diff --git a/tools/qfcc/include/flow.h b/tools/qfcc/include/flow.h
index 6f630c8a9..eb2d4ba20 100644
--- a/tools/qfcc/include/flow.h
+++ b/tools/qfcc/include/flow.h
@@ -44,7 +44,8 @@ typedef struct flowvar_s {
 	struct flowvar_s *next;		///< for ALLOC
 	struct set_s *use;			///< set of statements that use this var
 	struct set_s *define;		///< set of statements that define this var
-	struct set_s *udchains;		///< set of ud chains for this var
+	struct set_s *udchains;		///< set of ud-chains for this var
+	struct set_s *duchains;		///< set of du-chains for this var
 	struct operand_s *op;		///< an operand using this var
 	int         number;			///< number of variable in func's ref list
 	int         flowaddr;		///< psuedo address for local and temp vars
diff --git a/tools/qfcc/include/function.h b/tools/qfcc/include/function.h
index 4777d3668..a6f6d9dca 100644
--- a/tools/qfcc/include/function.h
+++ b/tools/qfcc/include/function.h
@@ -123,6 +123,7 @@ typedef struct function_s {
 	int                 num_statements;
 	int                 num_ud_chains;
 	struct udchain_s   *ud_chains;
+	struct udchain_s   *du_chains;
 	int                 pseudo_addr;///< pseudo address space for flow analysis
 	struct pseudoop_s  *pseudo_ops;///< pseudo operands used by this function
 } function_t;
diff --git a/tools/qfcc/include/statements.h b/tools/qfcc/include/statements.h
index d79f4d9d1..fab423f0c 100644
--- a/tools/qfcc/include/statements.h
+++ b/tools/qfcc/include/statements.h
@@ -119,6 +119,8 @@ typedef struct statement_s {
 	operand_t  *kill;			///< list of auxiliary operands killed
 	int         first_use;
 	int         num_use;
+	int         first_def;
+	int         num_def;
 } statement_t;
 
 typedef struct sblock_s {
diff --git a/tools/qfcc/source/dot_flow.c b/tools/qfcc/source/dot_flow.c
index 8440dd0d8..4a37156b4 100644
--- a/tools/qfcc/source/dot_flow.c
+++ b/tools/qfcc/source/dot_flow.c
@@ -204,11 +204,11 @@ print_flow_vars (dstring_t *dstr, flowgraph_t *graph, int level)
 	dasprintf (dstr, "%*sfv_%p [shape=none,label=<\n", indent, "", graph);
 	dasprintf (dstr, "%*s<table border=\"0\" cellborder=\"1\" "
 					 "cellspacing=\"0\">\n", indent + 2, "");
-	dasprintf (dstr, "%*s<tr><td colspan=\"6\">flow vars</td></tr>\n",
+	dasprintf (dstr, "%*s<tr><td colspan=\"7\">flow vars</td></tr>\n",
 			   indent + 4, "");
 	dasprintf (dstr, "%*s<tr><td>#</td><td>name</td>"
-			   "<td>addr</td><td>define</td><td>use</td><td>ud</td></tr>\n",
-			   indent + 4, "");
+			   "<td>addr</td><td>define</td><td>use</td><td>ud</td><td>du</td>"
+			   "</tr>\n", indent + 4, "");
 	for (i = 0; i < graph->func->num_vars; i++) {
 		var = graph->func->vars[i];
 		dasprintf (dstr, "%*s<tr><td>%d</td><td>%s</td><td>%d</td><td>%s</td>",
@@ -218,6 +218,7 @@ print_flow_vars (dstring_t *dstr, flowgraph_t *graph, int level)
 				   set_as_string (var->define));
 		dasprintf (dstr, "<td>%s</td>", set_as_string (var->use));
 		dasprintf (dstr, "<td>%s</td>", set_as_string (var->udchains));
+		dasprintf (dstr, "<td>%s</td>", set_as_string (var->duchains));
 		dasprintf (dstr, "</tr>\n");
 	}
 	dasprintf (dstr, "%*s</table>>];\n", indent + 2, "");
@@ -237,6 +238,22 @@ print_flow_vars (dstring_t *dstr, flowgraph_t *graph, int level)
 				   indent + 4, "", i, ud.var, ud.usest, ud.defst);
 	}
 	dasprintf (dstr, "%*s</table>>];\n", indent + 2, "");
+
+	dasprintf (dstr, "%*sdu_%p [shape=none,label=<\n", indent, "", graph);
+	dasprintf (dstr, "%*s<table border=\"0\" cellborder=\"1\" "
+					 "cellspacing=\"0\">\n", indent + 2, "");
+	dasprintf (dstr, "%*s<tr><td colspan=\"4\">du chains</td></tr>\n",
+			   indent + 4, "");
+	dasprintf (dstr, "%*s<tr><td>#</td><td>var</td>"
+			   "<td>def</td><td>use</td></tr>\n",
+			   indent + 4, "");
+	for (i = 0; i < graph->func->num_ud_chains; i++) {
+		udchain_t   du = graph->func->du_chains[i];
+		dasprintf (dstr, "%*s<tr><td>%d</td><td>%d</td>"
+				   "<td>%d</td><td>%d</td></tr>",
+				   indent + 4, "", i, du.var, du.defst, du.usest);
+	}
+	dasprintf (dstr, "%*s</table>>];\n", indent + 2, "");
 }
 
 static void
diff --git a/tools/qfcc/source/flow.c b/tools/qfcc/source/flow.c
index b5320a93e..8749e69db 100644
--- a/tools/qfcc/source/flow.c
+++ b/tools/qfcc/source/flow.c
@@ -40,7 +40,7 @@
 #include <stdlib.h>
 
 #include "QF/alloc.h"
-#include "QF/dstring.h"
+#include "QF/heapsort.h"
 #include "QF/set.h"
 #include "QF/va.h"
 
@@ -87,7 +87,7 @@ ALLOC_STATE (flowgraph_t, graphs);		///< flow graph pool
 
 /**	Allocate a new flow var.
  *
- *	The var's use, define and udchain sets are initialized to empty.
+ *	The var's use, define, udchain and duchain sets are initialized to empty.
  */
 static flowvar_t *
 new_flowvar (void)
@@ -97,6 +97,7 @@ new_flowvar (void)
 	var->use = set_new ();
 	var->define = set_new ();
 	var->udchains = set_new ();
+	var->duchains = set_new ();
 	return var;
 }
 
@@ -108,6 +109,7 @@ delete_flowvar (flowvar_t *var)
 	set_delete (var->use);
 	set_delete (var->define);
 	set_delete (var->udchains);
+	set_delete (var->duchains);
 	FREE (vars, var);
 }
 
@@ -1078,6 +1080,14 @@ live_set_def (set_t *stdef, set_t *use, set_t *def)
 	set_union (def, stdef);
 }
 
+static  int
+duchain_cmp (const void *_a, const void *_b)
+{
+	const udchain_t *a = _a;
+	const udchain_t *b = _b;
+	return a->defst - b->defst;
+}
+
 static void
 flow_build_chains (flowgraph_t *graph)
 {
@@ -1100,9 +1110,10 @@ flow_build_chains (flowgraph_t *graph)
 	for (int i = 0; i < graph->func->num_vars; i++) {
 		udchains[i] = set_new ();
 	}
+	int num_ud_chains;
 	while (1) {
 		udchain_t  *ud_chains = 0;
-		int num_ud_chains = 0;
+		num_ud_chains = 0;
 
 		// count use-def chain elements
 		for (int i = 0; i < graph->num_nodes; i++) {
@@ -1189,6 +1200,25 @@ flow_build_chains (flowgraph_t *graph)
 	set_delete (reach.gen);
 	set_delete (reach.kill);
 	set_delete (reach.stdef);
+
+	graph->func->du_chains = malloc (num_ud_chains * sizeof (udchain_t));
+	memcpy (graph->func->du_chains, graph->func->ud_chains,
+			num_ud_chains * sizeof (udchain_t));
+	heapsort (graph->func->du_chains, num_ud_chains, sizeof (udchain_t),
+			  duchain_cmp);
+	for (int i = 0; i < num_ud_chains; i++) {
+		udchain_t   du = graph->func->du_chains[i];
+
+		flowvar_t  *var = graph->func->vars[du.var];
+		set_add (var->duchains, i);
+
+		if (du.defst < graph->func->num_statements) {
+			statement_t *st = graph->func->statements[du.defst];
+			if (!st->num_def++) {
+				st->first_def = i;
+			}
+		}
+	}
 }
 
 static void