[Bug tree-optimization/125291] [16/17 Regression] libarchive-3.8.7 miscompiled on loongarch64-linux-gnu since r16-3562

[xry111 at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125291

--- Comment #4 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
Tentative untested patch:

diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index ecdef7529a6..b55ae0586c8 100644
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -1377,10 +1377,16 @@ 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 r = build_polynomial_chrec (loop->num, init_cond,
+                                          chrec_convert (TREE_TYPE (ev),
+                                                         right, NULL,
+                                                         false, NULL_TREE));
+         /* (u8)[6, 260, 514] is [6, 4, 2], not [6, 260, 514].  */
+         if (tree_int_cst_sign_bit (right)
+             == (scev_direction (r) == EV_DIR_DECREASES))
+           return r;
+       }
       return chrec_dont_know;
     }