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I am getting the following error sometimes when using my branch of GHC to build Cabal which compacts modifaces. Seems to be something to do with compactAddWithSharing.

Branch is here, https://gitlab.haskell.org/ghc/ghc/-/commits/wip/compact-modiface-new

The problem seems to happen quite late in the compilation so perhaps there is something overflowing. I would start looking in evacuate_hash_entry I think. It also doesn't always happen..

ghc: internal error: evacuate: strange closure type 20043649
Stack trace:
            0x7f84eb1801b2    set_initial_registers (rts/Libdw.c:294.5)
            0x7f84eacc1198    dwfl_thread_getframes (/nix/store/sv6f05ngaarba50ybr6fdfc7cciv6nbv-elfutils-0.176/lib/libdw-0.176.so)
            0x7f84eacc0c5b    get_one_thread_cb (/nix/store/sv6f05ngaarba50ybr6fdfc7cciv6nbv-elfutils-0.176/lib/libdw-0.176.so)
            0x7f84eacc0f97    dwfl_getthreads (/nix/store/sv6f05ngaarba50ybr6fdfc7cciv6nbv-elfutils-0.176/lib/libdw-0.176.so)
            0x7f84eacc1567    dwfl_getthread_frames (/nix/store/sv6f05ngaarba50ybr6fdfc7cciv6nbv-elfutils-0.176/lib/libdw-0.176.so)
            0x7f84eb180879    libdwGetBacktrace (rts/Libdw.c:265.8)
            0x7f84eb1888fd    rtsFatalInternalErrorFn (rts/RtsMessages.c:170.6)
            0x7f84eb188add    barf (rts/RtsMessages.c:49.3)
            0x7f84eb172681    evacuate1 (rts/sm/Evac.c:1061.5)
            0x7f84eb1ad730    evacuate_hash_entry (rts/sm/Scav.c:176.5)
            0x7f84eb17c2dc    mapHashTable (rts/Hash.c:466.76)
            0x7f84eb1ad900    scavenge_compact1 (rts/sm/Scav.c:196.9)
            0x7f84eb1ae1c5    scavenge_one (rts/sm/Scav.c:1573.9)
            0x7f84eb1aefcb    scavenge_loop1 (rts/sm/Scav.c:2039.12)
            0x7f84eb1a54ad    scavenge_until_all_done (rts/sm/GC.c:1274.13)
            0x7f84eb1a6c3b    GarbageCollect (rts/sm/GC.c:1498.22)
            0x7f84eb18bb09    scheduleDoGC (rts/Schedule.c:1892.9)
            0x7f84eb18c7ca    schedule (rts/Schedule.c:579.7)
            0x7f84eb18df31    scheduleWaitThread (rts/Schedule.c:2657.11)
            0x7f84eb18875d    hs_main (rts/RtsMain.c:73.18)
                  0x436902    (null) (/home/matt/ghc-compact-iface/_build/stage1/bin/ghc)
            0x7f84eae5bd8b    __libc_start_main (/nix/store/xg6ilb9g9zhi2zg1dpi4zcp288rhnvns-glibc-2.30/lib/libc-2.30.so)
                  0x43694a    _start (../sysdeps/x86_64/start.S:122.0)

    (GHC version 9.1.20210226 for x86_64_unknown_linux)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug