[OpenJDK 2D-Dev] RFR: 8266171: -Warray-bounds happens in imageioJPEG.c

[Yasumasa Suenaga]

On Thu, 29 Apr 2021 20:54:47 GMT, Phil Race <prr at openjdk.org> wrote:

>> We can see following compiler warnings in imageioJPEG.c on GCC 11.
>
> src/java.desktop/share/native/libjavajpeg/imageioJPEG.c line 673:
> 
>> 671:         if (info->is_decompressor) {
>> 672:             j_decompress_ptr dinfo = (j_decompress_ptr) info;
>> 673: #ifdef __GNUC__
> 
> I know how these structs are defined but I am not sure how gcc can decide anything like this.
> I'd almost worry if it were true that we had the other type despite what the flag said except I can't
> imagine gcc is doing even any static analysis of the code calling sequence and you may even need a dynamic analysis for this.
> 
> Have you submitted a gcc bug ?
> Why is it only complaining in this branch ?
> Have you considered disabling the warning in the make files - with broader scope of course - but a simpler change ?
> 
> Is 520 bytes the actual size of the compress struct ? And even then I am not sure I know what the compiler message means. so long as we have the right starting address free will only free what was allocated ... surely ...

I also think GCC does not do any static analysis of the code calling. And also the code uses `struct` like a C++ class, so I guess GCC reports incorrect warnings at this point.
Many -Warray-bounds related issues are reported in [GCC Bugzilla](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56456). Especially it does not seem to work when the value is accessed by a pointer https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99474 - it looks alike to me.

WebKit seems to encounter [similar issue](https://bugs.webkit.org/show_bug.cgi?id=224782), they has avoided it with pragma.

I concidered to disable it with a compiler option in makefile of course as you said, but I didn't do so because I heard in other review (I forget the JBS ticket) that we should disable warnings locally because they might be useful in future.

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PR: https://git.openjdk.java.net/jdk/pull/3788