Only copy small types in modernize-loop-convert.

Summary: If the size of the type is above a certain bound, we'll take a const reference. This bound can be set as an option. For now, the default value is 16 bytes.

Reviewers: klimek

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D14176

llvm-svn: 251694

clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.cpp

@@ -230,18 +230,18 @@ StatementMatcher makePseudoArrayLoopMatcher() {
   // FIXME: Also, a record doesn't necessarily need begin() and end(). Free
   // functions called begin() and end() taking the container as an argument
   // are also allowed.
-  TypeMatcher RecordWithBeginEnd = qualType(
-      anyOf(qualType(isConstQualified(),
-                     hasDeclaration(cxxRecordDecl(
-                         hasMethod(cxxMethodDecl(hasName("begin"), isConst())),
-                         hasMethod(cxxMethodDecl(hasName("end"),
-                                                 isConst())))) // hasDeclaration
-                     ),                                        // qualType
-            qualType(unless(isConstQualified()),
-                     hasDeclaration(
-                         cxxRecordDecl(hasMethod(hasName("begin")),
+  TypeMatcher RecordWithBeginEnd = qualType(anyOf(
+      qualType(isConstQualified(),
+               hasDeclaration(cxxRecordDecl(
+                   hasMethod(cxxMethodDecl(hasName("begin"), isConst())),
+                   hasMethod(cxxMethodDecl(hasName("end"),
+                                           isConst())))) // hasDeclaration
+               ),                                        // qualType
+      qualType(
+          unless(isConstQualified()),
+          hasDeclaration(cxxRecordDecl(hasMethod(hasName("begin")),
                                        hasMethod(hasName("end"))))) // qualType
-            ));
+      ));
 
   StatementMatcher SizeCallMatcher = cxxMemberCallExpr(
       argumentCountIs(0),
@@ -409,6 +409,7 @@ LoopConvertCheck::RangeDescriptor::RangeDescriptor()
 
 LoopConvertCheck::LoopConvertCheck(StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context), TUInfo(new TUTrackingInfo),
+      MaxCopySize(std::stoull(Options.get("MaxCopySize", "16"))),
       MinConfidence(StringSwitch<Confidence::Level>(
                         Options.get("MinConfidence", "reasonable"))
                         .Case("safe", Confidence::CL_Safe)
@@ -422,6 +423,7 @@ LoopConvertCheck::LoopConvertCheck(StringRef Name, ClangTidyContext *Context)
                       .Default(VariableNamer::NS_CamelCase)) {}
 
 void LoopConvertCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "MaxCopySize", std::to_string(MaxCopySize));
   SmallVector<std::string, 3> Confs{"risky", "reasonable", "safe"};
   Options.store(Opts, "MinConfidence", Confs[static_cast<int>(MinConfidence)]);
 
@@ -561,18 +563,20 @@ void LoopConvertCheck::doConversion(
   // If the new variable name is from the aliased variable, then the reference
   // type for the new variable should only be used if the aliased variable was
   // declared as a reference.
-  bool IsTriviallyCopyable =
+  bool IsCheapToCopy =
       !Descriptor.ElemType.isNull() &&
-      Descriptor.ElemType.isTriviallyCopyableType(*Context);
+      Descriptor.ElemType.isTriviallyCopyableType(*Context) &&
+      // TypeInfo::Width is in bits.
+      Context->getTypeInfo(Descriptor.ElemType).Width <= 8 * MaxCopySize;
   bool UseCopy =
       CanCopy && ((VarNameFromAlias && !AliasVarIsRef) ||
-                  (Descriptor.DerefByConstRef && IsTriviallyCopyable));
+                  (Descriptor.DerefByConstRef && IsCheapToCopy));
 
   if (!UseCopy) {
     if (Descriptor.DerefByConstRef) {
       Type = Context->getLValueReferenceType(Context->getConstType(Type));
     } else if (Descriptor.DerefByValue) {
-      if (!IsTriviallyCopyable)
+      if (!IsCheapToCopy)
         Type = Context->getRValueReferenceType(Type);
     } else {
       Type = Context->getLValueReferenceType(Type);

clang-tools-extra/clang-tidy/modernize/LoopConvertCheck.h

@@ -66,6 +66,7 @@ private:
                      const ForStmt *Loop, LoopFixerKind FixerKind);
 
   std::unique_ptr<TUTrackingInfo> TUInfo;
+  const unsigned long long MaxCopySize;
   const Confidence::Level MinConfidence;
   const VariableNamer::NamingStyle NamingStyle;
 };

clang-tools-extra/clang-tidy/modernize/ModernizeTidyModule.cpp

@@ -47,6 +47,10 @@ public:
   ClangTidyOptions getModuleOptions() override {
     ClangTidyOptions Options;
     auto &Opts = Options.CheckOptions;
+    // For types whose size in bytes is above this threshold, we prefer taking a
+    // const-reference than making a copy.
+    Opts["modernize-loop-convert.MaxCopySize"] = "16";
+
     Opts["modernize-loop-convert.MinConfidence"] = "reasonable";
     Opts["modernize-loop-convert.NamingStyle"] = "CamelCase";
     Opts["modernize-pass-by-value.IncludeStyle"] = "llvm";    // Also: "google".

clang-tools-extra/test/clang-tidy/Inputs/modernize-loop-convert/structures.h

@@ -23,6 +23,11 @@ struct NonTriviallyCopyable {
   int X;
 };
 
+struct TriviallyCopyableButBig {
+  int X;
+  char Array[16];
+};
+
 struct S {
   typedef MutableVal *iterator;
   typedef const MutableVal *const_iterator;

clang-tools-extra/test/clang-tidy/modernize-loop-convert-basic.cpp

@@ -113,6 +113,14 @@ const int *constArray() {
   // CHECK-FIXES: for (const auto & Elem : NonCopy)
   // CHECK-FIXES-NEXT: printf("2 * %d = %d\n", Elem.X, Elem.X + Elem.X);
 
+  const TriviallyCopyableButBig Big[N]{};
+  for (int I = 0; I < N; ++I) {
+    printf("2 * %d = %d\n", Big[I].X, Big[I].X + Big[I].X);
+  }
+  // CHECK-MESSAGES: :[[@LINE-3]]:3: warning: use range-based for loop instead
+  // CHECK-FIXES: for (const auto & Elem : Big)
+  // CHECK-FIXES-NEXT: printf("2 * %d = %d\n", Elem.X, Elem.X + Elem.X);
+
   bool Something = false;
   for (int I = 0; I < N; ++I) {
     if (Something)