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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/98425] Superfluous sign-extend for constrained
 integer
Date: Wed, 23 Dec 2020 10:40:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98425

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Sorry, in this case it isn't wrong (that is the passing of char/short case),
the __builtin_unreachable () says that negative values are undefined, there=
fore
the compiler can use both sign and zero extension interchangeably.
And the undefined signed overflow means when we have:
int i;
...
((unsigned long long) (i + 4)) * 4
we can compute it as
((unsigned long long) (i + 4)) * 4
or
((unsigned long long) i) * 4 + 16
because if i is in [INT_MAX - 3, INT_MAX] it is UB.=