[Bug tree-optimization/114396] [13/14 Regression] Vector: Runtime mismatch at -O2 with -fwrapv since r13-7988-g82919cf4cb2321

["liuhongt at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114396

--- Comment #15 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> (In reply to Robin Dapp from comment #8)
> > No fallout on x86 or aarch64.
> > 
> > Of course using false instead of TYPE_SIGN (utype) is also possible and
> > maybe clearer?
> 
> Well, wi::from_mpz doesn't take a sign argument.  It's comment says
> 
> /* Returns X converted to TYPE.  If WRAP is true, then out-of-range
>    values of VAL will be wrapped; otherwise, they will be set to the
>    appropriate minimum or maximum TYPE bound.  */
> wide_int
> wi::from_mpz (const_tree type, mpz_t x, bool wrap)
> 
> I'm not sure if we really want saturating behavior here, so 'true' is
> more correct?  Note if we want an unsigned result we should pass utype here,
> that might be the bug?  So
> 
>         begin = wi::from_mpz (utype, res, true);
> 
> ?
Yes, it should be.
> 
> The to_mpz args look like they could be mixing signs as well:
> 
>     case vect_step_op_mul:
>       {
>         tree utype = unsigned_type_for (type);
>         init_expr = gimple_convert (stmts, utype, init_expr);
>         wide_int skipn = wi::to_wide (skip_niters);
>         wide_int begin = wi::to_wide (step_expr);
>         auto_mpz base, exp, mod, res;
>         wi::to_mpz (begin, base, TYPE_SIGN (type));
> 
> TYPE_SIGN (step_expr)?
step_expr should have same type as init_expr.
> 
>         wi::to_mpz (skipn, exp, UNSIGNED);
> 
> TYPE_SIGN (skip_niters) (which should be UNSIGNED I guess)?
skipn must be a postive value, so I assume UNSIGNED/SIGNED doesn't make any
difference here.