unordered_set::emplace and unordered_map::emplace construct a node, then
try to insert it. If insertion fails, the node gets deleted.
To avoid this unnecessary malloc traffic, check to see if the argument
to emplace has the appropriate key_type. If so, we can use that key
directly and delay the malloc until we're sure we're inserting something
new.
Test updates by Eric Fiselier, who rewrote the old allocation tests to
include the new cases.
There are two orthogonal future directions:
1. Apply the same optimization to set and map.
2. Extend the optimization to when the argument is not key_type, but can
be converted to it without side effects. Ideally, we could do this
whenever key_type is trivially destructible and the argument is
trivially convertible to key_type, but in practise the relevant type
traits "blow up sometimes". At least, we should catch a few simple
cases (such as when both are primitive types).
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@263746 91177308-0d34-0410-b5e6-96231b3b80d8
Summary:
when `unordered_set::insert(value_type&&)` was called it would be treated like `unordered_set::emplace(Args&&)` and it would allocate and construct a node before trying to insert it.
This caused unnecessary allocations when the value was already in the set. This patch adds an overload to `__hash_table::__insert_unique` that specifically handles `value_type&&` more link `value_type const &`.
This patch also adds a single unified insert function for values into `__hash_table` called `__insert_unique_value` that handles the cases for `__insert_unique(value_type&&)` and `__insert_unique(value_type const &)`.
This patch fixes PR12999: http://llvm.org/bugs/show_bug.cgi?id=12999.
Reviewers: mclow.lists, titus, danalbert
Reviewed By: danalbert
Subscribers: cfe-commits
Differential Revision: http://reviews.llvm.org/D7570
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@239666 91177308-0d34-0410-b5e6-96231b3b80d8
