[clang-tidy] new check: bugprone-branch-clone

Summary:
Implement a check for detecting if/else if/else chains where two or more
branches are Type I clones of each other (that is, they contain identical code)
and for detecting switch statements where two or more consecutive branches are
Type I clones of each other.

Patch by donat.nagy.

Reviewers: alexfh, hokein, aaron.ballman, JonasToth

Reviewed By: JonasToth

Subscribers: MTC, lebedev.ri, whisperity, xazax.hun, Eugene.Zelenko, mgorny, rnkovacs, dkrupp, Szelethus, gamesh411, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D54757

llvm-svn: 348343

clang-tools-extra/clang-tidy/bugprone/BranchCloneCheck.cpp

@@ -0,0 +1,215 @@
+//===--- BranchCloneCheck.cpp - clang-tidy --------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "BranchCloneCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Analysis/CloneDetection.h"
+#include "llvm/Support/Casting.h"
+
+using namespace clang;
+using namespace clang::ast_matchers;
+
+/// Returns true when the statements are Type I clones of each other.
+static bool areStatementsIdentical(const Stmt *LHS, const Stmt *RHS,
+                                   const ASTContext &Context) {
+  llvm::FoldingSetNodeID DataLHS, DataRHS;
+  LHS->Profile(DataLHS, Context, false);
+  RHS->Profile(DataRHS, Context, false);
+  return (DataLHS == DataRHS);
+}
+
+namespace {
+/// A branch in a switch may consist of several statements; while a branch in
+/// an if/else if/else chain is one statement (which may be a CompoundStmt).
+using SwitchBranch = llvm::SmallVector<const Stmt *, 2>;
+} // anonymous namespace
+
+/// Determines if the bodies of two branches in a switch statements are Type I
+/// clones of each other. This function only examines the body of the branch
+/// and ignores the `case X:` or `default:` at the start of the branch.
+static bool areSwitchBranchesIdentical(const SwitchBranch LHS,
+                                       const SwitchBranch RHS,
+                                       const ASTContext &Context) {
+  if (LHS.size() != RHS.size())
+    return false;
+
+  for (size_t i = 0, Size = LHS.size(); i < Size; i++) {
+    // NOTE: We strip goto labels and annotations in addition to stripping
+    // the `case X:` or `default:` labels, but it is very unlikely that this
+    // would casue false positives in real-world code.
+    if (!areStatementsIdentical(LHS[i]->stripLabelLikeStatements(),
+                                RHS[i]->stripLabelLikeStatements(), Context)) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+namespace clang {
+namespace tidy {
+namespace bugprone {
+
+void BranchCloneCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      ifStmt(stmt().bind("if"),
+             hasParent(stmt(unless(ifStmt(hasElse(equalsBoundNode("if")))))),
+             hasElse(stmt().bind("else"))),
+      this);
+  Finder->addMatcher(switchStmt().bind("switch"), this);
+  Finder->addMatcher(conditionalOperator().bind("condOp"), this);
+}
+
+void BranchCloneCheck::check(const MatchFinder::MatchResult &Result) {
+  const ASTContext &Context = *Result.Context;
+
+  if (const auto *IS = Result.Nodes.getNodeAs<IfStmt>("if")) {
+    const Stmt *Then = IS->getThen();
+    assert(Then && "An IfStmt must have a `then` branch!");
+
+    const Stmt *Else = Result.Nodes.getNodeAs<Stmt>("else");
+    assert(Else && "We only look for `if` statements with an `else` branch!");
+
+    if (!isa<IfStmt>(Else)) {
+      // Just a simple if with no `else if` branch.
+      if (areStatementsIdentical(Then->IgnoreContainers(),
+                                 Else->IgnoreContainers(), Context)) {
+        diag(IS->getBeginLoc(), "if with identical then and else branches");
+        diag(IS->getElseLoc(), "else branch starts here", DiagnosticIDs::Note);
+      }
+      return;
+    }
+
+    // This is the complicated case when we start an if/else if/else chain.
+    // To find all the duplicates, we collect all the branches into a vector.
+    llvm::SmallVector<const Stmt *, 4> Branches;
+    const IfStmt *Cur = IS;
+    while (true) {
+      // Store the `then` branch.
+      Branches.push_back(Cur->getThen());
+
+      Else = Cur->getElse();
+      // The chain ends if there is no `else` branch.
+      if (!Else)
+        break;
+
+      // Check if there is another `else if`...
+      Cur = dyn_cast<IfStmt>(Else);
+      if (!Cur) {
+        // ...this is just a plain `else` branch at the end of the chain.
+        Branches.push_back(Else);
+        break;
+      }
+    }
+
+    size_t N = Branches.size();
+    llvm::BitVector KnownAsClone(N);
+
+    for (size_t i = 0; i + 1 < N; i++) {
+      // We have already seen Branches[i] as a clone of an earlier branch.
+      if (KnownAsClone[i])
+        continue;
+
+      int NumCopies = 1;
+
+      for (size_t j = i + 1; j < N; j++) {
+        if (KnownAsClone[j] ||
+            !areStatementsIdentical(Branches[i]->IgnoreContainers(),
+                                    Branches[j]->IgnoreContainers(), Context))
+          continue;
+
+        NumCopies++;
+        KnownAsClone[j] = true;
+
+        if (NumCopies == 2) {
+          // We report the first occurence only when we find the second one.
+          diag(Branches[i]->getBeginLoc(),
+               "repeated branch in conditional chain");
+          diag(Lexer::getLocForEndOfToken(Branches[i]->getEndLoc(), 0,
+                                          *Result.SourceManager, getLangOpts()),
+               "end of the original", DiagnosticIDs::Note);
+        }
+
+        diag(Branches[j]->getBeginLoc(), "clone %0 starts here",
+             DiagnosticIDs::Note)
+            << (NumCopies - 1);
+      }
+    }
+    return;
+  }
+
+  if (const auto *CO = Result.Nodes.getNodeAs<ConditionalOperator>("condOp")) {
+    // We do not try to detect chains of ?: operators.
+    if (areStatementsIdentical(CO->getTrueExpr(), CO->getFalseExpr(), Context))
+      diag(CO->getQuestionLoc(),
+           "conditional operator with identical true and false expressions");
+
+    return;
+  }
+
+  if (const auto *SS = Result.Nodes.getNodeAs<SwitchStmt>("switch")) {
+    const CompoundStmt *Body = dyn_cast_or_null<CompoundStmt>(SS->getBody());
+
+    // Code like
+    //   switch (x) case 0: case 1: foobar();
+    // is legal and calls foobar() if and only if x is either 0 or 1;
+    // but we do not try to distinguish branches in such code.
+    if (!Body)
+      return;
+
+    // We will first collect the branches of the switch statements. For the
+    // sake of simplicity we say that branches are delimited by the SwitchCase
+    // (`case:` or `default:`) children of Body; that is, we ignore `case:` or
+    // `default:` labels embedded inside other statements and we do not follow
+    // the effects of `break` and other manipulation of the control-flow.
+    llvm::SmallVector<SwitchBranch, 4> Branches;
+    for (const Stmt *S : Body->body()) {
+      // If this is a `case` or `default`, we start a new, empty branch.
+      if (isa<SwitchCase>(S))
+        Branches.emplace_back();
+
+      // There may be code before the first branch (which can be dead code
+      // and can be code reached either through goto or through case labels
+      // that are embedded inside e.g. inner compound statements); we do not
+      // store those statements in branches.
+      if (!Branches.empty())
+        Branches.back().push_back(S);
+    }
+
+    auto End = Branches.end();
+    auto BeginCurrent = Branches.begin();
+    while (BeginCurrent < End) {
+      auto EndCurrent = BeginCurrent + 1;
+      while (EndCurrent < End &&
+             areSwitchBranchesIdentical(*BeginCurrent, *EndCurrent, Context)) {
+        ++EndCurrent;
+      }
+      // At this point the iterator range {BeginCurrent, EndCurrent} contains a
+      // complete family of consecutive identical branches.
+      if (EndCurrent > BeginCurrent + 1) {
+        diag(BeginCurrent->front()->getBeginLoc(),
+             "switch has %0 consecutive identical branches")
+            << static_cast<int>(std::distance(BeginCurrent, EndCurrent));
+        diag(Lexer::getLocForEndOfToken((EndCurrent - 1)->back()->getEndLoc(),
+                                        0, *Result.SourceManager,
+                                        getLangOpts()),
+             "last of these clones ends here", DiagnosticIDs::Note);
+      }
+      BeginCurrent = EndCurrent;
+    }
+    return;
+  }
+
+  llvm_unreachable("No if statement and no switch statement.");
+}
+
+} // namespace bugprone
+} // namespace tidy
+} // namespace clang

clang-tools-extra/clang-tidy/bugprone/BranchCloneCheck.h

@@ -0,0 +1,39 @@
+//===--- BranchCloneCheck.h - clang-tidy ------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_BRANCHCLONECHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_BRANCHCLONECHECK_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace bugprone {
+
+/// A check for detecting if/else if/else chains where two or more branches are
+/// Type I clones of each other (that is, they contain identical code), for
+/// detecting switch statements where two or more consecutive branches are
+/// Type I clones of each other, and for detecting conditional operators where
+/// the true and false expressions are Type I clones of each other.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/bugprone-branch-clone.html
+class BranchCloneCheck : public ClangTidyCheck {
+public:
+  BranchCloneCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace bugprone
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BUGPRONE_BRANCHCLONECHECK_H

clang-tools-extra/clang-tidy/bugprone/BugproneTidyModule.cpp

@@ -14,6 +14,7 @@
 #include "ArgumentCommentCheck.h"
 #include "AssertSideEffectCheck.h"
 #include "BoolPointerImplicitConversionCheck.h"
+#include "BranchCloneCheck.h"
 #include "CopyConstructorInitCheck.h"
 #include "DanglingHandleCheck.h"
 #include "ExceptionEscapeCheck.h"
@@ -65,6 +66,8 @@ public:
         "bugprone-assert-side-effect");
     CheckFactories.registerCheck<BoolPointerImplicitConversionCheck>(
         "bugprone-bool-pointer-implicit-conversion");
+    CheckFactories.registerCheck<BranchCloneCheck>(
+        "bugprone-branch-clone");
     CheckFactories.registerCheck<CopyConstructorInitCheck>(
         "bugprone-copy-constructor-init");
     CheckFactories.registerCheck<DanglingHandleCheck>(

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

@@ -4,6 +4,7 @@ add_clang_library(clangTidyBugproneModule
   ArgumentCommentCheck.cpp
   AssertSideEffectCheck.cpp
   BoolPointerImplicitConversionCheck.cpp
+  BranchCloneCheck.cpp
   BugproneTidyModule.cpp
   CopyConstructorInitCheck.cpp
   DanglingHandleCheck.cpp

clang-tools-extra/docs/ReleaseNotes.rst

@@ -122,6 +122,13 @@ Improvements to clang-tidy
   Flags uses of ``absl::StrCat()`` to append to a ``std::string``. Suggests
   ``absl::StrAppend()`` should be used instead.
 
+- New :doc:`bugprone-branch-clone
+  <clang-tidy/checks/bugprone-branch-clone>` check.
+
+  Checks for repeated branches in ``if/else if/else`` chains, consecutive
+  repeated branches in ``switch`` statements and indentical true and false
+  branches in conditional operators.
+
 - New :doc:`bugprone-too-small-loop-variable
   <clang-tidy/checks/bugprone-too-small-loop-variable>` check.
 

clang-tools-extra/docs/clang-tidy/checks/bugprone-branch-clone.rst

@@ -0,0 +1,90 @@
+.. title:: clang-tidy - bugprone-branch-clone
+
+bugprone-branch-clone
+=====================
+
+Checks for repeated branches in ``if/else if/else`` chains, consecutive
+repeated branches in ``switch`` statements and indentical true and false
+branches in conditional operators.
+
+.. code-block:: c++
+
+    if (test_value(x)) {
+      y++;
+      do_something(x, y);
+    } else {
+      y++;
+      do_something(x, y);
+    }
+
+In this simple example (which could arise e.g. as a copy-paste error) the
+``then`` and ``else`` branches are identical and the code is equivalent the
+following shorter and cleaner code:
+
+.. code-block:: c++
+
+    test_value(x); // can be omitted unless it has side effects
+    y++;
+    do_something(x, y);
+
+
+If this is the inteded behavior, then there is no reason to use a conditional
+statement; otherwise the issue can be solved by fixing the branch that is
+handled incorrectly.
+
+The check also detects repeated branches in longer ``if/else if/else`` chains
+where it would be even harder to notice the problem.
+
+In ``switch`` statements the check only reports repeated branches when they are
+consecutive, because it is relatively common that the ``case:`` labels have
+some natural ordering and rearranging them would decrease the readability of
+the code. For example:
+
+.. code-block:: c++
+
+    switch (ch) {
+    case 'a':
+      return 10;
+    case 'A':
+      return 10;
+    case 'b':
+      return 11;
+    case 'B':
+      return 11;
+    default:
+      return 10;
+    }
+
+Here the check reports that the ``'a'`` and ``'A'`` branches are identical
+(and that the ``'b'`` and ``'B'`` branches are also identical), but does not
+report that the ``default:`` branch is also idenical to the first two branches.
+If this is indeed the correct behavior, then it could be implemented as:
+
+.. code-block:: c++
+
+    switch (ch) {
+    case 'a':
+    case 'A':
+      return 10;
+    case 'b':
+    case 'B':
+      return 11;
+    default:
+      return 10;
+    }
+
+Here the check does not warn for the repeated ``return 10;``, which is good if
+we want to preserve that ``'a'`` is before ``'b'`` and ``default:`` is the last
+branch.
+
+Finally, the check also examines conditional operators and reports code like:
+
+.. code-block:: c++
+
+    return test_value(x) ? x : x;
+
+Unlike if statements, the check does not detect chains of conditional
+operators.
+
+Note: This check also reports situations where branches become identical only
+after preprocession.

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

@@ -31,6 +31,7 @@ Clang-Tidy Checks
    bugprone-argument-comment
    bugprone-assert-side-effect
    bugprone-bool-pointer-implicit-conversion
+   bugprone-branch-clone
    bugprone-copy-constructor-init
    bugprone-dangling-handle
    bugprone-exception-escape