This is very odd and corner case scenario. We have a loop after epilog (with stress block layout, we might have something like that). If there are IGs after epilog, the epilog creates additional dummy IG to store GC state in it. We also update the GC vars global variables emitPrevGCrefVars and emitInitGCrefVars to have most up to date information. Whenever their values are different, we mark such IGs with IGF_GC_VARS stating that the IG contains gc information that the gcinfo should record.

The loop that is immediately after the epilog is marked for alignment, and so we create a new label IG for it. At this point, the GC vars information emitPrevGCrefVars == emitInitGCrefVars and so we skip marking this IG with IGF_GC_VARS. As a result, it inherits the GC var liveness information from the previous block (which is the dummy block after the epilog) which is wrong.

The fix is to force store the GC state on all IGs until we come across a valid labeled IG.

Attached is the dump from failing test. Here, IG14 is the epilog and IG15 is the dummy IG for which we add all the GC-vars like V73 etc. When we create IG16, we do not add any new GC vars and hence inherit those from IG15

617.dump.asm.zip

Here is the diff after the fix:

Thanks @BruceForstall for brainstorming this scenario with me.

Fixes: #113553