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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/108341] argument to `__builtin_ctz` should be
 assumed non-zero
Date: Mon, 09 Jan 2023 16:22:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108341

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Not always.
It depends on the definition of CTZ_DEFINED_VALUE_AT_ZERO.

/* The value at zero is only defined for the BMI instructions
   LZCNT and TZCNT, not the BSR/BSF insns in the original isa.  */
#define CTZ_DEFINED_VALUE_AT_ZERO(MODE, VALUE) \
        ((VALUE) =3D GET_MODE_BITSIZE (MODE), TARGET_BMI ? 2 : 0)
#define CLZ_DEFINED_VALUE_AT_ZERO(MODE, VALUE) \
        ((VALUE) =3D GET_MODE_BITSIZE (MODE), TARGET_LZCNT ? 2 : 0)



So assuming !=3D0 with -mbmi would be an invalid assumitation.=