From 9039b0cd25c0e617c06cbb19a59bdafbb476a94c Mon Sep 17 00:00:00 2001 From: Thiago Macieira Date: Thu, 19 Oct 2023 14:50:57 -0700 Subject: [PATCH] QArrayData: move AlignedQArrayData to the public header This allows us to use it in QTypedArrayData::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. Change-Id: I79e700614d034281bf55fffd178fa0aee2344307 Reviewed-by: Marc Mutz (cherry picked from commit 4d57b30194f31865132312c67abf796cb58a0f8f) Reviewed-by: Qt Cherry-pick Bot --- src/corelib/tools/qarraydata.cpp | 8 ++------ src/corelib/tools/qarraydata.h | 18 +++++++++++++++++- 2 files changed, 19 insertions(+), 7 deletions(-) diff --git a/src/corelib/tools/qarraydata.cpp b/src/corelib/tools/qarraydata.cpp index ff3bf2d6b88..3f417fc3903 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::AlignmentDummy. headerSize += alignment - headerAlignment; } Q_ASSERT(headerSize > 0); diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h index 2369a27e847..4b716190380 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 struct QTypedArrayData : QArrayData { - struct AlignmentDummy { QArrayData header; T data; }; + struct AlignmentDummy { QtPrivate::AlignedQArrayData header; T data; }; [[nodiscard]] static QPair allocate(qsizetype capacity, AllocationOption option = QArrayData::KeepSize) {