[clang-tidy] Add parallel execution by default in 'run-clang-tidy' and 'clang-tidy-diff' (#149739)

Change the default value of `-j` from `1` to `0` in `clang-tidy-diff.py`
script to autodetect number of CPU cores to run on.
Script `run-clang-tidy.py` already had this behavior by default.

Both scripts now also print the number of threads being used to
provide better visibility into their execution behavior.

Fixes #148624.

Author: vbvictor

clang-tools-extra/clang-tidy/tool/clang-tidy-diff.py

@@ -177,7 +177,7 @@ def main():
     parser.add_argument(
         "-j",
         type=int,
-        default=1,
+        default=0,
         help="number of tidy instances to be run in parallel.",
     )
     parser.add_argument(
@@ -318,6 +318,7 @@ def main():
     if max_task_count == 0:
         max_task_count = multiprocessing.cpu_count()
     max_task_count = min(len(lines_by_file), max_task_count)
+    print(f"Running clang-tidy in {max_task_count} threads...")
 
     combine_fixes = False
     export_fixes_dir = None

clang-tools-extra/clang-tidy/tool/run-clang-tidy.py

@@ -548,7 +548,7 @@ async def main() -> None:
     files = {f for f in files if file_name_re.search(f)}
 
     print(
-        "Running clang-tidy for",
+        f"Running clang-tidy in {max_task} threads for",
         len(files),
         "files out of",
         number_files_in_database,

clang-tools-extra/docs/ReleaseNotes.rst

@@ -93,6 +93,10 @@ Improvements to clang-query
 Improvements to clang-tidy
 --------------------------
 
+- The :program:`run-clang-tidy.py` and :program:`clang-tidy-diff.py` scripts
+  now run checks in parallel by default using all available hardware threads.
+  Both scripts display the number of threads being used in their output.
+
 New checks
 ^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/infrastructure/clang-tidy-diff.cpp

@@ -1,15 +1,19 @@
 // REQUIRES: shell
 // RUN: sed 's/placeholder_for_f/f/' %s > %t.cpp
 // RUN: clang-tidy -checks=-*,modernize-use-override %t.cpp -- -std=c++11 | FileCheck -check-prefix=CHECK-SANITY %s
-// RUN: not diff -U0 %s %t.cpp | %clang_tidy_diff -checks=-*,modernize-use-override -- -std=c++11 2>&1 | FileCheck %s
+// RUN: not diff -U0 %s %t.cpp | %clang_tidy_diff -checks=-*,modernize-use-override -- -std=c++11 2>&1 | FileCheck %s --check-prefixes=CHECK,CHECK-JMAX
 // RUN: not diff -U0 %s %t.cpp | %clang_tidy_diff -checks=-*,modernize-use-override -quiet -- -std=c++11 2>&1 | FileCheck -check-prefix=CHECK-QUIET %s
 // RUN: mkdir -p %T/compilation-database-test/
 // RUN: echo '[{"directory": "%T", "command": "clang++ -o test.o -std=c++11 %t.cpp", "file": "%t.cpp"}]' > %T/compilation-database-test/compile_commands.json
 // RUN: not diff -U0 %s %t.cpp | %clang_tidy_diff -checks=-*,modernize-use-override -path %T/compilation-database-test 2>&1 | FileCheck -check-prefix=CHECK %s
+
+// RUN: not diff -U0 %s %t.cpp | %clang_tidy_diff -checks=-*,modernize-use-override -j 1 -- -std=c++11 2>&1 | FileCheck %s --check-prefix=CHECK-J1
+// CHECK-J1: Running clang-tidy in 1 threads...
 struct A {
   virtual void f() {}
   virtual void g() {}
 };
+// CHECK-JMAX: Running clang-tidy in {{[1-9][0-9]*}} threads...
 // CHECK-NOT: warning:
 // CHECK-QUIET-NOT: warning:
 struct B : public A {

clang-tools-extra/test/clang-tidy/infrastructure/run-clang-tidy.cpp

@@ -8,7 +8,11 @@
 // RUN: echo "  modernize-use-auto.MinTypeNameLength: '0'" >> %t/.clang-tidy
 // RUN: cp "%s" "%t/test.cpp"
 // RUN: cd "%t"
-// RUN: not %run_clang_tidy "test.cpp"
+// RUN: not %run_clang_tidy "test.cpp" 2>&1 | FileCheck %s --check-prefix=CHECK-JMAX
+// CHECK-JMAX: Running clang-tidy in {{[1-9][0-9]*}} threads for
+
+// RUN: not %run_clang_tidy -j 1 "test.cpp" 2>&1 | FileCheck %s --check-prefix=CHECK-J1
+// CHECK-J1: Running clang-tidy in 1 threads for
 
 int main()
 {