devin.jeanpierre added a comment.
OK, while I'm struggling to set up a new Windows machine so I can make sure
this works on Windows... @Quuxplusone, after this is merged, do you want to
rebase D67524 <https://reviews.llvm.org/D67524> on top of this, or should I? I
can review it -- I think when I looked at it, I only had two ideas for changes:
1. May need to guard all of these optimizations on the allocator being the
standard allocator, since otherwise the change in the number of
constructor/destructor calls would be observable to more than just the
annotated type.
2. changing `std::swap` to correctly handle potentially-overlapping-objects. My
thought is we could test that there's no reusable tail padding.
First draft: `has_unique_object_representations` is conservative -- on the
Itanium ABI, "POD for the purposes of layout" types can have padding which
isn't reused when it's a potentially overlapping subobject.
Second draft: check by hand:
struct TestStruct {
[[no_unique_address]] T x;
// not sure if this needs to be a bitfield or anything like that, but the
idea is this.
char extra_byte;
};
bool has_padding = sizeof(TestStruct) == sizeof(T);
Third draft: we can improve on this further still by iterating over different
numbers of characters to determine the exact size of the padding, and memcpy
only the non-padding parts.
---
(D63620 <https://reviews.llvm.org/D63620> could in some form also be ported
over, but it needs to be guarded behind ABI stability, since
`[[clang::trivial_abi]]` is an ABI breaking change. For example, the same way
it was done for unique_ptr
<https://libcxx.llvm.org//DesignDocs/UniquePtrTrivialAbi.html>, with the same
benefits.)
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D114732/new/
https://reviews.llvm.org/D114732
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