Instead of adding it after the block size was calculated. This makes no
difference for non-growing (exact) blocks. For growing blocks, this
means we take that extra element into account before rounding to the
next power of two, instead of after. That results in a change of the
thresholds of when a block grows and also what capacity it will
contain.
For example, for a QString growing to 22-25 elements:
Request | Previously | Now |
elements | bytes | malloc()ed | capacity() | malloc()ed | capacity() |
22 | 44 | 66 | 24 | 64 | 23 |
23 | 46 | 66 | 24 | 64 | 23 |
24 | 48 | 66 | 24 | 128 | 55 |
25 | 50 | 130 | 56 | 128 | 55 |
To avoid wasting elementSize - 2 bytes in this footer, we only include
this footer if elementSize <= 2. Thus, for a QList<int> growing to 11-13
elements:
Request | Previously | Now |
elements | bytes | malloc()ed | capacity() | malloc()ed | capacity() |
11 | 44 | 66 | 12 | 64 | 12 |
12 | 48 | 66 | 12 | 128 | 28 |
13 | 52 | 130 | 28 | 128 | 28 |
In both cases, we now only allocate powers of two while growing, which
may be beneficial to some allocators.
Pick-to: 6.6
Change-Id: Ifa1111900d6945ea8e05fffd177dcb96e251d0a1
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
961620824c