> On 2015-Dec-16, at 14:42, Duncan P. N. Exon Smith <dexonsm...@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?).
0001-unordered_map-Avoid-unnecessary-mallocs-when-no-i-v2.patch
Description: Binary data
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