Re: [PATCH v5 2/2] Implement pahole-like 'ptype /o' option

[Sergio Durigan Junior <sergiodj@redhat.com>]

On Wednesday, December 13 2017, I wrote:

> On Wednesday, December 13 2017, Pedro Alves wrote:
>
>> I'm mildly worried about whether the bitfield handling is working
>> correctly on big endian machines.  We may want to lift this
>> x86-64-only restriction, by using e.g., alignas(N) or
>> __attribute__((aligned(N)) to take care of most of the differences
>> between architectures and end up with few per-arch code in
>> the .exp.  But I'm fine with starting with only x86-64 if you
>> confirm manually on e.g., a big endian PPC64 machine on the
>> compile farm, and we can extend the testcase in that direction
>> after this is merged.
>
> OK, I'll confirm on PPC64BE.

It seems like the algorithm for calculating bitfield offsets is not
working correctly on BE machines.  This is not only for "ptype /o", but
also for regular print commands.  For example, consider this test:

  struct tyu
  {
    int a1 : 1;

    int a2 : 3;

    int a3 : 23;

    char a4 : 2;

    int64_t a5;

    int a6 : 5;

    int64_t a7 : 3;
  };

  int
  main (int argc, char *argv[])
  {
    struct tyu e;

    e.a1 = e.a2 = e.a3 = e.a4 = e.a6 = e.a7 = -1;

    return 0;
  }

After stopping GDB at the "return 0;" line, here's what we see when we
print "e" on x86_64:

  (gdb) p e
  $1 = {a1 = -1, a2 = -1, a3 = -1, a4 = -1 '\377', a5 = 140737488344880, a6 = -1, a7 = -1}

While on PPC64BE:

  (gdb) p e
  $1 = {a1 = -1, a2 = 3, a3 = 3, a4 = 3 '\003', a5 = 70367536153528, a6 = -1, a7 = -1}

As for "ptype /o", the offsets printed on x86_64 and PPC64BE are the
same:

x86_64:

  /* offset    |  size */  type = struct tyu {
  /*    0:31   |     4 */    int a1 : 1;
  /*    0:28   |     4 */    int a2 : 3;
  /*    0: 5   |     4 */    int a3 : 23;
  /*    3: 3   |     1 */    char a4 : 2;
  /* XXX  3-bit hole   */
  /* XXX  4-byte hole  */
  /*    8      |     8 */    int64_t a5;
  /*   16:27   |     4 */    int a6 : 5;
  /*   16:56   |     8 */    int64_t a7 : 3;

                             /* total size (bytes):   24 */
                           }

PPC64BE:

  /* offset    |  size */  type = struct tyu {
  /*    0:31   |     4 */    int a1 : 1;
  /*    0:28   |     4 */    int a2 : 3;
  /*    0: 5   |     4 */    int a3 : 23;
  /*    3: 3   |     1 */    char a4 : 2;
  /* XXX  3-bit hole   */
  /* XXX  4-byte hole  */
  /*    8      |     8 */    int64_t a5;
  /*   16:27   |     4 */    int a6 : 5;
  /*   16:56   |     8 */    int64_t a7 : 3;

                             /* total size (bytes):   24 */
                           }

-- 
Sergio
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