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[base] Probe hashmap using indices rather than pointers

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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}
This commit is contained in:
leszeks 2016-11-14 07:01:18 -08:00 committed by Commit bot
parent 4e32419be4
commit 3abfc33709
1 changed files with 7 additions and 12 deletions
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19
src/base/hashmap.h
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@ -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,