[PATCH] D17920: Query the StringMap only once when creating MDString (NFC)

[David Blaikie via llvm-commits]

On Tue, Mar 22, 2016 at 9:02 AM, Mehdi AMINI via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> joker.eph added a comment.
>
> > std::unordered_map::emplace returns the classic <iterator, bool> pair.
> Perhaps emplace_second should do the same? Rather than relying on being
> able to detect the invalid default constructed state of the key?
>
>
> I don't understand what you mean: `std::pair<iterator, bool>
> emplace_second(StringRef Key, ArgsTy &&... Args) `
>

Right, to match/parallel with:

template< class... Args >
std::pair <http://en.cppreference.com/w/cpp/utility/pair><iterator,bool>
 emplace( Args&&... args )


>
>
> ================
> Comment at: include/llvm/IR/Metadata.h:595
> @@ -594,3 +594,3 @@
>    MDString() : Metadata(MDStringKind, Uniqued), Entry(nullptr) {}
> -  MDString(MDString &&) : Metadata(MDStringKind, Uniqued) {}
> +  MDString(MDString &&R) = default;
>
> ----------------
> dblaikie wrote:
> > Why do we need a move ctor at all if we're supporting emplace? Can we
> just delete it? (even if we omit it, it should be removed implicitly - same
> with the move assignment above, I think)
> StringMap requires movable types (for std::pair I think).
>

If we're supporting emplace, it's usually because the identity of the
object is important from the moment of construction - so perhaps we should
be making sure that identity is actually preserved, same as the standard
emplace.


>
>
>
> http://reviews.llvm.org/D17920
>
>
>
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