Refine {QString, QBA}::reallocData() logic

Fixed misleading naming of "slowReallocatePath". It's no longer "slow",
it's downright dangerous now to reallocate under certain conditions

While at it, added extra assert to QArrayData::reallocateUnaligned()
and cleaned up that function a bit

Change-Id: I05921fb5058eb563997e66107566c87fb4ea5599
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 4 insertions, 5 deletions

diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 32e5bb7b20..eed98e7c7e 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -2504,12 +2504,11 @@ void QString::reallocData(qsizetype alloc, QArrayData::AllocationOption option)
         return;
     }
 
-    // there's a case of slow reallocate path where we need to memmove the data
-    // before a call to ::realloc(), meaning that there's an extra "heavy"
-    // operation. just prefer ::malloc() branch in this case
-    const bool slowReallocatePath = d.freeSpaceAtBegin() > 0;
+    // don't use reallocate path when reducing capacity and there's free space
+    // at the beginning: might shift data pointer outside of allocated space
+    const bool cannotUseReallocate = d.freeSpaceAtBegin() > 0 && alloc < d.constAllocatedCapacity();
 
-    if (d->needsDetach() || slowReallocatePath) {
+    if (d->needsDetach() || cannotUseReallocate) {
         DataPointer dd(Data::allocate(alloc, option), qMin(alloc, d.size));
         if (dd.size > 0)
             ::memcpy(dd.data(), d.data(), dd.size * sizeof(QChar));