[llvm] [IR] Use alloc markers for operator delete variants (PR #138261)

[Nikita Popov via llvm-commits]

================
@@ -191,28 +191,70 @@ void *User::operator new(size_t Size, HungOffOperandsAllocMarker) {
 LLVM_NO_SANITIZE_MEMORY_ATTRIBUTE void User::operator delete(void *Usr) {
   // Hung off uses use a single Use* before the User, while other subclasses
   // use a Use[] allocated prior to the user.
-  User *Obj = static_cast<User *>(Usr);
+  const auto *Obj = static_cast<User *>(Usr);
   if (Obj->HasHungOffUses) {
-    assert(!Obj->HasDescriptor && "not supported!");
-
-    Use **HungOffOperandList = static_cast<Use **>(Usr) - 1;
-    // drop the hung off uses.
-    Use::zap(*HungOffOperandList, *HungOffOperandList + Obj->NumUserOperands,
-             /* Delete */ true);
-    ::operator delete(HungOffOperandList);
+    const HungOffOperandsAllocMarker Marker{
+        Obj->NumUserOperands,
+    };
+    operator delete(Usr, Marker);
   } else if (Obj->HasDescriptor) {
-    Use *UseBegin = static_cast<Use *>(Usr) - Obj->NumUserOperands;
-    Use::zap(UseBegin, UseBegin + Obj->NumUserOperands, /* Delete */ false);
-
-    auto *DI = reinterpret_cast<DescriptorInfo *>(UseBegin) - 1;
-    uint8_t *Storage = reinterpret_cast<uint8_t *>(DI) - DI->SizeInBytes;
-    ::operator delete(Storage);
+    const IntrusiveOperandsAndDescriptorAllocMarker Marker{
+        Obj->NumUserOperands,
+        Obj->HasDescriptor,
+    };
+    operator delete(Usr, Marker);
   } else {
-    Use *Storage = static_cast<Use *>(Usr) - Obj->NumUserOperands;
-    Use::zap(Storage, Storage + Obj->NumUserOperands,
-             /* Delete */ false);
-    ::operator delete(Storage);
+    const IntrusiveOperandsAllocMarker Marker{
+        Obj->NumUserOperands,
+    };
+    operator delete(Usr, Marker);
   }
 }
 
+// Repress memory sanitization, due to use-after-destroy by operator
+// delete. Bug report 24578 identifies this issue.
+LLVM_NO_SANITIZE_MEMORY_ATTRIBUTE void
+User::operator delete(void *Usr, const HungOffOperandsAllocMarker Marker) {
+  // Note: If a subclass manipulates the information which is required to
+  // calculate the Usr memory pointer, e.g. NumUserOperands, the operator
+  // delete of that subclass has to restore the changed information to the
+  // original value, since the dtor of that class is not called if the ctor
+  // fails.
----------------
nikic wrote:

I don't think these comments make sense anymore (as the data is now taken from the marker).

https://github.com/llvm/llvm-project/pull/138261