CLJS-1483: Minor DCE regression with advanced compilation mode

change :def case in compiler so that we once again only emit if init supplied.
The REPL is the only exception.

Without the above change foward declarations would prevent DCE.

fix minor DCE issue around murmur3 hashing.

src/main/cljs/cljs/core.cljs

@@ -1198,8 +1198,13 @@
         (next coll))
       (mix-collection-hash hash-code n))))
 
-(def ^:private empty-ordered-hash
-  (mix-collection-hash 1 0))
+(def ^{:private true :jsdoc ["@type {*}"]}
+  EMPTY_ORDERED_HASH nil)
+
+(defn- empty-ordered-hash []
+  (when (nil? EMPTY_ORDERED_HASH)
+    (set! EMPTY_ORDERED_HASH (mix-collection-hash 1 0)))
+  EMPTY_ORDERED_HASH)
 
 (defn ^number hash-unordered-coll
   "Returns the hash code, consistent with =, for an external unordered
@@ -1213,8 +1218,13 @@
       (recur (inc n) (bit-or (+ hash-code (hash (first coll))) 0) (next coll))
       (mix-collection-hash hash-code n))))
 
-(def ^:private empty-unordered-hash
-  (mix-collection-hash 0 0))
+(def ^{:private true :jsdoc ["@type {*}"]}
+  EMPTY_UNORDERED_HASH nil)
+
+(defn- empty-unordered-hash []
+  (when (nil? EMPTY_UNORDERED_HASH)
+    (set! EMPTY_UNORDERED_HASH (mix-collection-hash 0 0)))
+  EMPTY_UNORDERED_HASH)
 
 ;;;;;;;;;;;;;;;;;;; protocols on primitives ;;;;;;;;
 (declare hash-map list equiv-sequential)
@@ -2801,7 +2811,7 @@ reduces them without incurring seq initialization"
       false))
 
   IHash
-  (-hash [coll] empty-ordered-hash)
+  (-hash [coll] (empty-ordered-hash))
 
   ISeqable
   (-seq [coll] nil)
@@ -4916,7 +4926,7 @@ reduces them without incurring seq initialization"
 (set! (.-EMPTY-NODE PersistentVector) (VectorNode. nil (make-array 32)))
 
 (set! (.-EMPTY PersistentVector)
-  (PersistentVector. nil 0 5 (.-EMPTY-NODE PersistentVector) (array) empty-ordered-hash))
+  (PersistentVector. nil 0 5 (.-EMPTY-NODE PersistentVector) (array) (empty-ordered-hash)))
 
 (set! (.-fromArray PersistentVector)
   (fn [xs ^boolean no-clone]
@@ -5466,7 +5476,7 @@ reduces them without incurring seq initialization"
   ICounted
   (-count [coll] count))
 
-(set! (.-EMPTY PersistentQueue) (PersistentQueue. nil 0 nil [] empty-ordered-hash))
+(set! (.-EMPTY PersistentQueue) (PersistentQueue. nil 0 nil [] (empty-ordered-hash)))
 
 (es6-iterable PersistentQueue)
 
@@ -5643,7 +5653,7 @@ reduces them without incurring seq initialization"
   (-as-transient [coll]
     (transient (into (hash-map) coll))))
 
-(set! (.-EMPTY ObjMap) (ObjMap. nil (array) (js-obj) 0 empty-unordered-hash))
+(set! (.-EMPTY ObjMap) (ObjMap. nil (array) (js-obj) 0 (empty-unordered-hash)))
 
 (set! (.-HASHMAP_THRESHOLD ObjMap) 8)
 
@@ -5995,7 +6005,7 @@ reduces them without incurring seq initialization"
   (-as-transient [coll]
     (TransientArrayMap. (js-obj) (alength arr) (aclone arr))))
 
-(set! (.-EMPTY PersistentArrayMap) (PersistentArrayMap. nil 0 (array) empty-unordered-hash))
+(set! (.-EMPTY PersistentArrayMap) (PersistentArrayMap. nil 0 (array) (empty-unordered-hash)))
 
 (set! (.-HASHMAP-THRESHOLD PersistentArrayMap) 8)
 
@@ -6931,7 +6941,7 @@ reduces them without incurring seq initialization"
   (-as-transient [coll]
     (TransientHashMap. (js-obj) root cnt has-nil? nil-val)))
 
-(set! (.-EMPTY PersistentHashMap) (PersistentHashMap. nil 0 nil false nil empty-unordered-hash))
+(set! (.-EMPTY PersistentHashMap) (PersistentHashMap. nil 0 nil false nil (empty-unordered-hash)))
 
 (set! (.-fromArray PersistentHashMap)
   (fn [arr ^boolean no-clone]
@@ -7724,7 +7734,7 @@ reduces them without incurring seq initialization"
 
   (-comparator [coll] comp))
 
-(set! (.-EMPTY PersistentTreeMap) (PersistentTreeMap. compare nil 0 nil empty-unordered-hash))
+(set! (.-EMPTY PersistentTreeMap) (PersistentTreeMap. compare nil 0 nil (empty-unordered-hash)))
 
 (es6-iterable PersistentTreeMap)
 
@@ -8045,7 +8055,7 @@ reduces them without incurring seq initialization"
   (-as-transient [coll] (TransientHashSet. (-as-transient hash-map))))
 
 (set! (.-EMPTY PersistentHashSet)
-  (PersistentHashSet. nil (.-EMPTY PersistentArrayMap) empty-unordered-hash))
+  (PersistentHashSet. nil (.-EMPTY PersistentArrayMap) (empty-unordered-hash)))
 
 (set! (.-fromArray PersistentHashSet)
   (fn [items ^boolean no-clone]
@@ -8191,7 +8201,7 @@ reduces them without incurring seq initialization"
     (-lookup coll k not-found)))
 
 (set! (.-EMPTY PersistentTreeSet)
-  (PersistentTreeSet. nil (.-EMPTY PersistentTreeMap) empty-unordered-hash))
+  (PersistentTreeSet. nil (.-EMPTY PersistentTreeMap) (empty-unordered-hash)))
 
 (es6-iterable PersistentTreeSet)
 

src/main/clojure/cljs/compiler.cljc

@@ -602,37 +602,40 @@
 
 (defmethod emit* :def
   [{:keys [name var init env doc jsdoc export test var-ast]}]
-  (let [mname (munge name)]
-    (emit-comment env doc (concat jsdoc (:jsdoc init)))
-    (when (:def-emits-var env)
-      (when (= :return (:context env))
-        (emitln "return ("))
-      (emitln "(function (){"))
-    (emits var)
-    (when init
-      (emits " = "
-       (if-let [define (get-define mname jsdoc)]
-         define
-         init)))
-    (when (:def-emits-var env)
-      (emitln "; return (")
-      (emits (merge
-               {:op :var-special
-                :env (assoc env :context :expr)}
-               var-ast))
-      (emitln ");})()")
-      (when (= :return (:context env))
-        (emitln ")")))
-    ;; NOTE: JavaScriptCore does not like this under advanced compilation
-    ;; this change was primarily for REPL interactions - David
-    ;(emits " = (typeof " mname " != 'undefined') ? " mname " : undefined")
-    (when-not (= :expr (:context env)) (emitln ";"))
-    (when export
-      (emitln "goog.exportSymbol('" (munge export) "', " mname ");"))
-    (when (and ana/*load-tests* test)
-      (when (= :expr (:context env))
-        (emitln ";"))
-      (emitln var ".cljs$lang$test = " test ";"))))
+  ;; We only want to emit if an init is supplied, this is to avoid dead code
+  ;; elimination issues. The REPL is the exception to this rule.
+  (when (or init (:def-emits-var env))
+    (let [mname (munge name)]
+     (emit-comment env doc (concat jsdoc (:jsdoc init)))
+     (when (:def-emits-var env)
+       (when (= :return (:context env))
+         (emitln "return ("))
+       (emitln "(function (){"))
+     (emits var)
+     (when init
+       (emits " = "
+         (if-let [define (get-define mname jsdoc)]
+           define
+           init)))
+     (when (:def-emits-var env)
+       (emitln "; return (")
+       (emits (merge
+                {:op  :var-special
+                 :env (assoc env :context :expr)}
+                var-ast))
+       (emitln ");})()")
+       (when (= :return (:context env))
+         (emitln ")")))
+     ;; NOTE: JavaScriptCore does not like this under advanced compilation
+     ;; this change was primarily for REPL interactions - David
+     ;(emits " = (typeof " mname " != 'undefined') ? " mname " : undefined")
+     (when-not (= :expr (:context env)) (emitln ";"))
+     (when export
+       (emitln "goog.exportSymbol('" (munge export) "', " mname ");"))
+     (when (and ana/*load-tests* test)
+       (when (= :expr (:context env))
+         (emitln ";"))
+       (emitln var ".cljs$lang$test = " test ";")))))
 
 (defn emit-apply-to
   [{:keys [name params env]}]