Re: [java-gnome-hackers] release candidate

[Serkan K. <se...@ge...>]

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Mark Wielaard yazmış:
> On Tue, 2009-02-03 at 17:47 +1100, Andrew Cowie wrote:
>> On Tue, 2009-02-03 at 07:25 +0200, Serkan Kaba wrote:
>>
>>> Sorry I was wrong in the initial finding -O2 reveals the issue.
>> Yup, I can duplicate it now. Well done.
>>
>> Of course I'm none the wiser what that problem means. I wonder what
>> we're doing wrong.
>>
>>> I'll investigate why it does in the evening.
>> Thanks! This is just the sort of thing that can really screw up other
>> people, so I'd like to clear it up if we can.
>>
>> My guess would be that we're doing the idiom of using the JavaVM pointer
>> incorrectly, or are caching it incorrectly, or...
> 
> The problem is the definition of GetEnv which takes a (void **) as
> second argument, but you are pointing it to a (JNIEnv **). I am not sure
> why GetEnv is defined like that. It doesn't seem particular useful. But
> there is nothing you can do about that now since it is a standard
> interface definition. So, this is triggering alarm bells with gcc, with
> -O2 or higher you get:
> 
> -fstrict-aliasing
>      Allows the compiler to assume the strictest aliasing rules
>      applicable to the language being compiled.  For C (and C++), this
>      activates optimizations based on the type of expressions.  In
>      particular, an object of one type is assumed never to reside at
>      the same address as an object of a different type, unless the
>      types are almost the same.
>      [... more examples in the gcc info and man pages ...]
> http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fstrict_002daliasing-751
> 
> You can help gcc out by telling it "yes, this (void **) and (JNIEnv **)
> really point at the same thing" by putting them in a union and accessing
> both through that union:
> 
>   union env_union
>   {
>     void **void_env;
>     JNIEnv **jni_env;
>   };
> 
>   e.jni_env = &env;
>   result = (*cachedJavaVM)->GetEnv (cachedJavaVM,
>                                     e.void_env,
>                                     JNI_VERSION_1_4);
> 
> Then gcc knows that &env and e.void_env really point to the same thing.
> 
> Cheers,
> 
> Mark
Thanks for the remarks and hints. After googling I found 2 solutions.
One[1] being yours and the other[2] being typecasting to a intermediate
void pointer. I tried the latter which at least stops the warning. But
Diego Petteno said that might be just silencing and hiding the real problem)

1: http://osdir.com/ml/java.classpath.patches/2004-08/msg00034.html
2: http://mail.opensolaris.org/pipermail/tools-gcc/2005-August/000048.html
- --
Sincerely,
Serkan KABA
Gentoo/Java

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