Fix non-blocking assignment to an array error state handling

If either the index or part offset expressions generate an undefined
value then the assignment is skipped. This patch reworks the code that
handles the flags used to detect this. For some simple cases a global
flag is not needed, but for other cases one is needed since there are
two expressions that can generate an error and even when there is only
a variable expression this error state needs to be preserved if there
is a variable delay. An undefined delay value defaults to zero and is
not an error.

tgt-vvp/vvp_process.c

@@ -114,14 +114,12 @@ static void assign_to_array_word(ivl_signal_t lsig, ivl_expr_t word_ix,
 				 ivl_expr_t part_off_ex,
 				 unsigned nevents)
 {
-      unsigned skip_assign = transient_id++;
       int word_ix_reg = 3;
       int part_off_reg = 0;
       int delay_index;
       unsigned long part_off = 0;
 
       int error_flag = allocate_flag();
-      fprintf(vvp_out, "    %%flag_set/imm %d, 0;\n", error_flag);
 
 	/* Figure the constant part offset, if possible. If we can do
 	   so, then forget about the expression and use the calculated
@@ -143,11 +141,13 @@ static void assign_to_array_word(ivl_signal_t lsig, ivl_expr_t word_ix,
 
 	/* Calculate array word index into word index register */
       draw_eval_expr_into_integer(word_ix, word_ix_reg);
-      fprintf(vvp_out, "    %%flag_or %d, 4;\n", error_flag);
 
       if (part_off_ex) {
 	    part_off_reg = allocate_word();
+	      /* Save the index calculation error flag to a global. */
+	    fprintf(vvp_out, "    %%flag_mov %d, 4;\n", error_flag);
 	    draw_eval_expr_into_integer(part_off_ex, part_off_reg);
+	      /* Add the error state of the part select to the global. */
 	    fprintf(vvp_out, "    %%flag_or %d, 4;\n", error_flag);
 
       } else if (part_off != 0) {
@@ -158,26 +158,35 @@ static void assign_to_array_word(ivl_signal_t lsig, ivl_expr_t word_ix,
 		             part_off_reg, part_off);
       }
 
+	/* Calculated delay... */
       if (dexp != 0) {
-	      /* Calculated delay... */
 	    delay_index = allocate_word();
+	      /* If needed save the index calculation error flag. */
+	    if (! part_off_ex) {
+		  fprintf(vvp_out, "    %%flag_mov %d, 4;\n", error_flag);
+	    }
 	    draw_eval_expr_into_integer(dexp, delay_index);
-	    fprintf(vvp_out, "    %%flag_mov 4, %d;\n", error_flag);
 	    if (word_ix_reg != 3) {
 		  fprintf(vvp_out, "    %%ix/mov 3, %u;\n", word_ix_reg);
 		  clr_word(word_ix_reg);
 	    }
+	      /* Restore the error state since an undefined delay is okay. */
+	    fprintf(vvp_out, "    %%flag_mov 4, %d;\n", error_flag);
 	    fprintf(vvp_out, "    %%assign/vec4/a/d v%p, %d, %d;\n",
 		    lsig, part_off_reg, delay_index);
 
 	    clr_word(delay_index);
 
+	/* Event control delay... */
       } else if (nevents != 0) {
-	      /* Event control delay... */
+	      /* If needed use the global error state. */
+	    if (part_off_ex) {
+		  fprintf(vvp_out, "    %%flag_mov 4, %d;\n", error_flag);
+	    }
 	    fprintf(vvp_out, "    %%assign/vec4/a/e v%p, %d;\n", lsig, part_off_reg);
 
+	/* Constant delay... */
       } else {
-	      /* Constant delay... */
 	    unsigned long low_d = delay % UINT64_C(0x100000000);
 	    unsigned long hig_d = delay / UINT64_C(0x100000000);
 
@@ -187,13 +196,16 @@ static void assign_to_array_word(ivl_signal_t lsig, ivl_expr_t word_ix,
 	    if (word_ix_reg != 3) {
 		  fprintf(vvp_out, "    %%ix/mov 3, %u;\n", word_ix_reg);
 		  clr_word(word_ix_reg);
+	    }
+	      /* If needed use the global error state. */
+	    if (part_off_ex) {
+		  fprintf(vvp_out, "    %%flag_mov 4, %d;\n", error_flag);
 	    }
 	    fprintf(vvp_out, "    %%assign/vec4/a/d v%p, %d, %u;\n",
 		    lsig, part_off_reg, delay_index);
 	    clr_word(delay_index);
       }
 
-      fprintf(vvp_out, "t_%u ;\n", skip_assign);
       if (nevents != 0) fprintf(vvp_out, "    %%evctl/c;\n");
 
       clr_flag(error_flag);