[Bug tree-optimization/128140] [17 Regression] wrong code at -O2: vectorized x >= C ? x : C - 1 gives a wrong result in the lane where C wraps around to 0 since r17-3277-g729eb68a0021bd

[yinyuefengyi at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128140

Xionghu Luo (luoxhu at gcc dot gnu.org) <yinyuefengyi at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |yinyuefengyi at gmail dot com

--- Comment #2 from Xionghu Luo (luoxhu at gcc dot gnu.org) <yinyuefengyi at 
gmail dot com> ---
Got a fix candidate:


diff --git a/gcc/tree.cc b/gcc/tree.cc
index 7e5a53234ba..ba9daae7b52 100644
--- a/gcc/tree.cc
+++ b/gcc/tree.cc
@@ -10993,7 +10993,25 @@ uniform_vector_difference_p (const_tree t1, const_tree
t2)
   widest_int diff;
   bool diff_p = false;

-  if (VECTOR_CST_LOG2_NPATTERNS (t1) == VECTOR_CST_LOG2_NPATTERNS (t2)
+  unsigned HOST_WIDE_INT nelts;
+  bool nelts_constant_p = VECTOR_CST_NELTS (t1).is_constant (&nelts);
+
+  if (nelts_constant_p)
+    {
+      /* Compare every element rather than just the encoded ones whenever
+       the number of elements is known.  A stepped vector only
+       encodes the start of the step, which hides a wrap-around:
+       { 253, 254, 255, 0, ... } and { 252, 253, 254, 255, ... }
+       both encode to pairs that differ by one, whereas the lane
+       in which the step wraps differs by -255.
+       PR tree-optimization/128140.  */
+      for (unsigned HOST_WIDE_INT i = 0; i < nelts; ++i)
+       if (!record_uniform_integer_difference (vector_cst_elt (t1, i),
+                                               vector_cst_elt (t2, i),
+                                               &diff, &diff_p))
+         return NULL_TREE;
+    }
+  else if (VECTOR_CST_LOG2_NPATTERNS (t1) == VECTOR_CST_LOG2_NPATTERNS (t2)
       && (VECTOR_CST_NELTS_PER_PATTERN (t1)
          == VECTOR_CST_NELTS_PER_PATTERN (t2)))
     {
@@ -11007,17 +11025,7 @@ uniform_vector_difference_p (const_tree t1, const_tree
t2)
          return NULL_TREE;
     }
   else
-    {
-      unsigned HOST_WIDE_INT nelts;
-      if (!VECTOR_CST_NELTS (t1).is_constant (&nelts))
-       return NULL_TREE;
-
-      for (unsigned HOST_WIDE_INT i = 0; i < nelts; ++i)
-       if (!record_uniform_integer_difference (vector_cst_elt (t1, i),
-                                               vector_cst_elt (t2, i),
-                                               &diff, &diff_p))
-         return NULL_TREE;
-    }
+    return NULL_TREE;

   if (!diff_p || !wi::fits_shwi_p (diff))
     return NULL_TREE;