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[libc++] Check hash before calling __hash_table key_eq function

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Summary: The current implementations of __hash_table::find used by std::unordered_set/unordered_map call key_eq on each key that lands in the same bucket as the key you're looking for. However, since equal objects mush hash to the same value, you can short-circuit the possibly expensive call to key_eq by checking the hashes first.

Reviewers: EricWF

Subscribers: kmensah, cfe-commits

Differential Revision: http://reviews.llvm.org/D21510

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@274857 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Kwasi Mensah 2016-07-08 15:34:28 +00:00
parent 8e83446812
commit 95a22db0bb
1 changed files with 2 additions and 2 deletions
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4
include/__hash_table
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@ -2205,7 +2205,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::find(const _Key& __k)
|| __constrain_hash(__nd->__hash_, __bc) == __chash); || __constrain_hash(__nd->__hash_, __bc) == __chash);
__nd = __nd->__next_) __nd = __nd->__next_)
{ {
if (key_eq()(__nd->__value_, __k)) if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, __k))
#if _LIBCPP_DEBUG_LEVEL >= 2 #if _LIBCPP_DEBUG_LEVEL >= 2
return iterator(__nd, this); return iterator(__nd, this);
#else #else
@ -2235,7 +2235,7 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::find(const _Key& __k) const
|| __constrain_hash(__nd->__hash_, __bc) == __chash); || __constrain_hash(__nd->__hash_, __bc) == __chash);
__nd = __nd->__next_) __nd = __nd->__next_)
{ {
if (key_eq()(__nd->__value_, __k)) if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, __k))
#if _LIBCPP_DEBUG_LEVEL >= 2 #if _LIBCPP_DEBUG_LEVEL >= 2
return const_iterator(__nd, this); return const_iterator(__nd, this);
#else #else