[Bug c/38660] New: Pointer value changed to NULL

[Bug c/38660] New: Pointer value changed to NULL

[q at ping dot be]

Hi,

It seems that postgresql has a problem with a gcc 4.4 snapshot.  I've last
tested this with the gcc-snapshot package from Debian on x86_64 that has a
snapshot from 20081213.

It seems that a pointer gets changed into NULL that shouldn't.  I've first
asked about this on the pgsql list, see the thread starting at
http://archives.postgresql.org/pgsql-hackers/2008-12/msg01813.php

Kurt


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           Summary: Pointer value changed to NULL
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: q at ping dot be


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug c/38660] Pointer value changed to NULL

[q at ping dot be]

------- Comment #1 from q at ping dot be  2008-12-29 17:19 -------
Created an attachment (id=17001)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17001&action=view)
preprocessed file that has the problem


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2008-12-29 18:04 -------
plpython.c:2196: warning: dereferencing type-punned pointer will break
strict-aliasing rules


plpython.c:2196: warning: dereferencing pointer '_Py_TrueStruct.537' does break
strict-aliasing rules
plpython.c:2196: warning: dereferencing pointer '_Py_TrueStruct.537' does break
strict-aliasing rules


plpython.c:2956: warning: variable 'xmsg' might be clobbered by 'longjmp' or
'vfork'


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2008-12-29 18:05 -------
  ( (((PyObject *) &_Py_TrueStruct))->ob_refcnt++);


Yes that is obvious an alias violation.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[q at ping dot be]

------- Comment #4 from q at ping dot be  2008-12-29 18:13 -------
pgsql uses -fno-strict-aliasing to compile.

The command that is being used is:
gcc -O2 -Wall -Wmissing-prototypes -Wpointer-arith
-Wdeclaration-after-statement -Wendif-labels -fno-strict-aliasing -fwrapv -g
-fpic -I. -I/usr/include/python2.5 -I../../../src/include -D_GNU_SOURCE   -c -o
plpython.o plpython.c

And I only get those warnings:
plpython.c: In function 'PLyDict_FromTuple':
plpython.c:1733: warning: value computed is not used
plpython.c:1733: warning: value computed is not used


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2008-12-29 20:18 -------
And actually

plpython.c:2196: warning: dereferencing pointer '_Py_TrueStruct.537' does break
strict-aliasing rules

also means that it doesn't get miscompiled ;)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[q at ping dot be]

------- Comment #6 from q at ping dot be  2008-12-30 12:44 -------
So are you saying that because in an unrelated part of the code there is an
aliasing bug gcc can miscompile anything else, even if -fno-strict-aliasing is
used?

The problem is in the PLy_spi_execute_plan() function.  oldcontext ends up
being a NULL pointers while the only place it gets assigned something should
set it to a non-NULL value.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[q at ping dot be]

------- Comment #7 from q at ping dot be  2009-01-02 13:00 -------
Created an attachment (id=17021)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17021&action=view)
Reduced test case part 1


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[q at ping dot be]

------- Comment #8 from q at ping dot be  2009-01-02 13:01 -------
Created an attachment (id=17022)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17022&action=view)
Reduced test case part 2


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q at ping dot be changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #17001|0                           |1
        is obsolete|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[q at ping dot be]

------- Comment #9 from q at ping dot be  2009-01-02 13:04 -------
I've reduced the test case.  The call to siglongjmp() needs to be in a separate
file.

When the problem occurs the test program returns exit code 1.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[pinskia at gcc dot gnu dot org]

------- Comment #10 from pinskia at gcc dot gnu dot org  2009-01-02 20:16 -------
This code might turn out to be undefined ...


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[kurt at roeckx dot be]

------- Comment #11 from kurt at roeckx dot be  2009-01-06 23:44 -------
Which part do you think think is undefined and what would you recommend to
resolve it?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[pinskia at gcc dot gnu dot org]

------- Comment #12 from pinskia at gcc dot gnu dot org  2009-01-20 21:19 -------
THis is most likely the same issue as PR 38587.  Does -fno-ira fix the issue?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[kurt at roeckx dot be]

------- Comment #13 from kurt at roeckx dot be  2009-01-20 21:36 -------
My version of gcc doesn't seem to support the -fno-ira option.  Is that
something that needs to be enabled at compile time?  Can you try my test case
with that option?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[pinskia at gcc dot gnu dot org]

------- Comment #14 from pinskia at gcc dot gnu dot org  2009-01-20 21:41 -------
(In reply to comment #13)
> My version of gcc doesn't seem to support the -fno-ira option.  

Well then it is not a snapshot of GCC 4.4.0.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[kurt at roeckx dot be]

------- Comment #15 from kurt at roeckx dot be  2009-01-20 22:03 -------
I was still using:
gcc (Debian 20081213-1) 4.4.0 20081212 (experimental) [trunk revision 142725]

Which doesn't seem to have that option.

Upgrading to the latest in Debian gives this version:
gcc (Debian 20090107-1) 4.4.0 20090107 (experimental) [trunk revision 143170]

Using that version and the -fno-ira option changes the result of my test case.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660

[Bug middle-end/38660] Pointer value changed to NULL

[hjl dot tools at gmail dot com]

------- Comment #16 from hjl dot tools at gmail dot com  2009-01-21 20:03 -------
I am closing it as dup of PR 38587, which will be fixed by

http://gcc.gnu.org/ml/gcc-patches/2009-01/msg01067.html

Please re-open it if the patch above doesn't fix this bug.

*** This bug has been marked as a duplicate of 38587 ***


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hjl dot tools at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38660