580633 - Do less copying when adding elements to HashMap

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Luke Wagner [:luke]]

Attachment: fix

Although it boils away for builtin types, if you are storing a key or value with a non-trivial operator=/copy-constructor, HashMap will copy more than it needs to when inserting elements.  The solution is to make HashTable's interface a bit more fragile, which is fine since its an implementation detail of HashMap/HashSet, and let the caller do the assignment directly.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Comment on attachment 459012 [details] [diff] [review]
fix

Looks fine, but for HashMap (not HashSet, where the operation makes no sense), would it make sense to also add this function:

    bool add(AddPtr &p, const Key &k) {
        Entry *pentry;
        if (!impl.add(p, &pentry))
            return false;
        const_cast<Key &>(pentry->key) = k;
        return true;
    }

(relying on pentry->value being default-constructed already)?  Or would that not work for some reason?

Comment 2

[Luke Wagner [:luke]]

That would totally work

Comment 3

[Boris Zbarsky [:bzbarsky]]

Let's do that, then.  Then we get no-copy inserts as needed!

Comment 4

[Luke Wagner [:luke]]

http://hg.mozilla.org/tracemonkey/rev/116265442418
and with less fire
http://hg.mozilla.org/tracemonkey/rev/0b8353af4300

Comment 5

[Robert Sayre]

http://hg.mozilla.org/mozilla-central/rev/116265442418