QBitArray: refactor operator~() to write to an uninitialized buffer

No functionality change otherwise.

Change-Id: I85b3fc2dd45c4693be13fffd179627b2747c132b
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

2 files changed, 22 insertions, 11 deletions

diff --git a/src/corelib/tools/qbitarray.cpp b/src/corelib/tools/qbitarray.cpp
index 5c48b3b608..4a3e929909 100644
--- a/src/corelib/tools/qbitarray.cpp
+++ b/src/corelib/tools/qbitarray.cpp
@@ -594,18 +594,26 @@ QBitArray &QBitArray::operator^=(const QBitArray &other)
 
 QBitArray QBitArray::operator~() const
 {
-    qsizetype sz = size();
-    QBitArray a(sz);
-    const uchar *a1 = reinterpret_cast<const uchar *>(d.constData()) + 1;
-    uchar *a2 = reinterpret_cast<uchar *>(a.d.data()) + 1;
-    qsizetype n = d.size() - 1;
-
-    while (n-- > 0)
-        *a2++ = ~*a1++;
+    qsizetype n = d.size();
+    QBitArray result(QByteArrayData(n, n));
+    const uchar *src = reinterpret_cast<const uchar *>(data_ptr().data());
+    uchar *dst = reinterpret_cast<uchar *>(result.data_ptr().data());
+
+    uchar bitdiff = 8;
+    if (n)
+        bitdiff = dst[0] = src[0];      // copy the count of bits in the last byte
+
+    for (qsizetype i = 1; i < n; ++i)
+        dst[i] = ~src[i];
+
+    if (int tailCount = 16 - bitdiff; tailCount != 8) {
+        // zero the bits beyond our size in the last byte
+        Q_ASSERT(n > 1);
+        uchar tailMask = (1U << tailCount) - 1;
+        dst[n - 1] &= tailMask;
+    }
 
-    if (sz && sz % 8)
-        *(a2 - 1) &= (1 << (sz % 8)) - 1;
-    return a;
+    return result;
 }
 
 /*!
diff --git a/src/corelib/tools/qbitarray.h b/src/corelib/tools/qbitarray.h
index b5b6638507..a1dde3934c 100644
--- a/src/corelib/tools/qbitarray.h
+++ b/src/corelib/tools/qbitarray.h
@@ -18,6 +18,8 @@ class Q_CORE_EXPORT QBitArray
     friend Q_CORE_EXPORT size_t qHash(const QBitArray &key, size_t seed) noexcept;
     QByteArray d;
 
+    QBitArray(QByteArrayData &&dd) : d(std::move(dd)) {}
+
     template <typename BitArray> static auto bitLocation(BitArray &ba, qsizetype i)
     {
         Q_ASSERT(size_t(i) < size_t(ba.size()));
@@ -95,6 +97,7 @@ public:
 public:
     typedef QByteArray::DataPointer DataPtr;
     inline DataPtr &data_ptr() { return d.data_ptr(); }
+    inline const DataPtr &data_ptr() const { return d.data_ptr(); }
 };