[llvm-branch-commits] [libcxx] [libc++][format][2/7] Optimizes c-string arguments. (PR #101805)

[Louis Dionne via llvm-branch-commits]

================
@@ -64,32 +64,14 @@ struct _LIBCPP_TEMPLATE_VIS formatter<const _CharT*, _CharT> : public __formatte
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator format(const _CharT* __str, _FormatContext& __ctx) const {
     _LIBCPP_ASSERT_INTERNAL(__str, "The basic_format_arg constructor should have prevented an invalid pointer.");
-
-    __format_spec::__parsed_specifications<_CharT> __specs = _Base::__parser_.__get_parsed_std_specifications(__ctx);
-#  if _LIBCPP_STD_VER >= 23
-    if (_Base::__parser_.__type_ == __format_spec::__type::__debug)
-      return __formatter::__format_escaped_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);
-#  endif
-
-    // When using a center or right alignment and the width option the length
-    // of __str must be known to add the padding upfront. This case is handled
-    // by the base class by converting the argument to a basic_string_view.
+    // Converting the input to a basic_string_view means the data is looped over twice;
+    // - once to determine the lenght, and
+    // - once to process the data.
     //
-    // When using left alignment and the width option the padding is added
-    // after outputting __str so the length can be determined while outputting
-    // __str. The same holds true for the precision, during outputting __str it
-    // can be validated whether the precision threshold has been reached. For
-    // now these optimizations aren't implemented. Instead the base class
-    // handles these options.
-    // TODO FMT Implement these improvements.
-    if (__specs.__has_width() || __specs.__has_precision())
-      return __formatter::__write_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);
-
-    // No formatting required, copy the string to the output.
-    auto __out_it = __ctx.out();
-    while (*__str)
-      *__out_it++ = *__str++;
-    return __out_it;
+    // This sounds slower than writing the output directly. However internally
+    // the output algorithms have optimizations for "bulk" operations. This
+    // means processing the data twice is faster than processing it once.
----------------
ldionne wrote:

```suggestion
    // This sounds slower than writing the output directly. However internally
    // the output algorithms have optimizations for "bulk" operations, which make
    // this faster than a single-pass character-by-character output.
```

https://github.com/llvm/llvm-project/pull/101805