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From: "zsojka at seznam dot cz" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101403] [12 Regression] wrong code with
 __builtin_bswap16() at -O1 by r12-2137
Date: Sat, 10 Jul 2021 19:33:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101403

--- Comment #5 from Zdenek Sojka <zsojka at seznam dot cz> ---
(In reply to Jakub Jelinek from comment #4)
> Isn't the testcase UB? It reads uninitialized u (though it doesn't use the
> value for anything).

You are very right about that! The value is used, even though the use is
optimised out.
I am using -fsanitize=3Dundefined when reducing testcases, but this one was=
n't
caught. The only warning I see with -Wall -W is "warning: left-hand operand=
 of
comma expression has no effect [-Wunused-value]". valgrind also doesn't
complain because, obviously, the uninitialised read is optimised out.=