[Bug c/52291] New: __sync_fetch_and_add and friends poorly specified for pointer types

[Bug c/52291] New: __sync_fetch_and_add and friends poorly specified for pointer types

[hstong at ca dot ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52291

             Bug #: 52291
           Summary: __sync_fetch_and_add and friends poorly specified for
                    pointer types
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hstong@ca.ibm.com
                CC: michaelw@ca.ibm.com
              Host: powerpc64-unknown-linux-gnu
            Target: powerpc64-unknown-linux-gnu


The documentation states, regarding the __sync_fetch_and_X builtins:
These builtins perform the operation suggested by the name, and returns the
value that had previously been in memory. That is,

          { tmp = *ptr; *ptr op= value; return tmp; }
          { tmp = *ptr; *ptr = ~(tmp & value); return tmp; }   // nand

For pointer types (which GCC accepts for these builtins) the "operation
suggested by the name" leans toward pointer arithmetic; however, the pointer
types are not suitable for these operations in C/C++.

[Bug c/52291] __sync_fetch_and_add and friends poorly specified for pointer types

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52291

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-17 00:38:15 UTC ---
N.B. those built-ins are deprecated in favour of
http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

[Bug c/52291] __sync_fetch_and_add and friends poorly specified for pointer types

[hstong at ca dot ibm.com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52291

--- Comment #1 from Hubert Tong <hstong at ca dot ibm.com> 2012-02-17 00:37:12 UTC ---
I mean that the pseudo-code is not consistent with valid C/C++ when the
operands (`*ptr' and `value') are pointer types.

[Bug c/52291] __sync_fetch_and_add and friends poorly specified for pointer types

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52291

--- Comment #3 from Mikael Pettersson <mikpe at it dot uu.se> 2012-02-17 08:53:28 UTC ---
(In reply to comment #1)
> I mean that the pseudo-code is not consistent with valid C/C++ when the
> operands (`*ptr' and `value') are pointer types.

I would think it's implied that the pointer values are automatically converted
to uintptr_t and that the bit-twiddling or arithmetic is done on their
uintptr_t representations.