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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/112758] [13/14 Regression] Inconsistent
 Bitwise AND Operation Result between int and long long int
Date: Fri, 08 Dec 2023 17:37:57 +0000
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X-Bugzilla-Component: rtl-optimization
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X-Bugzilla-Keywords: needs-bisection, wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112758

--- Comment #4 from Segher Boessenkool <segher at gcc dot gnu.org> ---
WORD_REGISTER_OPERATIONS is extremely ill-defined.  Or, it is used for other
things than what it stands for, whichever way you want to look at it.

A backend that defines the macro to non-zero promises that for *any* operat=
ion
on any values in a smaller than full-register mode, the compiler can instead
do the operation in that full-register mode, and all the resulting bits will
be well-defined.  This is not true for most real non-trivial backends.

There is word_register_operation_p to filter out the most obvious and egreg=
ious
cases where WORD_REGISTER_OPERATIONS is just a foolish thing, but this func=
tion
isn't used nearly enough, and it doesn't filter out enough either.=