[libcxx-commits] [libcxx] [libc++][format] Adds print benchmarks. (PR #129765)

[Louis Dionne via libcxx-commits]

================
@@ -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
----------------
ldionne wrote:

```suggestion
// An example of a good test invocation.
// BENCHMARK_OUT=benchmark.txt BENCHMARK_OUT_FORMAT=console <exe> >/dev/null
```

What do you mean here? The canonical way of running benchmarks is

```
./libcxx/utils/libcxx-lit <build> -sv --param optimization=speed --show-all <benchmark>
```

Is this not how this benchmark should be run? If we should be adding additional environment variables to the benchmark when running it, we should find a way for a vanilla `lit` invocation to do it properly out of the box.

https://github.com/llvm/llvm-project/pull/129765