[libc++] Optimize std::getline

[philnik777]

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

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/121346.diff

4 Files Affected:

• (modified) libcxx/docs/ReleaseNotes/20.rst (+2)
• (modified) libcxx/include/istream (+34-26)
• (modified) libcxx/include/streambuf (+7)
• (added) libcxx/test/benchmarks/streams/getline.bench.cpp (+35)

diff --git a/libcxx/docs/ReleaseNotes/20.rst b/libcxx/docs/ReleaseNotes/20.rst
index c8a07fb8b73348..05881722672dda 100644
--- a/libcxx/docs/ReleaseNotes/20.rst
+++ b/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
 -------------------------
 
diff --git a/libcxx/include/istream b/libcxx/include/istream
index 4b177c41cc325e..76ddab04bbdcdd 100644
--- a/libcxx/include/istream
+++ b/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;
 }
 
diff --git a/libcxx/include/streambuf b/libcxx/include/streambuf
index 7f02a9b3314110..a3e1cf489d0efa 100644
--- a/libcxx/include/streambuf
+++ b/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>
diff --git a/libcxx/test/benchmarks/streams/getline.bench.cpp b/libcxx/test/benchmarks/streams/getline.bench.cpp
new file mode 100644
index 00000000000000..6a2215fe061167
--- /dev/null
+++ b/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();

[ldionne]

This is nice! LGTM with a few comments, importantly about validating our test coverage.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[fahadnayyar]

@philnik777 Should this change be cherry-picked on to release/20.x branch or will that happen automatically?

[philnik777]

Why would we cherry-pick this?

[ldionne]

It's release-noted in the LLVM 20 release notes, but I guess it landed later than expected so it should actually have been documented in the LLVM 21 release notes. Is that possible? If so, let's move the release notes to the right file.