RFR: 8298006: Build failure by maybe-uninitialized error on Linux s390x GCC8

[Phil Race]

On Mon, 5 Dec 2022 18:22:15 GMT, Phil Race <prr at openjdk.org> wrote:

>> I changed GCC toolchain from GCC6 to GCC8 on SLES12SP5 Linux s390x.
>> I could see following errors:
>> 
>> src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c: In function 'allocateRasterArray':
>> src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c:2944:73: error: 'roff[3]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>>                              (((*inP&rasterP->sppsm.maskArray[c]) >> roff[c])
>>                                                                      ~~~~^~~
>> 
>> 
>> According to error messages,
>> roff and loff may not be initialized.
>
>> src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c: In function 'allocateRasterArray':
>> src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c:2944:73: error: 'roff[3]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>>                              (((*inP&rasterP->sppsm.maskArray[c]) >> roff[c])
> 
> That's weird. allocateRasterArray is at lines 2360-2575 .. so why are you seeing it at line 2944 ????

> Thanks @prrace .
> 
> > That's weird. allocateRasterArray is at lines 2360-2575 .. so why are you seeing it at line 2944 ????
> 
> Ummm, I could not determine the reason...
> 
> I assume AddressSanitizer detected memory errors because of following 2 codes:
> 
> https://github.com/openjdk/jdk/blob/1e468320dc6e495343dbdf392cbcbda79672c7ad/src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c#L2937
> 
> 
> https://github.com/openjdk/jdk/blob/1e468320dc6e495343dbdf392cbcbda79672c7ad/src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c#L3118
> 
> So I'd like to put `memset()` on these parts.
> 
> But I checked `if` and `else` blocks on above part. These are exactly the same.
> 
> https://github.com/openjdk/jdk/blob/1e468320dc6e495343dbdf392cbcbda79672c7ad/src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c#L2937-L2965
> 
> 
> https://github.com/openjdk/jdk/blob/1e468320dc6e495343dbdf392cbcbda79672c7ad/src/java.desktop/share/native/libawt/awt/medialib/awt_ImagingLib.c#L3118-L3153
> 
> I tried to modify from `if .. else` block to flat. AddressSanitizer did not detect memory error.
> 
> Could you give me any advice ?

Oh you are running a tool to find these issues ? I thought it was supposed to be a build failure ?
Or are you running the tool to try to understand the build failure.
Tools are not always right in my experience.
I think there's just as much chance as both being false positives as both being correct.
I think at some point the tools arm-wave that this could be a problem but it is not provable. 
As Magnus pointed out "newer" compilers aren't complaining here. 

I don't know what kind of advice you are seeking. If you are asking me to somehow do better than the tools .. well I'd say its a whole lot easier to just memset these to zero. Zero may not be the correct offset either .. but it should be safer.

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PR: https://git.openjdk.org/jdk/pull/11475