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>

src/corelib/text/qbytearray.cpp

@@ -1708,12 +1708,11 @@ void QByteArray::reallocData(qsizetype alloc, QArrayData::AllocationOption optio
         return;
     }
 
-    // there's a case of slow reallocate path where we need to memmove the data
+    // don't use reallocate path when reducing capacity and there's free space
-    // before a call to ::realloc(), meaning that there's an extra "heavy"
+    // at the beginning: might shift data pointer outside of allocated space
-    // operation. just prefer ::malloc() branch in this case
+    const bool cannotUseReallocate = d.freeSpaceAtBegin() > 0 && alloc < d.constAllocatedCapacity();
-    const bool slowReallocatePath = d.freeSpaceAtBegin() > 0;
 
-    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);

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
+    // don't use reallocate path when reducing capacity and there's free space
-    // before a call to ::realloc(), meaning that there's an extra "heavy"
+    // at the beginning: might shift data pointer outside of allocated space
-    // operation. just prefer ::malloc() branch in this case
+    const bool cannotUseReallocate = d.freeSpaceAtBegin() > 0 && alloc < d.constAllocatedCapacity();
-    const bool slowReallocatePath = d.freeSpaceAtBegin() > 0;
 
-    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));

src/corelib/tools/qarraydata.cpp

@@ -233,10 +233,13 @@ QArrayData::reallocateUnaligned(QArrayData *data, void *dataPointer,
 {
     Q_ASSERT(!data || !data->isShared());
 
-    qsizetype headerSize = sizeof(QArrayData);
+    const qsizetype headerSize = sizeof(QArrayData);
     qsizetype allocSize = calculateBlockSize(capacity, objectSize, headerSize, option);
-    qptrdiff offset = dataPointer ? reinterpret_cast<char *>(dataPointer) - reinterpret_cast<char *>(data) : headerSize;
+    const qptrdiff offset = dataPointer
+            ? reinterpret_cast<char *>(dataPointer) - reinterpret_cast<char *>(data)
+            : headerSize;
     Q_ASSERT(offset > 0);
+    Q_ASSERT(offset <= allocSize); // == when all free space is at the beginning
 
     allocSize = reserveExtraBytes(allocSize);
     if (Q_UNLIKELY(allocSize < 0))  // handle overflow. cannot reallocate reliably
@@ -244,7 +247,7 @@ QArrayData::reallocateUnaligned(QArrayData *data, void *dataPointer,
 
     QArrayData *header = static_cast<QArrayData *>(::realloc(data, size_t(allocSize)));
     if (header) {
-        header->alloc = uint(capacity);
+        header->alloc = capacity;
         dataPointer = reinterpret_cast<char *>(header) + offset;
     } else {
         dataPointer = nullptr;