[libc++] Optimize std::getline

```
-----------------------------------------------
Benchmark                   old             new
-----------------------------------------------
BM_getline_string        318 ns         32.4 ns
```

Author: philnik777

libcxx/docs/ReleaseNotes/20.rst

@@ -73,6 +73,8 @@ Improvements and New Features
   optimized, resulting in a performance improvement of up to 2x for trivial element types (e.g., `std::vector<int>`),
   and up to 3.4x for non-trivial element types (e.g., `std::vector<std::vector<int>>`).
 
+- The performance of ``std::getline`` has been improved, resulting in a performance uplift of up to 10x.
+
 Deprecations and Removals
 -------------------------
 

libcxx/include/istream

@@ -1265,41 +1265,49 @@ _LIBCPP_HIDE_FROM_ABI basic_istream<_CharT, _Traits>&
 getline(basic_istream<_CharT, _Traits>& __is, basic_string<_CharT, _Traits, _Allocator>& __str, _CharT __dlm) {
   ios_base::iostate __state = ios_base::goodbit;
   typename basic_istream<_CharT, _Traits>::sentry __sen(__is, true);
-  if (__sen) {
+  if (!__sen)
+    return __is;
 #    if _LIBCPP_HAS_EXCEPTIONS
-    try {
+  try {
 #    endif
-      __str.clear();
-      streamsize __extr = 0;
-      while (true) {
-        typename _Traits::int_type __i = __is.rdbuf()->sbumpc();
-        if (_Traits::eq_int_type(__i, _Traits::eof())) {
-          __state |= ios_base::eofbit;
-          break;
-        }
-        ++__extr;
-        _CharT __ch = _Traits::to_char_type(__i);
-        if (_Traits::eq(__ch, __dlm))
-          break;
-        __str.push_back(__ch);
-        if (__str.size() == __str.max_size()) {
+    __str.clear();
+
+    auto& __buffer = *__is.rdbuf();
+
+    auto __next = __buffer.sgetc();
+    for (; !_Traits::eq_int_type(__next, _Traits::eof()); __next = __buffer.sgetc()) {
+      const auto* __first = __buffer.gptr();
+      const auto* __last  = __buffer.egptr();
+      const auto* __match = _Traits::find(__first, __last - __first, __dlm);
+      if (__match) {
+        if (auto __cap = __str.max_size() - __str.size(); __cap <= static_cast<size_t>(__match - __first)) {
+          __str.append(__first, __cap);
+          __buffer.__gbump_ptrdiff(__cap);
           __state |= ios_base::failbit;
           break;
         }
+        __str.append(__first, __match);
+        __buffer.__gbump_ptrdiff(__match - __first + 1);
+        break;
       }
-      if (__extr == 0)
-        __state |= ios_base::failbit;
+
+      __str.append(__first, __last);
+      __buffer.__gbump_ptrdiff(__last - __first);
+    }
+
+    if (_Traits::eq_int_type(__next, _Traits::eof()))
+      __state |= ios_base::eofbit | (__str.empty() ? ios_base::failbit : ios_base::goodbit);
+
 #    if _LIBCPP_HAS_EXCEPTIONS
-    } catch (...) {
-      __state |= ios_base::badbit;
-      __is.__setstate_nothrow(__state);
-      if (__is.exceptions() & ios_base::badbit) {
-        throw;
-      }
+  } catch (...) {
+    __state |= ios_base::badbit;
+    __is.__setstate_nothrow(__state);
+    if (__is.exceptions() & ios_base::badbit) {
+      throw;
     }
-#    endif
-    __is.setstate(__state);
   }
+#    endif
+  __is.setstate(__state);
   return __is;
 }
 

libcxx/include/streambuf

@@ -241,6 +241,9 @@ protected:
 
   inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 void gbump(int __n) { __ninp_ += __n; }
 
+  // gbump takes an int, so it might not be able to represent the offset we want to add.
+  _LIBCPP_HIDE_FROM_ABI void __gbump_ptrdiff(ptrdiff_t __n) { __ninp_ += __n; }
+
   inline _LIBCPP_HIDE_FROM_ABI_AFTER_V1 void setg(char_type* __gbeg, char_type* __gnext, char_type* __gend) {
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(std::__is_valid_range(__gbeg, __gnext), "[gbeg, gnext) must be a valid range");
     _LIBCPP_ASSERT_VALID_INPUT_RANGE(std::__is_valid_range(__gbeg, __gend), "[gbeg, gend) must be a valid range");
@@ -297,6 +300,10 @@ private:
   char_type* __bout_;
   char_type* __nout_;
   char_type* __eout_;
+
+  template <class _CharT2, class _Traits2, class _Allocator>
+  _LIBCPP_HIDE_FROM_ABI friend basic_istream<_CharT2, _Traits2>&
+  getline(basic_istream<_CharT2, _Traits2>&, basic_string<_CharT2, _Traits2, _Allocator>&, _CharT2);
 };
 
 template <class _CharT, class _Traits>

libcxx/test/benchmarks/streams/getline.bench.cpp

@@ -0,0 +1,35 @@
+
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+#include <istream>
+#include <sstream>
+
+#include <benchmark/benchmark.h>
+
+void BM_getline_string(benchmark::State& state) {
+  std::istringstream iss;
+
+  std::string str;
+  str.reserve(128);
+  iss.str("A long string to let getline do some more work, making sure that longer strings are parsed fast enough");
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(iss);
+
+    std::getline(iss, str);
+    benchmark::DoNotOptimize(str);
+    iss.seekg(0);
+  }
+}
+
+BENCHMARK(BM_getline_string);
+
+BENCHMARK_MAIN();