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ObjC bug on 64-bit architectures?


I'm trying to track down an Objective C testsuite failure on 64-bit
s390x.  However, the problem doesn't appear to be s390x specific from
what I can see; it looks to me like this bug should appear on every
big-endian 64-bit platform with a 32-bit 'int' type.

What happens is that the testcase (bycopy-3.m) crashes in main because
a class_pointer of an objc_protocol is not correctly initialized.

This is turn happens because the constructor __objc_init_protocols gets
called with an incorrect struct objc_protocol_list having a count field
of 0 (instead of 1), and hence does not initialize the protocol.

The objc_protocol_list is generated by the objc frontend; the problem
appears to be caused by a type mismatch.  The definition of the structure
objc_protocol_list says (from the file libobjc/objc/objc-api.h):

struct objc_protocol_list {
  struct objc_protocol_list *next;
  int count;
  Protocol *list[1];

However, the compiler generates this instance:

        .align 8
        .type   _OBJC_CLASS_PROTOCOLS_MyObject,@object
        .size   _OBJC_CLASS_PROTOCOLS_MyObject,24
        .quad   0
        .quad   1
        .quad   _OBJC_PROTOCOL_MyProtocol

The '.quad 1' is incorrect, because 'int' is a 32-bit data type; because
our platform is big-endian, this causes the 'count' field to be read as 0.

This assembly appears to be generated by the objc frontend routine
generate_protocol_list (in the file gcc/objc/objc-act.c).  Unfortunately,
my understanding of frontend code is rather limited, but it looks like that
routine is indeed doing a type cast from the integer constant that
the count to some pointer type.  This looks like a bug to me, but I'm not
sure how to fix it ...

Could someone familiar with the objc frontend have a look at this?


  Dr. Ulrich Weigand
  Linux for S/390 Design & Development
  IBM Deutschland Entwicklung GmbH, Schoenaicher Str. 220, 71032 Boeblingen
  Phone: +49-7031/16-3727   ---   Email:

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