[clang][bytecode] Implement placement-new (llvm#107033)

If we have a placement-new destination already, use that instead of
allocating a new one.
Tests are partially based on
`test/SemaCXX/cxx2c-constexpr-placement-new.cpp`.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -3097,12 +3097,11 @@ bool Compiler<Emitter>::VisitCXXNewExpr(const CXXNewExpr *E) {
   QualType ElementType = E->getAllocatedType();
   std::optional<PrimType> ElemT = classify(ElementType);
   unsigned PlacementArgs = E->getNumPlacementArgs();
+  const FunctionDecl *OperatorNew = E->getOperatorNew();
+  const Expr *PlacementDest = nullptr;
   bool IsNoThrow = false;
 
-  // FIXME: Better diagnostic. diag::note_constexpr_new_placement
   if (PlacementArgs != 0) {
-    // The only new-placement list we support is of the form (std::nothrow).
-    //
     // FIXME: There is no restriction on this, but it's not clear that any
     // other form makes any sense. We get here for cases such as:
     //
@@ -3111,27 +3110,43 @@ bool Compiler<Emitter>::VisitCXXNewExpr(const CXXNewExpr *E) {
     // (which should presumably be valid only if N is a multiple of
     // alignof(int), and in any case can't be deallocated unless N is
     // alignof(X) and X has new-extended alignment).
-    if (PlacementArgs != 1 || !E->getPlacementArg(0)->getType()->isNothrowT())
-      return this->emitInvalid(E);
+    if (PlacementArgs == 1) {
+      const Expr *Arg1 = E->getPlacementArg(0);
+      if (Arg1->getType()->isNothrowT()) {
+        if (!this->discard(Arg1))
+          return false;
+        IsNoThrow = true;
+      } else if (Ctx.getLangOpts().CPlusPlus26 &&
+                 OperatorNew->isReservedGlobalPlacementOperator()) {
+        // If we have a placement-new destination, we'll later use that instead
+        // of allocating.
+        PlacementDest = Arg1;
+      } else {
+        return this->emitInvalidNewDeleteExpr(E, E);
+      }
 
-    if (!this->discard(E->getPlacementArg(0)))
-      return false;
-    IsNoThrow = true;
+    } else {
+      return this->emitInvalid(E);
+    }
+  } else if (!OperatorNew->isReplaceableGlobalAllocationFunction()) {
+    return this->emitInvalidNewDeleteExpr(E, E);
   }
 
   const Descriptor *Desc;
-  if (ElemT) {
-    if (E->isArray())
-      Desc = nullptr; // We're not going to use it in this case.
-    else
-      Desc = P.createDescriptor(E, *ElemT, Descriptor::InlineDescMD,
-                                /*IsConst=*/false, /*IsTemporary=*/false,
-                                /*IsMutable=*/false);
-  } else {
-    Desc = P.createDescriptor(
-        E, ElementType.getTypePtr(),
-        E->isArray() ? std::nullopt : Descriptor::InlineDescMD,
-        /*IsConst=*/false, /*IsTemporary=*/false, /*IsMutable=*/false, Init);
+  if (!PlacementDest) {
+    if (ElemT) {
+      if (E->isArray())
+        Desc = nullptr; // We're not going to use it in this case.
+      else
+        Desc = P.createDescriptor(E, *ElemT, Descriptor::InlineDescMD,
+                                  /*IsConst=*/false, /*IsTemporary=*/false,
+                                  /*IsMutable=*/false);
+    } else {
+      Desc = P.createDescriptor(
+          E, ElementType.getTypePtr(),
+          E->isArray() ? std::nullopt : Descriptor::InlineDescMD,
+          /*IsConst=*/false, /*IsTemporary=*/false, /*IsMutable=*/false, Init);
+    }
   }
 
   if (E->isArray()) {
@@ -3148,26 +3163,42 @@ bool Compiler<Emitter>::VisitCXXNewExpr(const CXXNewExpr *E) {
 
     PrimType SizeT = classifyPrim(Stripped->getType());
 
-    if (!this->visit(Stripped))
-      return false;
-
-    if (ElemT) {
-      // N primitive elements.
-      if (!this->emitAllocN(SizeT, *ElemT, E, IsNoThrow, E))
+    if (PlacementDest) {
+      if (!this->visit(PlacementDest))
+        return false;
+      if (!this->visit(Stripped))
+        return false;
+      if (!this->emitCheckNewTypeMismatchArray(SizeT, E, E))
         return false;
     } else {
-      // N Composite elements.
-      if (!this->emitAllocCN(SizeT, Desc, IsNoThrow, E))
+      if (!this->visit(Stripped))
         return false;
+
+      if (ElemT) {
+        // N primitive elements.
+        if (!this->emitAllocN(SizeT, *ElemT, E, IsNoThrow, E))
+          return false;
+      } else {
+        // N Composite elements.
+        if (!this->emitAllocCN(SizeT, Desc, IsNoThrow, E))
+          return false;
+      }
     }
 
     if (Init && !this->visitInitializer(Init))
       return false;
 
   } else {
-    // Allocate just one element.
-    if (!this->emitAlloc(Desc, E))
-      return false;
+    if (PlacementDest) {
+      if (!this->visit(PlacementDest))
+        return false;
+      if (!this->emitCheckNewTypeMismatch(E, E))
+        return false;
+    } else {
+      // Allocate just one element.
+      if (!this->emitAlloc(Desc, E))
+        return false;
+    }
 
     if (Init) {
       if (ElemT) {
@@ -3194,6 +3225,11 @@ template <class Emitter>
 bool Compiler<Emitter>::VisitCXXDeleteExpr(const CXXDeleteExpr *E) {
   const Expr *Arg = E->getArgument();
 
+  const FunctionDecl *OperatorDelete = E->getOperatorDelete();
+
+  if (!OperatorDelete->isReplaceableGlobalAllocationFunction())
+    return this->emitInvalidNewDeleteExpr(E, E);
+
   // Arg must be an lvalue.
   if (!this->visit(Arg))
     return false;

clang/lib/AST/ByteCode/Interp.cpp

@@ -1286,6 +1286,80 @@ bool CallPtr(InterpState &S, CodePtr OpPC, uint32_t ArgSize,
   return Call(S, OpPC, F, VarArgSize);
 }
 
+bool CheckNewTypeMismatch(InterpState &S, CodePtr OpPC, const Expr *E,
+                          std::optional<uint64_t> ArraySize) {
+  const Pointer &Ptr = S.Stk.peek<Pointer>();
+
+  if (!CheckStore(S, OpPC, Ptr))
+    return false;
+
+  const auto *NewExpr = cast<CXXNewExpr>(E);
+  QualType StorageType = Ptr.getType();
+
+  if (isa_and_nonnull<CXXNewExpr>(Ptr.getFieldDesc()->asExpr())) {
+    // FIXME: Are there other cases where this is a problem?
+    StorageType = StorageType->getPointeeType();
+  }
+
+  const ASTContext &ASTCtx = S.getASTContext();
+  QualType AllocType;
+  if (ArraySize) {
+    AllocType = ASTCtx.getConstantArrayType(
+        NewExpr->getAllocatedType(),
+        APInt(64, static_cast<uint64_t>(*ArraySize), false), nullptr,
+        ArraySizeModifier::Normal, 0);
+  } else {
+    AllocType = NewExpr->getAllocatedType();
+  }
+
+  unsigned StorageSize = 1;
+  unsigned AllocSize = 1;
+  if (const auto *CAT = dyn_cast<ConstantArrayType>(AllocType))
+    AllocSize = CAT->getZExtSize();
+  if (const auto *CAT = dyn_cast<ConstantArrayType>(StorageType))
+    StorageSize = CAT->getZExtSize();
+
+  if (AllocSize > StorageSize ||
+      !ASTCtx.hasSimilarType(ASTCtx.getBaseElementType(AllocType),
+                             ASTCtx.getBaseElementType(StorageType))) {
+    S.FFDiag(S.Current->getLocation(OpPC),
+             diag::note_constexpr_placement_new_wrong_type)
+        << StorageType << AllocType;
+    return false;
+  }
+  return true;
+}
+
+bool InvalidNewDeleteExpr(InterpState &S, CodePtr OpPC, const Expr *E) {
+  assert(E);
+  const auto &Loc = S.Current->getSource(OpPC);
+
+  if (const auto *NewExpr = dyn_cast<CXXNewExpr>(E)) {
+    const FunctionDecl *OperatorNew = NewExpr->getOperatorNew();
+
+    if (!S.getLangOpts().CPlusPlus26 && NewExpr->getNumPlacementArgs() > 0) {
+      S.FFDiag(Loc, diag::note_constexpr_new_placement)
+          << /*C++26 feature*/ 1 << E->getSourceRange();
+    } else if (NewExpr->getNumPlacementArgs() == 1 &&
+               !OperatorNew->isReservedGlobalPlacementOperator()) {
+      S.FFDiag(Loc, diag::note_constexpr_new_placement)
+          << /*Unsupported*/ 0 << E->getSourceRange();
+    } else if (!OperatorNew->isReplaceableGlobalAllocationFunction()) {
+      S.FFDiag(Loc, diag::note_constexpr_new_non_replaceable)
+          << isa<CXXMethodDecl>(OperatorNew) << OperatorNew;
+    }
+  } else {
+    const auto *DeleteExpr = cast<CXXDeleteExpr>(E);
+    const FunctionDecl *OperatorDelete = DeleteExpr->getOperatorDelete();
+    if (!OperatorDelete->isReplaceableGlobalAllocationFunction()) {
+      S.FFDiag(Loc, diag::note_constexpr_new_non_replaceable)
+          << isa<CXXMethodDecl>(OperatorDelete) << OperatorDelete;
+    }
+  }
+
+  return false;
+}
+
 bool Interpret(InterpState &S, APValue &Result) {
   // The current stack frame when we started Interpret().
   // This is being used by the ops to determine wheter

clang/lib/AST/ByteCode/Interp.h

@@ -2947,6 +2947,17 @@ static inline bool IsConstantContext(InterpState &S, CodePtr OpPC) {
   return true;
 }
 
+/// Check if the initializer and storage types of a placement-new expression
+/// match.
+bool CheckNewTypeMismatch(InterpState &S, CodePtr OpPC, const Expr *E,
+                          std::optional<uint64_t> ArraySize = std::nullopt);
+
+template <PrimType Name, class T = typename PrimConv<Name>::T>
+bool CheckNewTypeMismatchArray(InterpState &S, CodePtr OpPC, const Expr *E) {
+  const auto &Size = S.Stk.pop<T>();
+  return CheckNewTypeMismatch(S, OpPC, E, static_cast<uint64_t>(Size));
+}
+bool InvalidNewDeleteExpr(InterpState &S, CodePtr OpPC, const Expr *E);
 //===----------------------------------------------------------------------===//
 // Read opcode arguments
 //===----------------------------------------------------------------------===//

clang/lib/AST/ByteCode/Opcodes.td

@@ -787,4 +787,18 @@ def Free : Opcode {
   let Args = [ArgBool];
 }
 
+def CheckNewTypeMismatch : Opcode {
+  let Args = [ArgExpr];
+}
+
+def InvalidNewDeleteExpr : Opcode {
+  let Args = [ArgExpr];
+}
+
+def CheckNewTypeMismatchArray : Opcode {
+  let Types = [IntegerTypeClass];
+  let Args = [ArgExpr];
+  let HasGroup = 1;
+}
+
 def IsConstantContext: Opcode;

clang/test/AST/ByteCode/new-delete.cpp

@@ -241,12 +241,10 @@ namespace std {
 
 
 
-/// FIXME: The new interpreter produces the wrong diagnostic.
 namespace PlacementNew {
   constexpr int foo() { // both-error {{never produces a constant expression}}
     char c[sizeof(int)];
-    new (c) int{12}; // ref-note {{this placement new expression is not supported in constant expressions before C++2c}} \
-                     // expected-note {{subexpression not valid in a constant expression}}
+    new (c) int{12}; // both-note {{this placement new expression is not supported in constant expressions before C++2c}}
     return 0;
   }
 }
@@ -305,41 +303,38 @@ namespace placement_new_delete {
   }
   static_assert(ok());
 
-  /// FIXME: Diagnosting placement new.
   constexpr bool bad(int which) {
     switch (which) {
     case 0:
-      delete new (placement_new_arg{}) int; // ref-note {{this placement new expression is not supported in constant expressions}} \
-                                            // expected-note {{subexpression not valid in a constant expression}}
+      delete new (placement_new_arg{}) int; // both-note {{this placement new expression is not supported in constant expressions}}
       break;
 
     case 1:
-      delete new ClassSpecificNew; // ref-note {{call to class-specific 'operator new'}}
+      delete new ClassSpecificNew; // both-note {{call to class-specific 'operator new'}}
       break;
 
     case 2:
-      delete new ClassSpecificDelete; // ref-note {{call to class-specific 'operator delete'}}
+      delete new ClassSpecificDelete; // both-note {{call to class-specific 'operator delete'}}
       break;
 
     case 3:
-      delete new DestroyingDelete; // ref-note {{call to class-specific 'operator delete'}}
+      delete new DestroyingDelete; // both-note {{call to class-specific 'operator delete'}}
       break;
 
     case 4:
       // FIXME: This technically follows the standard's rules, but it seems
       // unreasonable to expect implementations to support this.
-      delete new (std::align_val_t{64}) Overaligned; // ref-note {{this placement new expression is not supported in constant expressions}} \
-                                                     // expected-note {{subexpression not valid in a constant expression}}
+      delete new (std::align_val_t{64}) Overaligned; // both-note {{this placement new expression is not supported in constant expressions}}
       break;
     }
 
     return true;
   }
   static_assert(bad(0)); // both-error {{constant expression}} \
                          // both-note {{in call}}
-  static_assert(bad(1)); // ref-error {{constant expression}} ref-note {{in call}}
-  static_assert(bad(2)); // ref-error {{constant expression}} ref-note {{in call}}
-  static_assert(bad(3)); // ref-error {{constant expression}} ref-note {{in call}}
+  static_assert(bad(1)); // both-error {{constant expression}} both-note {{in call}}
+  static_assert(bad(2)); // both-error {{constant expression}} both-note {{in call}}
+  static_assert(bad(3)); // both-error {{constant expression}} both-note {{in call}}
   static_assert(bad(4)); // both-error {{constant expression}} \
                          // both-note {{in call}}
 }
@@ -586,7 +581,6 @@ constexpr void use_after_free_2() { // both-error {{never produces a constant ex
   p->f(); // both-note {{member call on heap allocated object that has been deleted}}
 }
 
-
 /// std::allocator definition
 namespace std {
   using size_t = decltype(sizeof(0));
@@ -758,6 +752,18 @@ namespace Limits {
 #endif
 }
 
+/// Just test that we reject placement-new expressions before C++2c.
+/// Tests for successful expressions are in placement-new.cpp
+namespace Placement {
+  consteval auto ok1() { // both-error {{never produces a constant expression}}
+    bool b;
+    new (&b) bool(true); // both-note 2{{this placement new expression is not supported in constant expressions before C++2c}}
+    return b;
+  }
+  static_assert(ok1()); // both-error {{not an integral constant expression}} \
+                        // both-note {{in call to}}
+}
+
 #else
 /// Make sure we reject this prior to C++20
 constexpr int a() { // both-error {{never produces a constant expression}}