[Bug c++/31268] New: Non-deterministic bug producing a run-time infinite loop

[Bug c++/31268] New: Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

It took me several hours to try to extract a usable test-case for this issue,
so I hope you will be able to make sense out of it.

The problem : compiling with current g++ 4.2, with -O2, makes the attached
program loop infinitely, while it is not expected to.

It does not loop with g++ 4.3 or older versions than 4.2.  It also does not
loop when adding -fno-strict-aliasing with g++ 4.2, or when compiling with
-O only.

Even more strange : it does not loop when I remove some unused bits of
the program (which is why I had a hard time shrinking it down), for example
unused typedefs (look for "limb2").

The relevant part of the program is a small class (MP_Float) containing an
std::vector<short>, and some code around it, namely the operator_minus()
function which is called, and which loops.

I attach the pre-processed file, as well as the main small file so that you
can see what is the relevant part of it, and decide if it is a compiler bug
or an issue with my program.  My program does some type conversions, which
may be the source of the problem (triggering undefined behavior ?), but they
seem fine to me.


-- 
           Summary: Non-deterministic bug producing a run-time infinite loop
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sylvain dot pion at sophia dot inria dot fr
  GCC host triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #1 from sylvain dot pion at sophia dot inria dot fr  2007-03-19 16:27 -------
I will happily create the attachements when bugzilla will work...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2007-03-19 16:32 -------
Just wild guessing - try -fwrapv.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2007-03-19 16:45 -------
(In reply to comment #2)
> Just wild guessing - try -fwrapv.

Well it already does not look for -fno-strict-aliasing so I am assuming an
aliasing bug in your code until you provide the sources.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #4 from sylvain dot pion at sophia dot inria dot fr  2007-03-19 16:50 -------
(sorry I still can't create attachments)

-ftrapw makes the program work (no loop).

Let me copy-paste here the non-preprocessed source files which show everything
which is executed, while waiting for bugzilla to allow me to add the large
pre-processed file.

-----------------------------
#ifndef CGAL_MP_FLOAT_H
#define CGAL_MP_FLOAT_H

#include <vector>

typedef short      limb;   // unused
typedef int        limb2;  // unused

struct MP_Float
{
  typedef std::vector<short>  V;
  typedef V::iterator        iterator; // unused

  V v;
  int exp; // unused

  MP_Float(short i)
    : v(1)
  {
    v[0] = i;
    canonicalize();
  }

  void remove_leading_zeros()
  {
    while ((!v.empty()) && (v.back() == 0))
      v.pop_back();
  }

  void remove_trailing_zeros()
  {
    if (v.empty() || (v.front() != 0))
      return;

    V::iterator i = v.begin();
    for (++i; *i == 0; ++i) ;
    //v.erase(v.begin(), i);
  }

  void canonicalize()
  {
    remove_leading_zeros();
    remove_trailing_zeros();
  }

  // replacing int by std::size_t appears to also fix the loop...
  int max_exp() const
  {
    return v.size();
  }

  short of_exp(int i) const
  {
    if (i >= max_exp()) return 0;
    return v[i];
  }
};

// This union is used to convert an unsigned short to a short with
// the same binary representation, without invoking implementation-defined
// behavior (standard 4.7.3).
union to_signed {
    unsigned short us;
    short s;
};

inline
void split(int l, short & high, short & low)
{
    to_signed l2 = {l};
    low = l2.s;
    high = (l - low) >> 16;
}

MP_Float
operator_minus(const MP_Float &a, const MP_Float & b /* unused */)
{
  int max_exp = std::max(a.max_exp(), b.max_exp());

  MP_Float r(0);
  r.v.resize(2);
  for(int i = 0; i < max_exp ; ++i)
  {
    int tmp = r.v[i] + (a.of_exp(i) - b.of_exp(i));
    split(tmp, r.v[i+1], r.v[i]);
  }
  r.canonicalize();
  return r;
}

#endif // CGAL_MP_FLOAT_H

// #include <CGAL/MP_Float_loop.h>
#include <CGAL/number_type_basic_loop.h> // this one pulls up unrelated stuff
but necessary for the bug to show up

int main()
{
  MP_Float a=2, b=1;
  MP_Float d= operator_minus(a, a);
}

-----------------------------


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #5 from sylvain dot pion at sophia dot inria dot fr  2007-03-19 16:55 -------
Subject: Re:  Non-deterministic bug producing a run-time infinite
 loop

Let me try to attach the pre-processed file through an email
attachement.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #6 from sylvain dot pion at sophia dot inria dot fr  2007-03-19 19:37 -------
Created an attachment (id=13235)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13235&action=view)
pre-processed source file


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[mmitchel at gcc dot gnu dot org]

------- Comment #7 from mmitchel at gcc dot gnu dot org  2007-03-26 01:44 -------
Richard, are you able to confirm this bug?


-- 

mmitchel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2007-03-26 01:59 -------
This works on the trunk at least on powerpc-darwin.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[rguenth at gcc dot gnu dot org]

------- Comment #9 from rguenth at gcc dot gnu dot org  2007-03-26 09:46 -------
Seems to work for me on x86_64 with -m32.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug c++/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #10 from sylvain dot pion at sophia dot inria dot fr  2007-03-26 18:03 -------
Let me mention that this is against 4.2.  The trunk works well for me.
I tried several times during March (including today), and the bug is still
here.

Note that varrying the conditions slightly (even removing an innocent unused
typedef) makes it work, so I'm not surprised that testing on x86_64 or powerpc
makes it also work.  It sounds like usage of uninitialized memory or some bad
non-deterministic bug of this kind, although a quick test with valgrind did not
show anything.

Here are more details about my configuration: Fedora Core 5 on x86, compiler
built with:
> .../g++ -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: /proj/geometrica/home/GCC/gcc-4_2-branch/configure
--disable-shared --disable-nls --enable-languages=c++
--prefix=/proj/geometrica/home/GCC/Linux-fc5-4.2
Thread model: posix
gcc version 4.2.0 20070324 (prerelease)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug middle-end/31268] Non-deterministic bug producing a run-time infinite loop

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2007-04-15 06:32 -------
I can reproduce this with -O2, with -O2 -fno-strict-aliasing, the infinite loop
goes away so this might be an aliasing violation.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug middle-end/31268] Non-deterministic bug producing a run-time infinite loop

[pinskia at gcc dot gnu dot org]

------- Comment #12 from pinskia at gcc dot gnu dot org  2007-04-15 06:32 -------
But note this was with a compiler from March 9th so this might already be
fixed.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug middle-end/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #13 from sylvain dot pion at sophia dot inria dot fr  2007-04-17 13:17 -------
I just built g++ 4.2 yesterday, and the failure is still there.
Note that if you want to check for an aliasing violation, even though the
preprocessed code is huge, the parts which are executed are relatively small
(see comment #4).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug middle-end/31268] Non-deterministic bug producing a run-time infinite loop

[rguenth at gcc dot gnu dot org]

------- Comment #14 from rguenth at gcc dot gnu dot org  2007-12-11 18:58 -------
I cannot reproduce this on native i686 either with

g++-4.2 --version
g++-4.2 (GCC) 4.2.3 20071123 (prerelease) (Debian 4.2.2-4)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug middle-end/31268] Non-deterministic bug producing a run-time infinite loop

[sylvain dot pion at sophia dot inria dot fr]

------- Comment #15 from sylvain dot pion at sophia dot inria dot fr  2007-12-23 13:34 -------
I also cannot reproduce it with todays' g++ 4.2.

That said, the original code from which the test-case is extracted is still
failing.  The original code is from the CGAL library.  I can give a way
to reproduce it if someone is interested in digging this issue.

As original submitter, I do not care that much about it, since g++ 4.3 is not
affected, and there is an easy workaround for g++ 4.2 (-fno-strict-aliasing).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268

[Bug middle-end/31268] Non-deterministic bug producing a run-time infinite loop

[pinskia at gcc dot gnu dot org]

------- Comment #16 from pinskia at gcc dot gnu dot org  2008-09-03 02:19 -------
Lets mark this as fixed, there have been some aliasing fixes between the
release of 4.2.0 and now.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
           Keywords|                            |alias, wrong-code
         Resolution|                            |FIXED
   Target Milestone|---                         |4.2.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31268