--- a/js/src/jit/ValueNumbering.cpp
+++ b/js/src/jit/ValueNumbering.cpp
@@ -51,17 +51,23 @@ bool
 ValueNumberer::VisibleValues::ValueHasher::match(Key k, Lookup l)
 {
     // If one of the instructions depends on a store, and the other instruction
     // does not depend on the same store, the instructions are not congruent.
     if (k->dependency() != l->dependency())
         return false;
 
     bool congruent = k->congruentTo(l); // Ask the values themselves what they think.
-    MOZ_ASSERT(congruent == l->congruentTo(k), "congruentTo relation is not symmetric");
+#ifdef DEBUG
+    if (congruent != l->congruentTo(k)) {
+       JitSpew(JitSpew_GVN, "      congruentTo relation is not symmetric between %s%u and %s%u!!",
+               k->opName(), k->id(),
+               l->opName(), l->id());
+    }
+#endif
     return congruent;
 }
 
 void
 ValueNumberer::VisibleValues::ValueHasher::rekey(Key &k, Key newKey)
 {
     k = newKey;
 }