diff --git a/libcxx/docs/ReleaseNotes/23.rst b/libcxx/docs/ReleaseNotes/23.rst
index 37aa972d22ea3..1852351cedc4e 100644
--- a/libcxx/docs/ReleaseNotes/23.rst
+++ b/libcxx/docs/ReleaseNotes/23.rst
@@ -69,6 +69,8 @@ Improvements and New Features
   ``"std::visit: variant is valueless"``, ``"std::get: variant is valueless"``, or
   ``"std::get: wrong alternative for variant"``. The standard only requires ``what()`` to return an
   unspecified non-null string, so user code that does not match on the exact message remains correct.
+- ``ifstream::read`` has been optimized to pass through large reads to system calls directly instead of copying them in
+  chunks.
 
 Deprecations and Removals
 -------------------------
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index 0c34c9149de0d..7b84bf6609086 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -309,6 +309,25 @@ protected:
     return basic_streambuf<_CharT, _Traits>::xsputn(__str, __len);
   }
 
+  _LIBCPP_HIDE_FROM_ABI_VIRTUAL streamsize xsgetn(char_type* __str, streamsize __len) override {
+    if (__file_ && __always_noconv_) {
+      const streamsize __n = std::min(this->egptr() - this->gptr(), __len);
+      if (__n != 0) {
+        traits_type::copy(__str, this->gptr(), __n);
+        this->__gbump_ptrdiff(__n);
+      }
+      const streamsize __remainder    = __len - __n;
+      const streamsize __buffer_space = this->egptr() - this->eback();
+
+      if (__remainder >= __buffer_space)
+        return std::fread(__str + __n, sizeof(char_type), __remainder, __file_) + __n;
+      else if (__remainder > 0)
+        return basic_streambuf<_CharT, _Traits>::xsgetn(__str + __n, __remainder) + __n;
+      return __n;
+    }
+    return basic_streambuf<_CharT, _Traits>::xsgetn(__str, __len);
+  }
+
 private:
   char* __extbuf_;
   const char* __extbufnext_;
diff --git a/libcxx/test/benchmarks/streams/ofstream.bench.cpp b/libcxx/test/benchmarks/streams/fstream.bench.cpp
similarity index 52%
rename from libcxx/test/benchmarks/streams/ofstream.bench.cpp
rename to libcxx/test/benchmarks/streams/fstream.bench.cpp
index 80827d59cdf01..2e16ae58749e6 100644
--- a/libcxx/test/benchmarks/streams/ofstream.bench.cpp
+++ b/libcxx/test/benchmarks/streams/fstream.bench.cpp
@@ -13,7 +13,7 @@
 
 #include <benchmark/benchmark.h>
 
-static void bm_write(benchmark::State& state) {
+static void bm_ofstream_write(benchmark::State& state) {
   std::vector<char> buffer;
   buffer.resize(16384);
 
@@ -22,6 +22,24 @@ static void bm_write(benchmark::State& state) {
   for (auto _ : state)
     stream.write(buffer.data(), buffer.size());
 }
-BENCHMARK(bm_write)->Name("std::ofstream::write(char*, size)");
+BENCHMARK(bm_ofstream_write)->Name("std::ofstream::write(char*, size)");
+
+static void bm_ifstream_read(benchmark::State& state) {
+  std::vector<char> buffer;
+  buffer.resize(16384);
+
+  std::ofstream gen_testfile("testfile");
+  gen_testfile.write(buffer.data(), buffer.size());
+
+  std::ifstream stream("testfile");
+  assert(stream);
+
+  for (auto _ : state) {
+    stream.read(buffer.data(), buffer.size());
+    benchmark::DoNotOptimize(buffer);
+    stream.seekg(0);
+  }
+}
+BENCHMARK(bm_ifstream_read)->Name("std::ifstream::read(char*, size)");
 
 BENCHMARK_MAIN();
diff --git a/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp b/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
index 283adbc057d1e..30e7b66d42325 100644
--- a/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
+++ b/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
@@ -19,4 +19,4 @@
 
 std::basic_filebuf<char, std::char_traits<wchar_t> > f;
 // expected-error-re@*:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
-// expected-error@*:* 10 {{only virtual member functions can be marked 'override'}}
+// expected-error@*:* 11 {{only virtual member functions can be marked 'override'}}
diff --git a/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp b/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp
index ba6f3c31d3e34..daafb36f9151a 100644
--- a/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp
+++ b/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp
@@ -21,7 +21,7 @@ std::basic_fstream<char, std::char_traits<wchar_t> > f;
 // expected-error-re@*:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
 // expected-error-re@*:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
 
-// expected-error@*:* 12 {{only virtual member functions can be marked 'override'}}
+// expected-error@*:* 13 {{only virtual member functions can be marked 'override'}}
 
 // FIXME: As of commit r324062 Clang incorrectly generates a diagnostic about mismatching
 // exception specifications for types which are already invalid for one reason or another.
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp
index de17cd4d8e9d3..d9ccc2fe62914 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp
@@ -68,6 +68,16 @@ int main(int, char**) {
     test_buffer.resize(24);
     assert(std::string(test_buffer.data(), 24) == "to test buffer behaviour");
   }
+  { // Ensure that the read fails gracefully with an unopened ifstream
+    // See https://llvm.org/PR168628
+    char buf[10];
+    std::ifstream ifs;
+    std::filebuf* filebuf = ifs.rdbuf();
+
+    assert(!filebuf->is_open());
+    assert(filebuf->sgetn(buf, sizeof(buf)) == 0);
+    assert(!filebuf->is_open());
+  }
 
   return 0;
 }