Bug 570663: turn a tableswitch on trace into a no-op if it has no cases, r=njn

js/src/jstracer.cpp

@@ -8731,13 +8731,22 @@ TraceRecorder::tableswitch()
         high = GET_JUMPX_OFFSET(pc);
     }
 
+    /* 
+     * If there are no cases, this is a no-op. The default case immediately
+     * follows in the bytecode and is always taken, so we need no special
+     * action to handle it.
+     */
+    int count = high + 1 - low;
+    if (count == 0)
+        return ARECORD_CONTINUE;
+
     /* Cap maximum table-switch size for modesty. */
-    if ((high + 1 - low) > MAX_TABLE_SWITCH)
+    if (count > MAX_TABLE_SWITCH)
         return InjectStatus(switchop());
 
     /* Generate switch LIR. */
     SwitchInfo* si = new (traceAlloc()) SwitchInfo();
-    si->count = high + 1 - low;
+    si->count = count;
     si->table = 0;
     si->index = (uint32) -1;
     LIns* diff = lir->ins2(LIR_subi, v_ins, lir->insImmI(low));

js/src/trace-test/tests/basic/bug570663-1.js

@@ -0,0 +1,4 @@
+// don't crash
+for (var a = 0; a < 4; a++) {
+  switch (NaN) {}
+}

js/src/trace-test/tests/basic/bug570663-2.js

@@ -0,0 +1,12 @@
+function f() {
+    var x;
+    for (var a = 0; a < 4; a++) {
+	switch (NaN) {
+	default:
+	    x = a;
+	}
+    }
+    assertEq(x, 3);
+}
+
+f();