[PATCH] Optimize inserting value_type into std::vector

[Jonathan Wakely <jwakely at redhat dot com>]

	* include/bits/stl_vector.h (vector::_S_insert_aux_assign): Define
	new overloaded functions.
	* include/bits/vector.tcc (vector::_M_insert_aux): Use new functions
	to avoid creating a redundant temporary.
	* testsuite/23_containers/vector/modifiers/insert_vs_emplace.cc: New
	test.

Tested x86_64-linux.

This improves our performance on Howard Hinnant's "insert vs emplace"
experiment at
http://htmlpreview.github.io/?https://github.com/HowardHinnant/papers/blob/master/insert_vs_emplace.html

With this small change there is no difference between emplacing or
using the relevant insert / push_back function. That also means we
beat libc++ in some cases, making us the bestest, whoo!

I originally wrote _S_insert_aux_arg functions which returned their
argument (either _Tp or _Tp&& as appropriate), relying on RVO to elide
the extra constructions, but that caused 23_containers/vector/40192.cc
to FAIL, so this patch passes the iterator into the new functions and
the assignment is done there.

Does anyone see any problem with this optimisation? I'm pretty sure
there are no cases where we actually need to create a temporary from
an expression that is already an rvalue of the correct type.

Howard's test code is CC BY 4.0, so I didn't add our usual GPL header
to the test file. I think the comments I added to the file meet the
requirements for attribution and indicating changes.

Attachment: patch.txt
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