ExprSequence.cpp

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//===---------- ExprSequence.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 "ExprSequence.h"
#include "clang/AST/ParentMapContext.h"
#include "llvm/ADT/SmallVector.h"
#include <optional>
 
namespace clang::tidy::utils {
 
// Returns the Stmt nodes that are parents of 'S', skipping any potential
// intermediate non-Stmt nodes.
//
// In almost all cases, this function returns a single parent or no parents at
// all.
//
// The case that a Stmt has multiple parents is rare but does actually occur in
// the parts of the AST that we're interested in. Specifically, InitListExpr
// nodes cause ASTContext::getParent() to return multiple parents for certain
// nodes in their subtree because RecursiveASTVisitor visits both the syntactic
// and semantic forms of InitListExpr, and the parent-child relationships are
// different between the two forms.
static SmallVector<const Stmt *, 1> getParentStmts(const Stmt *S,
                                                   ASTContext *Context) {
  SmallVector<const Stmt *, 1> Result;
 
  TraversalKindScope RAII(*Context, TK_AsIs);
  DynTypedNodeList Parents = Context->getParents(*S);
 
  SmallVector<DynTypedNode, 1> NodesToProcess(Parents.begin(), Parents.end());
 
  while (!NodesToProcess.empty()) {
    DynTypedNode Node = NodesToProcess.back();
    NodesToProcess.pop_back();
 
    if (const auto *S = Node.get<Stmt>()) {
      Result.push_back(S);
    } else {
      Parents = Context->getParents(Node);
      NodesToProcess.append(Parents.begin(), Parents.end());
    }
  }
 
  return Result;
}
 
namespace {
 
bool isDescendantOrEqual(const Stmt *Descendant, const Stmt *Ancestor,
                         ASTContext *Context) {
  if (Descendant == Ancestor)
    return true;
  return llvm::any_of(getParentStmts(Descendant, Context),
                      [Ancestor, Context](const Stmt *Parent) {
                        return isDescendantOrEqual(Parent, Ancestor, Context);
                      });
}
 
bool isDescendantOfArgs(const Stmt *Descendant, const CallExpr *Call,
                        ASTContext *Context) {
  return llvm::any_of(Call->arguments(),
                      [Descendant, Context](const Expr *Arg) {
                        return isDescendantOrEqual(Descendant, Arg, Context);
                      });
}
 
llvm::SmallVector<const InitListExpr *>
getAllInitListForms(const InitListExpr *InitList) {
  llvm::SmallVector<const InitListExpr *> result = {InitList};
  if (const InitListExpr *AltForm = InitList->getSyntacticForm())
    result.push_back(AltForm);
  if (const InitListExpr *AltForm = InitList->getSemanticForm())
    result.push_back(AltForm);
  return result;
}
 
} // namespace
 
ExprSequence::ExprSequence(const CFG *TheCFG, const Stmt *Root,
                           ASTContext *TheContext)
    : Context(TheContext), Root(Root) {
  for (const auto &SyntheticStmt : TheCFG->synthetic_stmts()) {
    SyntheticStmtSourceMap[SyntheticStmt.first] = SyntheticStmt.second;
  }
}
 
bool ExprSequence::inSequence(const Stmt *Before, const Stmt *After) const {
  Before = resolveSyntheticStmt(Before);
  After = resolveSyntheticStmt(After);
 
  // If 'After' is in the subtree of the siblings that follow 'Before' in the
  // chain of successors, we know that 'After' is sequenced after 'Before'.
  for (const Stmt *Successor = getSequenceSuccessor(Before); Successor;
       Successor = getSequenceSuccessor(Successor)) {
    if (isDescendantOrEqual(After, Successor, Context))
      return true;
  }
 
  SmallVector<const Stmt *, 1> BeforeParents = getParentStmts(Before, Context);
 
  // Since C++17, the callee of a call expression is guaranteed to be sequenced
  // before all of the arguments.
  // We handle this as a special case rather than using the general
  // `getSequenceSuccessor` logic above because the callee expression doesn't
  // have an unambiguous successor; the order in which arguments are evaluated
  // is indeterminate.
  for (const Stmt *Parent : BeforeParents) {
    // Special case: If the callee is a `MemberExpr` with a `DeclRefExpr` as its
    // base, we consider it to be sequenced _after_ the arguments. This is
    // because the variable referenced in the base will only actually be
    // accessed when the call happens, i.e. once all of the arguments have been
    // evaluated. This has no basis in the C++ standard, but it reflects actual
    // behavior that is relevant to a use-after-move scenario:
    //
    // ```
    // a.bar(consumeA(std::move(a));
    // ```
    //
    // In this example, we end up accessing `a` after it has been moved from,
    // even though nominally the callee `a.bar` is evaluated before the argument
    // `consumeA(std::move(a))`. Note that this is not specific to C++17, so
    // we implement this logic unconditionally.
    if (const auto *Call = dyn_cast<CXXMemberCallExpr>(Parent)) {
      if (is_contained(Call->arguments(), Before) &&
          isa<DeclRefExpr>(
              Call->getImplicitObjectArgument()->IgnoreParenImpCasts()) &&
          isDescendantOrEqual(After, Call->getImplicitObjectArgument(),
                              Context))
        return true;
 
      // We need this additional early exit so that we don't fall through to the
      // more general logic below.
      if (const auto *Member = dyn_cast<MemberExpr>(Before);
          Member && Call->getCallee() == Member &&
          isa<DeclRefExpr>(Member->getBase()->IgnoreParenImpCasts()) &&
          isDescendantOfArgs(After, Call, Context))
        return false;
    }
 
    if (!Context->getLangOpts().CPlusPlus17)
      continue;
 
    if (const auto *Call = dyn_cast<CallExpr>(Parent);
        Call && Call->getCallee() == Before &&
        isDescendantOfArgs(After, Call, Context))
      return true;
  }
 
  // If 'After' is a parent of 'Before' or is sequenced after one of these
  // parents, we know that it is sequenced after 'Before'.
  for (const Stmt *Parent : BeforeParents) {
    if (Parent == After || inSequence(Parent, After))
      return true;
  }
 
  return false;
}
 
bool ExprSequence::potentiallyAfter(const Stmt *After,
                                    const Stmt *Before) const {
  return !inSequence(After, Before);
}
 
const Stmt *ExprSequence::getSequenceSuccessor(const Stmt *S) const {
  for (const Stmt *Parent : getParentStmts(S, Context)) {
    // If a statement has multiple parents, make sure we're using the parent
    // that lies within the sub-tree under Root.
    if (!isDescendantOrEqual(Parent, Root, Context))
      continue;
 
    if (const auto *BO = dyn_cast<BinaryOperator>(Parent)) {
      // Comma operator: Right-hand side is sequenced after the left-hand side.
      if (BO->getLHS() == S && BO->getOpcode() == BO_Comma)
        return BO->getRHS();
    } else if (const auto *InitList = dyn_cast<InitListExpr>(Parent)) {
      // Initializer list: Each initializer clause is sequenced after the
      // clauses that precede it.
      for (const InitListExpr *Form : getAllInitListForms(InitList)) {
        for (unsigned I = 1; I < Form->getNumInits(); ++I) {
          if (Form->getInit(I - 1) == S) {
            return Form->getInit(I);
          }
        }
      }
    } else if (const auto *ConstructExpr = dyn_cast<CXXConstructExpr>(Parent)) {
      // Constructor arguments are sequenced if the constructor call is written
      // as list-initialization.
      if (ConstructExpr->isListInitialization()) {
        for (unsigned I = 1; I < ConstructExpr->getNumArgs(); ++I) {
          if (ConstructExpr->getArg(I - 1) == S) {
            return ConstructExpr->getArg(I);
          }
        }
      }
    } else if (const auto *Compound = dyn_cast<CompoundStmt>(Parent)) {
      // Compound statement: Each sub-statement is sequenced after the
      // statements that precede it.
      const Stmt *Previous = nullptr;
      for (const auto *Child : Compound->body()) {
        if (Previous == S)
          return Child;
        Previous = Child;
      }
    } else if (const auto *TheDeclStmt = dyn_cast<DeclStmt>(Parent)) {
      // Declaration: Every initializer expression is sequenced after the
      // initializer expressions that precede it.
      const Expr *PreviousInit = nullptr;
      for (const Decl *TheDecl : TheDeclStmt->decls()) {
        if (const auto *TheVarDecl = dyn_cast<VarDecl>(TheDecl)) {
          if (const Expr *Init = TheVarDecl->getInit()) {
            if (PreviousInit == S)
              return Init;
            PreviousInit = Init;
          }
        }
      }
    } else if (const auto *ForRange = dyn_cast<CXXForRangeStmt>(Parent)) {
      // Range-based for: Loop variable declaration is sequenced before the
      // body. (We need this rule because these get placed in the same
      // CFGBlock.)
      if (S == ForRange->getLoopVarStmt())
        return ForRange->getBody();
    } else if (const auto *TheIfStmt = dyn_cast<IfStmt>(Parent)) {
      // If statement:
      // - Sequence init statement before variable declaration, if present;
      //   before condition evaluation, otherwise.
      // - Sequence variable declaration (along with the expression used to
      //   initialize it) before the evaluation of the condition.
      if (S == TheIfStmt->getInit()) {
        if (TheIfStmt->getConditionVariableDeclStmt() != nullptr)
          return TheIfStmt->getConditionVariableDeclStmt();
        return TheIfStmt->getCond();
      }
      if (S == TheIfStmt->getConditionVariableDeclStmt())
        return TheIfStmt->getCond();
    } else if (const auto *TheSwitchStmt = dyn_cast<SwitchStmt>(Parent)) {
      // Ditto for switch statements.
      if (S == TheSwitchStmt->getInit()) {
        if (TheSwitchStmt->getConditionVariableDeclStmt() != nullptr)
          return TheSwitchStmt->getConditionVariableDeclStmt();
        return TheSwitchStmt->getCond();
      }
      if (S == TheSwitchStmt->getConditionVariableDeclStmt())
        return TheSwitchStmt->getCond();
    } else if (const auto *TheWhileStmt = dyn_cast<WhileStmt>(Parent)) {
      // While statement: Sequence variable declaration (along with the
      // expression used to initialize it) before the evaluation of the
      // condition.
      if (S == TheWhileStmt->getConditionVariableDeclStmt())
        return TheWhileStmt->getCond();
    }
  }
 
  return nullptr;
}
 
const Stmt *ExprSequence::resolveSyntheticStmt(const Stmt *S) const {
  if (SyntheticStmtSourceMap.count(S))
    return SyntheticStmtSourceMap.lookup(S);
  return S;
}
 
StmtToBlockMap::StmtToBlockMap(const CFG *TheCFG, ASTContext *TheContext)
    : Context(TheContext) {
  for (const auto *B : *TheCFG) {
    for (const auto &Elem : *B) {
      if (std::optional<CFGStmt> S = Elem.getAs<CFGStmt>())
        Map[S->getStmt()] = B;
    }
  }
}
 
const CFGBlock *StmtToBlockMap::blockContainingStmt(const Stmt *S) const {
  while (!Map.count(S)) {
    SmallVector<const Stmt *, 1> Parents = getParentStmts(S, Context);
    if (Parents.empty())
      return nullptr;
    S = Parents[0];
  }
 
  return Map.lookup(S);
}
 
} // namespace clang::tidy::utils