Add support for logical implication

The "->" operator is rarely used, but exists. Unfortunately, the syntax
is tied up in a horrible mess with the System Verilog constraint list
syntax. Do some flex magic to make it all work.

lexor.lex

@@ -249,6 +249,15 @@ TU [munpf]
 "'{" { return K_LP; }
 "::" { return K_SCOPE_RES; }
 
+  /* This is horrible. The Verilog systax uses "->" in a lot of places.
+     The trickiest is in constraints, where it is not an operator at all
+     but a constraint implication. This only turns up as a problem when
+     the "->" is followed by a constraint_expression_list. If that shows
+     up, then there will be a "{" to the right of the "->". In that case,
+     turn the "->" into a K_CONSTRAINT_IMPL so that the parser can be
+     written without the shift/reduce conflict. Ugh! */
+"->"/{W}*"{" { return gn_system_verilog()? K_CONSTRAINT_IMPL :  K_TRIGGER; }
+
   /* Watch out for the tricky case of (*). Cannot parse this as "(*"
      and ")", but since I know that this is really ( * ), replace it
      with "*" and return that. */

parse.y

@@ -502,6 +502,8 @@ static void current_function_set_statement(const YYLTYPE&loc, std::vector<Statem
 %token K_SCOPE_RES
 %token K_edge_descriptor
 
+%token K_CONSTRAINT_IMPL
+
  /* The base tokens from 1364-1995. */
 %token K_always K_and K_assign K_begin K_buf K_bufif0 K_bufif1 K_case
 %token K_casex K_casez K_cmos K_deassign K_default K_defparam K_disable
@@ -1148,12 +1150,16 @@ constraint_declaration /* IEEE1800-2005: A.1.9 */
 constraint_expression /* IEEE1800-2005 A.1.9 */
   : expression ';'
   | expression K_dist '{' '}' ';'
-  | expression K_TRIGGER constraint_set
+  | expression constraint_trigger
   | K_if '(' expression ')' constraint_set %prec less_than_K_else
   | K_if '(' expression ')' constraint_set K_else constraint_set
   | K_foreach '(' IDENTIFIER '[' loop_variables ']' ')' constraint_set
   ;
 
+constraint_trigger
+  : K_CONSTRAINT_IMPL '{' constraint_expression_list '}'
+  ;
+
 constraint_expression_list /* */
   : constraint_expression_list constraint_expression
   | constraint_expression
@@ -3632,14 +3638,12 @@ expression
 	FILE_NAME(tmp, @2);
 	$$ = tmp;
       }
-/*
-  FIXME: This creates shift/reduce issues that need to be solved
   | expression K_TRIGGER attribute_list_opt expression
       { PEBinary*tmp = new PEBLogic('q', $1, $4);
 	FILE_NAME(tmp, @2);
 	$$ = tmp;
       }
-*/
+
   | expression K_LEQUIV attribute_list_opt expression
       { PEBinary*tmp = new PEBLogic('Q', $1, $4);
 	FILE_NAME(tmp, @2);

tgt-vvp/eval_vec4.c

@@ -409,11 +409,26 @@ static void draw_binary_vec4_compare(ivl_expr_t expr)
       }
 }
 
+/*
+ * Handle the logical implication:
+ *    <le> -> <re>
+ * which is the same as the expression
+ *    !<le> || <re>
+ *
+ */
 static void draw_binary_vec4_limpl(ivl_expr_t expr)
 {
-      fprintf(stderr, "vvp.tgt sorry: No support for logical implication (%s:%u).\n",
-                      ivl_expr_file(expr), ivl_expr_lineno(expr));
-      assert(0);
+      ivl_expr_t le = ivl_expr_oper1(expr);
+      ivl_expr_t re = ivl_expr_oper2(expr);
+
+      /* The arguments should have alreacy been reduced. */
+      assert(ivl_expr_width(le) == 1);
+      assert(ivl_expr_width(re) == 1);
+
+      draw_eval_vec4(le);
+      fprintf(vvp_out, "    %%inv;\n");
+      draw_eval_vec4(re);
+      fprintf(vvp_out, "    %%or;\n");
 }
 
 static void draw_binary_vec4_lequiv(ivl_expr_t expr)