[clang-tidy] Add readability-avoid-unconditional-preprocessor-if check

Check flags always enabled or disabled code blocks in preprocessor '#if'
conditions, such as '#if 0' and '#if 1' etc.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D145617

clang-tools-extra/clang-tidy/readability/AvoidUnconditionalPreprocessorIfCheck.cpp

@@ -0,0 +1,104 @@
+//===--- AvoidUnconditionalPreprocessorIfCheck.cpp - clang-tidy -----------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "AvoidUnconditionalPreprocessorIfCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/Lex/PPCallbacks.h"
+#include "clang/Lex/Preprocessor.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+namespace {
+struct AvoidUnconditionalPreprocessorIfPPCallbacks : public PPCallbacks {
+
+  explicit AvoidUnconditionalPreprocessorIfPPCallbacks(ClangTidyCheck &Check,
+                                                       Preprocessor &PP)
+      : Check(Check), PP(PP) {}
+
+  void If(SourceLocation Loc, SourceRange ConditionRange,
+          ConditionValueKind ConditionValue) override {
+    if (ConditionValue == CVK_NotEvaluated)
+      return;
+    SourceManager &SM = PP.getSourceManager();
+    if (!isImmutable(SM, PP.getLangOpts(), ConditionRange))
+      return;
+
+    if (ConditionValue == CVK_True)
+      Check.diag(Loc, "preprocessor condition is always 'true', consider "
+                      "removing condition but leaving its contents");
+    else
+      Check.diag(Loc, "preprocessor condition is always 'false', consider "
+                      "removing both the condition and its contents");
+  }
+
+  bool isImmutable(SourceManager &SM, const LangOptions &LangOpts,
+                   SourceRange ConditionRange) {
+    SourceLocation Loc = ConditionRange.getBegin();
+    if (Loc.isMacroID())
+      return false;
+
+    Token Tok;
+    if (Lexer::getRawToken(Loc, Tok, SM, LangOpts, true)) {
+      std::optional<Token> TokOpt = Lexer::findNextToken(Loc, SM, LangOpts);
+      if (!TokOpt || TokOpt->getLocation().isMacroID())
+        return false;
+      Tok = *TokOpt;
+    }
+
+    while (Tok.getLocation() <= ConditionRange.getEnd()) {
+      if (!isImmutableToken(Tok))
+        return false;
+
+      std::optional<Token> TokOpt =
+          Lexer::findNextToken(Tok.getLocation(), SM, LangOpts);
+      if (!TokOpt || TokOpt->getLocation().isMacroID())
+        return false;
+      Tok = *TokOpt;
+    }
+
+    return true;
+  }
+
+  bool isImmutableToken(const Token &Tok) {
+    switch (Tok.getKind()) {
+    case tok::eod:
+    case tok::eof:
+    case tok::numeric_constant:
+    case tok::char_constant:
+    case tok::wide_char_constant:
+    case tok::utf8_char_constant:
+    case tok::utf16_char_constant:
+    case tok::utf32_char_constant:
+    case tok::string_literal:
+    case tok::wide_string_literal:
+    case tok::comment:
+      return true;
+    case tok::raw_identifier:
+      return (Tok.getRawIdentifier() == "true" ||
+              Tok.getRawIdentifier() == "false");
+    default:
+      return Tok.getKind() >= tok::l_square && Tok.getKind() <= tok::caretcaret;
+    }
+  }
+
+  ClangTidyCheck &Check;
+  Preprocessor &PP;
+};
+
+} // namespace
+
+void AvoidUnconditionalPreprocessorIfCheck::registerPPCallbacks(
+    const SourceManager &SM, Preprocessor *PP, Preprocessor *ModuleExpanderPP) {
+  PP->addPPCallbacks(
+      std::make_unique<AvoidUnconditionalPreprocessorIfPPCallbacks>(*this,
+                                                                    *PP));
+}
+
+} // namespace clang::tidy::readability

clang-tools-extra/clang-tidy/readability/AvoidUnconditionalPreprocessorIfCheck.h

@@ -0,0 +1,32 @@
+//===--- AvoidUnconditionalPreprocessorIfCheck.h - clang-tidy ---*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_AVOIDUNCONDITIONALPREPROCESSORIFCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_AVOIDUNCONDITIONALPREPROCESSORIFCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang::tidy::readability {
+
+/// Finds code blocks that are constantly enabled or disabled in preprocessor
+/// directives by analyzing `#if` conditions, such as `#if 0` and `#if 1`, etc.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/readability/avoid-unconditional-preprocessor-if.html
+class AvoidUnconditionalPreprocessorIfCheck : public ClangTidyCheck {
+public:
+  AvoidUnconditionalPreprocessorIfCheck(StringRef Name,
+                                        ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerPPCallbacks(const SourceManager &SM, Preprocessor *PP,
+                           Preprocessor *ModuleExpanderPP) override;
+};
+
+} // namespace clang::tidy::readability
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_AVOIDUNCONDITIONALPREPROCESSORIFCHECK_H

clang-tools-extra/clang-tidy/readability/CMakeLists.txt

@@ -5,6 +5,7 @@ set(LLVM_LINK_COMPONENTS
 
 add_clang_library(clangTidyReadabilityModule
   AvoidConstParamsInDecls.cpp
+  AvoidUnconditionalPreprocessorIfCheck.cpp
   BracesAroundStatementsCheck.cpp
   ConstReturnTypeCheck.cpp
   ContainerContainsCheck.cpp

clang-tools-extra/clang-tidy/readability/ReadabilityTidyModule.cpp

@@ -10,6 +10,7 @@
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
 #include "AvoidConstParamsInDecls.h"
+#include "AvoidUnconditionalPreprocessorIfCheck.h"
 #include "BracesAroundStatementsCheck.h"
 #include "ConstReturnTypeCheck.h"
 #include "ContainerContainsCheck.h"
@@ -60,6 +61,8 @@ class ReadabilityModule : public ClangTidyModule {
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
     CheckFactories.registerCheck<AvoidConstParamsInDecls>(
         "readability-avoid-const-params-in-decls");
+    CheckFactories.registerCheck<AvoidUnconditionalPreprocessorIfCheck>(
+        "readability-avoid-unconditional-preprocessor-if");
     CheckFactories.registerCheck<BracesAroundStatementsCheck>(
         "readability-braces-around-statements");
     CheckFactories.registerCheck<ConstReturnTypeCheck>(

clang-tools-extra/docs/ReleaseNotes.rst

@@ -133,6 +133,13 @@ New checks
   Checks that all implicit and explicit inline functions in header files are
   tagged with the ``LIBC_INLINE`` macro.
 
+- New :doc:`readability-avoid-unconditional-preprocessor-if
+  <clang-tidy/checks/readability/avoid-unconditional-preprocessor-if>` check.
+
+  Finds code blocks that are constantly enabled or disabled in preprocessor
+  directives by analyzing ``#if`` conditions, such as ``#if 0`` and
+  ``#if 1``, etc.
+
 New check aliases
 ^^^^^^^^^^^^^^^^^
 

clang-tools-extra/docs/clang-tidy/checks/list.rst

@@ -331,6 +331,7 @@ Clang-Tidy Checks
    `portability-simd-intrinsics <portability/simd-intrinsics.html>`_,
    `portability-std-allocator-const <portability/std-allocator-const.html>`_,
    `readability-avoid-const-params-in-decls <readability/avoid-const-params-in-decls.html>`_, "Yes"
+   `readability-avoid-unconditional-preprocessor-if <readability/avoid-unconditional-preprocessor-if.html>`_,
    `readability-braces-around-statements <readability/braces-around-statements.html>`_, "Yes"
    `readability-const-return-type <readability/const-return-type.html>`_, "Yes"
    `readability-container-contains <readability/container-contains.html>`_, "Yes"

clang-tools-extra/docs/clang-tidy/checks/readability/avoid-unconditional-preprocessor-if.rst

@@ -0,0 +1,32 @@
+.. title:: clang-tidy - readability-avoid-unconditional-preprocessor-if
+
+readability-avoid-unconditional-preprocessor-if
+===============================================
+
+Finds code blocks that are constantly enabled or disabled in preprocessor
+directives by analyzing ``#if`` conditions, such as ``#if 0`` and ``#if 1``,
+etc.
+
+.. code-block:: c++
+
+    #if 0
+        // some disabled code
+    #endif
+
+    #if 1
+       // some enabled code that can be disabled manually
+    #endif
+
+Unconditional preprocessor directives, such as ``#if 0`` for disabled code
+and ``#if 1`` for enabled code, can lead to dead code and always enabled code,
+respectively. Dead code can make understanding the codebase more difficult,
+hinder readability, and may be a sign of unfinished functionality or abandoned
+features. This can cause maintenance issues, confusion for future developers,
+and potential compilation problems.
+
+As a solution for both cases, consider using preprocessor macros or defines,
+like ``#ifdef DEBUGGING_ENABLED``, to control code enabling or disabling.
+This approach provides better coordination and flexibility when working with
+different parts of the codebase. Alternatively, you can comment out the entire
+code using ``/* */`` block comments and add a hint, such as ``@todo``,
+to indicate future actions.

clang-tools-extra/test/clang-tidy/checkers/readability/avoid-unconditional-preprocessor-if.cpp

@@ -0,0 +1,94 @@
+// RUN: %check_clang_tidy %s readability-avoid-unconditional-preprocessor-if %t
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'false', consider removing both the condition and its contents [readability-avoid-unconditional-preprocessor-if]
+#if 0
+// some code
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'true', consider removing condition but leaving its contents [readability-avoid-unconditional-preprocessor-if]
+#if 1
+// some code
+#endif
+
+#if test
+
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'true', consider removing condition but leaving its contents [readability-avoid-unconditional-preprocessor-if]
+#if 10>5
+
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'false', consider removing both the condition and its contents [readability-avoid-unconditional-preprocessor-if]
+#if 10<5
+
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'true', consider removing condition but leaving its contents [readability-avoid-unconditional-preprocessor-if]
+#if 10 > 5
+// some code
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'false', consider removing both the condition and its contents [readability-avoid-unconditional-preprocessor-if]
+#if 10 < 5
+// some code
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'false', consider removing both the condition and its contents [readability-avoid-unconditional-preprocessor-if]
+#if !(10 > \
+        5)
+// some code
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'true', consider removing condition but leaving its contents [readability-avoid-unconditional-preprocessor-if]
+#if !(10 < \
+        5)
+// some code
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'true', consider removing condition but leaving its contents [readability-avoid-unconditional-preprocessor-if]
+#if true
+// some code
+#endif
+
+// CHECK-MESSAGES: :[[@LINE+1]]:2: warning: preprocessor condition is always 'false', consider removing both the condition and its contents [readability-avoid-unconditional-preprocessor-if]
+#if false
+// some code
+#endif
+
+#define MACRO
+#ifdef MACRO
+// some code
+#endif
+
+#if !SOMETHING
+#endif
+
+#if !( \
+     defined MACRO)
+// some code
+#endif
+
+
+#if __has_include(<string_view>)
+// some code
+#endif
+
+#if __has_include(<string_view_not_exist>)
+// some code
+#endif
+
+#define DDD 17
+#define EEE 18
+
+#if 10 > DDD
+// some code
+#endif
+
+#if 10 < DDD
+// some code
+#endif
+
+#if EEE > DDD
+// some code
+#endif