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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/97960] [8/9/10/11 Regression] Wrong code at
 -O3 since r8-6511-g3ae129323d
Date: Tue, 24 Nov 2020 15:12:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97960

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Creating dr for b[_7]
        base_address: &b
        offset from base address: (ssizetype) ((sizetype) (signed char) _5 =
* 4)
        constant offset from base address: -1012
        step: 4
        base alignment: 32
        base misalignment: 0
        offset alignment: 4
        step alignment: 4
        base_object: b
        Access function 0: {(int) h_20, +, 1}_2

looks like the wrong sign for the constant offset.

#0  split_constant_offset_1 (type=3D<integer_type 0x7ffff68252a0 signed cha=
r>,=20
    op0=3D<ssa_name 0x7ffff6816f78 6>, code=3DNOP_EXPR, op1=3D<tree 0x0>,=20
    var=3D0x7fffffffba10, off=3D0x7fffffffba08, cache=3D..., limit=3D0x7fff=
ffffc19c)

now, var_min/max is UNSIGNED -3 / -1 (precision 8), woff is 3
we compute -3 + 3 =3D=3D 0 and overflow to true (UNSIGNED arithmetic)

_5 =3D (unsigned char) _35;
_6 =3D _5 + 3;
h_20 =3D (signed char) _6;

but then we continue with

                /* Calculate (ssizetype) OP0 - (ssizetype) TMP_VAR.  */
                widest_int diff =3D (widest_int::from (op0_min, sgn)
                                   - widest_int::from (var_min, sgn));

getting -253.  I remember this place has changed quite some times and
wide (sign-extended) vs. widest (signed) ints do not make it easier
to see what's correct ...

I'm defering to Richard here.  The C testcase trips at this point just
twice (ldist and vectorizer) so it's easy enough to 'catch' in a debugger.=