[libc++][format] Adds print benchmarks.

P3107R5 "Permit an efficient implementation of std::print" should
improve the performance of std::print. Before implementing these
improvements it adds the benchmark provided in the paper.

It replaces the `fmt::memory_buffer` with `std::vector<char>`, which
is not a stack buffer. However there is not a good Standard type.
(std::inplace_vector would be a better type.)

Author: mordante

libcxx/test/benchmarks/print.bench.cpp

@@ -0,0 +1,79 @@
+//===----------------------------------------------------------------------===//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++23
+
+// This benchmark writes the print and benchmark data to stdout. In order to
+// preserve the benchmark output it needs to be stored in a file (using the
+// console format). Another issue with the benchmark is the time it takes to
+// write output to the real terminal. In order to avoid that overhead write the
+// output to a fast "terminal", like /dev/null. For example, the printf
+//   console    1546   ns
+//   /dev/null    70.9 ns
+// An example of a good test invocation.
+// BENCHMARK_OUT=benchmark.txt BENCHMARK_OUT_FORMAT=console <exe> >/dev/null
+
+#include <print>
+
+#include <cstdio>
+#include <string>
+#include <vector>
+
+#include "benchmark/benchmark.h"
+
+void printf(benchmark::State& s) {
+  while (s.KeepRunning())
+    std::printf("The answer to life, the universe, and everything is %d.\n", 42);
+}
+BENCHMARK(printf);
+
+void vprint_string(std::string_view fmt, std::format_args args) {
+  auto s             = std::vformat(fmt, args);
+  std::size_t result = fwrite(s.data(), 1, s.size(), stdout);
+  if (result < s.size())
+    throw std::format_error("fwrite error");
+}
+
+template <typename... T>
+void print_string(std::format_string<T...> fmt, T&&... args) {
+  vprint_string(fmt.get(), std::make_format_args(args...));
+}
+
+void print_string(benchmark::State& s) {
+  while (s.KeepRunning()) {
+    print_string("The answer to life, the universe, and everything is {}.\n", 42);
+  }
+}
+BENCHMARK(print_string);
+
+void vprint_stack(std::string_view fmt, std::format_args args) {
+  auto buf = std::vector<char>{};
+  std::vformat_to(std::back_inserter(buf), fmt, args);
+  std::size_t result = fwrite(buf.data(), 1, buf.size(), stdout);
+  if (result < buf.size())
+    throw std::format_error("fwrite error");
+}
+
+template <typename... T>
+void print_stack(std::format_string<T...> fmt, T&&... args) {
+  vprint_stack(fmt.get(), std::make_format_args(args...));
+}
+
+void print_stack(benchmark::State& s) {
+  while (s.KeepRunning()) {
+    print_stack("The answer to life, the universe, and everything is {}.\n", 42);
+  }
+}
+BENCHMARK(print_stack);
+
+void print_direct(benchmark::State& s) {
+  while (s.KeepRunning())
+    std::print("The answer to life, the universe, and everything is {}.\n", 42);
+}
+BENCHMARK(print_direct);
+
+BENCHMARK_MAIN();