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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114032] ifcvt may introduce UB calls to
 __builtin_clz(0)
Date: Thu, 22 Feb 2024 11:03:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114032

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
In GCC 13 and earlier, .CLZ(0) was UB depending on CLZ_VALUE_DEFINED_AT_ZERO
(...) !=3D 2.
In GCC 14, it is always UB, and well defined is only .CLZ(0, 32) (but with =
the
current
requirement that the second argument, value at zero, matches
CLZ_VALUE_DEFINED_AT_ZERO (...) =3D=3D 2 filled in value for scalars (excep=
tion for
bitints).
If we wanted, we could (but probably stage1 material) allow any constant se=
cond
argument and just expand to conditional if the value doesn't match
CLZ_VALUE_DEFINED_AT_ZERO (...) !=3D 0 filled in value.
But, still, for the ifcvt and vectorization it would be useful to know what=
 is
the well defined value that will be cheapest to compute.
The backends could do that e.g. by supplying details for
CLZ_VALUE_DEFINED_AT_ZERO on vector modes even if scalar is undefined.
Or have some third way which would say this is not UB at 0, but we don't re=
ally
care what value at zero it has (but then e.g. VRP would need to give up on
that).=