[libcxx-commits] [libcxx] [libc++] Optimize num_put integral functions (PR #120859)

[Louis Dionne via libcxx-commits]

================
@@ -1259,30 +1262,68 @@ num_put<_CharT, _OutputIterator>::do_put(iter_type __s, ios_base& __iob, char_ty
 template <class _CharT, class _OutputIterator>
 template <class _Integral>
 _LIBCPP_HIDE_FROM_ABI inline _OutputIterator num_put<_CharT, _OutputIterator>::__do_put_integral(
-    iter_type __s, ios_base& __iob, char_type __fl, _Integral __v, char const* __len) const {
+    iter_type __s, ios_base& __iob, char_type __fl, _Integral __v) const {
   // Stage 1 - Get number in narrow char
-  char __fmt[8] = {'%', 0};
-  this->__format_int(__fmt + 1, __len, is_signed<_Integral>::value, __iob.flags());
+
   // Worst case is octal, with showbase enabled. Note that octal is always
   // printed as an unsigned value.
   using _Unsigned = typename make_unsigned<_Integral>::type;
-  _LIBCPP_CONSTEXPR const unsigned __nbuf =
+  _LIBCPP_CONSTEXPR const unsigned __buffer_size =
       (numeric_limits<_Unsigned>::digits / 3)          // 1 char per 3 bits
       + ((numeric_limits<_Unsigned>::digits % 3) != 0) // round up
       + 2;                                             // base prefix + terminating null character
-  char __nar[__nbuf];
-  _LIBCPP_DIAGNOSTIC_PUSH
-  _LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Wformat-nonliteral")
-  _LIBCPP_GCC_DIAGNOSTIC_IGNORED("-Wformat-nonliteral")
-  int __nc = __locale::__snprintf(__nar, sizeof(__nar), _LIBCPP_GET_C_LOCALE, __fmt, __v);
-  _LIBCPP_DIAGNOSTIC_POP
-  char* __ne = __nar + __nc;
-  char* __np = this->__identify_padding(__nar, __ne, __iob);
+
+  char __char_buffer[__buffer_size];
+  char* __buffer_ptr = __char_buffer;
+
+  auto __flags = __iob.flags();
+
+  auto __basefield = (__flags & ios_base::basefield);
+
+  // Extract base
+  int __base = 10;
+  if (__basefield == ios_base::oct)
+    __base = 8;
+  else if (__basefield == ios_base::hex)
+    __base = 16;
+
+  // Print '-' and make the argument unsigned
+  auto __uval = std::__to_unsigned_like(__v);
+  if (__basefield != ios_base::oct && __basefield != ios_base::hex && __v < 0) {
+    *__buffer_ptr++ = '-';
+    __uval          = std::__complement(__uval);
+  }
+
+  // Maybe add '+' prefix
+  if (std::is_signed<_Integral>::value && (__flags & ios_base::showpos) && __basefield != ios_base::oct &&
+      __basefield != ios_base::hex && __v > 0)
+    *__buffer_ptr++ = '+';
+
+  // Add base prefix
+  if (__flags & ios_base::showbase) {
+    if (__basefield == ios_base::oct) {
+      *__buffer_ptr++ = '0';
+    } else if (__basefield == ios_base::hex) {
+      *__buffer_ptr++ = '0';
+      *__buffer_ptr++ = (__flags & ios_base::uppercase ? 'X' : 'x');
+    }
+  }
+
+  auto __res = std::__to_chars_integral(__buffer_ptr, __char_buffer + __buffer_size, __uval, __base);
+  _LIBCPP_ASSERT_INTERNAL(__res.__ec == std::errc(), "to_chars failed!");
----------------
ldionne wrote:

```suggestion
  _LIBCPP_ASSERT_INTERNAL(__res.__ec == std::errc(), "to_chars: invalid maximum buffer size computed?");
```

https://github.com/llvm/llvm-project/pull/120859