On Fri, 15 May 2026, Xi Ruoyao wrote: > On Thu, 2026-05-14 at 13:01 +0200, Richard Biener wrote: > > > > > > > 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. > > I'd not pretend I really understand SCEV but after reading the code for > an hour my brain seems to agree.
Mine too, and yeah, I'm only spottily into SCEV (though I guess I'd call myself responsible given the original author is no longer around). It's also the reason I missed this when introducing this bug ... > And I've also bootstrapped & regtested > with a debug assert checking this. So I'll simply use TYPE_UNSIGNED in > v3. > > -- Richard Biener <[email protected]> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg, Germany; GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
