Hello,

I'm actually testing on the wip-rtl-cps branch, but this error involves
code that I believe is the same on that branch and on the wip-rtl branch.
Try opening a new Guile and doing the following:

scheme@(guile-user)> (use-modules (system vm rtl))
scheme@(guile-user)> (assemble-program '((begin-program foo)
 (assert-nargs-ge 0)
 (reserve-locals 4)
 (bind-rest 0)
 (box 1 0)
 (cache-current-module! 2 foo)
 (cached-toplevel-ref 2 foo car)
 (box-ref 3 1)
 (mov 0 3)
 (tail-call 1 2)
 (end-program)))
... ... ... ... ... ... ... ... ... ... $1 = #<rtl-program dcec90 609bc0>
scheme@(guile-user)> ($1 'hello)

The expected result is
$2 = hello

What I actually get is,

Program received signal SIGABRT, Aborted.
0x00007ffff7440425 in raise () from /lib/x86_64-linux-gnu/libc.so.6

The full backtrace is below. The interesting part is that it seems to be
tripping the check at libguile/vm-engine.c:1868, which checks whether an
object is a variable before doing a box-ref on it. When I look at it in
GDB, it seems that whatever is at register 1 does not satisfy
scm_variable_p, although I'm not very experienced with debugging Guile.
However, I am somewhat surprised at this, because I have used boxes and
box-ref before in the past with no trouble.

Another surprising thing is that if I open Guile, do some other things for
a while, and then run this code, the problem sometimes doesn't appear. That
is especially disturbing.

Does anyone have any idea where the issue is or how I should find it?

Thanks,
Noah

Here's the backtrace:

(gdb) bt
#0  0x00007ffff7440425 in raise ()
   from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7443b8b in abort ()
   from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007ffff7b30986 in rtl_vm_debug_engine (vm=0x6a6860,
    program=0xdcec90, argv=0x6a9548, nargs_=1) at vm-engine.c:1868
#3  0x00007ffff7b1aaf1 in vm_debug_engine (vm=0x6a6860,
    program=0xdcec90, argv=0x7fffffffd028, nargs=1) at vm-engine.c:419
#4  0x00007ffff7b38f6c in scm_c_vm_run (vm=0x6a6860,
    program=0x75dbe0, argv=0x7fffffffd028, nargs=1) at vm.c:791
#5  0x00007ffff7a5bff3 in scm_primitive_eval (exp=0x7fe7f0)
    at eval.c:691
#6  0x00007ffff7a5c0ad in scm_eval (exp=0x7fe7f0,
    module_or_state=0x7e0090) at eval.c:725
#7  0x00007ffff7acef2d in scm_shell (argc=1, argv=0x7fffffffe478)
    at script.c:441
#8  0x0000000000400bd0 in inner_main (closure=0x0, argc=1,
    argv=0x7fffffffe478) at guile.c:62
#9  0x00007ffff7a82663 in invoke_main_func (body_data=0x7fffffffe350)
    at init.c:336
#10 0x00007ffff7a563c9 in c_body (d=0x7fffffffe220)
    at continuations.c:513
#11 0x00007ffff7afc96c in apply_catch_closure (clo=0x81b360,
    args=0x304) at throw.c:146
#12 0x00007ffff7acf739 in apply_1 (smob=0x81b360, a=0x304)
    at smob.c:141
#13 0x00007ffff7b05cc8 in vm_regular_engine (vm=0x6a6860,
    program=0x7443e0, argv=0x7fffffffe0c0, nargs=2)
    at vm-i-system.c:873
#14 0x00007ffff7b38f6c in scm_c_vm_run (vm=0x6a6860,
    program=0x79d5d0, argv=0x7fffffffe0c0, nargs=4) at vm.c:791
#15 0x00007ffff7a5b793 in scm_call_4 (proc=0x79d5d0, arg1=0x404,
    arg2=0x81b360, arg3=0x81b340, arg4=0x81b320) at eval.c:513
#16 0x00007ffff7afc767 in scm_catch_with_pre_unwind_handler (
    key=0x404, thunk=0x81b360, handler=0x81b340,
    pre_unwind_handler=0x81b320) at throw.c:86
#17 0x00007ffff7afca44 in scm_c_catch (tag=0x404,
    body=0x7ffff7a563a1 <c_body>, body_data=0x7fffffffe220,
    handler=0x7ffff7a563d8 <c_handler>, handler_data=0x7fffffffe220,
    pre_unwind_handler=0x7ffff7a5642c <pre_unwind_handler>,
    pre_unwind_handler_data=0x751160) at throw.c:213
#18 0x00007ffff7a5623d in scm_i_with_continuation_barrier (
    body=0x7ffff7a563a1 <c_body>, body_data=0x7fffffffe220,
    handler=0x7ffff7a563d8 <c_handler>, handler_data=0x7fffffffe220,
    pre_unwind_handler=0x7ffff7a5642c <pre_unwind_handler>,
    pre_unwind_handler_data=0x751160) at continuations.c:451
#19 0x00007ffff7a564c3 in scm_c_with_continuation_barrier (
    func=0x7ffff7a82613 <invoke_main_func>, data=0x7fffffffe350)
    at continuations.c:547
#20 0x00007ffff7af97ba in with_guile_and_parent (base=0x7fffffffe290,
    base@entry=<error reading variable: value has been optimized out>,
data=0x7fffffffe2d0,
    data@entry=<error reading variable: value has been optimized out>)
    at threads.c:907
#21 0x00007ffff71b6f55 in GC_call_with_stack_base (
    fn=<optimized out>, arg=<optimized out>) at misc.c:1553
#22 0x00007ffff7af9894 in scm_i_with_guile_and_parent (
    func=0x7ffff7a82613 <invoke_main_func>, data=0x7fffffffe350,
    parent=0x0) at threads.c:950
#23 0x00007ffff7af98c0 in scm_with_guile (
    func=0x7ffff7a82613 <invoke_main_func>, data=0x7fffffffe350)
    at threads.c:956
#24 0x00007ffff7a825f4 in scm_boot_guile (argc=1,
    argv=0x7fffffffe478, main_func=0x400bac <inner_main>, closure=0x0)
    at init.c:319
#25 0x0000000000400c35 in main (argc=1, argv=0x7fffffffe478)
    at guile.c:81