[libcxx-commits] [libcxx] [libc++] fix minor performance issue in `basic_string<C>::append()` (PR #210078)

[Pavel Novikov via libcxx-commits]

toughengineer wrote:

This seems to work in my setup:
```c++
template <typename Iterator>
struct unoptimizable_forward_iterator : Iterator {
  using iterator_category = std::forward_iterator_tag;

  unoptimizable_forward_iterator(Iterator i) : Iterator{i} {}

  _LIBCPP_NOINLINE unoptimizable_forward_iterator& operator++() {
    ++static_cast<Iterator&>(*this);
    return *this;
  }
  unoptimizable_forward_iterator operator++(int) {
    auto it = *this;
    this->operator++();
    return it;
  }
};
```
And in the benchmark:
```c++
benchmark::DoNotOptimize(
    string.append(unoptimizable_forward_iterator(src.begin()), unoptimizable_forward_iterator(src.end())));
```
Any objections?
If everyone is happy with this, I'll create a PR for the benchmark tomorrow.
<details><summary>benchmark results</summary>

before:
```
2026-08-04T02:41:41+03:00
Running ./bench
Run on (20 X 3494.4 MHz CPU s)
CPU Caches:
  L1 Data 48 KiB (x10)
  L1 Instruction 32 KiB (x10)
  L2 Unified 2048 KiB (x10)
  L3 Unified 24576 KiB (x1)
Load Average: 0.11, 0.12, 0.05
***WARNING*** ASLR is enabled, the results may have unreproducible noise in them.
-------------------------------------------------------------------------------------------------------
Benchmark                                                             Time             CPU   Iterations
-------------------------------------------------------------------------------------------------------
std::string::append(ForwardIt, ForwardIt) (opaque)/5               16.7 ns         16.7 ns     41817708
std::u8string::append(ForwardIt, ForwardIt) (opaque)/5             16.7 ns         16.7 ns     40527928
std::wstring::append(ForwardIt, ForwardIt) (opaque)/5              22.1 ns         22.1 ns     32384438
std::string::append(ForwardIt, ForwardIt) (opaque)/30              65.7 ns         65.7 ns     10090497
std::u8string::append(ForwardIt, ForwardIt) (opaque)/30            66.9 ns         66.9 ns     10181715
std::wstring::append(ForwardIt, ForwardIt) (opaque)/30             65.4 ns         65.4 ns     10365533
std::string::append(ForwardIt, ForwardIt) (transparent)/5          16.6 ns         16.6 ns     42325432
std::u8string::append(ForwardIt, ForwardIt) (transparent)/5        16.9 ns         16.9 ns     40609161
std::wstring::append(ForwardIt, ForwardIt) (transparent)/5         21.7 ns         21.7 ns     31280904
std::string::append(ForwardIt, ForwardIt) (transparent)/30         66.1 ns         66.1 ns     10379597
std::u8string::append(ForwardIt, ForwardIt) (transparent)/30       67.4 ns         67.4 ns     10247057
std::wstring::append(ForwardIt, ForwardIt) (transparent)/30        65.1 ns         65.1 ns     10302284
```
after:
```
2026-08-04T02:42:29+03:00
Running ./bench
Run on (20 X 3494.4 MHz CPU s)
CPU Caches:
  L1 Data 48 KiB (x10)
  L1 Instruction 32 KiB (x10)
  L2 Unified 2048 KiB (x10)
  L3 Unified 24576 KiB (x1)
Load Average: 0.21, 0.15, 0.07
***WARNING*** ASLR is enabled, the results may have unreproducible noise in them.
-------------------------------------------------------------------------------------------------------
Benchmark                                                             Time             CPU   Iterations
-------------------------------------------------------------------------------------------------------
std::string::append(ForwardIt, ForwardIt) (opaque)/5               12.1 ns         12.1 ns     57699076
std::u8string::append(ForwardIt, ForwardIt) (opaque)/5             12.6 ns         12.6 ns     55272438
std::wstring::append(ForwardIt, ForwardIt) (opaque)/5              18.1 ns         18.1 ns     37924018
std::string::append(ForwardIt, ForwardIt) (opaque)/30              40.4 ns         40.4 ns     17080440
std::u8string::append(ForwardIt, ForwardIt) (opaque)/30            39.5 ns         39.5 ns     18209833
std::wstring::append(ForwardIt, ForwardIt) (opaque)/30             39.4 ns         39.4 ns     17736548
std::string::append(ForwardIt, ForwardIt) (transparent)/5          11.6 ns         11.6 ns     61564765
std::u8string::append(ForwardIt, ForwardIt) (transparent)/5        12.6 ns         12.6 ns     54926645
std::wstring::append(ForwardIt, ForwardIt) (transparent)/5         18.2 ns         18.2 ns     37916232
std::string::append(ForwardIt, ForwardIt) (transparent)/30         40.7 ns         40.7 ns     17099742
std::u8string::append(ForwardIt, ForwardIt) (transparent)/30       39.4 ns         39.4 ns     17974869
std::wstring::append(ForwardIt, ForwardIt) (transparent)/30        39.7 ns         39.7 ns     17597919
```

</details>

https://github.com/llvm/llvm-project/pull/210078