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Large graphical Qt applications heavily rely on heap allocations. Jemalloc is a general-purpose malloc(3) implementation designed to reduce heap fragmentation and improve scalability. It also provides extensive tuning options. Add a -jemalloc configure option, disabled by default. When enabled, Qt and user code link to jemalloc, overriding the system's default malloc(). Add cooperation with jemalloc for some Qt key classes: QArrayData (used by QByteArray, QString and QList<T>), QBindingStoragePrivate, QDataBuffer (used by the Qt Quick renderer), QDistanceFieldData, QImageData, QObjectPrivate::TaggedSignalVector, QVarLengthArray. This cooperation relies on two jemalloc-specific optimizations: 1. Efficient allocation via fittedMalloc(): Determine the actual allocation size using nallocx(), then adjust the container’s capacity to match. This minimizes future reallocations. Note: we round allocSize to a multiple of sizeof(T) to ensure that we can later recompute the exact allocation size during deallocation. 2. Optimized deallocation via sizedFree(): Use sdallocx(), which is faster than free when the allocation size is known, as it avoids internal size lookups. Adapt the QVarLengthArray auto tests on capacity. Non-standard functions docs are at https://jemalloc.net/jemalloc.3.html [ChangeLog][QtCore] Added optional support for the jemalloc allocator, and optimized memory allocations and deallocations in core Qt classes to cooperate with it. Change-Id: I6166e64e66876dee22662d3f3ea3e42a6647cfeb Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>