[Bug c/55658] New: bitfields and __attribute__((packed)) generate horrible code on x86_64

[tudorb at fb dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55658



             Bug #: 55658

           Summary: bitfields and __attribute__((packed)) generate

                    horrible code on x86_64

    Classification: Unclassified

           Product: gcc

           Version: 4.7.1

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: tud...@fb.com





Created attachment 28932

  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28932

Simple source code to exhibit the problem; disassembly (as produced by objdump

-d)



I have a structure defined as:



struct D {

  uint64_t id_ : 63;

  bool x_ : 1;

} __attribute__((packed));



uint64_t D_id(const struct D* p) { return p->id_; }



The generated code for D_id (using gcc 4.7.1 -O2 on Linux x86_64) builds the

return value byte by byte and masks out the most significant bit from the last

byte.



Removing __attribute__((packed)) and/or changing the structure so it is no

longer a bitfield generates sane code.



This happens in both C and C++ (unsurprisingly).