[clang-tidy] Do not move parameter if only DeclRefExpr occurs inside of a loop

[flx]

Summary

This fixes a bug where the performance-unnecessary-value-param check suggests a fix to move the parameter inside of a loop which could be invoked multiple times.

[alexfh]

clang-tidy/utils/DeclRefExprUtils.cpp

127 ↗

(On Diff #79497)

How will this work with lambdas / local classes declared inside a loop? Not sure if this case is going to happen in real code, but we'd better be clear about the limitations of the implementation.

129 ↗

(On Diff #79497)

Do you actually need to bind the node to "declRef"?

[flx]

clang-tidy/utils/DeclRefExprUtils.cpp

127 ↗

(On Diff #79497)

Why would this not work? Could you give an example? The way the function is written it handles my the use case for identifying when moving the parameter is not safe, so I could also just move it into the UnnecessaryValueParamCheck.

[alexfh]

clang-tidy/utils/DeclRefExprUtils.cpp

127 ↗

(On Diff #79497)

I was thinking about a case where a loop this matcher finds is outside of the function definition and shouldn't be considered:

void F() {
  for (;;) {
    struct C {
      void f(ExpensiveMovableType E) {
        auto F = E;
      }
    };
  }
}

[flx]

clang-tidy/utils/DeclRefExprUtils.cpp

127 ↗

(On Diff #79497)

This case is not an issue in the check as we're passing f's body statement to the hasLoopStmtAncestor function, so the search is scoped to f's body.

If you're concerned about the case where someone calls this with F's body but expects it to be scoped to f I can just move this function into the check and make it an implementation detail.

[flx]

Do you have any more comments, Alex?

[alexfh]

LG modulo comment.

clang-tidy/utils/DeclRefExprUtils.cpp

127 ↗

(On Diff #79497)

Making it an implementation detail until other potential users appear sounds good.

[alexfh]

Thanks, this is fine to commit.

clang-tidy/performance/UnnecessaryValueParamCheck.cpp

 bool isReferencedOutsideOfCallExpr(const FunctionDecl &Function,
                                    ASTContext &Context) {
   auto Matches = match(declRefExpr(to(functionDecl(equalsNode(&Function))),
                                    unless(hasAncestor(callExpr()))),
                        Context);
   return !Matches.empty();
 }
 
+bool hasLoopStmtAncestor(const DeclRefExpr &DeclRef, const Stmt &Stmt,
+                         ASTContext &Context) {
+  auto Matches =
+      match(findAll(declRefExpr(
+                equalsNode(&DeclRef),
+                unless(hasAncestor(stmt(anyOf(forStmt(), cxxForRangeStmt(),
+                                              whileStmt(), doStmt())))))),
+            Stmt, Context);
+  return Matches.empty();
+}
+
 } // namespace
 
 UnnecessaryValueParamCheck::UnnecessaryValueParamCheck(
     StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       IncludeStyle(utils::IncludeSorter::parseIncludeStyle(
           Options.get("IncludeStyle", "llvm"))) {}
 
void UnnecessaryValueParamCheck::check(const MatchFinder::MatchResult &Result) {
   // If the parameter is non-const, check if it has a move constructor and is
   // only referenced once to copy-construct another object or whether it has a
   // move assignment operator and is only referenced once when copy-assigned.
   // In this case wrap DeclRefExpr with std::move() to avoid the unnecessary
   // copy.
   if (!IsConstQualified) {
     auto CanonicalType = Param->getType().getCanonicalType();
     if (AllDeclRefExprs.size() == 1 &&
+        !hasLoopStmtAncestor(**AllDeclRefExprs.begin(), *Function->getBody(),
+                             *Result.Context) &&
         ((utils::type_traits::hasNonTrivialMoveConstructor(CanonicalType) &&
           utils::decl_ref_expr::isCopyConstructorArgument(
               **AllDeclRefExprs.begin(), *Function->getBody(),
               *Result.Context)) ||
          (utils::type_traits::hasNonTrivialMoveAssignment(CanonicalType) &&
           utils::decl_ref_expr::isCopyAssignmentArgument(
               **AllDeclRefExprs.begin(), *Function->getBody(),
               *Result.Context)))) {

test/clang-tidy/performance-unnecessary-value-param.cpp

 
 void PositiveMoveOnCopyAssignment(ExpensiveMovableType E) {
   ExpensiveMovableType F;
   F = E;
   // CHECK-MESSAGES: [[@LINE-1]]:7: warning: parameter 'E' is passed by value
   // CHECK-FIXES: F = std::move(E);
 }
 
+// The argument could be moved but is not since copy statement is inside a loop.
+void PositiveNoMoveInsideLoop(ExpensiveMovableType E) {
+  // CHECK-MESSAGES: [[@LINE-1]]:52: warning: the parameter 'E' is copied
+  // CHECK-FIXES: void PositiveNoMoveInsideLoop(const ExpensiveMovableType& E) {
+  for (;;) {
+    auto F = E;
+  }
+}
+
 void PositiveConstRefNotMoveConstructible(ExpensiveToCopyType T) {
   // CHECK-MESSAGES: [[@LINE-1]]:63: warning: the parameter 'T' is copied
   // CHECK-FIXES: void PositiveConstRefNotMoveConstructible(const ExpensiveToCopyType& T) {
   auto U = T;
 }
 
 void PositiveConstRefNotMoveAssignable(ExpensiveToCopyType A) {
   // CHECK-MESSAGES: [[@LINE-1]]:60: warning: the parameter 'A' is copied