[BOLT][Utils] Make nfc-check-setup compatible with ninja

[Amir]

Summary

Preserve llvm-bolt timestamps to trick Ninja into not rebuilding the binary
after it has been replaced by a symlink.
This makes it possible to use the script and then run ninja check-bolt as
usual, instead of resorting to llvm-lit invocation.

Test Plan:

nfc-check-setup.py
ninja check-bolt
ls -l bin/llvm-bolt # still a symlink

[Amir]

Huh, it didn't work. Checked too early before llvm-bolt was replaced as a dependency of check-bolt target.

bolt/utils/nfc-check-setup.py

if not source_dir:
         sys.exit("Source directory is not found")
 
     script_dir = os.path.dirname(os.path.abspath(__file__))
     wrapper_path = f"{script_dir}/llvm-bolt-wrapper.py"
     # build the current commit
     subprocess.run(
         shlex.split("cmake --build . --target llvm-bolt"), cwd=args.build_dir
     )
+    # preserve llvm-bolt's timestamp to trick Ninja not to rebuild it
+    stat = os.stat(bolt_path)
     # rename llvm-bolt
     os.replace(bolt_path, f"{bolt_path}.new")
     # memorize the old hash for logging
     old_ref = get_git_ref_or_rev(source_dir)
 
     # determine whether a stash is needed
     stash = subprocess.run(
         shlex.split("git status --porcelain"),
def main():
     ini = subprocess.check_output(
         shlex.split(f"{wrapper_path} {bolt_path}.old {bolt_path}.new") + wrapper_args,
         text=True,
     )
     with open(f"{args.build_dir}/bin/llvm-bolt-wrapper.ini", "w") as f:
         f.write(ini)
     # symlink llvm-bolt-wrapper
     os.symlink(wrapper_path, bolt_path)
+    # restore original [am]time on symlink
+    os.utime(bolt_path, (stat.st_atime, stat.st_mtime))
     if args.switch_back:
         if stash:
             subprocess.run(shlex.split("git stash pop"), cwd=source_dir)
         subprocess.run(shlex.split(f"git checkout {old_ref}"), cwd=source_dir)
     else:
         print(
             f"The repository {source_dir} has been switched from {old_ref} "
             f"to {new_ref}. Local changes were stashed. Switch back using\n\t"
             f"git checkout {old_ref}\n"
         )
     print(
         f"Build directory {args.build_dir} is ready to run BOLT tests, e.g.\n"
-        "\tbin/llvm-lit -sv tools/bolt/test\nor\n"
-        "\tbin/llvm-lit -sv tools/bolttests"
+        "\tninja check-bolt"
     )
 
 
 if __name__ == "__main__":
     main()