Add QList::emplaceFront() that fixes huge performance issue in prepend

Prepend in QList was using insert() logic that always uses append-aware
functions. This results in the fact that freeSpaceAtBegin() is always 0
and forces us to actually allocate on *every* call (with the same
capacity!). Vicious cycle is hot-fixable with introduction of
emplaceFront (or anything prepend-aware, really)

This brings me from 632ms to 0.65ms for 100k iterations of
list.prepend(int(0)). Still ~3x worse than QList in 5.15 but much
faster than QVector, which takes 382ms in the same workload

Not addressed:
- QString/QBA
- Other prepend functions in QList e.g. prepend(it1, it2)
- Lower-level array operations that should just be extended

Task-number: QTBUG-86583
Change-Id: Ie82b07d81a67605cd308d9fabf9532d57935647f
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions