Warn that a signed part select may not work for signals < 32 bits.

If a signed signal is driving a part select in a CA and the width
is less than 32 bits. the value will be zero extended and will
not work for negative values. This patch adds a warning that this
could happen. This will be fixed in development.

expr_synth.cc

@@ -1089,7 +1089,8 @@ NetNet* NetESelect::synthesize(Design *des, NetScope*scope, NetExpr*root)
       if (base_ != 0) {
 	    off = base_->synthesize(des, scope, root);
 
-	    NetPartSelect*sel = new NetPartSelect(sub, off, expr_width());
+	    NetPartSelect*sel = new NetPartSelect(sub, off, expr_width(),
+	                                          base_->has_sign());
 	    sel->set_line(*this);
 	    des->add_node(sel);
 

netlist.cc

@@ -885,7 +885,7 @@ const NetDelaySrc* NetNet::delay_path(unsigned idx) const
 NetPartSelect::NetPartSelect(NetNet*sig, unsigned off, unsigned wid,
 			     NetPartSelect::dir_t dir__)
 : NetNode(sig->scope(), sig->scope()->local_symbol(), 2),
-    off_(off), wid_(wid), dir_(dir__)
+    off_(off), wid_(wid), dir_(dir__), signed_flag_(false)
 {
       set_line(*sig);
 
@@ -904,9 +904,9 @@ NetPartSelect::NetPartSelect(NetNet*sig, unsigned off, unsigned wid,
 }
 
 NetPartSelect::NetPartSelect(NetNet*sig, NetNet*sel,
-			     unsigned wid)
+			     unsigned wid, bool signed_flag)
 : NetNode(sig->scope(), sig->scope()->local_symbol(), 3),
-    off_(0), wid_(wid), dir_(VP)
+    off_(0), wid_(wid), dir_(VP), signed_flag_(signed_flag)
 {
       switch (dir_) {
 	  case NetPartSelect::VP:

netlist.h

@@ -1795,12 +1795,14 @@ class NetPartSelect  : public NetNode {
       explicit NetPartSelect(NetNet*sig,
 			     unsigned off, unsigned wid, dir_t dir);
       explicit NetPartSelect(NetNet*sig, NetNet*sel,
-			     unsigned wid);
+			     unsigned wid, bool signed_flag = false);
       ~NetPartSelect();
 
       unsigned base()  const;
       unsigned width() const;
       dir_t    dir()   const;
+	/* Is the select signal signed? */
+      bool signed_flag() const { return signed_flag_; }
 
       virtual void dump_node(ostream&, unsigned ind) const;
       bool emit_node(struct target_t*tgt) const;
@@ -1809,6 +1811,7 @@ class NetPartSelect  : public NetNode {
       unsigned off_;
       unsigned wid_;
       dir_t    dir_;
+      bool signed_flag_;
 };
 
 /*

t-dll.cc

@@ -2153,8 +2153,12 @@ bool dll_target::part_select(const NetPartSelect*net)
       obj->scope = find_scope(des_, net->scope());
       assert(obj->scope);
 
-	/* Part selects are always unsigned. */
+	/* Part selects are always unsigned, so we use this to indicate
-      obj->u_.part.signed_flag = 0;
+	 * if the part select base signal is signed or not. */
+      if (net->signed_flag())
+	    obj->u_.part.signed_flag = 1;
+      else
+	    obj->u_.part.signed_flag = 0;
 
 	/* Choose the width of the part select. */
       obj->width = net->width();

tgt-vvp/vvp_scope.c

@@ -1654,6 +1654,14 @@ static void draw_lpm_part(ivl_lpm_t net)
 	    fprintf(vvp_out, ", %u, %u;\n", base, width);
       } else {
 	    const char*sel_symbol = draw_net_input(sel);
+	      /* We need to enhance .part/v to support a signed index. */
+	    if (ivl_lpm_signed(net) && width_of_nexus(sel) < 8*sizeof(int)) {
+		  fprintf(stderr, "%s:%u: tgt-vvp warning: V0.9 may give "
+		                  "incorrect results for a select with a "
+		                  "signed index less than %zu bits.\n",
+		                  ivl_lpm_file(net), ivl_lpm_lineno(net),
+		                  8*sizeof(int));
+	    }
 	    fprintf(vvp_out, "L_%p%s .part/v %s",
 		    net, dly, draw_net_input(ivl_lpm_data(net,0)));
 	    fprintf(vvp_out, ", %s", sel_symbol);