> Am 14.05.2026 um 10:59 schrieb Xi Ruoyao <[email protected]>:
>
> On Thu, 2026-05-14 at 09:51 +0200, Richard Biener wrote:
>
>
> /* snip */
>
>>> diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
>>> index f524786f33b..ec3c9ab4f3d 100644
>>> --- a/gcc/tree-scalar-evolution.cc
>>> +++ b/gcc/tree-scalar-evolution.cc
>>> @@ -1391,10 +1391,22 @@ simplify_peeled_chrec (class loop *loop, tree arg,
>>> tree init_cond)
>>> && wi::to_widest (init_cond) == wi::to_widest (left_before)
>>> && !scev_probably_wraps_p (NULL_TREE, left_before, right, NULL,
>>> loop, false))
>>> - return build_polynomial_chrec (loop->num, init_cond,
>>> - chrec_convert (TREE_TYPE (ev),
>>> - right, NULL,
>>> - false, NULL_TREE));
>>> + {
>>> + tree tp = TREE_TYPE (right);
>>> +
>>> + /* We likely need a sign-extension to make things like
>>> + u8(6, 4, 2) => i32(6, 4, 2), instead of i32(6, 260, 514).
>>> + But u8(1, 129) shouldn't be i32(1, -127) either. */
>>> + if (TYPE_UNSIGNED (tp)
>>> + && wi::to_widest (TYPE_MAX_VALUE (tp)) <
>>> + (wi::to_widest (init_cond) + wi::to_widest (right)))
>>
>> I’d say we just want the TYPE_UNSIGNED check here, esp. it shouldn’t depend
>> on init_cond and also work for symbolic right.
>
> So wouldn't doing so make u8(1, 129) => i32(1, -127) and misoptimize
> that? Or maybe we cannot have a loop iterating only twice here?
I think SCEV probably warp_p guarantees the unsigned IV doesn’t underflow when
the step is interpreted as signed (underflow zero). So yes, I think we know
u8(1, 128) does not actually iterate.
Richard
>>
>
> --
> Xi Ruoyao <[email protected]>
