Add script for profiling self check (Linux only)

The script compiles mypy and profiles self check using the 'perf'
profiler.

Example of how to use this:
```
$ python misc/profile_self_check.py
... [will take several minutes]

CPU profile collected. You can now analyze the profile:
  perf report -i mypy.profile.tmpdir/perf.data
```

Author: JukkaL

misc/perf_compare.py

@@ -35,7 +35,7 @@ def heading(s: str) -> None:
     print()
 
 
-def build_mypy(target_dir: str, multi_file: bool) -> None:
+def build_mypy(target_dir: str, multi_file: bool, *, cflags: str | None = None) -> None:
     env = os.environ.copy()
     env["CC"] = "clang"
     env["MYPYC_OPT_LEVEL"] = "2"

misc/profile_self_check.py

@@ -0,0 +1,128 @@
+"""Compile mypy using mypyc and profile self-check using perf.
+
+Notes:
+ - Only Linux is supported for now (TODO: add support for other profilers)
+ - The profile is collected at C level
+   - It includes C functions compiled by mypyc and CPython runtime functions
+ - The names of mypy functions are mangled to C names, but usually it's clear what they mean
+   - Generally CPyDef_ prefix for native functions and CPyPy_ prefix for wrapper functions
+ - It's important to compile CPython using special flags (see below) to get good results
+ - Generally use the latest Python feature release (or the most recent beta if supported by mypyc)
+ - The tool prints a command that can be used to analyze the profile afterwards
+
+You may need to adjust kernel parameters temporarily, e.g. this (note that this has security
+impliciations):
+
+  sudo sysctl kernel.perf_event_paranoid=-1
+
+This is the recommended way to configure CPython for profiling:
+
+  ./configure \
+      --enable-optimizations \
+      --with-lto \
+      CFLAGS="-O2 -g -fno-omit-frame-pointer"
+"""
+
+import argparse
+import glob
+import os
+import shutil
+import subprocess
+import sys
+import time
+
+from perf_compare import build_mypy, clone
+
+# Use these C compiler flags when compiling mypy (important). Note that it's strongly recommended
+# to also compile CPython using similar flags, but we don't enforce it in this script.
+CFLAGS = "-O2 -fno-omit-frame-pointer -g"
+
+
+def _profile_self_check(target_dir: str) -> None:
+    cache_dir = os.path.join(target_dir, ".mypy_cache")
+    if os.path.exists(cache_dir):
+        shutil.rmtree(cache_dir)
+    files = []
+    for pat in "mypy/*.py", "mypy/*/*.py", "mypyc/*.py", "mypyc/test/*.py":
+        files.extend(glob.glob(pat))
+    self_check_cmd = ["python", "-m", "mypy", "--config-file", "mypy_self_check.ini"] + files
+    cmdline = ["perf", "record", "-g"] + self_check_cmd
+    t0 = time.time()
+    subprocess.run(cmdline, cwd=target_dir, check=True)
+    elapsed = time.time() - t0
+    print(f"{elapsed:.2f}s elapsed")
+
+
+def profile_self_check(target_dir: str) -> None:
+    try:
+        _profile_self_check(target_dir)
+    except subprocess.CalledProcessError:
+        print("\nProfiling failed! You may missing some permissions.")
+        print("\nThis may help (note that it has security implications):")
+        print("  sudo sysctl kernel.perf_event_paranoid=-1")
+        sys.exit(1)
+
+
+def check_requirements() -> None:
+    if sys.platform != "linux":
+        # TODO: How to make this work on other platforms?
+        sys.exit("error: Only Linux is supported")
+
+    try:
+        subprocess.run(["perf", "-h"], capture_output=True)
+    except subprocess.CalledProcessError:
+        print("error: The 'perf' profiler is not installed")
+        sys.exit(1)
+
+    try:
+        subprocess.run(["clang", "--version"], capture_output=True)
+    except subprocess.CalledProcessError:
+        print("error: The clang compiler is not installed")
+        sys.exit(1)
+
+    if not os.path.isfile("mypy_self_check.ini"):
+        print("error: Run this in the mypy repository root")
+        sys.exit(1)
+
+
+def main() -> None:
+    check_requirements()
+
+    parser = argparse.ArgumentParser(
+        description="Compile mypy and profile self checking using 'perf'."
+    )
+    parser.add_argument(
+        "--multi-file",
+        action="store_true",
+        help="compile mypy into one C file per module (to reduce RAM use during compilation)",
+    )
+    parser.add_argument(
+        "--skip-compile", action="store_true", help="use compiled mypy from previous run"
+    )
+    args = parser.parse_args()
+    multi_file: bool = args.multi_file
+    skip_compile: bool = args.skip_compile
+
+    target_dir = "."
+
+    target_dir = "mypy.profile.tmpdir"
+
+    if not skip_compile:
+        clone(target_dir, "HEAD")
+
+        print(f"Building mypy in {target_dir}...")
+        build_mypy(target_dir, multi_file, cflags=CFLAGS)
+    elif not os.path.isdir(target_dir):
+        sys.exit("error: Can't find compile mypy from previous run -- can't use --skip-compile")
+
+    profile_self_check(target_dir)
+
+    print()
+    print('NOTE: Compile CPython using CFLAGS="-O2 -g -fno-omit-frame-pointer" for good results')
+    print()
+    print("CPU profile collected. You can now analyze the profile:")
+    print(f"  perf report -i {target_dir}/perf.data ")
+
+
+if __name__ == "__main__":
+    main()