QArrayData: move AlignedQArrayData to the public header

This allows us to use it in QTypedArrayData<T>::AlignmentDummy, which is
used in __builtin_assume_aligned() as a hint to the compiler. This
increases the value we were passing as the compiler hint from 4 to 8 on
32-bit platforms and from 8 to 16 on 64-bit platforms. We actually do
align to a bit higher than even that, but that's not an ABI guarantee
we're making.

However, it looks like GCC on 32-bit platforms is buggy, so we work
around it. Commit r240248 ("Make max_align_t respect _Float128.",
63012d9a57edc950c5f30242d1e19318b5708060 in Git) increased
std::max_align_t to 16 bytes on all platforms, saying "Such an increase
is of course an ABI change" and "I think glibc malloc alignment should
also increase to 16-byte 32-bit x86", but such a change to glibc was
never implemented. Moreover, there are systems that don't use glibc,
such as when using GCC on other OSes.

This is not a change in Qt behavior anywhere. i386 does not (usually) do
alignment checks, so most code wouldn't notice this at all.

Pick-to: 6.7
Change-Id: I79e700614d034281bf55fffd178fa0aee2344307
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

1 files changed, 17 insertions, 1 deletions

diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h
index 2369a27e84..4b71619038 100644
--- a/src/corelib/tools/qarraydata.h
+++ b/src/corelib/tools/qarraydata.h
@@ -102,11 +102,27 @@ struct QArrayData
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(QArrayData::ArrayOptions)
 
+namespace QtPrivate {
+// QArrayData with strictest alignment requirements supported by malloc()
+#if defined(Q_PROCESSOR_X86_32) && defined(Q_CC_GNU)
+// GCC's definition is incorrect since GCC 8 (commit r240248 in SVN; commit
+// 63012d9a57edc950c5f30242d1e19318b5708060 in Git). This is applied to all
+// GCC-like compilers in case they decide to follow GCC's lead in being wrong.
+constexpr size_t MaxPrimitiveAlignment = 2 * sizeof(void *);
+#else
+constexpr size_t MaxPrimitiveAlignment = alignof(std::max_align_t);
+#endif
+
+struct alignas(MaxPrimitiveAlignment) AlignedQArrayData : QArrayData
+{
+};
+}
+
 template <class T>
 struct QTypedArrayData
     : QArrayData
 {
-    struct AlignmentDummy { QArrayData header; T data; };
+    struct AlignmentDummy { QtPrivate::AlignedQArrayData header; T data; };
 
     [[nodiscard]] static QPair<QTypedArrayData *, T *> allocate(qsizetype capacity, AllocationOption option = QArrayData::KeepSize)
     {