[Bug tree-optimization/97960] [8/9/10/11 Regression] Wrong code at -O3 since r8-6511-g3ae129323d

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97960

--- 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=<integer_type 0x7ffff68252a0 signed char>, 
    op0=<ssa_name 0x7ffff6816f78 6>, code=NOP_EXPR, op1=<tree 0x0>, 
    var=0x7fffffffba10, off=0x7fffffffba08, cache=..., limit=0x7fffffffc19c)

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

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

but then we continue with

                /* Calculate (ssizetype) OP0 - (ssizetype) TMP_VAR.  */
                widest_int diff = (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.