[🍒] Unconditionally lower std::string's alignment requirement from 16 to 8 (#68925)

libcxx/docs/ReleaseNotes/18.rst

@@ -275,11 +275,10 @@ ABI Affecting Changes
   results in an ABI break, however in practice we expect uses of ``std::projected`` in ABI-sensitive places to be
   extremely rare. Any error resulting from this change should result in a link-time error.
 
-- Under the unstable ABI, the internal alignment requirements for heap allocations
-  inside ``std::string`` has decreased from 16 to 8. This saves memory since string requests fewer additional
-  bytes than it did previously. However, this also changes the return value of ``std::string::max_size``
-  and can cause code compiled against older libc++ versions but linked at runtime to a new version
-  to throw a different exception when attempting allocations that are too large
+- The internal alignment requirements for heap allocations inside ``std::string`` has decreased from 16 to 8. This
+  saves memory since string requests fewer additional bytes than it did previously. However, this also changes the
+  return value of ``std::string::max_size`` and can cause code compiled against older libc++ versions but linked at
+  runtime to a new version to throw a different exception when attempting allocations that are too large
   (``std::bad_alloc`` vs ``std::length_error``).
 
 - The layout of some range adaptors that use the ``movable-box`` exposition-only type as an implementation

libcxx/include/__config

@@ -174,11 +174,6 @@
 // The implementation moved to the header, but we still export the symbols from
 // the dylib for backwards compatibility.
 #    define _LIBCPP_ABI_DO_NOT_EXPORT_TO_CHARS_BASE_10
-// Save memory by providing the allocator more freedom to allocate the most
-// efficient size class by dropping the alignment requirements for std::string's
-// 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
 #  elif _LIBCPP_ABI_VERSION == 1
 #    if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
 // Enable compiling copies of now inline methods into the dylib to support

libcxx/include/string

@@ -1937,12 +1937,7 @@ private:
     return (__s + (__a - 1)) & ~(__a - 1);
   }
   enum {
-    __alignment =
-#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
-        8
-#else
-        16
-#endif
+    __alignment = 8
   };
   static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 size_type __recommend(size_type __s) _NOEXCEPT {
     if (__s < __min_cap) {

libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx{{10.13|10.15|11.0}}
+
 // <string>
 
 // This test demonstrates the smaller allocation sizes when the alignment
@@ -17,14 +19,8 @@
 
 #include "test_macros.h"
 
-// alignment of the string heap buffer is hardcoded to either 16 or 8
-
-const std::size_t alignment =
-#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
-    8;
-#else
-    16;
-#endif
+// alignment of the string heap buffer is hardcoded to 8
+const std::size_t alignment = 8;
 
 int main(int, char**) {
   std::string input_string;

libcxx/test/libcxx/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -17,14 +17,8 @@
 
 #include "test_macros.h"
 
-// alignment of the string heap buffer is hardcoded to 16
-
-static const std::size_t alignment =
-#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
-    8;
-#else
-    16;
-#endif
+// alignment of the string heap buffer is hardcoded to 8
+static const std::size_t alignment = 8;
 
 template <class = int>
 TEST_CONSTEXPR_CXX20 void full_size() {

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -7,6 +7,12 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-exceptions
+
+// After changing the alignment of the allocated pointer from 16 to 8, the exception thrown is no longer `bad_alloc`
+// but instead length_error on systems using new headers but older dylibs.
+//
+// XFAIL: stdlib=apple-libc++ && target={{.+}}-apple-macosx{{10.13|10.15|11.0}}
+
 // <string>
 
 // size_type max_size() const; // constexpr since C++20