Git HEAD (incl. udef-immediates) hangs in SAVE-LISP-AND-DIE

Hi,

5249a30f980f1e plus my UDEF-IMMEDIATE patches
(no, can't test that application without them)
hangs when writing an image;
older SBCLs (say, 4-6 months ago) including udef did work fine;
SAVE-LISP-AND-DIE took 1-2 minutes to produce a ~7GB image,
now that step doesn't terminate after 15+ minutes.

"perf record -g -p <pid>" with a few seconds measurement
(symbols created via sb-perf contrib) reports

      main
      initialize_lisp
      create_main_lisp_thread
      Lcall
      SB-IMPL__START-LISP
      SB-IMPL__START-LISP
      SB-IMPL__START-LISP
      CHKSUM-TO-PKG_MAIN
      CHKSUM-TO-PKG__MAKE-EXECUTABLE
      SAVE-LISP-AND-DIE
      SB-IMPL__DEINIT
      SB-IMPL__TUNE-IMAGE-FOR-DUMP
      SB-C__COALESCE-DEBUG-INFO
      SB-VM_MAP-ALLOCATED-OBJECTS
    - SB-VM__WALK-DYNAMIC-SPACE
       - 94.09% SB-VM__MAP-OBJECTS-IN-RANGE
            18.14% SB-C__COALESCE-DEBUG-INFO
            11.35% PRIMITIVE-OBJECT-SIZE
            2.77% SB-VM__MAP-OBJECTS-IN-RANGE
         2.96% SB-C__COALESCE-DEBUG-INFO
         2.95% PRIMITIVE-OBJECT-SIZE

Can somebody reproduce that on plain SBCL?

The base udef patch didn't change since 2024;
I renamed the contrib and had a few small additions there,
but I've no idea what kind of interaction might break
SAVE-LISP-AND-DIE here.


Thank you for any help!