Seeking advice on creating reduced testcase

[Jeff Law]

On 09/24/2015 03:40 PM, Johan Alfredsson wrote:
> Hi,
>
> I have come across something which I believe is a bug in g++. I've
> seen this issue both in 4.9.1 and 5.2.0, but I'm struggling to create
> a small testcase. (The output of g++ -v for 5.2.0 is below.)
>
> The issue is a deadlock that seems to be due to there being (runtime)
> the equivalent of _two_ different variables __gthread_active_ptr. It
> just so happens that in the libstdc++ pool allocator template, the
> allocate() function, for a certain instantiation, is compiled so that
> the lock() method of the __scoped_lock (in ext/concurrence.h) is
> getting 'true' as the result of its call to __gthread_active_p(),
> while in the unlock() method it gets 'false'. The next allocation of
> the same type will of course deadlock due to not releasing the mutex
> after the previous allocation.
For a locking or race condition issue, you might consider using 
hellgrind to gain an understanding of what's going wrong without having 
to further reduce the source.    Once you have that understanding, it 
may be possible to show the problem with simpler source or just explain 
what's going wrong (if indeed it is a compiler issue).

>
> Looking at the assembly code, in the unlock() method the code
>
>    if (__gthread_active_p())
>
> is actually reduced to
>
>    mov $0x0,%eax
>    test %eax,%eax
>
> so somehow g++ managed (erroneously) to fold this runtime-test during
> compile time (at least that is my guess). In the lock() method it
> looks like this:
That would be an indication that the compiler thinks all the other tests 
were redundant.  The compiler is quite aggressive at trying to track 
this kind of stuff.

jeff