GH-132508: Use tagged integers on the evaluation stack for the last instruction offset

[markshannon]

When reraising in a finally block, the exception needs to look as if it were raised from an earlier point in the code.
To do this we save the earlier instruction offset as an integer on the evaluation stack.
Currently, this requires boxing the integer, which can (extremely rarely) fail.
By using a tagged integer we can avoid that failure mode.

This is might seem like an elaborate fix for a very minor issue, and it is, but we will want tagged integers/pointers for many other things and this is a nice small step to that larger change.

See #132509

• Issue: Exception unwinding might be broken when handling a memory error. #132508

[Fidget-Spinner]

2 comments

[markshannon]

As expected, performance is neutral.

[brandtbucher]

I believe the magic number needs to be updated (and everything regenerated too) since the exception_unwind label now pushes tagged ints.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[markshannon]

I believe the magic number needs to be updated (and everything regenerated too) since the exception_unwind label now pushes tagged ints.

Nothing has changed in terms of stack effects. The only change is that we push a tagged int instead of a boxed one.

Ah, nevermind.

[markshannon]

The failure on JIT/ARM64 look like a JIT bug. The assertion errors seem impossible and do not occur on any other platform.
Neither the ARM64 (JIT) interpreter or the other JIT builds show any of the failures.

[Fidget-Spinner]

I think I've seen that JIT failure before on my old PR when I used a different tagging scheme.

[markshannon]

The JIT ARM 64 windows failure is just a timeout for the int repr test that sometimes happens.