1 //===------------- ExprSequence.h - clang-tidy ----------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_EXPRSEQUENCE_H 10 #define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_EXPRSEQUENCE_H 11 12 #include "clang/Analysis/CFG.h" 13 #include "clang/Lex/Lexer.h" 14 #include "llvm/ADT/DenseMap.h" 15 #include "llvm/ADT/SmallPtrSet.h" 16 #include "llvm/ADT/SmallVector.h" 17 18 #include "../ClangTidy.h" 19 20 namespace clang::tidy::utils { 21 22 /// Provides information about the evaluation order of (sub-)expressions within 23 /// a `CFGBlock`. 24 /// 25 /// While a `CFGBlock` does contain individual `CFGElement`s for some 26 /// sub-expressions, the order in which those `CFGElement`s appear reflects 27 /// only one possible order in which the sub-expressions may be evaluated. 28 /// However, we want to warn if any of the potential evaluation orders can lead 29 /// to a use-after-move, not just the one contained in the `CFGBlock`. 30 /// 31 /// This class implements only a simplified version of the C++ sequencing 32 /// rules. The main limitation is that we do not distinguish between value 33 /// computation and side effect -- see the "Implementation" section for more 34 /// details. 35 /// 36 /// Note: `SequenceChecker` from SemaChecking.cpp does a similar job (and much 37 /// more thoroughly), but using it would require 38 /// - Pulling `SequenceChecker` out into a header file (i.e. making it part of 39 /// the API), 40 /// - Removing the dependency of `SequenceChecker` on `Sema`, and 41 /// - (Probably) modifying `SequenceChecker` to make it suitable to be used in 42 /// this context. 43 /// For the moment, it seems preferable to re-implement our own version of 44 /// sequence checking that is special-cased to what we need here. 45 /// 46 /// Implementation 47 /// -------------- 48 /// 49 /// `ExprSequence` uses two types of sequencing edges between nodes in the AST: 50 /// 51 /// - Every `Stmt` is assumed to be sequenced after its children. This is 52 /// overly optimistic because the standard only states that value computations 53 /// of operands are sequenced before the value computation of the operator, 54 /// making no guarantees about side effects (in general). 55 /// 56 /// For our purposes, this rule is sufficient, however, because this check is 57 /// interested in operations on objects, which are generally performed through 58 /// function calls (whether explicit and implicit). Function calls guarantee 59 /// that the value computations and side effects for all function arguments 60 /// are sequenced before the execution of the function. 61 /// 62 /// - In addition, some `Stmt`s are known to be sequenced before or after 63 /// their siblings. For example, the `Stmt`s that make up a `CompoundStmt`are 64 /// all sequenced relative to each other. The function 65 /// `getSequenceSuccessor()` implements these sequencing rules. 66 class ExprSequence { 67 public: 68 /// Initializes this `ExprSequence` with sequence information for the given 69 /// `CFG`. `Root` is the root statement the CFG was built from. 70 ExprSequence(const CFG *TheCFG, const Stmt *Root, ASTContext *TheContext); 71 72 /// Returns whether \p Before is sequenced before \p After. 73 bool inSequence(const Stmt *Before, const Stmt *After) const; 74 75 /// Returns whether \p After can potentially be evaluated after \p Before. 76 /// This is exactly equivalent to `!inSequence(After, Before)` but makes some 77 /// conditions read more naturally. 78 bool potentiallyAfter(const Stmt *After, const Stmt *Before) const; 79 80 private: 81 // Returns the sibling of \p S (if any) that is directly sequenced after \p S, 82 // or nullptr if no such sibling exists. For example, if \p S is the child of 83 // a `CompoundStmt`, this would return the Stmt that directly follows \p S in 84 // the `CompoundStmt`. 85 // 86 // As the sequencing of many constructs that change control flow is already 87 // encoded in the `CFG`, this function only implements the sequencing rules 88 // for those constructs where sequencing cannot be inferred from the `CFG`. 89 const Stmt *getSequenceSuccessor(const Stmt *S) const; 90 91 const Stmt *resolveSyntheticStmt(const Stmt *S) const; 92 93 ASTContext *Context; 94 const Stmt *Root; 95 96 llvm::DenseMap<const Stmt *, const Stmt *> SyntheticStmtSourceMap; 97 }; 98 99 /// Maps `Stmt`s to the `CFGBlock` that contains them. Some `Stmt`s may be 100 /// contained in more than one `CFGBlock`; in this case, they are mapped to the 101 /// innermost block (i.e. the one that is furthest from the root of the tree). 102 class StmtToBlockMap { 103 public: 104 /// Initializes the map for the given `CFG`. 105 StmtToBlockMap(const CFG *TheCFG, ASTContext *TheContext); 106 107 /// Returns the block that \p S is contained in. Some `Stmt`s may be contained 108 /// in more than one `CFGBlock`; in this case, this function returns the 109 /// innermost block (i.e. the one that is furthest from the root of the tree). 110 const CFGBlock *blockContainingStmt(const Stmt *S) const; 111 112 private: 113 ASTContext *Context; 114 115 llvm::DenseMap<const Stmt *, const CFGBlock *> Map; 116 }; 117 118 } // namespace clang::tidy::utils 119 120 #endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_EXPRSEQUENCE_H 121