[clang][bytecode][NFC] Add a FIXME comment for heap allocations

clang/lib/AST/ByteCode/DynamicAllocator.cpp

@@ -13,6 +13,25 @@
 using namespace clang;
 using namespace clang::interp;
 
+// FIXME: There is a peculiar problem with the way we track pointers
+// to blocks and the way we allocate dynamic memory.
+//
+// When we have code like this:
+// while (true) {
+//   char *buffer = new char[1024];
+//   delete[] buffer;
+// }
+//
+// We have a local variable 'buffer' pointing to the heap allocated memory.
+// When deallocating the memory via delete[], that local variable still
+// points to the memory, which means we will create a DeadBlock for it and move
+// it over to that block, essentially duplicating the allocation. Moving
+// the data is also slow.
+//
+// However, when we actually try to access the allocation after it has been
+// freed, we need the block to still exist (alive or dead) so we can tell
+// that it's a dynamic allocation.
+
 DynamicAllocator::~DynamicAllocator() { cleanup(); }
 
 void DynamicAllocator::cleanup() {