jyknight added a comment. > For C++, I confess I have some problems interpreting this sentence: > > (https://eel.is/c++draft/cpp.predefined) > >> `__STDCPP_DEFAULT_NEW_ALIGNMENT__`: An integer literal of type >> std::size_t whose value is the alignment guaranteed by a call to operator >> new(std::size_t) or operator new[](std::size_t). [ Note: Larger alignments >> will be passed to operator new(std::size_t, std::align_val_t), etc. (8.3.4). >> — end note ] > > It seems to suggest that a large alignment may be assumed, but practically, I > know at least mimalloc uses 8-byte alignment for 64-bit Darwin, not 16-byte > alignment.
See https://eel.is/c++draft/basic.stc.dynamic.allocation#3 (this text was moved around a bit since C++17, but it hasn't changed meaning). `operator new(std::size_t)` requires that "the storage is aligned for any object that does not have new-extended alignment and is of the requested size." ()That is, if you have a size 8 allocation, it is not required to be 16-byte aligned. > [...] This needs to be very careful since > `__STDCPP_DEFAULT_NEW_ALIGNMENT__` doesn't seem to match the practice. I don't think that's true? I believe that macro is generally being set / handled correctly. Reintroducing an optimization like this with an additional check that the allocation size is large enough should be valid everywhere. > mimalloc uses 8-byte alignment for 64-bit Darwin, not 16-byte alignment. It uses 8-byte alignment for 8-byte allocations, you mean. It uses 16-byte alignment for 16-byte (or larger) allocations (doing otherwise would be a clear bug, given that max_align_t is 16). In D118804#3291060 <https://reviews.llvm.org/D118804#3291060>, @xbolva00 wrote: > Or can we bailout just for gnu? and preserve this for darwin? This isn't a GNU-only issue, so I don't think that makes sense. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D118804/new/ https://reviews.llvm.org/D118804 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
