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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/115005] [gcc-13] bogus -Warray-bounds warning
Date: Thu, 09 May 2024 16:29:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115005

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Folding statement: _336 =3D cmd_8bits[numbits_181];
Folded into: _336 =3D cmd_8bits[4294967295];


Which comes from:

                uint32 d =3D get_bits(in, numbits+1, &bb);

                if (d >=3D cmd_8bits[numbits]) {

get_bits:
static uint32 get_bits(HIO_HANDLE *f, int n, struct bit_buffer *bb)
{
        uint32 bits;

        if (n =3D=3D 0) {
                return 0;
        }

There is a jump threading there handling n=3D=3D0 (aka numbits=3D=3D-1u) an=
d that is
causing the warning. Now if I understand it numbits can't be more than 128 =
(or
even 16 in the 16bit case).

Adding `if (numbits >=3D 128) __builtin_unreachable();` right before the gi=
t_bits
gets rid of the warning and seems like would produce better code.

The reason why using smaller types works is well (numbits+1) can never be 0=
.=