[Bug c++/66364] New: poor optimization of packed structs containing bitfields

["rogero at howzatt dot demon.co.uk" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66364

            Bug ID: 66364
           Summary: poor optimization of packed structs containing
                    bitfields
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rogero at howzatt dot demon.co.uk
  Target Milestone: ---
            Target: x86_64-redhat-linux

Created attachment 35667
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35667&action=edit
Simple program demonstrating the issue

Code accessing bitfields in packed structures seem to load the value a byte at
a time and build the value up using shifts and ors even with -O3.

On x64 it could load the value using a single mov instruction as it does when
the struct is not packed or when the field is not a bitfield.