https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127814
Drea Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=111280
--- Comment #8 from Drea Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #7)
> So looking myself when we apply versioning we are doing the cost check
> before checking the assumptions and glob everything into a single BB:
>
> _2439 = (unsigned int) cb_838;
> _2438 = _4 + -1993837876;
> _2437 = _2438 * -520093677;
> _2436 = (unsigned int) _2437;
> _2435 = _2439 - _2436;
> _2434 = _2435 + 1040187353;
> _2433 = _2434 > 1040187353; <- cost check
> _2432 = _4 + -1993837876;
> _2431 = _2432 * -520093677;
> _2430 = _2431 >= -1107296295;
> _2429 = _4 + -1993837876;
> _2428 = _2429 * -520093677;
> _2427 = _2428 + -1040187353;
> _2426 = cb_838 >= _2427;
> _2425 = _2426 & _2430; <- assumption
> _2424 = _2425;
> _2423 = _2424 & _2433;
>
> we then split off the cost check but not the assumption. There are many more
> tests (version for alias checks, etc.) that could be similarly affected.
>
> We still have to avoid the may_be_zero combination with the assumptions,
> neither the assumptions nor the may_be_zero check are using unsigned
> arithmetic.
>
> I'll note the existing combination of may_be_zero with niter suffers from
> a similar issue. It does
>
> niter = fold_build3 (COND_EXPR, TREE_TYPE (niter),
> may_be_zero,
> build_int_cst (TREE_TYPE (niter), 0),
> rewrite_to_non_trapping_overflow
> (niter));
>
> so caring for -ftrapv but not UB. I think all of the callers of that
> function want to avoid UB, so we should mutate that function to the GENERIC
> variant of rewrite_to_defined_unconditional. We might want to think
> of even adjusting the gimplifier to handle COND_EXPR / TRUTH_{AND,OR}IF_EXPR
> gimplification by rewriting to defined.
a related COND_EXPR issue with the gimplifier has come up before in PR 111280
and there rewrtiting to defined does not help though (though I can think of a
way to make CLZ/CTZ defined in that case, BIT_IOR in either INT_MIN or 1).
Let me ponder on this for a few days.
