Lower std::string's alignment requirement from 16 to 8.

libcxx/docs/ReleaseNotes/18.rst

@@ -133,6 +133,13 @@ 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 save 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``).
+
 Build System Changes
 --------------------
 

libcxx/include/__config

@@ -167,6 +167,11 @@
 // 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

@@ -1851,7 +1851,14 @@ private:
         _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
         size_type __align_it(size_type __s) _NOEXCEPT
             {return (__s + (__a-1)) & ~(__a-1);}
-    enum {__alignment = 16};
+    enum {
+      __alignment =
+#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
+      8
+#else
+      16
+#endif
+    };
     static _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
     size_type __recommend(size_type __s) _NOEXCEPT
     {

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

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// This test demonstrates the smaller allocation sizes when the alignment
+// requirements of std::string are dropped from 16 to 8.
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <string>
+
+#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
+
+int main(int, char**) {
+  std::string input_string;
+  input_string.resize(64, 'a');
+
+  // Call a constructor which selects its size using __recommend.
+  std::string test_string(input_string.data());
+  const std::size_t expected_align8_size = 71;
+
+  // Demonstrate the lesser capacity/allocation size when the alignment requirement is 8.
+  if (alignment == 8) {
+    assert(test_string.capacity() == expected_align8_size);
+  } else {
+    assert(test_string.capacity() == expected_align8_size + 8);
+  }
+
+  return 0;
+}

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

@@ -18,7 +18,13 @@
 #include "test_macros.h"
 
 // alignment of the string heap buffer is hardcoded to 16
-static const std::size_t alignment = 16;
+
+static const std::size_t alignment =
+#ifdef _LIBCPP_ABI_STRING_8_BYTE_ALIGNMENT
+    8;
+#else
+    16;
+#endif
 
 template <class = int>
 TEST_CONSTEXPR_CXX20 void full_size() {