Bug#630441: g++-4.6 miscompilation

[Philip Ashmore <contact@philipashmore.com>]

On 04/09/11 15:21, Matthias Klose wrote:
> tag 630441 moreinfo help
> thanks
>
> On 07/21/2011 12:01 PM, Philip Ashmore wrote:
> > Sorry if I wasn't clear.
> > All the tests pass in the debug (-0O) build.
> >
> > I've got gcc/g++ 4.4.6-6 installed and all the tests pass in debug 
> > and release
> > mode.
> > This is a problem with the g++ 4.6 release (-03) optimization.
>
> unproven.
Well you could try it yourself, but that might be unproven too =)

> Repeating my questions here:
>
> how do you know that it's not undefined behaviour exposed by the new 
> compiler
> version?
I reported this bug because of undefined behaviour of the compiler.

> Some more information is needed:
>
>  - is this seen on amd64 only, or on other architectures too?
See Message 27 above 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=630441#27

A 32 bit chroot means 32 bit, unless I'm mistaken.

>  - which optimization flags are used? does lowering the optimization
>    level works around the issue?
See message 22 above 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=630441#22

>  - does it build using gcc-snapshot, gcc-4.5 or gcc-4.4?
I have gcc and g++ locked to version 4:4.4.5-1, where they work.

time sh build.sh (from Message 12 above 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=630441#12)
...

real    6m52.589s
user    5m37.477s
sys     0m31.150s

The test fails with gcc-snapshot-20110816-1 (attached) at the same place.

time sh build-with-gcc-snapshot.sh
...
real    12m9.970s
user    10m57.633s
sys     0m29.250s


>  - if you have a working and a non-working build, can you try
The build fails due to a test failure caused by the compiler generating 
incorrect code.
If we want to fix this bug then working around it won't help.
The same problem may happen at any time in anyones code.
The fact that my code caught the problem in a test where it could be 
spotted is sheer good/bad
luck, and is reproducible.

>    to combine object files to determine the problematic object
>    file?
Philip

Attachment: build-with-gcc-snapshot.sh
Description: Bourne shell script