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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97976] Optimization relating to NULL pointer assumptions in
 gcc 9.1
Date: Wed, 25 Nov 2020 18:18:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97976

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |INVALID

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Peter Bisroev from comment #5)
> So my question is, is gcc allowed to assume that the target will always
> match?

Yes. If p is not null then it points to a value object. It is undefined to
decrement a pointer "before" the start of an object (you can only increment=
 to
one position past the end). That means the compiler can assume that a non-n=
ull
pointer never becomes null, because it would have to point before the start=
 of
the object to do so. So if it can't become null, the !p check is always fal=
se,
and it must return true.=