QVersionNumber: don't allocate in fromString() in the common case

Use QVarLengthArray instead of QList to avoid allocations in the case
where the result fits into inline storage.

Of course, we now perform one allocation more when we need the QList
backend, but a) that should be the rarer case and b) we use 32 (more
than InlineSegmentsCount) Prealloc for the QVarLengthArray to reliably
skip the first few QList non-reserved capacity jumps (measured to be
{4, 12, 28} for back-insertion, not all of which is probably available
for append()s) to come out ahead either way.

Reviewers may object, saying that we could just reserve(32) the QList,
too, but while that would skip over the first few QList reallocations
alright, it means we'd be carrying the extra capacity around for the
duration of the QVersionNumber's lifetime (unless we'd shrink it at
the end, bringing back the additional allocation and that solution
back to par compared to this one).

As a consequence:

[ChangeLog][QtCore][QVersionNumber] Can now be also be constructed
from QVarLengthArray.

Change-Id: I4016367f64f6cefa6ed9147d33b06636f36b02cb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

0 files changed, 0 insertions, 0 deletions