[clang][bytecode] Add Descriptor::hasTrivialDtor() (#146286)

We sometimes used to have a long list of 

```
  GetLocalPtr
  PopPtr
  [...]
```

ops at the end of scopes, because we first got a pointer to a local
variable and only then did we figure out that we didn't actually want to
call the destructor for it. Add a new function that allows us to just
ask the `Descriptor` whether we need to call its destructor.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.h

@@ -574,17 +574,17 @@ template <class Emitter> class LocalScope : public VariableScope<Emitter> {
     // Emit destructor calls for local variables of record
     // type with a destructor.
     for (Scope::Local &Local : llvm::reverse(this->Ctx->Descriptors[*Idx])) {
-      if (!Local.Desc->isPrimitive() && !Local.Desc->isPrimitiveArray()) {
-        if (!this->Ctx->emitGetPtrLocal(Local.Offset, E))
-          return false;
+      if (Local.Desc->hasTrivialDtor())
+        continue;
+      if (!this->Ctx->emitGetPtrLocal(Local.Offset, E))
+        return false;
 
-        if (!this->Ctx->emitDestruction(Local.Desc, Local.Desc->getLoc()))
-          return false;
+      if (!this->Ctx->emitDestruction(Local.Desc, Local.Desc->getLoc()))
+        return false;
 
-        if (!this->Ctx->emitPopPtr(E))
-          return false;
-        removeIfStoredOpaqueValue(Local);
-      }
+      if (!this->Ctx->emitPopPtr(E))
+        return false;
+      removeIfStoredOpaqueValue(Local);
     }
     return true;
   }

clang/lib/AST/ByteCode/Descriptor.cpp

@@ -502,6 +502,21 @@ SourceInfo Descriptor::getLoc() const {
   llvm_unreachable("Invalid descriptor type");
 }
 
+bool Descriptor::hasTrivialDtor() const {
+  if (isPrimitive() || isPrimitiveArray() || isDummy())
+    return true;
+
+  if (isRecord()) {
+    assert(ElemRecord);
+    const CXXDestructorDecl *Dtor = ElemRecord->getDestructor();
+    return !Dtor || Dtor->isTrivial();
+  }
+
+  // Composite arrays.
+  assert(ElemDesc);
+  return ElemDesc->hasTrivialDtor();
+}
+
 bool Descriptor::isUnion() const { return isRecord() && ElemRecord->isUnion(); }
 
 InitMap::InitMap(unsigned N)

clang/lib/AST/ByteCode/Descriptor.h

@@ -281,6 +281,9 @@ struct Descriptor final {
   /// Checks if this is a dummy descriptor.
   bool isDummy() const { return IsDummy; }
 
+  /// Whether variables of this descriptor need their destructor called or not.
+  bool hasTrivialDtor() const;
+
   void dump() const;
   void dump(llvm::raw_ostream &OS) const;
   void dumpFull(unsigned Offset = 0, unsigned Indent = 0) const;