[clang][bytecode] Avoid copies with elidable CXXConstructExprs (#166931)

To fix the newly added cwg6.cpp.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -3235,7 +3235,8 @@ bool Compiler<Emitter>::VisitCXXConstructExpr(const CXXConstructExpr *E) {
       return this->visitInitializer(E->getArg(0));
 
     // Zero initialization.
-    if (E->requiresZeroInitialization()) {
+    bool ZeroInit = E->requiresZeroInitialization();
+    if (ZeroInit) {
       const Record *R = getRecord(E->getType());
 
       if (!this->visitZeroRecordInitializer(R, E))
@@ -3246,6 +3247,19 @@ bool Compiler<Emitter>::VisitCXXConstructExpr(const CXXConstructExpr *E) {
         return true;
     }
 
+    // Avoid materializing a temporary for an elidable copy/move constructor.
+    if (!ZeroInit && E->isElidable()) {
+      const Expr *SrcObj = E->getArg(0);
+      assert(SrcObj->isTemporaryObject(Ctx.getASTContext(), Ctor->getParent()));
+      assert(Ctx.getASTContext().hasSameUnqualifiedType(E->getType(),
+                                                        SrcObj->getType()));
+      if (const auto *ME = dyn_cast<MaterializeTemporaryExpr>(SrcObj)) {
+        if (!this->emitCheckFunctionDecl(Ctor, E))
+          return false;
+        return this->visitInitializer(ME->getSubExpr());
+      }
+    }
+
     const Function *Func = getFunction(Ctor);
 
     if (!Func)

clang/lib/AST/ByteCode/Interp.cpp

@@ -919,45 +919,19 @@ bool CheckInit(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   return true;
 }
 
-static bool CheckCallable(InterpState &S, CodePtr OpPC, const Function *F) {
-
-  if (F->isVirtual() && !S.getLangOpts().CPlusPlus20) {
-    const SourceLocation &Loc = S.Current->getLocation(OpPC);
-    S.CCEDiag(Loc, diag::note_constexpr_virtual_call);
-    return false;
-  }
-
-  if (S.checkingPotentialConstantExpression() && S.Current->getDepth() != 0)
-    return false;
-
-  if (F->isValid() && F->hasBody() && F->isConstexpr())
-    return true;
-
-  const FunctionDecl *DiagDecl = F->getDecl();
-  const FunctionDecl *Definition = nullptr;
-  DiagDecl->getBody(Definition);
-
-  if (!Definition && S.checkingPotentialConstantExpression() &&
-      DiagDecl->isConstexpr()) {
-    return false;
-  }
-
-  // Implicitly constexpr.
-  if (F->isLambdaStaticInvoker())
-    return true;
-
+static bool diagnoseCallableDecl(InterpState &S, CodePtr OpPC,
+                                 const FunctionDecl *DiagDecl) {
   // Bail out if the function declaration itself is invalid.  We will
   // have produced a relevant diagnostic while parsing it, so just
   // note the problematic sub-expression.
   if (DiagDecl->isInvalidDecl())
     return Invalid(S, OpPC);
 
   // Diagnose failed assertions specially.
-  if (S.Current->getLocation(OpPC).isMacroID() &&
-      F->getDecl()->getIdentifier()) {
+  if (S.Current->getLocation(OpPC).isMacroID() && DiagDecl->getIdentifier()) {
     // FIXME: Instead of checking for an implementation-defined function,
     // check and evaluate the assert() macro.
-    StringRef Name = F->getDecl()->getName();
+    StringRef Name = DiagDecl->getName();
     bool AssertFailed =
         Name == "__assert_rtn" || Name == "__assert_fail" || Name == "_wassert";
     if (AssertFailed) {
@@ -1004,7 +978,7 @@ static bool CheckCallable(InterpState &S, CodePtr OpPC, const Function *F) {
     // for a constant expression. It might be defined at the point we're
     // actually calling it.
     bool IsExtern = DiagDecl->getStorageClass() == SC_Extern;
-    bool IsDefined = F->isDefined();
+    bool IsDefined = DiagDecl->isDefined();
     if (!IsDefined && !IsExtern && DiagDecl->isConstexpr() &&
         S.checkingPotentialConstantExpression())
       return false;
@@ -1027,6 +1001,35 @@ static bool CheckCallable(InterpState &S, CodePtr OpPC, const Function *F) {
   return false;
 }
 
+static bool CheckCallable(InterpState &S, CodePtr OpPC, const Function *F) {
+  if (F->isVirtual() && !S.getLangOpts().CPlusPlus20) {
+    const SourceLocation &Loc = S.Current->getLocation(OpPC);
+    S.CCEDiag(Loc, diag::note_constexpr_virtual_call);
+    return false;
+  }
+
+  if (S.checkingPotentialConstantExpression() && S.Current->getDepth() != 0)
+    return false;
+
+  if (F->isValid() && F->hasBody() && F->isConstexpr())
+    return true;
+
+  const FunctionDecl *DiagDecl = F->getDecl();
+  const FunctionDecl *Definition = nullptr;
+  DiagDecl->getBody(Definition);
+
+  if (!Definition && S.checkingPotentialConstantExpression() &&
+      DiagDecl->isConstexpr()) {
+    return false;
+  }
+
+  // Implicitly constexpr.
+  if (F->isLambdaStaticInvoker())
+    return true;
+
+  return diagnoseCallableDecl(S, OpPC, DiagDecl);
+}
+
 static bool CheckCallDepth(InterpState &S, CodePtr OpPC) {
   if ((S.Current->getDepth() + 1) > S.getLangOpts().ConstexprCallDepth) {
     S.FFDiag(S.Current->getSource(OpPC),
@@ -1500,6 +1503,21 @@ bool CheckDestructor(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   return CheckActive(S, OpPC, Ptr, AK_Destroy);
 }
 
+/// Opcode. Check if the function decl can be called at compile time.
+bool CheckFunctionDecl(InterpState &S, CodePtr OpPC, const FunctionDecl *FD) {
+  if (S.checkingPotentialConstantExpression() && S.Current->getDepth() != 0)
+    return false;
+
+  const FunctionDecl *Definition = nullptr;
+  const Stmt *Body = FD->getBody(Definition);
+
+  if (Definition && Body &&
+      (Definition->isConstexpr() || Definition->hasAttr<MSConstexprAttr>()))
+    return true;
+
+  return diagnoseCallableDecl(S, OpPC, FD);
+}
+
 static void compileFunction(InterpState &S, const Function *Func) {
   const FunctionDecl *Definition = Func->getDecl()->getDefinition();
   if (!Definition)

clang/lib/AST/ByteCode/Interp.h

@@ -117,6 +117,7 @@ bool CheckBitCast(InterpState &S, CodePtr OpPC, bool HasIndeterminateBits,
                   bool TargetIsUCharOrByte);
 bool CheckBCPResult(InterpState &S, const Pointer &Ptr);
 bool CheckDestructor(InterpState &S, CodePtr OpPC, const Pointer &Ptr);
+bool CheckFunctionDecl(InterpState &S, CodePtr OpPC, const FunctionDecl *FD);
 
 bool handleFixedPointOverflow(InterpState &S, CodePtr OpPC,
                               const FixedPoint &FP);

clang/lib/AST/ByteCode/Opcodes.td

@@ -53,6 +53,7 @@ def ArgBool : ArgType { let Name = "bool"; }
 def ArgFixedPoint : ArgType { let Name = "FixedPoint"; let AsRef = true; }
 
 def ArgFunction : ArgType { let Name = "const Function *"; }
+def ArgFunctionDecl : ArgType { let Name = "const FunctionDecl *"; }
 def ArgRecordDecl : ArgType { let Name = "const RecordDecl *"; }
 def ArgRecordField : ArgType { let Name = "const Record::Field *"; }
 def ArgFltSemantics : ArgType { let Name = "const llvm::fltSemantics *"; }
@@ -422,6 +423,8 @@ def CheckLiteralType : Opcode {
 
 def CheckArraySize : Opcode { let Args = [ArgUint64]; }
 
+def CheckFunctionDecl : Opcode { let Args = [ArgFunctionDecl]; }
+
 // [] -> [Value]
 def GetGlobal : AccessOpcode;
 def GetGlobalUnchecked : AccessOpcode;

clang/test/AST/ByteCode/cxx11.cpp

@@ -379,3 +379,11 @@ namespace DiscardedAddrLabel {
   }
 }
 
+struct Counter {
+  int copies;
+  constexpr Counter(int copies) : copies(copies) {}
+  constexpr Counter(const Counter& other) : copies(other.copies + 1) {}
+};
+// Passing an lvalue by value makes a non-elidable copy.
+constexpr int PassByValue(Counter c) { return c.copies; }
+static_assert(PassByValue(Counter(0)) == 0, "expect no copies");

clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -fcxx-exceptions %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify -fcxx-exceptions %s -fexperimental-new-constant-interpreter
 
 // An explicitly-defaulted function may be declared constexpr only if it would
 // have been implicitly declared as constexpr.