[Bug 817768] Re: GCC optimizer removes necessary code

[kornelix]

I reported it years ago and never heard anything. I don't even know if the
bug is still there in more recent releases of gcc. Once I recognized the
problem, it was easy to get around by adding do-nothing statements to
reference variables whose code was getting optimized out. I also tried to
make a small test case to illustrate the problem but this test always
worked OK.

If you delete the bug from Ubuntu tracking that is OK with me.

regards
Mike


2012/3/6 Václav Šmilauer <eu at doxos.eu>

> Has this bug been reported to gcc developers? In that case, it can be
> tracked automatically.
>
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> https://bugs.launchpad.net/bugs/817768
>
> Title:
>  GCC optimizer removes necessary code
>
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https://bugs.launchpad.net/bugs/817768

Title:
  GCC optimizer removes necessary code

Status in “gcc-4.5” package in Ubuntu:
  New

Bug description:
  GCC 4.5.2 in Ubuntu 11.04

  The program below crashes when compiled with GCC 4.5.2 with optimization
  level -O3. This is apparently a bug in GCC.

  The crash does not occur if the program is compiled with an earlier
  release of GCC, also with -O3 optimization.

  The crash does not occur if the program is compiled without
  optimization.

  The crash does not occur if a do-nothing line of code is added which
  makes a reference to the variable involved. Apparently this stops GCC
  from optimizing-out necessary code. 

  Crash matrix

  GCC level    Optimization    dummy code    result
  4.4.5             -O3                      no                     OK
  4.5.2             -O0                      no                     OK
  4.5.2             -O3                      no                     crash
  4.5.2             -O3                      yes                    OK

  There is the crash report from GDB:

  Program received signal SIGSEGV, Segmentation fault.
  0x0000000000498051 in pvlist_remove (pv=0x306b6fc, ii=<value optimized out>) at zfuncs.cc:1774
  1774	      pv->list[ii-1] = pv->list[ii];
  (gdb) q

  Here is the relevant part of the program. The commented line will never
  execute the printf() call but it does stop the program from crashing.

  struct  pvlist {
     int      max;                          //  max. entries
     int      act;                          //  actual entries
     char   **list;                         //  entries
  };

  pvlist * pvlist_create(int max);                                           //  create pvlist
  void pvlist_free(pvlist *pv);                                              //  free pvlist
  int pvlist_append(pvlist *pv, cchar *entry, int unique = 0);               //  append new entry (opt. if unique)
  int pvlist_prepend(pvlist *pv, cchar *entry, int unique = 0);              //  prepend new entry (opt. if unique)
  int pvlist_find(pvlist *pv, cchar *entry);                                 //  find entry by name
  int pvlist_remove(pvlist *pv, cchar *entry);                               //  remove entry by name
  int pvlist_remove(pvlist *pv, int Nth);                                    //  remove entry by number (0...)
  int pvlist_count(pvlist *pv);                                              //  return entry count
  int pvlist_replace(pvlist *pv, int Nth, cchar *entry);                     //  replace Nth entry (0...)
  char * pvlist_get(pvlist *pv, int Nth);                                    //  return Nth entry (0...)
  int pvlist_sort(pvlist *pv);                                               //  sort list, ascending

  
  //  remove an entry by number and repack list
     
  int pvlist_remove(pvlist *pv, int ii)
  {
     if (ii < 0 || ii >= pv->act) return -1;
     zfree(pv->list[ii]);
     for (ii++; ii < pv->act; ii++) {
        if (! pv->act) printf("meaningless reference %d",ii);                //  get around GCC optimization bug
        pv->list[ii-1] = pv->list[ii];
     }
     pv->act--;
     return 0;
  }

  ProblemType: Bug
  DistroRelease: Ubuntu 11.04
  Package: gcc 4:4.5.2-1ubuntu3
  ProcVersionSignature: Ubuntu 2.6.38-10.46-generic 2.6.38.7
  Uname: Linux 2.6.38-10-generic x86_64
  NonfreeKernelModules: nvidia
  Architecture: amd64
  Date: Fri Jul 29 00:00:30 2011
  InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
  ProcEnviron:
   LANGUAGE=en_US:en
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: gcc-defaults
  UpgradeStatus: No upgrade log present (probably fresh install)

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