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Subject: Re: gcc 11 weird bug
To: Hannes Domani <ssbssa AT yahoo DOT de>, "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
References: <248361fa-f16d-cebb-eefe-be78e09f4c10 AT towo DOT net>
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From: Thomas Wolff <towo AT towo DOT net>
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Date: Wed, 27 Oct 2021 20:52:34 +0200
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Am 27.10.2021 um 12:55 schrieb Hannes Domani:
>   Am Mittwoch, 27. Oktober 2021, 11:19:19 MESZ hat Thomas Wolff <towo AT towo DOT net> Folgendes geschrieben:
>
>> I noticed that mintty did not compile anymore after upgrade to gcc 11,
>> but only on cygwin 32-bit.
>> I tried to minimize the test case as much as possible without having the
>> bug vanish, to the attached standalone file.
>> Compile this with
>> cc -O2 -Wall -Werror m0.c
>> and it gives a false positive warning about possible uninitialized data
>> usage.
>> While data flow analysis is not perfect, it is weird that this used to
>> happen on 32 bit but not on 64 bit.
>> Meanwhile, after updating some other packages (not sure which), but
>> still the same gcc version, the report on the test case also happens on
>> 64 bit, while the original, unstripped file, as part of mintty, still
>> works without error on 64 bit, which is even weirder.
>> I have not yet had the opportunity to test this on Linux, sorry, so I'm
>> reporting it here.
>> Thomas
> If you mean this warning:
>
> m0.c: In function 'do_bidi':
> m0.c:256:12: error: '*types[0]' may be used uninitialized [-Werror=maybe-uninitialized]
>
> This warning is correct, because as far as gcc is concerned, count could
> be 0, and in this case types[0] will be uninitialized (and doesn't even
> exist, since it's declared as 'uchar types[count];').
Thanks for the hint. I acknowledge that the analyser cannot know that 
count > 0 here. But if types[0] exists, it cannot be unitialized so the 
wording of the warning is not correct in this case. Anyway, this leads 
to a less obtrusive workaround than the current one.
Thomas

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