[libcxx] [PATCH] unordered_map: Avoid unnecessary mallocs when no insert occurs

[Duncan P. N. Exon Smith via cfe-commits]

> On 2015-Dec-16, at 14:42, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> This is a follow-up to r239666: "Fix PR12999 - unordered_set::insert
> calls operator new when no insert occurs".  That fix didn't apply to
> `unordered_map` because unordered_map::value_type gets packed inside:
> --
> union __value_type {
>  pair<key_type, mapped_type> __nc;       // Only C++11 or higher.
>  pair<const key_type, mapped_type> __cc; // Always.
>  // Constructors...
> };
> --
> and the underlying __hash_table only knows about __value_type.

Sorry for the quick ping, but I realized this morning that my approach
was still leaving mallocs on the table.

I've attached a new patch that handles more cases.

This patch should avoid unnecessary mallocs whenever the caller passes
in a pair<T, U> such that T is trivially convertible to key_type.

Since __hash_table's value_type is really *never* a pair (for
unordered_map, it's a union of two pairs) the static dispatch is all in
unordered_map.  It's doing this:
  - If the argument isn't a pair<>, alloc.
  - If argument.first can be referenced as const key_type&, don't alloc.
  - If argument.first can be trivially converted to key_type, don't
    alloc.
  - Else alloc.

In the pre-C++11 world the caller has already converted to
unordered_map::value_type.  We can always avoid the alloc.

To support all of this:
  - In C++03, __unordered_map_equal and __unordered_map_hasher need to
    handle unordered_map::value_type.
  - In C++03, __hash_table::__insert_unique_value() now takes its
    argument by template.
  - In C++11, __hash_table::__insert_unique_value() is now a one-liner
    that forwards to __insert_unique_key_value() for the real work.
  - The versions of __hash_table::__construct_node() that take a
    pre-computed hash have been renamed to __construct_node_hash(), and
    these versions use perfect forwarding.

Most of the following still apply:

> This is one of my first patches for libc++, and I'm not sure of a few
> things:
>  - Did I successfully match the coding style?  (I'm kind of lost
>    without clang-format TBH.)
>  - Should I separate the change to __construct_node_hash() into a
>    separate prep commit?  (I would if this were LLVM, but I'm not sure
>    if the common practice is different for libc++.)
>  - Most of the overloads I added to __unordered_map_hasher and
>    __unordered_map_equal aren't actually used by
>    __hash_table::__insert_unique_value().  Should I omit the unused
>    ones?  (Again, for LLVM I would have omitted them.)

(For the updated patch, I went with the LLVM approach of only adding
the used API.  It seems more appropriate in this case.)

> After this I'll fix the same performance issue in std::map (and I
> assume std::set?).

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