[CIR] Handle non-empty null base class initialization (#168646)

This implements null base class initialization for non-empty bases.

Author: andykaylor

clang/include/clang/CIR/Dialect/IR/CIRTypes.td

@@ -657,6 +657,9 @@ def CIR_RecordType : CIR_Type<"Record", "record", [
       }
       llvm_unreachable("Invalid value for RecordType::getKind()");
     }
+    mlir::Type getElementType(size_t idx) {
+      return getMembers()[idx];
+    }
     std::string getPrefixedName() {
       return getKindAsStr() + "." + getName().getValue().str();
     }

clang/lib/CIR/CodeGen/CIRGenBuilder.h

@@ -212,6 +212,16 @@ class CIRGenBuilderTy : public cir::CIRBaseBuilderTy {
                  &ignored);
       return fv.bitwiseIsEqual(fpVal);
     }
+    if (const auto recordVal = mlir::dyn_cast<cir::ConstRecordAttr>(attr)) {
+      for (const auto elt : recordVal.getMembers()) {
+        // FIXME(cir): the record's ID should not be considered a member.
+        if (mlir::isa<mlir::StringAttr>(elt))
+          continue;
+        if (!isNullValue(elt))
+          return false;
+      }
+      return true;
+    }
 
     if (const auto arrayVal = mlir::dyn_cast<cir::ConstArrayAttr>(attr)) {
       if (mlir::isa<mlir::StringAttr>(arrayVal.getElts()))

clang/lib/CIR/CodeGen/CIRGenExprCXX.cpp

@@ -240,8 +240,46 @@ static void emitNullBaseClassInitialization(CIRGenFunction &cgf,
   if (base->isEmpty())
     return;
 
-  cgf.cgm.errorNYI(base->getSourceRange(),
-                   "emitNullBaseClassInitialization: not empty");
+  const ASTRecordLayout &layout = cgf.getContext().getASTRecordLayout(base);
+  CharUnits nvSize = layout.getNonVirtualSize();
+
+  // We cannot simply zero-initialize the entire base sub-object if vbptrs are
+  // present, they are initialized by the most derived class before calling the
+  // constructor.
+  SmallVector<std::pair<CharUnits, CharUnits>, 1> stores;
+  stores.emplace_back(CharUnits::Zero(), nvSize);
+
+  // Each store is split by the existence of a vbptr.
+  // TODO(cir): This only needs handling for the MS CXXABI.
+  assert(!cir::MissingFeatures::msabi());
+
+  // If the type contains a pointer to data member we can't memset it to zero.
+  // Instead, create a null constant and copy it to the destination.
+  // TODO: there are other patterns besides zero that we can usefully memset,
+  // like -1, which happens to be the pattern used by member-pointers.
+  // TODO: isZeroInitializable can be over-conservative in the case where a
+  // virtual base contains a member pointer.
+  mlir::TypedAttr nullConstantForBase = cgf.cgm.emitNullConstantForBase(base);
+  if (!cgf.getBuilder().isNullValue(nullConstantForBase)) {
+    cgf.cgm.errorNYI(
+        base->getSourceRange(),
+        "emitNullBaseClassInitialization: base constant is not null");
+  } else {
+    // Otherwise, just memset the whole thing to zero.  This is legal
+    // because in LLVM, all default initializers (other than the ones we just
+    // handled above) are guaranteed to have a bit pattern of all zeros.
+    // TODO(cir): When the MS CXXABI is supported, we will need to iterate over
+    // `stores` and create a separate memset for each one. For now, we know that
+    // there will only be one store and it will begin at offset zero, so that
+    // simplifies this code considerably.
+    assert(stores.size() == 1 && "Expected only one store");
+    assert(stores[0].first == CharUnits::Zero() &&
+           "Expected store to begin at offset zero");
+    CIRGenBuilderTy builder = cgf.getBuilder();
+    mlir::Location loc = cgf.getLoc(base->getBeginLoc());
+    builder.createStore(loc, builder.getConstant(loc, nullConstantForBase),
+                        destPtr);
+  }
 }
 
 void CIRGenFunction::emitCXXConstructExpr(const CXXConstructExpr *e,

clang/lib/CIR/CodeGen/CIRGenExprConstant.cpp

@@ -1521,6 +1521,101 @@ ConstantEmitter::~ConstantEmitter() {
          "not finalized after being initialized for non-abstract emission");
 }
 
+static mlir::TypedAttr emitNullConstantForBase(CIRGenModule &cgm,
+                                               mlir::Type baseType,
+                                               const CXXRecordDecl *baseDecl);
+
+static mlir::TypedAttr emitNullConstant(CIRGenModule &cgm, const RecordDecl *rd,
+                                        bool asCompleteObject) {
+  const CIRGenRecordLayout &layout = cgm.getTypes().getCIRGenRecordLayout(rd);
+  mlir::Type ty = (asCompleteObject ? layout.getCIRType()
+                                    : layout.getBaseSubobjectCIRType());
+  auto recordTy = mlir::cast<cir::RecordType>(ty);
+
+  unsigned numElements = recordTy.getNumElements();
+  SmallVector<mlir::Attribute> elements(numElements);
+
+  auto *cxxrd = dyn_cast<CXXRecordDecl>(rd);
+  // Fill in all the bases.
+  if (cxxrd) {
+    for (const CXXBaseSpecifier &base : cxxrd->bases()) {
+      if (base.isVirtual()) {
+        // Ignore virtual bases; if we're laying out for a complete
+        // object, we'll lay these out later.
+        continue;
+      }
+
+      const auto *baseDecl = base.getType()->castAsCXXRecordDecl();
+      // Ignore empty bases.
+      if (isEmptyRecordForLayout(cgm.getASTContext(), base.getType()) ||
+          cgm.getASTContext()
+              .getASTRecordLayout(baseDecl)
+              .getNonVirtualSize()
+              .isZero())
+        continue;
+
+      unsigned fieldIndex = layout.getNonVirtualBaseCIRFieldNo(baseDecl);
+      mlir::Type baseType = recordTy.getElementType(fieldIndex);
+      elements[fieldIndex] = emitNullConstantForBase(cgm, baseType, baseDecl);
+    }
+  }
+
+  // Fill in all the fields.
+  for (const FieldDecl *field : rd->fields()) {
+    // Fill in non-bitfields. (Bitfields always use a zero pattern, which we
+    // will fill in later.)
+    if (!field->isBitField() &&
+        !isEmptyFieldForLayout(cgm.getASTContext(), field)) {
+      unsigned fieldIndex = layout.getCIRFieldNo(field);
+      elements[fieldIndex] = cgm.emitNullConstantAttr(field->getType());
+    }
+
+    // For unions, stop after the first named field.
+    if (rd->isUnion()) {
+      if (field->getIdentifier())
+        break;
+      if (const auto *fieldRD = field->getType()->getAsRecordDecl())
+        if (fieldRD->findFirstNamedDataMember())
+          break;
+    }
+  }
+
+  // Fill in the virtual bases, if we're working with the complete object.
+  if (cxxrd && asCompleteObject) {
+    for ([[maybe_unused]] const CXXBaseSpecifier &vbase : cxxrd->vbases()) {
+      cgm.errorNYI(vbase.getSourceRange(), "emitNullConstant: virtual base");
+      return {};
+    }
+  }
+
+  // Now go through all other fields and zero them out.
+  for (unsigned i = 0; i != numElements; ++i) {
+    if (!elements[i]) {
+      cgm.errorNYI(rd->getSourceRange(), "emitNullConstant: field not zeroed");
+      return {};
+    }
+  }
+
+  mlir::MLIRContext *mlirContext = recordTy.getContext();
+  return cir::ConstRecordAttr::get(recordTy,
+                                   mlir::ArrayAttr::get(mlirContext, elements));
+}
+
+/// Emit the null constant for a base subobject.
+static mlir::TypedAttr emitNullConstantForBase(CIRGenModule &cgm,
+                                               mlir::Type baseType,
+                                               const CXXRecordDecl *baseDecl) {
+  const CIRGenRecordLayout &baseLayout =
+      cgm.getTypes().getCIRGenRecordLayout(baseDecl);
+
+  // Just zero out bases that don't have any pointer to data members.
+  if (baseLayout.isZeroInitializableAsBase())
+    return cgm.getBuilder().getZeroInitAttr(baseType);
+
+  // Otherwise, we can just use its null constant.
+  return emitNullConstant(cgm, baseDecl, /*asCompleteObject=*/false);
+}
+
 mlir::Attribute ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &d) {
   // Make a quick check if variable can be default NULL initialized
   // and avoid going through rest of code which may do, for c++11,
@@ -1820,23 +1915,32 @@ mlir::Attribute ConstantEmitter::tryEmitPrivate(const APValue &value,
 }
 
 mlir::Value CIRGenModule::emitNullConstant(QualType t, mlir::Location loc) {
-  if (t->getAs<PointerType>()) {
-    return builder.getNullPtr(getTypes().convertTypeForMem(t), loc);
-  }
+  return builder.getConstant(loc, emitNullConstantAttr(t));
+}
+
+mlir::TypedAttr CIRGenModule::emitNullConstantAttr(QualType t) {
+  if (t->getAs<PointerType>())
+    return builder.getConstNullPtrAttr(getTypes().convertTypeForMem(t));
 
   if (getTypes().isZeroInitializable(t))
-    return builder.getNullValue(getTypes().convertTypeForMem(t), loc);
+    return builder.getZeroInitAttr(getTypes().convertTypeForMem(t));
 
   if (getASTContext().getAsConstantArrayType(t)) {
-    errorNYI("CIRGenModule::emitNullConstant ConstantArrayType");
+    errorNYI("CIRGenModule::emitNullConstantAttr ConstantArrayType");
+    return {};
   }
 
-  if (t->isRecordType())
-    errorNYI("CIRGenModule::emitNullConstant RecordType");
+  if (const RecordType *rt = t->getAs<RecordType>())
+    return ::emitNullConstant(*this, rt->getDecl(), /*asCompleteObject=*/true);
 
   assert(t->isMemberDataPointerType() &&
          "Should only see pointers to data members here!");
 
-  errorNYI("CIRGenModule::emitNullConstant unsupported type");
+  errorNYI("CIRGenModule::emitNullConstantAttr unsupported type");
   return {};
 }
+
+mlir::TypedAttr
+CIRGenModule::emitNullConstantForBase(const CXXRecordDecl *record) {
+  return ::emitNullConstant(*this, record, false);
+}

clang/lib/CIR/CodeGen/CIRGenModule.h

@@ -461,6 +461,12 @@ class CIRGenModule : public CIRGenTypeCache {
   /// expression of the given type.
   mlir::Value emitNullConstant(QualType t, mlir::Location loc);
 
+  mlir::TypedAttr emitNullConstantAttr(QualType t);
+
+  /// Return a null constant appropriate for zero-initializing a base class with
+  /// the given type. This is usually, but not always, an LLVM null constant.
+  mlir::TypedAttr emitNullConstantForBase(const CXXRecordDecl *record);
+
   llvm::StringRef getMangledName(clang::GlobalDecl gd);
 
   void emitTentativeDefinition(const VarDecl *d);

clang/lib/CIR/CodeGen/CIRGenRecordLayout.h

@@ -184,6 +184,11 @@ class CIRGenRecordLayout {
     return fieldIdxMap.lookup(fd);
   }
 
+  unsigned getNonVirtualBaseCIRFieldNo(const CXXRecordDecl *rd) const {
+    assert(nonVirtualBases.count(rd) && "Invalid non-virtual base!");
+    return nonVirtualBases.lookup(rd);
+  }
+
   /// Check whether this struct can be C++ zero-initialized
   /// with a zeroinitializer.
   bool isZeroInitializable() const { return zeroInitializable; }

clang/test/CIR/CodeGen/ctor-null-init.cpp

@@ -29,3 +29,63 @@ void test_empty_base_null_init() {
 // OGCG-NEXT: entry:
 // OGCG-NEXT:   %[[B:.*]] = alloca %struct.B
 // OGCG-NEXT:   ret void
+
+
+struct C {
+  int c;
+  C() = default;
+  C(int); // This constructor triggers the null base class initialization.
+};
+
+struct D : C {
+};
+
+void test_non_empty_base_null_init() {
+  D{};
+}
+
+// CIR: cir.func {{.*}} @_Z29test_non_empty_base_null_initv()
+// CIR:   %[[TMP:.*]] = cir.alloca !rec_D, !cir.ptr<!rec_D>, ["agg.tmp.ensured"]
+// CIR:   %[[BASE:.*]] = cir.base_class_addr %[[TMP]] : !cir.ptr<!rec_D> nonnull [0] -> !cir.ptr<!rec_C>
+// CIR:   %[[ZERO:.*]] = cir.const #cir.const_record<{#cir.int<0> : !s32i}> : !rec_C
+// CIR:   cir.store{{.*}} %[[ZERO]], %[[BASE]]
+
+// LLVM: define{{.*}} void @_Z29test_non_empty_base_null_initv()
+// LLVM:   %[[TMP:.*]] = alloca %struct.D
+// LLVM:   store %struct.C zeroinitializer, ptr %[[TMP]]
+
+// OGCG: define {{.*}} void @_Z29test_non_empty_base_null_initv()
+// OGCG:   %[[TMP:.*]] = alloca %struct.D
+// OGCG:   %[[BASE:.*]] = getelementptr inbounds i8, ptr %[[TMP]], i64 0
+// OGCG:   call void @llvm.memset.p0.i64(ptr{{.*}} %[[BASE]], i8 0, i64 4, i1 false)
+
+struct E {
+  int e;
+};
+
+struct F : E {
+  F() = default;
+  F(int);
+};
+
+struct G : F {
+};
+
+void test_base_chain_null_init() {
+  G{};
+}
+
+// CIR: cir.func {{.*}} @_Z25test_base_chain_null_initv()
+// CIR:   %[[TMP:.*]] = cir.alloca !rec_G, !cir.ptr<!rec_G>, ["agg.tmp.ensured"]
+// CIR:   %[[BASE:.*]] = cir.base_class_addr %[[TMP]] : !cir.ptr<!rec_G> nonnull [0] -> !cir.ptr<!rec_F>
+// CIR:   %[[ZERO:.*]] = cir.const #cir.const_record<{#cir.zero : !rec_E}> : !rec_F
+// CIR:   cir.store{{.*}} %[[ZERO]], %[[BASE]]
+
+// LLVM: define{{.*}} void @_Z25test_base_chain_null_initv()
+// LLVM:   %[[TMP:.*]] = alloca %struct.G
+// LLVM:   store %struct.F zeroinitializer, ptr %[[TMP]]
+
+// OGCG: define {{.*}} void @_Z25test_base_chain_null_initv()
+// OGCG:   %[[TMP:.*]] = alloca %struct.G
+// OGCG:   %[[BASE:.*]] = getelementptr inbounds i8, ptr %[[TMP]], i64 0
+// OGCG:   call void @llvm.memset.p0.i64(ptr{{.*}} %[[BASE]], i8 0, i64 4, i1 false)