Add Flang-Tidy: A Fortran Static Analysis Tool

[Originns]

Summary

This PR introduces flang-tidy, a new Fortran static analysis tool for the LLVM project. flang-tidy is a flang-based "linter" that provides an extensible framework for diagnosing and fixing typical programming errors in Fortran code, including style violations, interface misuse, and bugs detectable through static analysis.

This PR includes only the core infrastructure of flang-tidy with only a single example check. All the other checks that we have implemented will be presented in a follow-up PR to not overwhelm the reviewers.

Motivation

The Fortran ecosystem has lacked a comprehensive, modern static analysis tool comparable to clang-tidy for C++. With the maturation of flang as LLVM's Fortran frontend, we now have the infrastructure needed to build such a tool.

Design

The below UML diagram shows the rough architecture of flang-tidy to give a brief overview of our design. This diagram is not exactly reflected as-is in the code, but should give reviewers a general idea of the layout.

Key Features

Check Categories

• bugprone- - Detects bug-prone code constructs
• modernize- - Advocates modern Fortran language features
• openmp- - Analyzes OpenMP constructs for correctness
• performance- - Identifies performance-related issues
• readability- - Improves code readability and maintainability

Usage Examples

# Basic usage with compilation flags
flang-tidy test.f90 -- -Imy_project/include -DMY_DEFINES

# Enable specific check categories
flang-tidy test.f90 -checks=-*,bugprone-*,-bugprone-arithmetic-*

Future Work

• Additional check categories (security, HPC-specific patterns)
• Automatic fix suggestions for more check types
• Enhanced OpenMP analysis capabilities

Reviewers

@mjklemm @skatrak

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⚠️ Python code formatter, darker found issues in your code. ⚠️

You can test this locally with the following command:

darker --check --diff -r HEAD~1...HEAD flang/tools/docs/conf.py flang/tools/test/flang-tidy/check_flang_tidy.py flang/tools/test/lit.cfg.py

View the diff from darker here.

--- test/flang-tidy/check_flang_tidy.py	2025-06-12 11:37:10.000000 +0000
+++ test/flang-tidy/check_flang_tidy.py	2025-06-12 12:49:17.758998 +0000
@@ -104,13 +104,11 @@
 
             if not has_check_message:
                 self.expect_no_diagnosis = True
 
         if self.expect_no_diagnosis and self.has_check_messages:
-            sys.exit(
-                "%s not found in the input" % self.messages.prefix
-            )
+            sys.exit("%s not found in the input" % self.messages.prefix)
         assert self.has_check_messages or self.expect_no_diagnosis
 
     def prepare_test_inputs(self):
         cleaned_test = re.sub("// *CHECK-[A-Z0-9\\-]*:[^\r\n]*", "//", self.input_text)
         write_file(self.temp_file_name, cleaned_test)
@@ -121,12 +119,12 @@
                 "flang-tidy",
                 self.temp_file_name,
             ]
             + ["--checks=" + self.check_name]
             + self.extra_args
-            #+ ["--"]
-            #+ self.flang_extra_args
+            # + ["--"]
+            # + self.flang_extra_args
         )
         if self.expect_flang_tidy_error:
             args.insert(0, "not")
         print("Running " + repr(args) + "...")
         flang_tidy_output = try_run(args)
--- test/lit.cfg.py	2025-06-12 11:37:10.000000 +0000
+++ test/lit.cfg.py	2025-06-12 12:49:17.778699 +0000
@@ -14,19 +14,11 @@
 
 # testFormat: The test format to use to interpret tests.
 config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)
 
 # suffixes: A list of file extensions to treat as test files.
-config.suffixes = [
-    ".f",
-    ".f90",
-    ".f95",
-    ".f03",
-    ".f08",
-    ".mod",
-    ".test"
-]
+config.suffixes = [".f", ".f90", ".f95", ".f03", ".f08", ".mod", ".test"]
 
 # Exclude 'Inputs' directories as they are for test dependencies.
 config.excludes = ["Inputs"]
 
 # test_source_root: The root path where tests are located.

[mjklemm]

This is a massive PR. Would it be possible to carve it up further into infrastructure changes to introduce the new tool, documentation updates, and addition of the analysis? This would make reviewing easier.

[klausler]

This seems like an awful lot of code to add one new warning message that could easily be part of the flang-new compiler itself, perhaps controlled by a new UsageWarning.

[tarunprabhu]

This is a massive PR. Would it be possible to carve it up further into infrastructure changes to introduce the new tool, documentation updates, and addition of the analysis? This would make reviewing easier.

I agree. This is a lot to review at once.