[IMPALA-9781] Fix GCC 7 runtime issue: Unaligned loads and stores for int128_t types

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 4.0.0
• Fix Version/s: Impala 4.0.0
• Component/s: Backend
• Epic Link: GCC7_newabi_upgrade
• Target Version: Impala 4.0.0
• Epic Color: ghx-label-2
• Assignee: Joe McDonnell
• Reporter: Joe McDonnell
• Created: 27/May/20 23:33
• Updated: 01/Jun/20 22:35
• Resolved: 01/Jun/20 22:35

Description

When running with a release binary built with GCC 7, Impala crashes with multiple distinct unaligned memory errors. They are all related to manipulating 128-bit values. Presumably GCC 7 added an optimization to use an instruction that requires alignment for setting 128-bit values. The locations impacted are:

SlotRef::GetDecimalValInterpreted():

 

    case 16:
      return DecimalVal(*reinterpret_cast<int128_t*>(t->GetSlot(slot_offset_)));

DecimalUtil::DecodeFromFixedLenByteArray():

 

 

  template<typename T>
  static inline void DecodeFromFixedLenByteArray(
      const uint8_t* buffer, int fixed_len_size, T* v) {
...
    *v = 0; <--- unaligned store

DictDecoder::GetValue():

  virtual void GetValue(int index, void* buffer) {
    T* val_ptr = reinterpret_cast<T*>(buffer);
    ...
    *val_ptr = dict_[index]; <--- unaligned store
  }

These can be converted to use memcpy / memset to avoid the unaligned stores/loads.