Regression test for vvp scheduler fix.

This is for the latent bug exposed when testing the original fix for
issue #1041. Contributed by gatk555 in PR #1065.

ivtest/vpi/br_gh1041b.c

@@ -0,0 +1,74 @@
+#include <vpi_user.h>
+
+static void step(void);
+
+static vpiHandle w;
+
+static PLI_INT32 tick_cb(struct t_cb_data *cb)
+{
+    static struct t_vpi_value val = { .format = vpiIntVal };
+    static int                idx;
+
+    (void)cb;
+
+    ++idx;
+    val.value.integer = idx & 1;
+    vpi_put_value(w, &val, NULL, vpiNoDelay);
+    step();
+    return 0;
+}
+
+/* Request a callback after a delay. */
+
+static void step(void)
+{
+    static struct t_vpi_time now = { .type = vpiSimTime, .low = 2 };
+    static struct t_cb_data  cbd =
+        { .reason = cbAfterDelay, .cb_rtn = tick_cb, .time = &now };
+
+    /* Callback after delay. */
+
+    vpi_register_cb(&cbd);
+}
+
+/* Callback function - simulation is starting. */
+
+static PLI_INT32 start_cb(struct t_cb_data *cb)
+{
+    static struct t_vpi_value val = { .format = vpiIntVal };
+
+    (void)cb;
+
+    w = vpi_handle_by_name("test.w", NULL);
+    if (!w)
+        vpi_printf("No handle!\n");
+    vpi_printf("Got handle for %s\n", vpi_get_str(vpiFullName, w));
+    val.value.integer = 0;
+    vpi_put_value(w, &val, NULL, vpiNoDelay);
+    step();
+    return 0;
+}
+
+/* VPI initialisation. */
+
+static void start(void)
+{
+    static struct t_vpi_time now = { .type = vpiSimTime };
+    static struct t_cb_data  cbd = { .reason = cbStartOfSimulation,
+                                     .time = &now, .cb_rtn = start_cb };
+
+    /* At this point VPI objects do not exist,
+     * so request a callback once they do.
+     */
+
+    vpi_register_cb(&cbd);
+}
+
+/* This is a table of registration functions. This table is the external
+ * symbol that the VVP simulator looks for when loading this .vpi module.
+ */
+
+void (*vlog_startup_routines[])(void) = {
+      start,
+      0
+};

ivtest/vpi/br_gh1041b.v

@@ -0,0 +1,18 @@
+module test(w);
+   input wire w;
+   wire a, b;
+
+   
+   initial begin
+      #11 $finish;
+   end
+
+   assign b = 0;
+   
+   assign a = !w | b;
+   
+   always @(a) begin
+      $display($time, ": Wire a is now ", a);
+   end
+endmodule
+   

ivtest/vpi_gold/br_gh1041b.gold

@@ -0,0 +1,10 @@
+Compiling vpi/br_gh1041b.c...
+Making br_gh1041b.vpi from  br_gh1041b.o...
+Got handle for test.w
+                   0: Wire a is now 1
+                   2: Wire a is now 0
+                   4: Wire a is now 1
+                   6: Wire a is now 0
+                   8: Wire a is now 1
+                  10: Wire a is now 0
+vpi/br_gh1041b.v:7: $finish called at 11 (1s)

ivtest/vpi_regress.list

@@ -72,6 +72,7 @@ br_gh317		normal			br_gh317.c		br_gh317.gold
 br_gh496		normal,-g2009		br_gh496.c		br_gh496.gold
 br_gh1037		normal,-g2009		br_gh1037.c		br_gh1037.gold
 br_gh1041		normal			br_gh1041.c		br_gh1041.gold
+br_gh1041b		normal			br_gh1041b.c		br_gh1041b.gold
 br_ml20191013		normal			br_ml20191013.c		br_ml20191013.gold
 by_index		normal			by_index.c		by_index.gold
 by_name			normal			by_name.c		by_name.log