(gdb) walkstk 
#0  {inline frame} @ 0x00ebe658: std::__detail::_Hash_code_base<unsigned long, std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> >, std::__detail::_Select1st, std::hash<unsigned long>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>::_M_bucket_index() at /usr/include/c++/4.9/bits/hashtable_policy.h:1186
#1  {inline frame} @ 0x00ebe658: std::_Hashtable<unsigned long, std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> >, HPHP::req::Allocator<std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> >, HPHP::type_scan::Action::Conservative<std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> > > >, std::__detail::_Select1st, std::equal_to<unsigned long>, std::hash<unsigned long>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >::_M_bucket_index() at /usr/include/c++/4.9/bits/hashtable.h:610
#2  0x7ffe3e587230 @ 0x00ebe658: std::_Hashtable<unsigned long, std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> >, HPHP::req::Allocator<std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> >, HPHP::type_scan::Action::Conservative<std::pair<unsigned long const, HPHP::req::weak_ptr<HPHP::WeakRefData> > > >, std::__detail::_Select1st, std::equal_to<unsigned long>, std::hash<unsigned long>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >::_M_find_before_node() at /usr/include/c++/4.9/bits/hashtable.h:1448
#3  {inline frame} @ 0x0101126e: std::_Hashtable<HPHP::ObjectData const*, std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor>, HPHP::req::Allocator<std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor>, HPHP::type_scan::Action::Conservative<std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor> > >, std::__detail::_Select1st, std::equal_to<HPHP::ObjectData const*>, std::hash<HPHP::ObjectData const*>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >::_M_find_node() at /usr/include/c++/4.9/bits/hashtable.h:625
#4  0x7ffe3e587260 @ 0x0101126e: std::__detail::_Map_base<HPHP::ObjectData const*, std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor>, HPHP::req::Allocator<std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor>, HPHP::type_scan::Action::Conservative<std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor> > >, std::__detail::_Select1st, std::equal_to<HPHP::ObjectData const*>, std::hash<HPHP::ObjectData const*>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true>, true>::operator[](HPHP::ObjectData const*&&)() at /usr/include/c++/4.9/bits/hashtable_policy.h:623
#5  {inline frame} @ 0x010113af: std::unordered_map<HPHP::ObjectData const*, HPHP::ArrayNoDtor, std::hash<HPHP::ObjectData const*>, std::equal_to<HPHP::ObjectData const*>, HPHP::req::Allocator<std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor>, HPHP::type_scan::Action::Conservative<std::pair<HPHP::ObjectData const* const, HPHP::ArrayNoDtor> > > >::operator[](HPHP::ObjectData const*&&)() at /usr/include/c++/4.9/bits/unordered_map.h:631
#6  0x7ffe3e5872a0 @ 0x010113af: HPHP::ObjectData::setDynPropArray() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/object-data.cpp:302
#7  0x7ffe3e5872e0 @ 0x02130a38: HPHP::ObjectData::reserveProperties() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/object-data.cpp:294
#8  0x7ffe3e587330 @ 0x021307e7: HPHP::ObjectData::setProp() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/object-data.cpp:1394
#9  {inline frame} @ 0x021d3c93: HPHP::SetPropObj<(HPHP::KeyType)0>() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/vm/member-operations.h:2850
#10 {inline frame} @ 0x021d3c93: HPHP::SetProp<true, (HPHP::KeyType)0>() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/vm/member-operations.h:2890
#11 {inline frame} @ 0x021d3c93: HPHP::iopSetM() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/vm/bytecode.cpp:3510
#12 {inline frame} @ 0x021d3c93: HPHP::iopWrapper() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/vm/bytecode.cpp:6565
#13 0x7ffe3e587890 @ 0x021d3c93: HPHP::dispatchImpl<false>() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/vm/bytecode.cpp:6905
#14 0x7ffe3e5878c0 @ 0x00f4484f: HPHP::exception_handler<void (*)()>() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/vm/unwind-inl.h:30
#15 {inline frame} @ 0x00f44cfa: HPHP::enterVMCustomHandler<HPHP::enterVM(HPHP::ActRec*, Action) [with Action = HPHP::ExecutionContext::invokeFunc(const HPHP::Func*, const HPHP::Variant&, HPHP::ObjectData*, HPHP::Class*, HPHP::VarEnv*, HPHP::StringData*, HPHP::ExecutionContext::InvokeFlags, bool)::<lambda(HPHP::ActRec*)>::<lambda()>]::<lambda()> >() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/execution-context.cpp:1574
#16 {inline frame} @ 0x00f44cfa: HPHP::enterVM<HPHP::ExecutionContext::invokeFunc(const HPHP::Func*, const HPHP::Variant&, HPHP::ObjectData*, HPHP::Class*, HPHP::VarEnv*, HPHP::StringData*, HPHP::ExecutionContext::InvokeFlags, bool)::<lambda(HPHP::ActRec*)>::<lambda()> >() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/execution-context.cpp:1580
#17 {inline frame} @ 0x00f44cfa: HPHP::ExecutionContext::<lambda(HPHP::ActRec*)>::operator()() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/execution-context.cpp:1651
#18 {inline frame} @ 0x00f44cfa: HPHP::ExecutionContext::invokeFuncImpl<HPHP::ExecutionContext::invokeFunc(const HPHP::Func*, const HPHP::Variant&, HPHP::ObjectData*, HPHP::Class*, HPHP::VarEnv*, HPHP::StringData*, HPHP::ExecutionContext::InvokeFlags, bool)::<lambda(HPHP::TypedValue&)>, HPHP::ExecutionContext::invokeFunc(const HPHP::Func*, const HPHP::Variant&, HPHP::ObjectData*, HPHP::Class*, HPHP::VarEnv*, HPHP::StringData*, HPHP::ExecutionContext::InvokeFlags, bool)::<lambda(HPHP::ActRec*, HPHP::TypedValue&)>, HPHP::ExecutionContext::invokeFunc(const HPHP::Func*, const HPHP::Variant&, HPHP::ObjectData*, HPHP::Class*, HPHP::VarEnv*, HPHP::StringData*, HPHP::ExecutionContext::InvokeFlags, bool)::<lambda(HPHP::ActRec*)> >() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/execution-context.cpp:1542
#19 0x7ffe3e587980 @ 0x00f44cfa: HPHP::ExecutionContext::invokeFunc() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/execution-context.cpp:1655
#20 0x7ffe3e5879c0 @ 0x00f45255: HPHP::ExecutionContext::invokeUnit() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/execution-context.cpp:1272
#21 {inline frame} @ 0x021a3b7c: HPHP::invoke_file_impl() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/builtin-functions.cpp:909
#22 0x7ffe3e587a30 @ 0x021a3b7c: HPHP::invoke_file() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/builtin-functions.cpp:922
#23 0x7ffe3e587aa0 @ 0x021a3dd2: HPHP::include_impl_invoke() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/builtin-functions.cpp:948
#24 0x7ffe3e587c80 @ 0x021a2ad3: HPHP::hphp_invoke() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/program-functions.cpp:2254
#25 0x7ffe3e587d50 @ 0x00fd8dd4: HPHP::hphp_invoke_simple() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/program-functions.cpp:2208
#26 0x7ffe3e589940 @ 0x00fe2daf: HPHP::execute_program_impl() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/program-functions.cpp:1827
#27 0x7ffe3e589990 @ 0x00fe50fe: HPHP::execute_program() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/runtime/base/program-functions.cpp:1148
#28 0x7ffe3e589a40 @ 0x00b56d6a: main() at /tmp/buildd/hhvm-3.18.2+dfsg/hphp/hhvm/main.cpp:83
#29 0x0 @ 0x7ff99baa3b45: ? __libc_start_main