[libc++] Replace __resize_default_init with resize_and_overwrite (#157121)

Since `__resize_default_init` is only ever used inside the dylib we can
remove the libc++-internal API and switch to the public one. This patch
inlines a bunch of functions that aren't required anymore and simplifies
the code that way.

Author: philnik777

libcxx/include/string

@@ -1312,13 +1312,17 @@ public:
 #  if _LIBCPP_STD_VER >= 23
   template <class _Op>
   _LIBCPP_HIDE_FROM_ABI constexpr void resize_and_overwrite(size_type __n, _Op __op) {
-    __resize_default_init(__n);
-    __erase_to_end(std::move(__op)(data(), _LIBCPP_AUTO_CAST(__n)));
+    size_type __sz  = size();
+    size_type __cap = capacity();
+    if (__n > __cap)
+      __grow_by_without_replace(__cap, __n - __cap, __sz, __sz, 0);
+    __annotate_delete();
+    __set_size(__n);
+    __annotate_new(__n);
+    __erase_to_end(std::move(__op)(data(), auto(__n)));
   }
 #  endif
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __resize_default_init(size_type __n);
-
 #  if _LIBCPP_STD_VER < 26 || defined(_LIBCPP_ENABLE_CXX26_REMOVED_STRING_RESERVE)
   _LIBCPP_DEPRECATED_IN_CXX20 _LIBCPP_HIDE_FROM_ABI void reserve() _NOEXCEPT { shrink_to_fit(); }
 #  endif
@@ -1410,8 +1414,6 @@ public:
   _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(const value_type* _LIBCPP_DIAGNOSE_NULLPTR __s);
   _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string& append(size_type __n, value_type __c);
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __append_default_init(size_type __n);
-
   template <class _InputIterator, __enable_if_t<__has_exactly_input_iterator_category<_InputIterator>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 basic_string&
   append(_InputIterator __first, _InputIterator __last) {
@@ -3081,22 +3083,6 @@ basic_string<_CharT, _Traits, _Allocator>::append(size_type __n, value_type __c)
   return *this;
 }
 
-template <class _CharT, class _Traits, class _Allocator>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 inline void
-basic_string<_CharT, _Traits, _Allocator>::__append_default_init(size_type __n) {
-  if (__n == 0)
-    return;
-  size_type __cap = capacity();
-  size_type __sz  = size();
-  if (__cap - __sz < __n)
-    __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __sz, 0);
-  __annotate_increase(__n);
-  pointer __p = __get_pointer();
-  __sz += __n;
-  __set_size(__sz);
-  traits_type::assign(__p[__sz], value_type());
-}
-
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c) {
   bool __is_short = !__is_long();
@@ -3435,16 +3421,6 @@ _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::re
     __erase_to_end(__n);
 }
 
-template <class _CharT, class _Traits, class _Allocator>
-_LIBCPP_CONSTEXPR_SINCE_CXX20 inline void
-basic_string<_CharT, _Traits, _Allocator>::__resize_default_init(size_type __n) {
-  size_type __sz = size();
-  if (__n > __sz) {
-    __append_default_init(__n - __sz);
-  } else
-    __erase_to_end(__n);
-}
-
 template <class _CharT, class _Traits, class _Allocator>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void basic_string<_CharT, _Traits, _Allocator>::reserve(size_type __requested_capacity) {
   if (__requested_capacity > max_size())

libcxx/src/filesystem/format_string.h

@@ -34,20 +34,19 @@ inline _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 1, 0) string vformat_string(const ch
 
   va_list apcopy;
   va_copy(apcopy, ap);
-  int ret = ::vsnprintf(buf.data(), buf.size(), msg, apcopy);
+  int size = ::vsnprintf(buf.data(), buf.size(), msg, apcopy);
   va_end(apcopy);
 
   string result;
-  if (static_cast<size_t>(ret) < buf.size()) {
-    result.assign(buf.data(), static_cast<size_t>(ret));
+  if (static_cast<size_t>(size) < buf.size()) {
+    result.assign(buf.data(), static_cast<size_t>(size));
   } else {
     // we did not provide a long enough buffer on our first attempt. The
     // return value is the number of bytes (excluding the null byte) that are
     // needed for formatting.
-    size_t size_with_null = static_cast<size_t>(ret) + 1;
-    result.__resize_default_init(size_with_null - 1);
-    ret = ::vsnprintf(&result[0], size_with_null, msg, ap);
-    _LIBCPP_ASSERT_INTERNAL(static_cast<size_t>(ret) == (size_with_null - 1), "TODO");
+    result.resize_and_overwrite(size, [&](char* res, size_t n) { return ::vsnprintf(res, n, msg, ap); });
+    _LIBCPP_ASSERT_INTERNAL(static_cast<size_t>(size) == result.size(),
+                            "vsnprintf did not result in the same number of characters as the first attempt?");
   }
   return result;
 }

libcxx/test/benchmarks/containers/string.bench.cpp

@@ -6,8 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 
+#include <algorithm>
 #include <cstdint>
 #include <cstdlib>
 #include <new>
@@ -67,6 +68,21 @@ static void BM_StringCtorDefault(benchmark::State& state) {
 }
 BENCHMARK(BM_StringCtorDefault);
 
+static void BM_StringResizeAndOverwrite(benchmark::State& state) {
+  std::string str;
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(str);
+    str.resize_and_overwrite(10, [](char* ptr, size_t n) {
+      std::fill_n(ptr, n, 'a');
+      return n;
+    });
+    benchmark::DoNotOptimize(str);
+    str.clear();
+  }
+}
+BENCHMARK(BM_StringResizeAndOverwrite);
+
 enum class Length { Empty, Small, Large, Huge };
 struct AllLengths : EnumValuesAsTuple<AllLengths, Length, 4> {
   static constexpr const char* Names[] = {"Empty", "Small", "Large", "Huge"};

libcxx/test/libcxx-03/strings/basic.string/string.modifiers/resize_default_initialized.pass.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 // UNSUPPORTED: availability-filesystem-missing
 // UNSUPPORTED: no-filesystem
 // ADDITIONAL_COMPILE_FLAGS: -I %{libcxx-dir}/src

libcxx/test/libcxx/input.output/filesystems/class.directory_entry/directory_entry.mods/last_write_time.pass.cpp

@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// UNSUPPORTED: c++03, c++11, c++14
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 // UNSUPPORTED: availability-filesystem-missing
 
 // <filesystem>