[libcxx-commits] [libcxx] [libc++] Replace `__compressed_pair` with `[[no_unique_address]]` (PR #76756)

[Louis Dionne via libcxx-commits]

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@@ -179,6 +179,8 @@
 // pointer from 16 to 8. This changes the output of std::string::max_size,
 // which makes it ABI breaking
 #    define _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
+// Don't add padding from __compressed_pair
----------------
ldionne wrote:

```suggestion
// Historically, libc++ used a type called __compressed_pair to reduce storage
// needs in cases of empty types (e.g. an empty allocator in std::vector). We switched
// to using [[no_unique_address]], however for ABI compatibility reasons we had to
// add artificial padding in a few places.
//
// This setting disables the addition of such artificial padding, leading to a more
// optimal representation for several types.
```

https://github.com/llvm/llvm-project/pull/76756