 +1762,7 @@ loop_statement /* IEEE1800-2005: A.6.8 */
       }
 
       // The initialization statement is optional.
-  | K_for '(' ';' expression ';' for_step_opt ')'
+  | K_for '(' ';' expression_opt ';' for_step_opt ')'
     statement_or_null
       { PForStatement*tmp = new PForStatement(nullptr, nullptr, $4, $6, $8);
 	FILE_NAME(tmp, @1);
@@ -1773,7 +1773,7 @@ loop_statement /* IEEE1800-2005: A.6.8 */
       // statement in a synthetic named block. We can name the block
       // after the variable that we are creating, that identifier is
       // safe in the controlling scope.
-  | K_for '(' K_var_opt data_type IDENTIFIER '=' expression ';' expression ';' for_step_opt ')'
+  | K_for '(' K_var_opt data_type IDENTIFIER '=' expression ';' expression_opt ';' for_step_opt ')'
       { static unsigned for_counter = 0;
 	char for_block_name [64];
 	snprintf(for_block_name, sizeof for_block_name, "$ivl_for_loop%u", for_counter);
@@ -1858,7 +1858,7 @@ loop_statement /* IEEE1800-2005: A.6.8 */
 
   /* Error forms for loop statements. */
 
-  | K_for '(' lpvalue '=' expression ';' expression ';' error ')'
+  | K_for '(' lpvalue '=' expression ';' expression_opt ';' error ')'
     statement_or_null
       { $$ = 0;
 	yyerror(@1, "error: Error in for loop step assignment.");
@@ -3682,6 +3682,11 @@ expression
       }
   ;
 
+expression_opt
+  : expression { $$ = $1; }
+  | { $$ = nullptr; }
+  ;
+
 expr_mintypmax
   : expression
       { $$ = $1; }

t-dll-api.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000-2021 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2000-2024 Stephen Williams (steve@icarus.com)
  * Copyright CERN 2013 / Stephen Williams (steve@icarus.com)
  * Copyright (c) 2016 CERN Michele Castellana (michele.castellana@cern.ch)
  *
@@ -2822,7 +2822,6 @@ extern "C" ivl_expr_t ivl_stmt_cond_expr(ivl_statement_t net)
 	    return net->u_.while_.cond_;
 
 	  case IVL_ST_FORLOOP:
-	    assert(net->u_.forloop_.condition); // XXXX
 	    return net->u_.forloop_.condition;
 
 	  default:

tgt-vvp/vvp_proc_loops.c

@@ -121,10 +121,12 @@ int show_stmt_forloop(ivl_statement_t net, ivl_scope_t scope)
 
       /* Top of the loop, draw the condition test. */
       fprintf(vvp_out, "T_%u.%u ; Top of for-loop\n", thread_count, top_label);
+      if (ivl_stmt_cond_expr(net)) {
 	    int use_flag = draw_eval_condition(ivl_stmt_cond_expr(net));
 	    fprintf(vvp_out, "	  %%jmp/0xz T_%u.%u, %d;\n",
 		    thread_count, out_label, use_flag);
 	    clr_flag(use_flag);
+      }
 
       /* Draw the body of the loop. */
       rc += show_statement(ivl_stmt_sub_stmt(net), scope);