diff --git a/js/src/jscntxt.cpp b/js/src/jscntxt.cpp
index 6d07b7fb1c7..98e9666ea85 100644
--- a/js/src/jscntxt.cpp
+++ b/js/src/jscntxt.cpp
@@ -1557,7 +1557,7 @@ js_ReportValueErrorFlags(JSContext *cx, uintN flags, const uintN errorNumber,
 
 #if defined DEBUG && defined XP_UNIX
 /* For gdb usage. */
-void js_traceon(JSContext *cx)  { cx->tracefp = stderr; }
+void js_traceon(JSContext *cx)  { cx->tracefp = stderr; cx->tracePrevOp = JSOP_LIMIT; }
 void js_traceoff(JSContext *cx) { cx->tracefp = NULL; }
 #endif
 
diff --git a/js/src/jscntxt.h b/js/src/jscntxt.h
index 27a290177ec..05ed2aca1fd 100644
--- a/js/src/jscntxt.h
+++ b/js/src/jscntxt.h
@@ -922,6 +922,7 @@ struct JSContext {
     char                *lastMessage;
 #ifdef DEBUG
     void                *tracefp;
+    JSOp                tracePrevOp;
 #endif
 
     /* Per-context optional error reporter. */
diff --git a/js/src/jsinterp.cpp b/js/src/jsinterp.cpp
index e7ad75cb28b..d4ecbc6bc4a 100644
--- a/js/src/jsinterp.cpp
+++ b/js/src/jsinterp.cpp
@@ -2045,12 +2045,11 @@ js_DoIncDec(JSContext *cx, const JSCodeSpec *cs, jsval *vp, jsval *vp2)
 #ifdef DEBUG
 
 JS_STATIC_INTERPRET JS_REQUIRES_STACK void
-js_TraceOpcode(JSContext *cx, jsint len)
+js_TraceOpcode(JSContext *cx)
 {
     FILE *tracefp;
     JSStackFrame *fp;
     JSFrameRegs *regs;
-    JSOp prevop;
     intN ndefs, n, nuses;
     jsval *siter;
     JSString *str;
@@ -2060,10 +2059,22 @@ js_TraceOpcode(JSContext *cx, jsint len)
     JS_ASSERT(tracefp);
     fp = cx->fp;
     regs = fp->regs;
-    if (len != 0) {
-        prevop = (JSOp) regs->pc[-len];
-        ndefs = js_CodeSpec[prevop].ndefs;
-        if (ndefs != 0) {
+
+    /*
+     * Operations in prologues don't produce interesting values, and
+     * js_DecompileValueGenerator isn't set up to handle them anyway.
+     */
+    if (cx->tracePrevOp != JSOP_LIMIT && regs->pc >= fp->script->main) {
+        ndefs = js_GetStackDefs(cx, &js_CodeSpec[cx->tracePrevOp],
+                                cx->tracePrevOp, fp->script, regs->pc);
+
+        /*
+         * If there aren't that many elements on the stack, then 
+         * we have probably entered a new frame, and printing output
+         * would just be misleading.
+         */
+        if (ndefs != 0 &&
+            ndefs < regs->sp - fp->slots) {
             for (n = -ndefs; n < 0; n++) {
                 char *bytes = js_DecompileValueGenerator(cx, n, regs->sp[n],
                                                          NULL);
@@ -2108,6 +2119,10 @@ js_TraceOpcode(JSContext *cx, jsint len)
         }
         fprintf(tracefp, " @ %u\n", (uintN) (regs->sp - StackBase(fp)));
     }
+    cx->tracePrevOp = op;
+
+    /* It's nice to have complete traces when debugging a crash.  */
+    fflush(tracefp);
 }
 
 #endif /* DEBUG */
@@ -2534,6 +2549,30 @@ js_Interpret(JSContext *cx)
 # define JS_EXTENSION_(s) s
 #endif
 
+# ifdef DEBUG
+    /*
+     * We call this macro from BEGIN_CASE in threaded interpreters,
+     * and before entering the switch in non-threaded interpreters.
+     * However, reaching such points doesn't mean we've actually
+     * fetched an OP from the instruction stream: some opcodes use
+     * 'op=x; DO_OP()' to let another opcode's implementation finish
+     * their work, and many opcodes share entry points with a run of
+     * consecutive BEGIN_CASEs.
+     * 
+     * Take care to trace OP only when it is the opcode fetched from
+     * the instruction stream, so the trace matches what one would
+     * expect from looking at the code.  (We do omit POPs after SETs;
+     * unfortunate, but not worth fixing.)
+     */
+#  define TRACE_OPCODE(OP)  JS_BEGIN_MACRO                                    \
+                                if (JS_UNLIKELY(cx->tracefp != NULL) &&       \
+                                    (OP) == *regs.pc)                         \
+                                    js_TraceOpcode(cx);                       \
+                            JS_END_MACRO
+# else
+#  define TRACE_OPCODE(OP)  ((void) 0)
+# endif
+
 #if JS_THREADED_INTERP
     static void *const normalJumpTable[] = {
 # define OPDEF(op,val,name,token,length,nuses,ndefs,prec,format) \
@@ -2572,15 +2611,6 @@ js_Interpret(JSContext *cx)
                                 DO_OP();                                      \
                             JS_END_MACRO
 
-# ifdef DEBUG
-#  define TRACE_OPCODE(OP)  JS_BEGIN_MACRO                                    \
-                                if (cx->tracefp)                              \
-                                    js_TraceOpcode(cx, len);                  \
-                            JS_END_MACRO
-# else
-#  define TRACE_OPCODE(OP)  (void)0
-# endif
-
 # define BEGIN_CASE(OP)     L_##OP: TRACE_OPCODE(OP); CHECK_RECORDER();
 # define END_CASE(OP)       DO_NEXT_OP(OP##_LENGTH);
 # define END_VARLEN_CASE    DO_NEXT_OP(len);
@@ -2846,13 +2876,10 @@ js_Interpret(JSContext *cx)
       advance_pc:
         regs.pc += len;
         op = (JSOp) *regs.pc;
-# ifdef DEBUG
-        if (cx->tracefp)
-            js_TraceOpcode(cx, len);
-# endif
 
       do_op:
         CHECK_RECORDER();
+        TRACE_OPCODE(op);
         switchOp = intN(op) | switchMask;
       do_switch:
         switch (switchOp) {
diff --git a/js/src/jsinterp.h b/js/src/jsinterp.h
index 6d5653b4231..d7046b63dc3 100644
--- a/js/src/jsinterp.h
+++ b/js/src/jsinterp.h
@@ -611,7 +611,7 @@ js_DoIncDec(JSContext *cx, const JSCodeSpec *cs, jsval *vp, jsval *vp2);
  * previous opcode.
  */
 extern JS_REQUIRES_STACK void
-js_TraceOpcode(JSContext *cx, jsint len);
+js_TraceOpcode(JSContext *cx);
 
 /*
  * JS_OPMETER helper functions.
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index b36680958fb..efd1b9fd3fc 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -1859,13 +1859,8 @@ DisassWithSrc(JSContext *cx, JSObject *obj, uintN argc, jsval *argv,
 static JSBool
 Tracing(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval)
 {
-    JSBool bval;
-    JSString *str;
+    FILE *file;
 
-#if JS_THREADED_INTERP
-    JS_ReportError(cx, "tracing not supported in JS_THREADED_INTERP builds");
-    return JS_FALSE;
-#else
     if (argc == 0) {
         *rval = BOOLEAN_TO_JSVAL(cx->tracefp != 0);
         return JS_TRUE;
@@ -1873,24 +1868,39 @@ Tracing(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval)
 
     switch (JS_TypeOfValue(cx, argv[0])) {
       case JSTYPE_NUMBER:
-        bval = JSVAL_IS_INT(argv[0])
-               ? JSVAL_TO_INT(argv[0])
-               : (jsint) *JSVAL_TO_DOUBLE(argv[0]);
+      case JSTYPE_BOOLEAN: {
+        JSBool bval;
+        if (!JS_ValueToBoolean(cx, argv[0], &bval))
+            goto bad_argument;
+        file = bval ? stderr : NULL;
         break;
-      case JSTYPE_BOOLEAN:
-        bval = JSVAL_TO_BOOLEAN(argv[0]);
-        break;
-      default:
-        str = JS_ValueToString(cx, argv[0]);
-        if (!str)
+      }
+      case JSTYPE_STRING: {
+        char *name = JS_GetStringBytes(JSVAL_TO_STRING(argv[0]));
+        file = fopen(name, "w");
+        if (!file) {
+            JS_ReportError(cx, "tracing: couldn't open output file %s: %s", 
+                           name, strerror(errno));
             return JS_FALSE;
-        JS_ReportError(cx, "tracing: illegal argument %s",
-                       JS_GetStringBytes(str));
-        return JS_FALSE;
+        }
+        break;
+      }
+      default:
+          goto bad_argument;
     }
-    cx->tracefp = bval ? stderr : NULL;
+    if (cx->tracefp && cx->tracefp != stderr)
+      fclose((FILE *)cx->tracefp);
+    cx->tracefp = file;
+    cx->tracePrevOp = JSOP_LIMIT;
     return JS_TRUE;
-#endif
+
+ bad_argument:
+    JSString *str = JS_ValueToString(cx, argv[0]);
+    if (!str)
+        return JS_FALSE;
+    JS_ReportError(cx, "tracing: illegal argument %s",
+                   JS_GetStringBytes(str));
+    return JS_FALSE;
 }
 
 static void
@@ -3676,7 +3686,8 @@ static const char *const shell_help_messages[] = {
 "dumpHeap([fileName[, start[, toFind[, maxDepth[, toIgnore]]]]])\n"
 "  Interface to JS_DumpHeap with output sent to file",
 "notes([fun])             Show source notes for functions",
-"tracing([toggle])        Turn tracing on or off",
+"tracing([true|false|filename]) Turn bytecode execution tracing on/off.\n"
+"                         With filename, send to file.\n",
 "stats([string ...])      Dump 'arena', 'atom', 'global' stats",
 #endif
 #ifdef TEST_CVTARGS