[PATCH] D21510: [libc++] Check hash before calling __hash_table key_eq function

[Kwasi Mensah via cfe-commits]

kmensah created this revision.
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kmensah set the repository for this revision to rL LLVM.

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.

Repository:
  rL LLVM

http://reviews.llvm.org/D21510

Files:
  include/__hash_table

Index: include/__hash_table
===================================================================
--- include/__hash_table
+++ include/__hash_table
@@ -2200,7 +2200,7 @@
                                        __constrain_hash(__nd->__hash_, __bc) == __chash;
                                                            __nd = __nd->__next_)
             {
-                if (key_eq()(__nd->__value_, __k))
+                if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, __k))
 #if _LIBCPP_DEBUG_LEVEL >= 2
                     return iterator(__nd, this);
 #else
@@ -2229,7 +2229,7 @@
                                            __constrain_hash(__nd->__hash_, __bc) == __chash;
                                                            __nd = __nd->__next_)
             {
-                if (key_eq()(__nd->__value_, __k))
+                if ((__nd->__hash_ == __hash) && key_eq()(__nd->__value_, __k))
 #if _LIBCPP_DEBUG_LEVEL >= 2
                     return const_iterator(__nd, this);
 #else


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