[Bug 817768] Re: GCC optimizer removes necessary code
kornelix
mikecorn at arcor.de
Wed Aug 10 21:49:46 UTC 2011
I conducted these experiments and here are the results:
+ GCC 4.4.5 in Ubuntu 10.04: NO BUG
+ GCC 4.5.2 in Ubuntu 11.04: BUG PRESENT
+ GCC 4.6.1 in Ubuntu 11.10 (alpha 3): NO BUG
Other factors:
+ The bug goes away if optimization is -O0 instead of -O3
+ The bug goes away if a do-nothing line of code is inserted to reference the variable "ii" whose initialization is apparently being removed with -O3 optimization.
I was not able to make a minimal program to demonstrate the bug. The bug
went away in the minimal program. Therefore I have attached the full
application which demonstrates the bug.
How to demonstrate the bug:
0. Use Ubuntu 11.04 with GCC 4.5.2 and libgtk2.0-dev installed
1. unpack the tarball: $ tar -xzf mashup-2.9.tar.gz
2. build the application: $ cd mashup; $ make
3. run the application: $ ./mashup
4. press the toolbar button with the printer icon (3rd from last)
5. choose the last "paper format" in the drop-down list: "custom N.N x N.N cm"
6. edit this item by deleting characters from the end.
This should be permitted with no crashing.
7. $ gedit zfuncs.cc
8. search for "////".
This is a line in the function pvlist_remove(), commented "stop g++ optimization bug".
This is a do-nothing line of code whose purpose is to reference the variable "ii".
Make sure you are in pvlist_remove() since "////" is present in a few other places.
9. comment this line away by adding "//" before the line
10. rebuild: $ make clean; $ make
11. repeat steps 3-6. After deleting two characters, the program crashes with seg-fault
12. $ gedit Makefile
13. replace the GCC optimization -O3 with -O0
14. repeat steps 3-6. The bug is gone.
There is another instance of this bug in another function,
image_navi::image_navigate(). See the line of code in that function
containing the comment "stop g++ optimization bug". This is an unrelated
function demonstrating the same bug, but testing in this case is more
complex.
On 08/08/2011 03:21 PM, Matthias Klose wrote:
> - please recheck with GCC 4.5 and GCC 4.6 in oneiric.
> - please attach a self-contained example, including the command options used
> - include the warnings building the file.
>
> ** Changed in: gcc-defaults (Ubuntu)
> Status: New => Incomplete
>
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https://bugs.launchpad.net/bugs/817768
Title:
GCC optimizer removes necessary code
Status in “gcc-defaults” package in Ubuntu:
Incomplete
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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