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, 2 insertions, 6 deletions

diff --git a/src/corelib/tools/qarraydata.cpp b/src/corelib/tools/qarraydata.cpp
index ff3bf2d6b8..3f417fc390 100644
--- a/src/corelib/tools/qarraydata.cpp
+++ b/src/corelib/tools/qarraydata.cpp
@@ -145,18 +145,13 @@ static QArrayData *allocateData(qsizetype allocSize)
     return header;
 }
 
-
 namespace {
-// QArrayData with strictest alignment requirements supported by malloc()
-struct alignas(std::max_align_t) AlignedQArrayData : QArrayData
-{
-};
-
 struct AllocationResult {
     void *data;
     QArrayData *header;
 };
 }
+using QtPrivate::AlignedQArrayData;
 
 static inline AllocationResult
 allocateHelper(qsizetype objectSize, qsizetype alignment, qsizetype capacity,
@@ -172,6 +167,7 @@ allocateHelper(qsizetype objectSize, qsizetype alignment, qsizetype capacity,
         // Allocate extra (alignment - Q_ALIGNOF(AlignedQArrayData)) padding
         // bytes so we can properly align the data array. This assumes malloc is
         // able to provide appropriate alignment for the header -- as it should!
+        // Effectively, we allocate one QTypedArrayData<T>::AlignmentDummy.
         headerSize += alignment - headerAlignment;
     }
     Q_ASSERT(headerSize > 0);