Bug 246248 – [GLib] JPEG2000 decoder produces warning about maybe uninitialized variable

• Component: Images
• Status: RESOLVED FIXED

[Adrian Perez]

Caught by Clang with -Werror=conditional-uninitialized, inside JPEG2000Decoder::decode():

/app/webkit/Source/WebCore/platform/image-decoders/jpeg2000/JPEG2000ImageDecoder.cpp:546:56: error: variable 'a' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
            int adjustedAlpha = image->numcomps > 3 ? (a >> adjust[3]) + ((a >> (adjust[3] - 1)) % 2) : 0xFF;
                                                       ^
/app/webkit/Source/WebCore/platform/image-decoders/jpeg2000/JPEG2000ImageDecoder.cpp:506:27: note: initialize the variable 'a' to silence this warning
            int r, g, b, a;
                          ^
                           = 0
1 error generated.

[Adrian Perez]

Now that I think of it... The mishandling of alpha might related to the
decoding artifacts reported in bug #244528 (or at least be part of the
issue).

[Adrian Perez]

Pull request: https://github.com/WebKit/WebKit/pull/5166

[Adrian Perez]

(In reply to Adrian Perez from comment #1)
> Now that I think of it... The mishandling of alpha might related to the
> decoding artifacts reported in bug #244528 (or at least be part of the
> issue).

*Narrator voice*: It wasn´t.

[Carlos Garcia Campos]

I don't see how this can happen because a is initialized when image->numcomps > 3 and only used when image->numcomps > 3 too.

[Adrian Perez]

(In reply to Carlos Garcia Campos from comment #4)
> I don't see how this can happen because a is initialized when
> image->numcomps > 3 and only used when image->numcomps > 3 too.

I took another look at the code and you are right, but of course
the compiler cannot completely reason about the code and produces
a warning... so in the end I am only silencing the warning in my
patch 🤦️

Still, I think we can merge it to contribute a bit to cleaner
build output logs.

[Carlos Garcia Campos]

Sure, it doesn't hurt, but let's change the bug title and commit message to make it clear it's not fixing anything, but just silencing a warning.

[EWS]

Committed 255674@main (9a76ccc77f6e): <https://commits.webkit.org/255674@main>

Reviewed commits have been landed. Closing PR #5166 and removing active labels.

[Radar WebKit Bug Importer]

<rdar://problem/101290537>