jeaye / cling-singleton-bug

Cling singleton bug

This appears to be a bug related to static globals and an odd combination of unique_ptr<T> and T* fields. The result is that non-JIT code sees one value for the T* field, while JIT code sees another. All other fields show the same value.

Building and running

$ mkdir build ; cd build
$ cmake .. -DCling_DIR=/path/to/cling-build/builddir
$ make
$ ./cling-demo
before: 0
cling: (int *) 0x1 <invalid memory address>
after: 0

As shown in the output above, this demo outputs the same value from non-JIT code and JIT code. It doesn't match up. However, if the unique_ptr field before the T* field is a T* or even a shared_ptr<T>, this issue doesn't manifest.

Environment

I have reproduced this in the following configurations:

• Arch Linux, GCC 12.2.0, Cling stable debug build (from cpt)
• Arch Linux, Clang 14.0.6, Cling stable debug build (from cpt)
• Arch Linux, Clang 14.0.6, Cling llvm-13 debug build (from cpt)
• Ubuntu 22.04, Clang 14, Cling stable release build (from cpt)

Curiously, I have not been able to reproduce it on:

• macOS 12, AppleClang 12, Cling stable release build (from cpt)

Bug

I suspect that this is a bug, since I do not believe this to be UB and I haven't seen any Cling documentation stating this should not work. Also, the oddities around unique_ptr are perplexing.