diff --git a/src/cmd/9g/ggen.c b/src/cmd/9g/ggen.c
index c41d8eb414..7d9cf5050d 100644
--- a/src/cmd/9g/ggen.c
+++ b/src/cmd/9g/ggen.c
@@ -900,7 +900,7 @@ ret:
 void
 clearfat(Node *nl)
 {
-	uint64 w, c, q, t;
+	uint64 w, c, q, t, boff;
 	Node dst, end, r0, *f;
 	Prog *p, *pl;
 
@@ -944,6 +944,8 @@ clearfat(Node *nl)
 		patch(gbranch(ABNE, T, 0), pl);
 
 		regfree(&end);
+		// The loop leaves R3 on the last zeroed dword
+		boff = 8;
 	} else if(q >= 4) {
 		p = gins(ASUB, N, &dst);
 		p->from.type = D_CONST;
@@ -953,17 +955,21 @@ clearfat(Node *nl)
 		afunclit(&p->to, f);
 		// 4 and 128 = magic constants: see ../../runtime/asm_power64x.s
 		p->to.offset = 4*(128-q);
-	} else
-	for(t = 0; t < q; t++) {
-		p = gins(AMOVD, &r0, &dst);
-		p->to.type = D_OREG;
-		p->to.offset = 8*t;
+		// duffzero leaves R3 on the last zeroed dword
+		boff = 8;
+	} else {
+		for(t = 0; t < q; t++) {
+			p = gins(AMOVD, &r0, &dst);
+			p->to.type = D_OREG;
+			p->to.offset = 8*t;
+		}
+		boff = 8*q;
 	}
 
 	for(t = 0; t < c; t++) {
 		p = gins(AMOVB, &r0, &dst);
 		p->to.type = D_OREG;
-		p->to.offset = t;
+		p->to.offset = t+boff;
 	}
 	reg[REGRT1]--;
 }
diff --git a/src/runtime/asm_power64x.s b/src/runtime/asm_power64x.s
index ab2db061c2..2ad3e56e94 100644
--- a/src/runtime/asm_power64x.s
+++ b/src/runtime/asm_power64x.s
@@ -829,7 +829,7 @@ notfound:
 // in ../../cmd/9g/ggen.c:/^clearfat.
 // R0: always zero
 // R3 (aka REGRT1): ptr to memory to be zeroed - 8
-// R3 is updated as a side effect.
+// On return, R3 points to the last zeroed dword.
 TEXT runtime·duffzero(SB), NOSPLIT, $-8-0
 	MOVDU	R0, 8(R3)
 	MOVDU	R0, 8(R3)
diff --git a/test/clearfat.go b/test/clearfat.go
new file mode 100644
index 0000000000..45d539306e
--- /dev/null
+++ b/test/clearfat.go
@@ -0,0 +1,68 @@
+// runoutput
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check that {5,6,8,9}g/ggen.c:clearfat is zeroing the entire object.
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+const ntest = 1100
+
+func main() {
+	var decls, calls bytes.Buffer
+
+	for i := 1; i <= ntest; i++ {
+		s := strconv.Itoa(i)
+		decls.WriteString(strings.Replace(decl, "$", s, -1))
+		calls.WriteString(strings.Replace("poison$()\n\tclearfat$()\n\t", "$", s, -1))
+	}
+
+	program = strings.Replace(program, "$DECLS", decls.String(), 1)
+	program = strings.Replace(program, "$CALLS", calls.String(), 1)
+	fmt.Print(program)
+}
+
+var program = `package main
+
+var count int
+
+$DECLS
+
+func main() {
+	$CALLS
+	if count != 0 {
+		println("failed", count, "case(s)")
+	}
+}
+`
+
+const decl = `
+func poison$() {
+	// Grow and poison the stack space that will be used by clearfat$
+	var t [2*$]byte
+	for i := range t {
+		t[i] = 0xff
+	}
+}
+
+func clearfat$() {
+	var t [$]byte
+
+	for _, x := range t {
+		if x != 0 {
+//			println("clearfat$: index", i, "expected 0, got", x)
+			count++
+			break
+		}
+	}
+}
+`