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Date: Wed, 27 Oct 2021 10:55:43 +0000 (UTC)
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>, Thomas Wolff <towo AT towo DOT net>
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Subject: Re: gcc 11 weird bug
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From: Hannes Domani via Cygwin <cygwin AT cygwin DOT com>
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 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];').


Hannes

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