Their are several bugs I have noticed when running pypy-zebu: (unless otherwise specified I have confirmed the errors for both architectures, I have also confirmed that these errors don't occur with the normal pypy):

1. Pypy in interactive mode (but not when reading from a file) segfaults when their is a syntax error:

$ echo '.' | pypy-zebu -i
Python 2.7.12 (e8da6780d84e, Oct 04 2017, 01:16:24)
[PyPy 5.6.0 with Mu] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>> Segmentation fault (core dumped)

LLDB reports:

* thread #2, name = 'Mu Thread #7595', stop reason = signal SIGSEGV: invalid address (fault address: 0x20)
    frame #0: 0x0000000000f1a2b2 pypy-zebu`__mu_W_SyntaxError_descr_repr_0Zdblk15ZdZa821942Zccallsite_26
pypy-zebu`__mu_W_SyntaxError_descr_repr_0Zdblk15ZdZa821942Zccallsite_26:
->  0xf1a2b2 <+0>: movq   0x20(%rax), %rax
    0xf1a2b6 <+4>: movq   0x28(%rax), %rdi

1. Pypy only reads the first commandline argument, e.g. pypy-zebu <(echo 'print "hello"')' prints 'hello', but 'pypy-zebu -v <(echo 'print "hello"')' dosn't (it can bee seen that the argument is not considered their at all:

pypy-zebu -c pass
Argument expected for the '-c' option
usage: /home/isaacg/mu-client-pypy/pypy/bin/pypy-zebu [options]
Try `/home/isaacg/mu-client-pypy/pypy/bin/pypy-zebu -h` for more information.

As well as

$ echo 'import sys; print sys.argv[1]' | pypy-zebu - 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

1. Pybench segfaults (when it dosn't run out of gc space) (e.g. MU_IMMIX_SPACE=40000000000 pypy-zebu pybench.py), lldb reports:

* thread #2, name = 'Mu Thread #7595', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
    frame #0: 0x0000000001002cf1 pypy-zebu`__mu_descr_file_fdopen_0_v1Zcepilogue + 24
pypy-zebu`__mu_descr_file_fdopen_0_v1Zcepilogue:
->  0x1002cf1 <+24>: retq

EDIT: On aarch64 GDB reports:

Thread 2 "Mu Thread #9020" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xfffb509db030 (LWP 31518)]
0x0000000000000000 in ?? ()

or

Thread 2 "Mu Thread #9020" received signal SIGBUS, Bus error.
[Switching to Thread 0xfffb509db030 (LWP 31535)]
0x005f5f7472617473 in ?? ()

2. help() throws a ValueError:

$ MU_IMMIX_SPACE=40000000000 pypy-zebu
Python 2.7.12 (e8da6780d84e, Oct 04 2017, 01:16:24)
[PyPy 5.6.0 with Mu] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>> help()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/site.py", line 472, in __call__
    import pydoc
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/pydoc.py", line 56, in <module>
    import sys, imp, os, re, types, inspect, __builtin__, pkgutil, warnings
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/inspect.py", line 39, in <module>
    import tokenize
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/tokenize.py", line 103, in <module>
    re.compile, (Token, PseudoToken, Single3, Double3))
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/re.py", line 194, in compile
    return _compile(pattern, flags)
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/re.py", line 249, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/home/isaacg/mu-client-pypy/lib-python/2.7/sre_compile.py", line 595, in compile
    groupindex, indexgroup
ValueError: cannot convert negative integer to unsigned
>>>>

3. (aarch64 only) when it runs out of memory and panics, it segfaults:

$ MU_IMMIX_SPACE=1 pypy-zebu
thread 'Mu Thread #9020349' panicked at 'Triggering GC when GC is disabled', /home/isaacg/mu-impl-fast-git/src/gc/src/heap/gc/mod.rs:259:8
Segmentation fault (core dumped)

GDB reports:

Thread 2 "Mu Thread #9020" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xffffb1b31030 (LWP 2811)]
0x0000ffffb75909e0 in ?? () from /lib/aarch64-linux-gnu/libgcc_s.so.1
(gdb) bt
#0  0x0000ffffb75909e0 in ?? () from /lib/aarch64-linux-gnu/libgcc_s.so.1
#1  0x0000ffffb7591234 in _Unwind_Backtrace () from /lib/aarch64-linux-gnu/libgcc_s.so.1
#2  0x0000ffffb7c274d8 in std::sys::imp::backtrace::tracing::imp::unwind_backtrace () at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
#3  0x0000ffffb7c22678 in std::sys_common::backtrace::_print () at /checkout/src/libstd/sys_common/backtrace.rs:71
#4  0x0000ffffb7c32d58 in std::sys_common::backtrace::print () at /checkout/src/libstd/sys_common/backtrace.rs:60
#5  std::panicking::default_hook::{{closure}} () at /checkout/src/libstd/panicking.rs:380
#6  0x0000ffffb7c32968 in std::panicking::default_hook () at /checkout/src/libstd/panicking.rs:396
#7  0x0000ffffb7c33270 in std::panicking::rust_panic_with_hook () at /checkout/src/libstd/panicking.rs:611
#8  0x0000ffffb7a2994c in std::panicking::begin_panic_new::h91c15d79ddd4536b () from /home/isaacg/mu-impl-fast-git/target/release/libmu.so
#9  0x0000ffffb7a3811c in mu_gc::heap::gc::sync_barrier::hebcd16541e55b150 () from /home/isaacg/mu-impl-fast-git/target/release/libmu.so
#10 0x0000ffffb7a3466c in mu_gc::heap::immix::immix_mutator::ImmixMutatorLocal::yieldpoint_slow::h9cccb390ce79aedf () from /home/isaacg/mu-impl-fast-git/target/release/libmu.so
#11 0x0000ffffb7a34888 in mu_gc::heap::immix::immix_mutator::ImmixMutatorLocal::alloc_from_global::h102601e3553ebff9 () from /home/isaacg/mu-impl-fast-git/target/release/libmu.so
#12 0x0000ffffb7a407f4 in muentry_alloc_fast () from /home/isaacg/mu-impl-fast-git/target/release/libmu.so
#13 0x0000000000cdc6e8 in pypy_mu_main_0 ()
#14 0x0000000000cdc69c in entry_point_0 ()
Backtrace stopped: previous frame inner to this frame (corrupt stack?)

1. pypy is allocating zero sized object with Zebu. This does not seem correct. The only zero sized type in Mu is void. Either the client is allocating void object, or there is a bug somewhere.