Ubuntu
gcc-4.5 package

  1. Bug #817768
  2. Comment #5

Comment 5 for bug 817768

Revision history for this message
kornelix (mikecorn-deactivatedaccount) wrote : Re: [Bug 817768] Re: GCC optimizer removes necessary code

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
>