Re: Need help creating a small test case for g++ 4.0.0 bug

["Paul C. Leopardi" <leopardi at bigpond dot net dot au>]

Hi Janis,
A belated reply to your message is below.
Best regards, Paul Leopardi

On Tue, 17 May 2005 03:23 am, Janis Johnson wrote:
> On Sat, May 14, 2005 at 12:16:54PM +1000, Paul C. Leopardi wrote:
> > Hi all,
> > I originally posted these messages to gcc-help, but had no reply, so I am
> > re-posting links to them here.
> >
> > I think I have found a bug in g++ 4.0.0, but need help in reporting it.
> > Maintainers like their bug reports to include short test cases, but I
> > don't know how to generate a short test case involving inlining. I
> > discovered the original problem by compiling GluCat (
> > http://glucat.sf.net ) and the preprocessor output from a short GluCat
> > test program contains over 66 000 lines of libstdc++, uBLAS and Glucat
> > code.
> >
> > Can anyone help, or should I just file a bug report using the huge test
> > case?
>
> The information in http://gcc.gnu.org/bugs/minimize.html might help.

I have now downloaded, bootstapped and installed gcc 4.0.1. The bug in g++ 
optimization is still there. I've made an attempt to follow the instructions 
on minimizing test cases and have so far accomplished:
wc of old preprocessed source:
  99412  260586 2965538 peg01.ii
wc of new preprocessed source:
  69309  241979 2668391 peg01.ii
As you can see, this is not much of a reduction. The bug I'm seeing keeps 
disappearing as I try to reduce the source code. It seems to be a subtle 
interaction between the Barton-Nackman trick, Boost uBLAS, GNU hash_map and 
the g++ flags -fstrict-aliasing and -finline-functions. If I try to eliminate 
any of these, the bug disappears. 
[ See http://en.wikipedia.org/wiki/Barton-Nackman ]

So I seem to be left with a large ( >2.5MB ) preprocessed source file. Should 
I try to report the bug using this large file as a test case? Or maybe I 
could get one of the gcc developers interested in using GluCat as a test bed 
for detecting bugs in g++ ? 

I am planning to release GluCat 0.1.9 fairly soon in any case, using g++ -O3 
-fno-strict-aliasing as a workaround. Since GluCat is a template library, the 
compiler flags are only used to build test programs, and as a guideline to 
users of the library. So the bug is not holding up GluCat. 

It's just that I'd like to see how a nightmare corner case bug like this gets 
fixed, and I'd like to contribute something to that fix, even though I don't 
know enough about gcc internals to provide a patch.