[base] Probe hashmap using indices rather than pointers

Using indices rather than pointers to probe the hashmap lets us
unconditionally mask the index to ensure it wraps around, rather than
branching on the pointer value. This produces slightly more optimal
code.

Review-Url: https://codereview.chromium.org/2488423003
Cr-Commit-Position: refs/heads/master@{#40967}

src/base/hashmap.h

@@ -229,9 +229,8 @@ template <typename Key, typename Value, typename MatchFun,
           class AllocationPolicy>
 void TemplateHashMapImpl<Key, Value, MatchFun, AllocationPolicy>::Clear() {
   // Mark all entries as empty.
-  const Entry* end = map_end();
-  for (Entry* entry = map_; entry < end; entry++) {
-    entry->clear();
+  for (size_t i = 0; i < capacity_; ++i) {
+    map_[i].clear();
   }
   occupancy_ = 0;
 }
@@ -264,19 +263,15 @@ typename TemplateHashMapImpl<Key, Value, MatchFun, AllocationPolicy>::Entry*
 TemplateHashMapImpl<Key, Value, MatchFun, AllocationPolicy>::Probe(
     const Key& key, uint32_t hash) const {
   DCHECK(base::bits::IsPowerOfTwo32(capacity_));
-  Entry* entry = map_ + (hash & (capacity_ - 1));
-  const Entry* end = map_end();
-  DCHECK(map_ <= entry && entry < end);
+  size_t i = hash & (capacity_ - 1);
+  DCHECK(i < capacity_);
 
   DCHECK(occupancy_ < capacity_);  // Guarantees loop termination.
-  while (entry->exists() && !match_(hash, entry->hash, key, entry->key)) {
-    entry++;
-    if (entry >= end) {
-      entry = map_;
-    }
+  while (map_[i].exists() && !match_(hash, map_[i].hash, key, map_[i].key)) {
+    i = (i + 1) & (capacity_ - 1);
   }
 
-  return entry;
+  return &map_[i];
 }
 
 template <typename Key, typename Value, typename MatchFun,