From 5e1546faaf9567f14776bf5d3a6933c551ca9219 Mon Sep 17 00:00:00 2001
From: Cary R <cygcary@yahoo.com>
Date: Sun, 5 Sep 2010 12:30:03 -0700
Subject: [PATCH] Don't elide a BUFZ that represents a continuous assignment.

To prevent a force from back propagating we need to keep a BUFZ
that represents a continuous assignment between two nets. This
only effects continuous assignments of the form assign out = in.
In general these are fairly rare so keeping them has minimal
impact on the simulation speed.
---
 tgt-vvp/vvp_scope.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/tgt-vvp/vvp_scope.c b/tgt-vvp/vvp_scope.c
index aed28c035..da38e9647 100644
--- a/tgt-vvp/vvp_scope.c
+++ b/tgt-vvp/vvp_scope.c
@@ -260,9 +260,43 @@ const char*drive_string(ivl_drive_t drive)
  * on. The last net is selected as the output of the nexus.
  */
 
+/*
+ * When checking if we can elide a buffer we need to keep the buffer
+ * if both the input and output for the buffer are connected only
+ * to netlist signals. This routine performs this check on the
+ * given nexus.
+ */
+static unsigned is_netlist_signal(ivl_net_logic_t net, ivl_nexus_t nex)
+{
+      unsigned idx, rtn;
+
+	/* Assume that this is a netlist signal. */
+      rtn = 1;
+
+      for (idx = 0; idx < ivl_nexus_ptrs(nex); idx += 1) {
+	    ivl_nexus_ptr_t nptr;
+	    ivl_signal_t sptr;
+
+	    nptr = ivl_nexus_ptr(nex, idx);
+
+	      /* Skip a pointer to the buffer we're checking. */
+            if (ivl_nexus_ptr_log(nptr) == net) continue;
+
+	      /* Check to see if this is a netlist signal. */
+	    sptr = ivl_nexus_ptr_sig(nptr);
+	    if (sptr && !ivl_signal_local(sptr)) continue;
+
+	      /* If we get here then this is not just a netlist signal. */
+	    rtn = 0;
+	    break;
+      }
+
+      return rtn;
+}
+
 /*
  * This tests a bufz device against an output receiver, and determines
- * if the device can be skipped. If this function returns true, then a
+ * if the device can be skipped. If this function returns false, then a
  * gate will be generated for this node. Otherwise, the code generator
  * will connect its input to its output and skip the gate.
  */
@@ -311,6 +345,14 @@ int can_elide_bufz(ivl_net_logic_t net, ivl_nexus_ptr_t nptr)
       if (drive_count != 1)
 	    return 0;
 
+	/* If both the input and output are netlist signal then we cannot
+	   elide a BUFZ since it represents a continuous assignment. */
+      if (is_netlist_signal(net, ivl_logic_pin(net, 0)) &&
+          is_netlist_signal(net, ivl_logic_pin(net, 1)) &&
+          (ivl_logic_type(net) == IVL_LO_BUFZ)) {
+	     return 0;
+      }
+
       return 1;
 }