Remove unreachable code (#535)

* Remove unreachable code

This was noticed by MSVC in a debug build which produced the following
warning:

    warning C4702: unreachable code

* Update slot outside of the loop in MPMCQueue

As suggested by @absurdfarce this change keeps finding a slot in the
loop but moves the use of the slot past the loop. That way there is a
return after the loop which should make clear to any code analysis that
this function will always return something.

Author: SeverinLeonhardt

src/execution_profile.cpp

@@ -108,7 +108,6 @@ CassError cass_execution_profile_set_latency_aware_routing_settings(
 CassError cass_execution_profile_set_whitelist_filtering(CassExecProfile* profile,
                                                          const char* hosts) {
   return cass_execution_profile_set_whitelist_filtering_n(profile, hosts, SAFE_STRLEN(hosts));
-  return CASS_OK;
 }
 
 CassError cass_execution_profile_set_whitelist_filtering_n(CassExecProfile* profile,

src/mpmc_queue.hpp

@@ -56,9 +56,10 @@ class MPMCQueue : public Allocated {
     // buffer must be a size which is a power of 2. this also allows
     // the sequence to double as a ticket/lock.
     size_t pos = tail_.load(MEMORY_ORDER_RELAXED);
+    Node* node;
 
     for (;;) {
-      Node* node = &buffer_[pos & mask_];
+      node = &buffer_[pos & mask_];
       size_t node_seq = node->seq.load(MEMORY_ORDER_ACQUIRE);
       intptr_t dif = (intptr_t)node_seq - (intptr_t)pos;
 
@@ -69,11 +70,7 @@ class MPMCQueue : public Allocated {
         // weak compare is faster, but can return spurious results
         // which in this instance is OK, because it's in the loop
         if (tail_.compare_exchange_weak(pos, pos + 1, MEMORY_ORDER_RELAXED)) {
-          // set the data
-          node->data = data;
-          // increment the sequence so that the tail knows it's accessible
-          node->seq.store(pos + 1, MEMORY_ORDER_RELEASE);
-          return true;
+          break;
         }
       } else if (dif < 0) {
         // if seq is less than head seq then it means this slot is
@@ -85,15 +82,19 @@ class MPMCQueue : public Allocated {
       }
     }
 
-    // never taken
-    return false;
+    // set the data
+    node->data = data;
+    // increment the sequence so that the tail knows it's accessible
+    node->seq.store(pos + 1, MEMORY_ORDER_RELEASE);
+    return true;
   }
 
   bool dequeue(T& data) {
     size_t pos = head_.load(MEMORY_ORDER_RELAXED);
+    Node* node;
 
     for (;;) {
-      Node* node = &buffer_[pos & mask_];
+      node = &buffer_[pos & mask_];
       size_t node_seq = node->seq.load(MEMORY_ORDER_ACQUIRE);
       intptr_t dif = (intptr_t)node_seq - (intptr_t)(pos + 1);
 
@@ -104,12 +105,7 @@ class MPMCQueue : public Allocated {
         // weak compare is faster, but can return spurious results
         // which in this instance is OK, because it's in the loop
         if (head_.compare_exchange_weak(pos, pos + 1, MEMORY_ORDER_RELAXED)) {
-          // set the output
-          data = node->data;
-          // set the sequence to what the head sequence should be next
-          // time around
-          node->seq.store(pos + mask_ + 1, MEMORY_ORDER_RELEASE);
-          return true;
+          break;
         }
       } else if (dif < 0) {
         // if seq is less than head seq then it means this slot is
@@ -121,8 +117,12 @@ class MPMCQueue : public Allocated {
       }
     }
 
-    // never taken
-    return false;
+    // set the output
+    data = node->data;
+    // set the sequence to what the head sequence should be next
+    // time around
+    node->seq.store(pos + mask_ + 1, MEMORY_ORDER_RELEASE);
+    return true;
   }
 
   bool is_empty() const {